- Email: [email protected]
HLA class I specific inhibitory receptors
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HLA class I specific inhibitory receptors Alessandro Moretta* and Lorenzo Morettat All human natural killer cells and some memory T cells express HLA class I receptors, so-called natural killer cell receptors (NKRs), a receptor class that in the past few years has been shown to include several members of the immunoglobulin superfamily and the C-type lectin CDg4-NKG2A complex. NKR ligand mediated cross-linking leads to the recruitment and activation of a tyrosine phosphatase involved in downregulating the phosphorylation of effector molecules involved in cell triggering. Thus, NKR engagement leads to the inhibition of different NK and T cell functions.
Addresses *Dipartimento di Scienze Biomediche e Biotecnologie, Universit& di Brescia e Istituto di Istologia ed Embriologia Generale, Universitt, degli Studi di Genova, Via Giovanni Battista, Marsano 10, 16132 Ganova, Italy tlstituto di Patologia Generale, Universit~ degli Studi di Genova e Istituto Sciantifico Tumori, Laboratorio di Immunopatologia, Centro Biotecnologie Avanzate, Largo go Rosanna, Benzi 1O, 16132 Ganova, Italy; e*mail: [email protected] Correspondence: Lorenzo Moretta Current Opinion in Immunology 1997, 9:694-701 http://biomednet.com/elecref/0952791500900694 © Current Biology Ltd ISSN 0952-7915 Abbreviations CTL cytolytic T lymphocyte HCMV humancytomegalovirus HSV herpessimplex virus ITIM immunoreceptortyrosine based inhibition motif mAb monoclonalantibody NK natural killer NKR naturalkiller cell receptor PTK protein tyrosine kinase SH Src homology SHP SH2 domain containing protein tyrosine phosphatase V variable
Introduction
Natural killer (NK) cells are thought to play an important role in the host defenses because they kill virally-infected or tumor cells but spare normal self cells [1]. Why NK cells do not kill indiscriminately has recently been elucidated at the molecular level. Thus, NK cells have been shown to express receptors that recognize MHC molecules expressed on normal cells [2]. As predicted by the 'missing self' hypothesis [3], the lack of expression of MHC molecules on target cells leads to the susceptibility of such cells to NK cell mediated lysis [3,4]. It soon became evident, however, that NK cells do not just express an 'universal receptor' capable of recognizing any MHC class I molecule, but rather they express several receptors discriminating among different allelic forms of MHC class I molecules [2,4-71. In humans, different
receptor types, specific for groups of HLA-C or HLA-B molecules were identified [2,5-7]. Molecular cloning of the genes encoding these receptors revealed new members of the Ig superfamily, each characterized by two or three Ig-like domains in their extracellular region [8,9]. Although only a relatively limited number of receptor molecules is presently known, as many as 29 natural killer cell receptor (NKR) sequences have been identified [10"]. Furthermore, whereas the known Ig-superfamily NKRs were found to recognize only a limited number of HLA class I alleles, it has become clear that HLA class I recognition represents a general mechanism which involves all NK cells [11]. Therefore, the search for the other 'missing receptors' has been the object of intensive investigation during the past year and the results are presented in this review. Other important progress that will be discussed in this review includes the following: the identification of the molecular mechanism involved in NKR signal transduction and the generation of the inhibitory signal; the phenotypic and functional characterization of T lymphocytes expressing NKRs in normal and cancer subjects; the analysis of viral strategies to elude NK recognition; and, finally, the cytokine involvement in the induction of NKR expression during NK cell maturation. Identification cell receptor
of a novel Ig-like natural
killer
As outlined above, direct evidence for HLA class I specificity existed only for a limited number of NKRs belonging to the Ig superfamily. These include the NKRs p58.1 and p58.2 [12] and their corresponding activating forms, p50.1 and p50.2 respectively [13,14°']. p58.1 (p50.1) has been shown to recognize one distinct group of HLA-C alleles and p58.2 (p50.2) to recognize another. Together these alleles encompass all the expressed alleles of the HLA-C locus. A third member of the p50 family of activating receptors (termed p50.3) has recently been identified at the molecular level [151. So far, however, no precise information exists regarding the HLA class I ligands recognized by this new receptor [15]. In addition, the p70 (NKB1) molecule, another Ig superfamily member, recognizes alleles belonging to the HLA-Bw4 supertypic specificity [16,17,18",19]. Although, the molecules encoded by the different NKR mRNA transcripts may display the same HLA allele specificity and monoclonal antibody (mAb) reactivity, the gap between the large number of identified transcripts of Ig superfamily NKRs and the number of identified receptors is still wide. In addition, for a large proportion of the HLA-B alleles and for all of the HLA-A alleles no corresponding NKR has been identified [11]. In the search
HLA class I specific inhibitory receptors Moretta and Moretta
for a novel NKR, Pende et ai. [20°] identified a clonally distributed molecule, termed p140, of either 70kDa (monomer) or 140kDa (homodimer), that functioned as a receptor for HLA-A3 and -All, but did not recognize HLA-A1, -A2 or -A24. Isolation of the corresponding eDNA for p140 revealed a three Ig domain structure on the basis of its predicted amino acid sequence. Dtihring etal. [21 °] using a different antibody to that used in [20"], but one which also reacted with the HLA-Bw4 specific p70 receptor, identified only the monomeric form of p140. As this receptor, unlike p58 and p70, has two cysteines (amino acid positions 302 and 336) in the extracellular region of the protein proximal to the transmembrane portion, it is possible that at least one of these residues may be involved in the formation of a disulphide bridge [20°]. Whether the dimeric form is necessary for binding HLA-A3 molecules and for providing an efficient signal to the cell remains to be defined. Table 1 summarizes the principal molecular features of humans NKRs. The
CD94-NKG2A
receptor
complex
Humans and mice appeared to use different types of receptors for MHC class I recognition. Thus, whereas the human NKRs all appeared to belong to the Ig superfamily, the murine receptors (members of the Ly49 molecular family) were found to be type II membrane proteins related to the C-type lectins [22]. It is noteworthy that both mouse and human NK cells express a family of C-type lectins which are encoded by a group of linked genes, the so called 'NK gene complex'. Could molecules encoded by the human NK gene complex also function as MHC-specific receptors? A possible candidate to fulfill this function was CD94 [23,24]. Thus, anti-CD94 monoclonal antibodies were found to specifically reconstitute the cytolytic activity of HLA-Bw6 specific, but not HLA-Bw4 specific, NK cells [25]. Although CD94 is expressed by most NK cells, its level of expression varies greatly
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among NK cells. Moreover, CD94 molecules were found to be heterogeneous in function, as both inhibitory and activating forms of CD94 receptors have been described [24]. T h e recent selection of mAbs specific for the 'inhibitory form' of CD94 molecules [26°°] allowed us to identify the actual molecular structure of the receptor and to better define its HLA class I specificity. Similar conclusions regarding the molecular structure of the inhibitory form of CD94 molecules were drawn in another study [27°°]. T h e novel mAbs were found to be specific, not for the CD94 molecule itself, but rather for the CD94-associated molecules typical of the 'inhibitory form' [26°°]. T h e CD94-associated molecules were identified as being the molecular product of the NKG2A eDNA [28°°]. Similar data were obtained by another laboratory using a different experimental approach [29°°]. A general agreemeent has now been reached on the fact that the CD94--NKG2A heterodimer functions as an HLA class I specific inhibitory receptor [26°°-29°°]. T h e HLA class i specificity of CD94-NKG2A is, however, broader than that of the Ig superfamily NKRs and it encompasses the HLA-Bw6, HLA-C and HLA-A alleles. This group of alleles has a closely related sequence at amino acid residues 77-83 in the etl helix, which differs from that of the HLA-Bw4 alleles [10°]. Taken together, these data indicate that human NK cells express lectin-like MHC class I specific receptors, characterized by a broader specificity than that of the NKRs of the Ig superfamily. Ig-like NKRs have probably adapted to HLA class I recognition more recently than the lectins, namely after the divergence of the rodents and primates, as Ig-NKRs have not yet been detected in mice. Moreover the ability of the Ig-like NKRs to precisely recognize polymorphic MHC class I determinants appears
Table 1 The human NK]Rs for HLA class I antigens. Reactive mAbs EB6, XA141 EB6, XA141 EB6, XA141 GL183, y249 GL183, y249 GL183, y249 PAX180, FES172 Dxg, Z27 DXg, Z27 Q66, Q241 Q66, Q241 3B1, XA185 Z199
Putative ligands* HLA-C HLA-C HLA-C HLA-C HLA-C HLA-C
'group 1' 'group 1' 'group 1' 'group 2' 'group 2' 'group 2' ? HLA-Bw4 HLA-Bw4 HLA-A3 and -A11 HLA-A3 and -A11 Various HLA class I molecules$
Function
ITIM~
eDNA
Protein structure
Molecular mass (kDa)
Protein denomination
Inhibitory Inhibitory Activating Inhibitory Inhibitory Activating Activating Inhibitory Inhibitory Inhibitory Inhibitory Inhibitory
+ + + + + + + + +
EB6-cl.42 EB6-cl.47.11 EB6-act I 183-cl.6 183-cl.43 183-act I cl.39 (KKA3) c1.11 (AMB11 ) cl.2 (NKB1) d.5 (AMC5) c1.1.1 (17.1 C) CD94-LL288 NKG2A
2 Ig domains 2 Ig domains 2 Ig domains 2 Ig domains 2 Ig domains 2 Ig domains 2 Ig domains 3 Ig domains 3 Ig domains 3 Ig domains 3 Ig domains Dimeric, C-type lectins$
58 58 50 58 58 50 50 70 70 140 140 30 43
)58.1 )58, I )50,1 )58.2 )58.2 )50.2 )50.3 )70 pTO (NKBI) p140 p140 CDg4 NKG2A
*HLA-C 'group 1' includes the HLA-Cw2, -Cw4, -Cw5, -Cw6 alleles and the HLA-C 'group 2' includes the HLA-Cwl, -Cw3,-Cw7 and -Cw8 alleles. tThe immunoreceptor tyrosine-based inhibitory motif (mM) is characterized by the amino acid sequence V/IXYXXL (in single-letter amino acid code, where X represents any amino acid) and is involved in the recruitment of SHPs responsible for the inhibition of NK cell triggering. SThe inhibitory receptor consists of the dimeric comolex CD94-NKG2A.
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to be a further adaptation of these NKRs keeping pace with the rapid evolution of the HLA class I genes [10"]. That CD94 may represent a more ancient form of HLA class I specific NKRs is also suggested by recent data by Mingari et al. [30"]. These authors showed that CD34÷CD7 ÷ postnatal thymic precursors undergo maturation towards mature NK cells in the presence of IL-15 [30"°]. More importantly, they showed that these cells expressed de novo CD94-NKG2A as the only detectable HLA class I specific inhibitory NKR. T h e acquisition of CD94-NKG2A paralleled the appearance of NK activity thus providing a powerful means to prevent lysis of autologous cells [30°']. Remarkably, no NKR members of the Ig superfamily could be induced by IL-15 [30°']. This may reflect the inability of the thymic precursors to express these NKRs or, rather, the requirement for a more complex signalling mechanism and/or different cytokines to induce their expression.
the I T I M s in the NKR intracytoplasmic tail, followed by the recruitment of SHP-1, and by SHP-1 dependent inhibition of crucial phosphorylation events involved in cell activation [31,37"]. A simplified model of the molecular mechanisms involved in NKR-mediated activation is shown in Figure 1. Figure I
Activating receptors 'On signal'
kill
H2
(a) Coaggregation Ts?hain Membrane
Mechanism of natural killer cell receptor mediated inhibition Given that ligand-mediated cross-linking of NKRs prevents cytolytic activity and cytokine production by NK cells (or cytolytic T lymphocytes [CTLs], see below), it was important to define the mechanism(s) leading to the inhibition of the cell activation pathways As cell activation involves a number of protein tyrosine kinases (PTKs), it appeared conceivable that these kinases could represent the molecular targets of NKR-mediated inhibition [31]. Several laboratories contributed to the identification of the inhibitory mechanisms. T h e presence of tyrosine based motifs, in the cytoplasmic tail of inhibitory NKRs (both those of the Ig superfamily and NKG2A), provided potential sites for interaction with Src homology (SH)2 domains. These motifs, termed 'immunoreceptor tyrosine based inhibition motifs' (ITIMs), are spaced, in different NKRs, by 26-29 amino acids [31]. It was found that NKR cross-linking leads to NKR tyrosine phosphorylation, and a similar effect was detected using phosphatase inhibitors [32"'-35"']. In addition, it has been shown that two tyrosines are required for NKR inhibitory activity in both T and NK cell activation. T h e protein involved in NKR-mediated inhibition was identified as being the hemopoietic SH2 domain containing protein tyrosine phosphatase (SHP)-I which is characterized by its two tandem SH2 domains [32"'-35"']. A direct association of SHP-1 with NKRs upon NKR phosphorylation has been documented. In this context, it has been shown that NKRs require co-engagement with an activatory receptor for their actions [36"]. This may reflect the need for NKRs to be phosphorylated by a nondiffusible P T K associated with the activating receptor [31]. Indeed the tyrosine phosphorylation of NKRs is strictly required for their recruitment of SHP-1 [34"',35"']. A model has been proposed in which NKR signalling involves an Lck-dependent tyrosine phosphorytation of
~
(b)
(f) ITAM [ ~
ITIM ~ Current Opinion in Immunology
Model of NKR-mediated inhibition of NK cell activation. The white blocks indicate elements that are part of the NKR triggering (activating) pathway, whereas the shaded blocks are part of the inhibitory pathway. (a) Coaggregation of an activating (CD16) and an inhibitory (p58) NKR, induced by either ligand or mAb, leads to (b) the activation of the Src-PTK associated with CD16 which then phosphorylates both (¢) the ITAMs of the y chains of CD16 and (d) the ITIMs of p58. This leads to (e) the recruitment and activation of SH2-containing P'I'Ks, such as ZAP-70 and p72-Syk, to the ITAMs, and of SHP-1 and -2 to the raMs. (f) The SHPs dephosphorylate the adaptor/effector molecules ZAP-70 and p72-Syk, which are part of the ITAM-associated cascade, thereby inhibiting activation. The scheme is simplified as it shows only one triggering and one inhibitory receptor. In addition, only ZAP-70 and p72-Syk are shown of the several adaptor/effector molecules which may be the molecular targets of the SHPs. Although not shown, a similar phenomenon occurs in T lymphocytes triggered via TCRs or other triggering receptors, and inhibitory receptors. Notably, the coaggregation of activating and inhibitory receptors may facilitate the interaction between phosphorylated molecules involved in the activating pathway and the ITIM-bound SHPs. This molecular model and, in particular, the need for co-aggregation of activating and inhibitory receptors explains why NKR-mediated NK cell inhibition is 'polarized', that is, limited to only those cell compartments bearing NKR ligands [2]. S-S, disulfide bond; C2, Ig-like constant domain; P, phosphorylation.
HLA-G recognition by natural killer cells T h e lack of classical HLA molecules in the human placenta prevents allorecognition and lysis of the placenta by maternal T lymphocytes but it poses the problem
HLA class I specificinhibitoryreceptors Morettaand Moretta 697
of susceptibility to NK cell mediated lysis [38,39]. T h e nonclassical and relatively nonpolymorphic HLA class I molecule HLA-G may mediate protection from NK cells [40,41]. HLA-G molecules are primarily expressed on the trophoblast and could thus play a key role in protecting the placenta from NK cell mediated damage. A possible candidate NKR required for this protective function was the CD94-NKG2A receptor, in view of its broad specificity for different HLA class I molecules and its expression on relatively large proportions of NK cells. Studies using NK cell clones, selected on the basis of the expression of appropriate NKRs, showed that CD94-NKG2A, but not other known NKRs, including p58.1, p58.2, p70 or p140, mediated HLA-G recognition. These data have been reported independently by two groups [42",43°]. Pazmany et al. [44"'], however, reported that both p58.1 and p58.2 can mediate HLA-G recognition. In addition, according to Munz et al. [45"], HLA-G recognition would be mediated by the pT0 receptor. At present, it is difficult to reconcile these conflicting results. It should be stressed that whatever the correct explanation may be, none of these reports [42°-45 °] provides a satisfactory explanation for how trophoblast cells which do not express the-putative HLA-G specific NKR, escape attack by NK cells (remarkably, these cells always represent a sizable fraction of total NK cells). One may speculate that, during pregnancy, there may be de novo expression by all NK cells of either the CD94-NKG2A complex or of other still undefined HLA-G specific NKRs. In this context, evidence has been provided for the existence of another HLA-G specific inhibitory NKR [43°], however, no information is available on its molecular nature. Additional work focused on the phenotypic and functional analysis of NK cells in pregnant women is clearly needed.
Viral strategies to elude natural killer cell recognition Cytolytic CD8 + T lymphocytes are known to play a major role in the control of viral infections. Although the lysis of infected cells does not necessarily represent the primary mechanism of defense, which may rather involve the production of cytokines such as IFN-y [46], the crucial defense event is the CTL-mediated recognition of viral peptides in an MHC class I restricted fashion. Therefore, it is not surprising that different viruses have evolved strategies to interfere with antigen processing, peptide presentation, and MHC class I expression [47]. It is important to note, however, that although the downregulation of MHC class I expression will prevent the CTL-mediated lysis of infected cells, it should also render them susceptible to NK cell mediated attack. Viruses, such as adenoviruses and herpesviruses, including herpes simplex virus (HSV) and human cytomegalovirus (HCMV), have evolved different strategies to interfere with MHC class I recognition [47]. Of particular interest is HCMV which synthesizes a number of proteins
('stealth' proteins of the US [unique short] family of early genes) which inhibit, by different mechanisms, the surface expression of HLA class I molecules [47]. It is interesting that NK cells play an important role in controlling both HSV and HMCV infections [48]; however, a potential mechanism to escape the NK cell recognition may be the expression of UL18, a 132-microglobulin binding protein with a structural homology to MHC class I molecules [49]. UL18 has been shown to function by protecting HCMV-infected cells against NK cell lysis [50"°,51°']. HLA class I negative cells transfected with UL18 became resistant to NK cell mediated lysis [50"']. T h e CD94--NKG2A receptor complex is apparently involved in this recognition. Therefore, HCMV has evolved two complementary strategies, interference with MHC class I presentation and MHC class I imitation, to avoid identification in infected cells by CD8 ÷ C T L s and NK cells respectively. The fact that HCMV has developed a mechanism for evading NK cell mediated detection suggests that indeed NK cells represent an important component of the defense against viral infections.
T lymphocytes expressing natural killer cell receptors At the time of the discovery of p58 molecules in NK cells [5], it was also reported that a small subset of peripheral T lymphocytes expressed NKRs [5,11]. It was only after the formal demonstration that p58 molecules functioned as inhibitory receptors [12], however, that the obvious question of whether NKRs could also inhibit T cell function was posed. The finding, by different groups, that NKRs can sharply inhibit TCR-induced T cell functions, including cytolytic activity [52-54] and cytokine production [53,55"], encouraged further studies on NKR ÷ T lymphocytes. Analysis of NKR ÷ T lymphocytes in healthy donors revealed that essentially all NKRs, whether belonging to the Ig superfamily or to a C-type lectin family could be expressed by T cells [56"]. Moreover, analysis of polymerase chain reaction amplified full length cDNAs o f T lymphocyte NKRs showed sequences that are identical to those of the corresponding NKRs expressed by NK cells [56°]. Analysis of the tissue distribution of NKR ÷ T lymphocytes indicated that they were present in peripheral lymphoid tissues, but were absent in the thymus and in umbilical cord blood [56"]. Consistent with this distribution, NKR ÷ T lymphocytes were found to express a memory phenotype and a skewed T C R variable (V)I3 repertoire [57°']. Moreover, at least in healthy donors, the represented V[3 segments were found to be oligoclonally or monoclonally expanded [57"']. T h e expression of NKRs which inhibit T cell function, poses relevant questions as to the meaning of this event. As NKR ÷ T lymphocytes are represented by CD8 ÷ CTLs in most donors, the expression of NKRs may represent a failsafe mechanism to avoid autoimmunity, particularly for those CTLs which have acquired NK like activity and could thus lyse normal cells independently of T C R
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specificity [53]. On the other hand, it is clear that the expression of NKRs, which work across the main function (to kill cells that present MHC class I associated foreign peptides) of CTLs, could be harmful to the host in chronic viral infections or in tumors. Indeed, a tumor specific C T L clone isolated from a melanoma patient was found to express p58.2 molecules [58°°]. This clone did not lyse the original autologous tumor (expressing the complete set of HLA class I alleles), but it lysed a loss variant which did not express several alleles, including HLA-Cw7, a iigand for the p58.2 receptor [58°°]. In addition, NKR expression in virus-specific C T L s was found in HIV-infected patients and mAb-mediated masking of these NKRs restored the lysis of target cells expressing HIV antigens [59°']. Similarly to inhibitory NKRs, activating forms of NKRs have also been detected on T lymphocytes. Activating p50 molecules were previously shown to display the same HLA-C specificity as the corresponding inhibitory p58 molecule [13,14"]. Moreover, molecular cloning revealed a high degree of homology in the extracellular domain of the corresponding p58 and p50 molecules, but there were found to be important differences in the transmembrane and intracytoplasmic domains [14°']. Activating p50 molecules have been shown to act as co-stimulatory molecules able to enhance T cell responses; p50 + T lymphocytes were found to respond to a lower dose of superantigen, depending on the expression of the appropriate (for p50 recognition) HLA-C ligand on target cells [60°].
lymphocytes, in addition to NK cells, it will be important to analyze further its ability to prevent graft rejection. It is evident that these studies, together with the identification of cytokines able to modulate the surface expression of inhibitory NKR may lead to the development of novel strategies to prevent allograft rejection or graft versus host disease.
Acknowledgements We wish to thank our colleagues R Biassoni, MC IS,ligari and C Bottino for discussion and for freely sharing their ideas with us. We are grateful to C Miriello for the secretarial assistance. This work was supported by grants awarded by the Associazione Italiana per la Ricerca sul Cancro (AIRC), lstituto Superiore di SanitY, Consiglio Nazionale delle Ricerche, Progetto Finalizzato Applicazioni Cliniche della Ricerca Oncologica and Ministero delrUniversit/t e della Ricerca Scientifica e Tecnologica (MURST) to L Moretta and grants awarded by MURST and AIRC to A Moretta
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: = =. 1.
Trinchieri G: Biology of natural killer cells. Adv Immunol 1989, 47:187-376.
2.
Moretta L, Ciccone E, Mingari Me, Biassoni R, Moretta A: Human HK cells: origin, cionality, specificity and receptors. Adv Immuno11994, 55:341-380.
3.
Ljunggren H-G, Karre K: In search of the 'missing self': MHC molecules and NK cell recognition. Imrnunol Today 1990, 11:237-244.
4.
Moretta L, Ciccone E, Moretta A, H6glund P, Ohlen C, K~rre K: AIIorecegnition by NK cells: nonself or no self? Immuno/Today 1992, 13:300-306.
Condusions There is no doubt that in the past few years we have witnessed major progress towards the understanding of the molecular mechanisms involved in NK cell function. At the present, however, we know more about NK cell inactivation than we do about their activation. Although HLA class I specific activating receptors may play a role in NK cell triggering by MHC class I positive target cells (see [13,14"]), other receptors must play a predominant role against target cells which lack MHC class I molecules. A recent study by Sivori et al. [61 °°] describes a novel triggering surface molecule (p46) which, unlike all other NK cell markers, is strictly NK cell specific being expressed on 100% of resting or activated NK cells but not on other cell types. The molecular characterization of p46 and other molecules involved in NK cell triggering as well as of their ligands will be a major goal of future investigation. Another important aspect will be to define the in vivo relevance of the inhibitory effect involving human NKRs. In this context, important information has been provided by recent studies in transgenic mice [62°°]. Thus, the transgenic expression of the p58.2 NKR prevented the rejection of transgenic, H-2 mismatched, bone marrow grafts which express the HLA-Cw3 allele (recognized by p58.2). This study demonstrates, for the first time, an in vivo effect of human inhibitory NKR expression [62°']. As the NKR transgene is expressed in T
of special interest of outstanding interest
5.
Moretta A, Bottino C, Pande D, Tripodi G, Tambuasi G, Viale O, Orengo A, Barbaresi M, Merli A, Ciccone E, Moretta L: Identification of four subsets of human CD3-CD16+ NK cells by the expression of clonally distributed functional surface molecules. Correlation between subset assignment of NK clones end ability to mediate specific elloantigen recognition. J Exp Med 1990, 172:1589-1598.
6.
Ciccone E, Pende D, Vials O, Di Donato C, Tripodi G, Orengo AM, Guardiola J, Moretta A, Moretta L: Evidence of a natural killer (NIO cell repertoire for (silo)antigen recognition: definition of five distinct NK-determined allospecificities in humans. J Exp Med 1992, 175:709-718.
7.
Litwin V, Gumperz J, Parham P, Phillips JH, Lanier LL: Specificity of HLA class I antigen recognition by human NK clones: evidence for cionel heterogeneity, protection by self end nonself alleles, and influence of the target cell type, J Exp Mad 1993, 178:1321-1336.
8.
s. 10. •
Wagtmann N, Biassoni R, Cantoni C, Verdiani S, Malnati MS, Vitale M, Bottino C, Moretta L, Moretta A, Long EO: Molecular clones of the p58 NK cell receptor reveal immunoglobulin related molecules with diversity in both the extra- end intracellular domains. Immunity 1995, 2:439-449. Golonna M, Samaridis J: Cloning of immunoglobulin-superfamily members associated with HLA-C and HLA-B recognition by human natural killer cells. Science 1995, 268:405-408.
Valiante NM, Uenert K, Shilling HG, Smits BJ, Parham P: Killer cell receptors: keeping pace with MHC class I evolution. /mmunol Rev 1997, 155:155-164. An interesting analysis of the co-evolution of the Ig superfamily natural killer cell receptors (NKRs) and the MHC class I epitopes they recognize. NKRs are of relatively recent origin in primate evolution.
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11.
Moretta A, Bottino C, Vitale M, Pende D, Biassoni R, Mingari MC, Moretta L: Receptors for HI.A-class I-molecules in human Natural Killer cells. Annu Ray Immuno11996, 14:619-648.
22.
Yokoyama WM, Seaman WE: The Ly49 and NKR-P1 gene families encoding lectin-like receptors on natural killer cells: the NK gene complex. Annu Rev Immunol 1993, 11:613-635.
12.
Moretta A, Vital• M, Bottino C, Orengo AM, Morelli L, Augugliaro R, Barbaresi M, Ciccone E, Moretta L: I)58 molecules as putaUve receptors for MHC class I molecules in human Natural Killer (NK) cells. Anti-p58 antibodies reconstitute lysis of MHC class I-protected cells in NK clones displaying different speclficitles. J Exp Med 1993, 178:597-604.
23.
Aramburu J, Balboa MA, Ramirez A, Silva A, Acevedo A, SanchezMadrid F, de Land~zuri MO, Lbpez-Botet M: A novel functional cell surface dlmer (K1>43) expressed by natural killer cells and T cell receptor-y/6-1- T lymphocytes. Inhibition of IL-2 dependent proliferation by enU-Kp43 monodonal antibody. J Immuno11990, 44:3238-3247.
13.
Moretta A, Sivori S, Vitale M, Pende D, Morelli L, Augugliaro R, Bottino C, Moretta L: Existence of both inhibitory (p58) and activatory (I)50) receptors for HLA-C molecules in human Natural Killer cells. J Exp Med 1995, 182:875-884.
24.
Perez-Vitlar JJ, Melero I, Rodr;guez A, Carretaro M, Aramburu J, Sivori S, Orengo AM, Moretta A, Lbpez-Botet M: Functional ambivalence of the Kp43 (CD94) NK cell-associated surface antigen. J Immuno11995, 154:5779-5788.
25.
Moretta A, Vitale M, Sivod S, Bottino C, Mar•Ill L, Augugliaro R, Barbaresi M, Pende D, Ciccone E, Lbpez-Botet M, Moretta L: Human Natural Killer cell receptors for HLA-class I molecules. Evidence that the Kp43 (CD94) molecule functions as a receptor for HLA-B alleles. J Exp Med 1994, 180:545-555.
14. =e
Biassoni R, Cantoni C, Falco M, Verdiani S, Bottino C, Vitale M, Conte R, Poggi A, Moretta A, Moretta L: The HLA-C-specific 'activatory' or 'inhibitory' Natural Killer cell receptors display highly homologous extracellular domains but differ in their transmembrane and intrecytoplasmic portions. J Exp Mad 1996, 183:645-650. A follow up to the previous discovery [13] of the activating forms of HLAC-specific natural killer cell receptors (NKRs). The molecular cloning of the genes encoding the activating receptors shows an amino acid sequence identity of the activating receptors with the inhibitory receptors in the extracellular domains, but there are important differences in the transmembrane and intracytoplasmic regions. 15.
Bottino C, Sivori S, Vitale M, Cantoni C, Falco M, Panda D, Morelli L, Augugliaro R, Semenzato GP, Biassoni Ret al.: A novel surface molecule homologous to the p58/p50 family of receptors is selectively expressed on a subset of human Natural Killer cells and induces both tdggedng of cell functions and proliferation. Eur J Immunol 1996, 26:1816-1824.
16.
Litwin V, Gumperz JE, Parham P, Phillips JH, Lanier LL: NKB1 : • Natural Killer cell receptor involved in the recognition of polymorphic HLA-B molecules. J Exp Med 1994, 180:537-543.
17.
Lanier LL, Gumperz JE, Parham P, Melero I, Lbpez-Botet M, Phillips JH: The NKB1 and HP-3E4 NK cell receptors are structurally distinct glycoproteins and independently recognize polymorphic HLA-B and HLA-C molecules. J Immunol 1995, 154:3320-3327.
18. •
Vitale M, Sivori S, Pende D, Augugliaro R, Di Donato C, Amoroso A, Malnati M, Bottino C, Moretta L, Moretta A: Physical and functional independency of p70 and p58 NK cell receptors for HI.A-class I. Their role in the definition of different groups of alloreimtive NK cell clones. Proc Natl Acad Sci USA 1996, 93:1453-1457. The HLA-Bw4 specific natural killer cell receptor (NKR) pTO corresponds to the previously descdbed NKB1 molecule [15]. The authors of this paper show that when p70 is co-expressed with another inhibitory NKR, p58, both receptors function independently. Furthermore, expression of pT0 either alone or in combination with other NKRs is shown to determine the ability of the NKR-bearing cell to lyse allogeneic cells. This study reaches similar conclusions as those in [17]. 19.
Gumperz JE, Litwin V, Phillips JH, Lanier LL, Parham R The Bw4 public epitope of HLA-B molecules confers reactivity with Natural Killer cell clones that express NKB1, a putaUve HLA receptor. J Exp Med 1995, 181:1133-1144.
20. •
Pende D, Biassoni R, Cantoni C, Vardiani S, Falco M, Di Donato C, Accame L, Bottino C, Moretta A, Moretta L. The natural killer cell receptor specific for HLA-A ellotypes: • novel member of the p58/pTO family of inhibitory receptors which is charactodzed by three Igolike domains and is expressed as a 140 kD disulphide-linked dimer. J Exp Mad 1996, 184:505-518. A novel three Ig-like dorr~n natural killer cell receptor (NKR) is described that is specific for a defined group of HLA-A alleles. Unlike pTO, this NKR can be cell-surface expressed as both a monomer and homodimer. 21. •
26. ••
Sivori S, Vital• M, Bottino C, Marcenaro E, Sansevsrino L, Parolini S, Moretta L, Moretta A: CD94 functions as a natural killer cell inhibitory receptor for different HLA-dase-I alleles. IdenUficaUon of the inhibitory form of CD94 by the use of novel monodonal antibodies. Eur J Immunol 1996, 26:24872492. The authors demonstrate that the CD94 natural killer cell receptor (NKR) has a broad specificity for different HLA class I alleles and they cleerly distinguish the inhibitory form from the activating form by the aid of mAbs specific for a molecule associated with the inhibitory form. 27. •,
Phillips JH, Chang C, Mattson J, Gumperz JE, Parham P, Lanier LL: CD94 and a novel associated protein (94AP) form of • NK cell receptor involved in the recogniUon of HLA-A, HLA-B, and HLA-C allotypes. Immunity 1996, 5:163-172. Using a slightly different approach to that of [26"°], the authors reach a similar conclusion, namely that the CDg4 natural killer cell receptor (NKR) has a broad specificity for different HLA class I alleles. Again, the inhibitory form of CD94 is clearly distinguished from the activating form. 28. •-
Carretero M, Cantoni C, Belln T, Bottino C, Biassoni R, Rodriguez A, Perez-Villar JJ, Moretta L, Moretta A, Lbpez-Botet M: The CD94 and NKG2-A C-type lecUns covalenUy assemble to form a NK cell inhibitory receptor for HLA class I molecules. Eur J Immunol 1997, 27:563-567. Thanks to mAbs specific for the inhibitory form of the CD94 natural killer cell receptor (NKR) [26"], this study shows that the 'inhibitory form' is actually composed of CD94 associated with NKG2A. Although the corresponding cDNA of NKG2A, a type II molecule, has bQen known for several years, its molecular product and function had remained elusive. See also [29°°]. 29. **
Lazetic S, Chang C, Houchins JP, Lanier LL, Philips JH: Human natural killer cell receptors involved in MHC class I recognition are disulphide-linked heterodimers of CD94 and NKG2 subunits. J Immuno/1996, 157:4741-4745. The authors reach the same conclusion as that of [28"'], that is, that CD94 and NKG2A assemble to form heterodimers that are an actual HLA class I specific inhibitory receptor. 30.
Mingari MC, Vital• C, Cantoni C, Bellomo R, Pont• M, Schiavetti F, Bertone S, Moretta A, Moretta L: Interleukin-15-induced maturaUon of human Natural Killer cells from early thymic precursors. Selective expression of CD94/NKG2A as the only HI.A-class I specific inhibitory receptor. Eur J Immunol 1997, 27:1374-1380. In an attempt to identify the mechanisms involved in the de nova expression of inhibitory natural killer cell receptors (NKRs) by CD7 + CD34 + precursors undergoing maturation towards mature NK cells, the authors show that IL* 15 is capable of inducing the expression of CD94-NKG2A in parallel with the acquisition of cytolytic function. This mechanism prevents the lysis of autologous cells. •e
31.
Rbnard V, Cambiaggi A, Vely F, Blery M, Olcese L, Olivero S, Bouchet M, Vivier E: Trensduction of cytotoxic signals in Natural Killer cells: a general model of fine tuning between ectivatory end inhibitory pathways in lymphocytes. Immunol Ray 1997, 155:205-221.
32. ,o
Olcese L, Lang P, V61y F, Cambiaggi A, Marguet D, Blery M, HiDDenKL. Biassoni R. Moretta A. Moretta L e t al.: Human and
D6hring C, Scheidegger D, Samaridis J, Celia M, Colonna M: A human killer inhibitory receptor specific for HLA-A. J Immunol 1996, 156:3098-3101.
Describes the same HLA-A specific natural killer cell receptor (NKR) as that of [20"]. The authors in this paper describe only the monomeric form of the receptor.
700
Cancer
mouse Natural Killer cell inhibitory receptors recruit the PTPIC and PTP1 D protein tyrosine phosphatases. J Immuno/1996, 156:4531-4534.
This study allows us to explain the molecular mechanism by which the crosslinking of inhibitory natural killer cell receptors (NKRs) leads to the downregulation of NKR+ NK cell or T lymphocyte cell killing activity. The mechanism involves the recruitment of the Src homology 2 domain containing protein tyrosine phosphatase (SHP)-I hemopoietic phosphstase to the NKR immunoreceptor tyrosine based inhibition motifs. See also [ 3 3 " - 3 5 " ] . 33. •o
Burshtyn DN, Scharenberg AM, Wagtmann N, Rajagopalan S, Berrada K, Yi T, Kinet JP, Long EO. Recruitment of tyrosine phosphatase HCP by the killer cell inhibitory receptor. Immunity 1996, 4:77-85. This study allows us to explain the molecular mechanism by which the crosslinking of inhibitory natural killer cell receptors (NKRs) leads to the downregulation of NKR + NK cell or T lymphocyte cell killing activity. The mechanism involves the recruitment of the Src homology 2 domain containing protein tyrosine phosphatase (SHP)-I hemopoietic phosphatase to the NKR immunoreceptor tyrosine based inhibition motifs. See also [32°',34°°,35"°]. 34. ••
Fry AM, Lanier LL, Weiss A. Phosphotyrosines in the killer cell inhibitory receptor motif of NKB1 are required for negative signalling and for association with protein tyrosine phosphatase 1C. J Exp Med 1996, 184:295-300. This study allows us to explain the molecular mechanism by which the crosslinking of inhibitory natural killer cell receptors (NKRs) leads to the downregulation of NKR + NK cell or T lymphocyte cell killing activity. The mechanism involves the recruitment of the Src homology 2 domain containing protein tyrosine phosphatase (SHP)-I hemopoietic phosphatase to the NKR immunoreceptor tyrosine based inhibition motifs. See also [32",33"',35"']. 35. •=
Campbell KS, Dessing M, Lbpez-Botet, M, Celia M, Colonna M: Tyrosine phosphorylation of a human killer inhibitory receptor recruits protein phosphatase 1C. J Exp Med 1996, 184:93-100. This study allows us to explain the molecular mechanism by which the crosslinking of inhibitory natural killer cell receptors (NKRs) leads to the downregulation of NKR + NK cell or T lymphocyte cell killing activity. The mechanism involves the recruitment of the Src homology 2 domain containing protein tyrosine phosphatase (SHP)-I hemopoietic phosphatase to the NKR immunoreceptor tyrosine based inhibition motifs. See also [32"°-34"']. 36. •
Blery M, Delon J, Trautmann A, Cambiaggi A, Olcese L, Biassoni R, Moretta L, Chavrier P, Moretta A, Daron M, Vivier E: Reconstituted killer-cell inhibitory receptors for MHC class I molecules control mast cell activation induced via immunoroceptor tyrosine-based activation motifs. J Biol Chem 1997, 272:8989-8996. The authors show that inhibitory natural killer cell receptors (NKRs) need to be cross-linked with activating receptors involved in the activation of kinases. A likely explanation for this requirement is that Tyr phospho~lation of ITIMs in the cytoplasmic tail of NKRs is required for the recruitment of Src homology 2 domain containing protein tyrosine phosphatases. 37. •
Binstadt BA, Brumbaugh KM, Leibson PJ: Signal transduction by human NK cell MHC-recognizing receptors. Immuno/Rev 1997, 155:197-203. An interesting review that also contains new data and a model for natural killer cell receptor (NKR) signalling. 38.
King A, Boocock C, Sharkey AM, Gardner L, Beretta A, Siccardi AG, Loke YW: Evidence for the expression of HLA-C class I mRNA and protein by human first trimester trophoblast. J Immuno/1996, 156:2068-2076.
39.
Parham P: Keeping mother at bay. Curt Biol 1996, 6:638-641.
40.
Le Bouteiller P, Lenfant E Antigen-presenting function(s) of the non-classical HLA-E, -F and -G class I molecules: the beginning of • story. Res Immunol 1996, 147:301-313.
41.
Geraghty DE. Structure of the HLA class I region and expression of its resident genes. Curr Opin Immunol 1993, 5:3-7.
42.
Perez-Villar JJ, Melero I, Navarro F, Carretero M, Bellon T, Llano M, Colonna M, Geraghty DE, Lbpez-Botet M: The CD94/NKG2A inhibitory receptor complex is involved in the Natural Killer cell-mediated recognition of cells expressing HLA-G1. J /mmuno/1997, in press.
According to these authors, the recognition of HLA-G molecules is mediated by the CD94-NKG2A complex and not by other known natural killer cell receptors (NKRs). See also [43°-45°]. 43. •
Pende D, Sivori S, Accame L, Pareti L, Falco M, Geraghty D, Le Bouteiller P, Moretta L, Moretta A: HLA-G recognition by human natural killer ceils. Involvement of CD94 both as inhibitory and as activating receptor complex. Eur J Immunol 1997, 27:1875-1880. As in [42"], recognition of HLA-G molecules is shown to be mediated by the CD94-NKG2A coomplex. In addition, the authors show that the activating form of the CD94 receptor can recognize HLA-G. See also [44",45"]. 44.
Pazmany L, Mandelboim O, Val6s-Gbmez M, Davis DM, Reyburn HT, Strominger JL: Protection from Natural killer cellsmediated lysis by HLA-G expression on target cells. Science 1996, 274:792-795. The authors identify the p58 molecules p58.1 and p58.2 as the putative HLA-G-specific receptors. See also [42",43",45"]. •
45.
Munz C, Holmes N, King A, Loke WY, Colonna M, Schild H, Rammensee HG: Human histocompatibility leukocyte antigen (HLA-G) molecules inhibit NKAT3 expressing natural killer cells. J Exp Mad 1997, 185:385-391. The authors suggest a role for the HLA-Bw4 specific p70 natural killer cell receptor (NKR) in the recognition of HLA-G. See also [42"-44"]. •
46.
Kagi D, Hengartner H: Different roles for cytotoxic T cells in the control of infections with cytopathic versus noncytopathic viruses. Curr Opin Immuno11996, 8:472-477.
47.
Fr(ih A, Ahn K, Paterson PA: Inhibition of MHC class I antigen presentation by viral proteins. J Mol Mad 1997, 75:18-27.
48.
Biron CA, Byron KS, Sullivan JL Severe herpes virus infections in an adolescent without natural killer cells. New Engl J Med 1989, 320:1731-1735.
49.
Beck S, Barrell BG. Human cytomegalovirus encodes a glycoprotein homologous to MHC class I-antigens. Nature 1988, 331:269-272.
50. •-
Reybum HT, Mandelboim O, Val~s-Gomez M, Davis DM, Pazmani L, Strominger JL: The class I MHC homologue of human cytomegalovirus inhibits attack by natural killer cells. Nature 1997, 386:514-516. The authors show that an MHC class I homologous, HCMV-encoded, protein protects HCMV-infected cells from NK cell mediated lysis. 51. ••
Farrell HE, Hally H, Lynch DM, Reming P, She,am GR, Scalzo hA, Davis-Poynter NJ. Inhibition of natural killer cells by a cytomegalovirus MHC class I homologoue in vivo. Nature 1997, 386:510-514. The authors show that an MHC class I homologous, HCMV-ancoded, protein protects HCMV-infected cells from NK cell mediated lysis. 52.
Ferrini S, Cambiaggi A, Meazza R, Sforzini S, Marciano S, Mingari MC, Moretta L: T cell clones expressing the NK-related p58 receptor molecule display heterogeneity in phenotypic properties and p58 function. Eur J Immuno11994, 24:22942298.
53.
Mingari MC, Vitale C, Cambiaggi A, Schiavetti F, Melioli G, Ferrini S, Poggi A: Cytolytic T lymphocytes displaying natural killer (NK)-Iike activity: expression of NK-relatad functional receptors for HLA class I molecules (p58 and CD94) and inhibitory effect on the TCR-mediated target cell lysis or lymphokine production. Int Immunol 1995, 4:697-703.
54.
Phillips JH, Gumperz JE, Parham P, Lanier LL: Superantigendependent, cell-mediated cytotoxicity inhibited by MHC class I receptor on T lymphocytes. Science 1995, 128:403-405.
55. •
D'Andrea A, Chang C, Phillips JH, Lanier LL: Regulation of T cell lymphokine production by killer cell inhibitory receptor recognition of self HLA class I alleles. J Exp Med 1996, 184:789-794. This study confirms and extends the previous data by Mingari et el. showing that cross-linking of natural killer cell receptors (NKRs) leads to inhibition of lymphokine production by CD8 + NKR+ cells [53].
HE& class I specific inhibitory receptors Moretta and Moretta
56. •
Mingari MC, Pont• M, Cantoni C, Vitale C, Schiavetti F, Bertone S, Bellomo R, Tradori Cappai A, Biassoni R: HI.A-class I-specific inhibitory receptors in human cytolytic T lymphocytes: molecular characterization, distribution in lymphoid tissues and co-expression by individual T cells. Int Immunol 1997, 9:485-491. This paper shows that natural killer cell receptor (NKR) + T lymphocytes are present in tonsils, lymph nodes whereas they are absent in thymus and umbilical cord blood. In addition, polymerase chain reaction amplified T lymo phocyte NKR cDNAs are shown to be identical to those of NK cells. 57. •,
Mingari MC, Schiavetti F, Pont• M, Vital• M, Maggi E, Romagnani S, Demarest J, Pantaleo G, Fauci AS, Moretta L: Human CD8 + T lymphocyte subsets that express HLA-class I specific inhibitory receptors represent oligoclonally or monoclonally expanded cell populations. Proc Nat/Acad Sci USA 1996, 93:12433-12438. In this study the authors show that natural killer cell receptor (NKR) + T cells isolated from healthy donors display a memory phenotype and are oligoclonal or monoclonal in nature. 58. =.
Ikeda H, Lath6 B, Lehmann F, Van Baren N, Baurain JF, De Smet C, Chambost H, Vitaie M, Moretta A, Boon T, Coulie PG: Characterization of an antigen that is recognized on a melanoma showing partial HLA loss by CTL expressing an NK inhibitory receptor./mmunity 1997, 6:199-208. This study demonstrates that tumor antigen specific CTLs may express inhibitory natural killer cell receptors (NKRs) and that this may lead to impaired antitumor responses. 59. ••
De Maria A, Ferraris A, Guastella M, Pilia S, Cantoni C, Polero L, Mingari MC, Bassetti D, Fauci AS, Moretta L: Expression of
701
HI.A-class I-specific inhibitory Natural Killer cell receptors in HIV-specific cytolytic T lymphocytes. Impairment of specific cytolytic functions. Proc Natl Acad Sci USA 1997, in press. This study shows that different inhibitory natural killer cell receptor (NKRs) are highly expressed on CD8 + T cells from HIV patients. In addition, the authors show that these NKRs are expressed on HIV-specific CTLs and prevent lysis of autologous HIV antigen expressing target cells. 60.
Mandelboim O, Davis DM, Reybum HT, Vales-Gomez M, Sheu EG, Pazmany L, Strominger JL: Enhancement of class II-restdcted T cell responses by costimulatory NK receptors for d a n I MHC proteins. Science 1996, 274:2097-2100. The activating p50 molecules, when expressed in T lymphocytes, may allow responses to lower doses of superantigens, provided that target cells express the appropriate HLA-C ligand. •
61. ••
Sivori S, Vital• M, Morelli L, Sanasevarino L, Augugliaro R, Bottino C, Moretta L, Moretta A: p46, a novel Natural Killer cellspecific surface molecule which mediates cell activation. J Exp Mad 1997, in press. This study describes p46, a novel surface molecule which is strictly NKspecific. More importantly, p46 induces potent NK cell triggering and may be involved in the process of HLA class I independent NK cell triggering. 62. ••
Cambiaggi A, Verthuy C, Naquet P, Romagn6 F, Farrier P, Biassoni R, Moretta A, Moretta L, Vivier E: NK-cell acceptance of H-2 mismatch bone-marrow grafts in transgenic mice expressing HLA-Cw3 specific killer-cell inhibitory receptor (CD158b). Proc Natl Acad Sci USA 1997, 94:8088-8092. This study provides the first evidence for an in vivo role for human natural killer cell receptors (NKRs) in the immune response against mistmatched bone marrow grafts in a transgenic mice model.
HLA class I specific inhibitory receptors Alessandro Moretta* and Lorenzo Morettat All human natural killer cells and some memory T cells express HLA class I receptors, so-called natural killer cell receptors (NKRs), a receptor class that in the past few years has been shown to include several members of the immunoglobulin superfamily and the C-type lectin CDg4-NKG2A complex. NKR ligand mediated cross-linking leads to the recruitment and activation of a tyrosine phosphatase involved in downregulating the phosphorylation of effector molecules involved in cell triggering. Thus, NKR engagement leads to the inhibition of different NK and T cell functions.
Addresses *Dipartimento di Scienze Biomediche e Biotecnologie, Universit& di Brescia e Istituto di Istologia ed Embriologia Generale, Universitt, degli Studi di Genova, Via Giovanni Battista, Marsano 10, 16132 Ganova, Italy tlstituto di Patologia Generale, Universit~ degli Studi di Genova e Istituto Sciantifico Tumori, Laboratorio di Immunopatologia, Centro Biotecnologie Avanzate, Largo go Rosanna, Benzi 1O, 16132 Ganova, Italy; e*mail: [email protected] Correspondence: Lorenzo Moretta Current Opinion in Immunology 1997, 9:694-701 http://biomednet.com/elecref/0952791500900694 © Current Biology Ltd ISSN 0952-7915 Abbreviations CTL cytolytic T lymphocyte HCMV humancytomegalovirus HSV herpessimplex virus ITIM immunoreceptortyrosine based inhibition motif mAb monoclonalantibody NK natural killer NKR naturalkiller cell receptor PTK protein tyrosine kinase SH Src homology SHP SH2 domain containing protein tyrosine phosphatase V variable
Introduction
Natural killer (NK) cells are thought to play an important role in the host defenses because they kill virally-infected or tumor cells but spare normal self cells [1]. Why NK cells do not kill indiscriminately has recently been elucidated at the molecular level. Thus, NK cells have been shown to express receptors that recognize MHC molecules expressed on normal cells [2]. As predicted by the 'missing self' hypothesis [3], the lack of expression of MHC molecules on target cells leads to the susceptibility of such cells to NK cell mediated lysis [3,4]. It soon became evident, however, that NK cells do not just express an 'universal receptor' capable of recognizing any MHC class I molecule, but rather they express several receptors discriminating among different allelic forms of MHC class I molecules [2,4-71. In humans, different
receptor types, specific for groups of HLA-C or HLA-B molecules were identified [2,5-7]. Molecular cloning of the genes encoding these receptors revealed new members of the Ig superfamily, each characterized by two or three Ig-like domains in their extracellular region [8,9]. Although only a relatively limited number of receptor molecules is presently known, as many as 29 natural killer cell receptor (NKR) sequences have been identified [10"]. Furthermore, whereas the known Ig-superfamily NKRs were found to recognize only a limited number of HLA class I alleles, it has become clear that HLA class I recognition represents a general mechanism which involves all NK cells [11]. Therefore, the search for the other 'missing receptors' has been the object of intensive investigation during the past year and the results are presented in this review. Other important progress that will be discussed in this review includes the following: the identification of the molecular mechanism involved in NKR signal transduction and the generation of the inhibitory signal; the phenotypic and functional characterization of T lymphocytes expressing NKRs in normal and cancer subjects; the analysis of viral strategies to elude NK recognition; and, finally, the cytokine involvement in the induction of NKR expression during NK cell maturation. Identification cell receptor
of a novel Ig-like natural
killer
As outlined above, direct evidence for HLA class I specificity existed only for a limited number of NKRs belonging to the Ig superfamily. These include the NKRs p58.1 and p58.2 [12] and their corresponding activating forms, p50.1 and p50.2 respectively [13,14°']. p58.1 (p50.1) has been shown to recognize one distinct group of HLA-C alleles and p58.2 (p50.2) to recognize another. Together these alleles encompass all the expressed alleles of the HLA-C locus. A third member of the p50 family of activating receptors (termed p50.3) has recently been identified at the molecular level [151. So far, however, no precise information exists regarding the HLA class I ligands recognized by this new receptor [15]. In addition, the p70 (NKB1) molecule, another Ig superfamily member, recognizes alleles belonging to the HLA-Bw4 supertypic specificity [16,17,18",19]. Although, the molecules encoded by the different NKR mRNA transcripts may display the same HLA allele specificity and monoclonal antibody (mAb) reactivity, the gap between the large number of identified transcripts of Ig superfamily NKRs and the number of identified receptors is still wide. In addition, for a large proportion of the HLA-B alleles and for all of the HLA-A alleles no corresponding NKR has been identified [11]. In the search
HLA class I specific inhibitory receptors Moretta and Moretta
for a novel NKR, Pende et ai. [20°] identified a clonally distributed molecule, termed p140, of either 70kDa (monomer) or 140kDa (homodimer), that functioned as a receptor for HLA-A3 and -All, but did not recognize HLA-A1, -A2 or -A24. Isolation of the corresponding eDNA for p140 revealed a three Ig domain structure on the basis of its predicted amino acid sequence. Dtihring etal. [21 °] using a different antibody to that used in [20"], but one which also reacted with the HLA-Bw4 specific p70 receptor, identified only the monomeric form of p140. As this receptor, unlike p58 and p70, has two cysteines (amino acid positions 302 and 336) in the extracellular region of the protein proximal to the transmembrane portion, it is possible that at least one of these residues may be involved in the formation of a disulphide bridge [20°]. Whether the dimeric form is necessary for binding HLA-A3 molecules and for providing an efficient signal to the cell remains to be defined. Table 1 summarizes the principal molecular features of humans NKRs. The
CD94-NKG2A
receptor
complex
Humans and mice appeared to use different types of receptors for MHC class I recognition. Thus, whereas the human NKRs all appeared to belong to the Ig superfamily, the murine receptors (members of the Ly49 molecular family) were found to be type II membrane proteins related to the C-type lectins [22]. It is noteworthy that both mouse and human NK cells express a family of C-type lectins which are encoded by a group of linked genes, the so called 'NK gene complex'. Could molecules encoded by the human NK gene complex also function as MHC-specific receptors? A possible candidate to fulfill this function was CD94 [23,24]. Thus, anti-CD94 monoclonal antibodies were found to specifically reconstitute the cytolytic activity of HLA-Bw6 specific, but not HLA-Bw4 specific, NK cells [25]. Although CD94 is expressed by most NK cells, its level of expression varies greatly
695
among NK cells. Moreover, CD94 molecules were found to be heterogeneous in function, as both inhibitory and activating forms of CD94 receptors have been described [24]. T h e recent selection of mAbs specific for the 'inhibitory form' of CD94 molecules [26°°] allowed us to identify the actual molecular structure of the receptor and to better define its HLA class I specificity. Similar conclusions regarding the molecular structure of the inhibitory form of CD94 molecules were drawn in another study [27°°]. T h e novel mAbs were found to be specific, not for the CD94 molecule itself, but rather for the CD94-associated molecules typical of the 'inhibitory form' [26°°]. T h e CD94-associated molecules were identified as being the molecular product of the NKG2A eDNA [28°°]. Similar data were obtained by another laboratory using a different experimental approach [29°°]. A general agreemeent has now been reached on the fact that the CD94--NKG2A heterodimer functions as an HLA class I specific inhibitory receptor [26°°-29°°]. T h e HLA class i specificity of CD94-NKG2A is, however, broader than that of the Ig superfamily NKRs and it encompasses the HLA-Bw6, HLA-C and HLA-A alleles. This group of alleles has a closely related sequence at amino acid residues 77-83 in the etl helix, which differs from that of the HLA-Bw4 alleles [10°]. Taken together, these data indicate that human NK cells express lectin-like MHC class I specific receptors, characterized by a broader specificity than that of the NKRs of the Ig superfamily. Ig-like NKRs have probably adapted to HLA class I recognition more recently than the lectins, namely after the divergence of the rodents and primates, as Ig-NKRs have not yet been detected in mice. Moreover the ability of the Ig-like NKRs to precisely recognize polymorphic MHC class I determinants appears
Table 1 The human NK]Rs for HLA class I antigens. Reactive mAbs EB6, XA141 EB6, XA141 EB6, XA141 GL183, y249 GL183, y249 GL183, y249 PAX180, FES172 Dxg, Z27 DXg, Z27 Q66, Q241 Q66, Q241 3B1, XA185 Z199
Putative ligands* HLA-C HLA-C HLA-C HLA-C HLA-C HLA-C
'group 1' 'group 1' 'group 1' 'group 2' 'group 2' 'group 2' ? HLA-Bw4 HLA-Bw4 HLA-A3 and -A11 HLA-A3 and -A11 Various HLA class I molecules$
Function
ITIM~
eDNA
Protein structure
Molecular mass (kDa)
Protein denomination
Inhibitory Inhibitory Activating Inhibitory Inhibitory Activating Activating Inhibitory Inhibitory Inhibitory Inhibitory Inhibitory
+ + + + + + + + +
EB6-cl.42 EB6-cl.47.11 EB6-act I 183-cl.6 183-cl.43 183-act I cl.39 (KKA3) c1.11 (AMB11 ) cl.2 (NKB1) d.5 (AMC5) c1.1.1 (17.1 C) CD94-LL288 NKG2A
2 Ig domains 2 Ig domains 2 Ig domains 2 Ig domains 2 Ig domains 2 Ig domains 2 Ig domains 3 Ig domains 3 Ig domains 3 Ig domains 3 Ig domains Dimeric, C-type lectins$
58 58 50 58 58 50 50 70 70 140 140 30 43
)58.1 )58, I )50,1 )58.2 )58.2 )50.2 )50.3 )70 pTO (NKBI) p140 p140 CDg4 NKG2A
*HLA-C 'group 1' includes the HLA-Cw2, -Cw4, -Cw5, -Cw6 alleles and the HLA-C 'group 2' includes the HLA-Cwl, -Cw3,-Cw7 and -Cw8 alleles. tThe immunoreceptor tyrosine-based inhibitory motif (mM) is characterized by the amino acid sequence V/IXYXXL (in single-letter amino acid code, where X represents any amino acid) and is involved in the recruitment of SHPs responsible for the inhibition of NK cell triggering. SThe inhibitory receptor consists of the dimeric comolex CD94-NKG2A.
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Cancer
to be a further adaptation of these NKRs keeping pace with the rapid evolution of the HLA class I genes [10"]. That CD94 may represent a more ancient form of HLA class I specific NKRs is also suggested by recent data by Mingari et al. [30"]. These authors showed that CD34÷CD7 ÷ postnatal thymic precursors undergo maturation towards mature NK cells in the presence of IL-15 [30"°]. More importantly, they showed that these cells expressed de novo CD94-NKG2A as the only detectable HLA class I specific inhibitory NKR. T h e acquisition of CD94-NKG2A paralleled the appearance of NK activity thus providing a powerful means to prevent lysis of autologous cells [30°']. Remarkably, no NKR members of the Ig superfamily could be induced by IL-15 [30°']. This may reflect the inability of the thymic precursors to express these NKRs or, rather, the requirement for a more complex signalling mechanism and/or different cytokines to induce their expression.
the I T I M s in the NKR intracytoplasmic tail, followed by the recruitment of SHP-1, and by SHP-1 dependent inhibition of crucial phosphorylation events involved in cell activation [31,37"]. A simplified model of the molecular mechanisms involved in NKR-mediated activation is shown in Figure 1. Figure I
Activating receptors 'On signal'
kill
H2
(a) Coaggregation Ts?hain Membrane
Mechanism of natural killer cell receptor mediated inhibition Given that ligand-mediated cross-linking of NKRs prevents cytolytic activity and cytokine production by NK cells (or cytolytic T lymphocytes [CTLs], see below), it was important to define the mechanism(s) leading to the inhibition of the cell activation pathways As cell activation involves a number of protein tyrosine kinases (PTKs), it appeared conceivable that these kinases could represent the molecular targets of NKR-mediated inhibition [31]. Several laboratories contributed to the identification of the inhibitory mechanisms. T h e presence of tyrosine based motifs, in the cytoplasmic tail of inhibitory NKRs (both those of the Ig superfamily and NKG2A), provided potential sites for interaction with Src homology (SH)2 domains. These motifs, termed 'immunoreceptor tyrosine based inhibition motifs' (ITIMs), are spaced, in different NKRs, by 26-29 amino acids [31]. It was found that NKR cross-linking leads to NKR tyrosine phosphorylation, and a similar effect was detected using phosphatase inhibitors [32"'-35"']. In addition, it has been shown that two tyrosines are required for NKR inhibitory activity in both T and NK cell activation. T h e protein involved in NKR-mediated inhibition was identified as being the hemopoietic SH2 domain containing protein tyrosine phosphatase (SHP)-I which is characterized by its two tandem SH2 domains [32"'-35"']. A direct association of SHP-1 with NKRs upon NKR phosphorylation has been documented. In this context, it has been shown that NKRs require co-engagement with an activatory receptor for their actions [36"]. This may reflect the need for NKRs to be phosphorylated by a nondiffusible P T K associated with the activating receptor [31]. Indeed the tyrosine phosphorylation of NKRs is strictly required for their recruitment of SHP-1 [34"',35"']. A model has been proposed in which NKR signalling involves an Lck-dependent tyrosine phosphorytation of
~
(b)
(f) ITAM [ ~
ITIM ~ Current Opinion in Immunology
Model of NKR-mediated inhibition of NK cell activation. The white blocks indicate elements that are part of the NKR triggering (activating) pathway, whereas the shaded blocks are part of the inhibitory pathway. (a) Coaggregation of an activating (CD16) and an inhibitory (p58) NKR, induced by either ligand or mAb, leads to (b) the activation of the Src-PTK associated with CD16 which then phosphorylates both (¢) the ITAMs of the y chains of CD16 and (d) the ITIMs of p58. This leads to (e) the recruitment and activation of SH2-containing P'I'Ks, such as ZAP-70 and p72-Syk, to the ITAMs, and of SHP-1 and -2 to the raMs. (f) The SHPs dephosphorylate the adaptor/effector molecules ZAP-70 and p72-Syk, which are part of the ITAM-associated cascade, thereby inhibiting activation. The scheme is simplified as it shows only one triggering and one inhibitory receptor. In addition, only ZAP-70 and p72-Syk are shown of the several adaptor/effector molecules which may be the molecular targets of the SHPs. Although not shown, a similar phenomenon occurs in T lymphocytes triggered via TCRs or other triggering receptors, and inhibitory receptors. Notably, the coaggregation of activating and inhibitory receptors may facilitate the interaction between phosphorylated molecules involved in the activating pathway and the ITIM-bound SHPs. This molecular model and, in particular, the need for co-aggregation of activating and inhibitory receptors explains why NKR-mediated NK cell inhibition is 'polarized', that is, limited to only those cell compartments bearing NKR ligands [2]. S-S, disulfide bond; C2, Ig-like constant domain; P, phosphorylation.
HLA-G recognition by natural killer cells T h e lack of classical HLA molecules in the human placenta prevents allorecognition and lysis of the placenta by maternal T lymphocytes but it poses the problem
HLA class I specificinhibitoryreceptors Morettaand Moretta 697
of susceptibility to NK cell mediated lysis [38,39]. T h e nonclassical and relatively nonpolymorphic HLA class I molecule HLA-G may mediate protection from NK cells [40,41]. HLA-G molecules are primarily expressed on the trophoblast and could thus play a key role in protecting the placenta from NK cell mediated damage. A possible candidate NKR required for this protective function was the CD94-NKG2A receptor, in view of its broad specificity for different HLA class I molecules and its expression on relatively large proportions of NK cells. Studies using NK cell clones, selected on the basis of the expression of appropriate NKRs, showed that CD94-NKG2A, but not other known NKRs, including p58.1, p58.2, p70 or p140, mediated HLA-G recognition. These data have been reported independently by two groups [42",43°]. Pazmany et al. [44"'], however, reported that both p58.1 and p58.2 can mediate HLA-G recognition. In addition, according to Munz et al. [45"], HLA-G recognition would be mediated by the pT0 receptor. At present, it is difficult to reconcile these conflicting results. It should be stressed that whatever the correct explanation may be, none of these reports [42°-45 °] provides a satisfactory explanation for how trophoblast cells which do not express the-putative HLA-G specific NKR, escape attack by NK cells (remarkably, these cells always represent a sizable fraction of total NK cells). One may speculate that, during pregnancy, there may be de novo expression by all NK cells of either the CD94-NKG2A complex or of other still undefined HLA-G specific NKRs. In this context, evidence has been provided for the existence of another HLA-G specific inhibitory NKR [43°], however, no information is available on its molecular nature. Additional work focused on the phenotypic and functional analysis of NK cells in pregnant women is clearly needed.
Viral strategies to elude natural killer cell recognition Cytolytic CD8 + T lymphocytes are known to play a major role in the control of viral infections. Although the lysis of infected cells does not necessarily represent the primary mechanism of defense, which may rather involve the production of cytokines such as IFN-y [46], the crucial defense event is the CTL-mediated recognition of viral peptides in an MHC class I restricted fashion. Therefore, it is not surprising that different viruses have evolved strategies to interfere with antigen processing, peptide presentation, and MHC class I expression [47]. It is important to note, however, that although the downregulation of MHC class I expression will prevent the CTL-mediated lysis of infected cells, it should also render them susceptible to NK cell mediated attack. Viruses, such as adenoviruses and herpesviruses, including herpes simplex virus (HSV) and human cytomegalovirus (HCMV), have evolved different strategies to interfere with MHC class I recognition [47]. Of particular interest is HCMV which synthesizes a number of proteins
('stealth' proteins of the US [unique short] family of early genes) which inhibit, by different mechanisms, the surface expression of HLA class I molecules [47]. It is interesting that NK cells play an important role in controlling both HSV and HMCV infections [48]; however, a potential mechanism to escape the NK cell recognition may be the expression of UL18, a 132-microglobulin binding protein with a structural homology to MHC class I molecules [49]. UL18 has been shown to function by protecting HCMV-infected cells against NK cell lysis [50"°,51°']. HLA class I negative cells transfected with UL18 became resistant to NK cell mediated lysis [50"']. T h e CD94--NKG2A receptor complex is apparently involved in this recognition. Therefore, HCMV has evolved two complementary strategies, interference with MHC class I presentation and MHC class I imitation, to avoid identification in infected cells by CD8 ÷ C T L s and NK cells respectively. The fact that HCMV has developed a mechanism for evading NK cell mediated detection suggests that indeed NK cells represent an important component of the defense against viral infections.
T lymphocytes expressing natural killer cell receptors At the time of the discovery of p58 molecules in NK cells [5], it was also reported that a small subset of peripheral T lymphocytes expressed NKRs [5,11]. It was only after the formal demonstration that p58 molecules functioned as inhibitory receptors [12], however, that the obvious question of whether NKRs could also inhibit T cell function was posed. The finding, by different groups, that NKRs can sharply inhibit TCR-induced T cell functions, including cytolytic activity [52-54] and cytokine production [53,55"], encouraged further studies on NKR ÷ T lymphocytes. Analysis of NKR ÷ T lymphocytes in healthy donors revealed that essentially all NKRs, whether belonging to the Ig superfamily or to a C-type lectin family could be expressed by T cells [56"]. Moreover, analysis of polymerase chain reaction amplified full length cDNAs o f T lymphocyte NKRs showed sequences that are identical to those of the corresponding NKRs expressed by NK cells [56°]. Analysis of the tissue distribution of NKR ÷ T lymphocytes indicated that they were present in peripheral lymphoid tissues, but were absent in the thymus and in umbilical cord blood [56"]. Consistent with this distribution, NKR ÷ T lymphocytes were found to express a memory phenotype and a skewed T C R variable (V)I3 repertoire [57°']. Moreover, at least in healthy donors, the represented V[3 segments were found to be oligoclonally or monoclonally expanded [57"']. T h e expression of NKRs which inhibit T cell function, poses relevant questions as to the meaning of this event. As NKR ÷ T lymphocytes are represented by CD8 ÷ CTLs in most donors, the expression of NKRs may represent a failsafe mechanism to avoid autoimmunity, particularly for those CTLs which have acquired NK like activity and could thus lyse normal cells independently of T C R
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specificity [53]. On the other hand, it is clear that the expression of NKRs, which work across the main function (to kill cells that present MHC class I associated foreign peptides) of CTLs, could be harmful to the host in chronic viral infections or in tumors. Indeed, a tumor specific C T L clone isolated from a melanoma patient was found to express p58.2 molecules [58°°]. This clone did not lyse the original autologous tumor (expressing the complete set of HLA class I alleles), but it lysed a loss variant which did not express several alleles, including HLA-Cw7, a iigand for the p58.2 receptor [58°°]. In addition, NKR expression in virus-specific C T L s was found in HIV-infected patients and mAb-mediated masking of these NKRs restored the lysis of target cells expressing HIV antigens [59°']. Similarly to inhibitory NKRs, activating forms of NKRs have also been detected on T lymphocytes. Activating p50 molecules were previously shown to display the same HLA-C specificity as the corresponding inhibitory p58 molecule [13,14"]. Moreover, molecular cloning revealed a high degree of homology in the extracellular domain of the corresponding p58 and p50 molecules, but there were found to be important differences in the transmembrane and intracytoplasmic domains [14°']. Activating p50 molecules have been shown to act as co-stimulatory molecules able to enhance T cell responses; p50 + T lymphocytes were found to respond to a lower dose of superantigen, depending on the expression of the appropriate (for p50 recognition) HLA-C ligand on target cells [60°].
lymphocytes, in addition to NK cells, it will be important to analyze further its ability to prevent graft rejection. It is evident that these studies, together with the identification of cytokines able to modulate the surface expression of inhibitory NKR may lead to the development of novel strategies to prevent allograft rejection or graft versus host disease.
Acknowledgements We wish to thank our colleagues R Biassoni, MC IS,ligari and C Bottino for discussion and for freely sharing their ideas with us. We are grateful to C Miriello for the secretarial assistance. This work was supported by grants awarded by the Associazione Italiana per la Ricerca sul Cancro (AIRC), lstituto Superiore di SanitY, Consiglio Nazionale delle Ricerche, Progetto Finalizzato Applicazioni Cliniche della Ricerca Oncologica and Ministero delrUniversit/t e della Ricerca Scientifica e Tecnologica (MURST) to L Moretta and grants awarded by MURST and AIRC to A Moretta
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: = =. 1.
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2.
Moretta L, Ciccone E, Mingari Me, Biassoni R, Moretta A: Human HK cells: origin, cionality, specificity and receptors. Adv Immuno11994, 55:341-380.
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Ljunggren H-G, Karre K: In search of the 'missing self': MHC molecules and NK cell recognition. Imrnunol Today 1990, 11:237-244.
4.
Moretta L, Ciccone E, Moretta A, H6glund P, Ohlen C, K~rre K: AIIorecegnition by NK cells: nonself or no self? Immuno/Today 1992, 13:300-306.
Condusions There is no doubt that in the past few years we have witnessed major progress towards the understanding of the molecular mechanisms involved in NK cell function. At the present, however, we know more about NK cell inactivation than we do about their activation. Although HLA class I specific activating receptors may play a role in NK cell triggering by MHC class I positive target cells (see [13,14"]), other receptors must play a predominant role against target cells which lack MHC class I molecules. A recent study by Sivori et al. [61 °°] describes a novel triggering surface molecule (p46) which, unlike all other NK cell markers, is strictly NK cell specific being expressed on 100% of resting or activated NK cells but not on other cell types. The molecular characterization of p46 and other molecules involved in NK cell triggering as well as of their ligands will be a major goal of future investigation. Another important aspect will be to define the in vivo relevance of the inhibitory effect involving human NKRs. In this context, important information has been provided by recent studies in transgenic mice [62°°]. Thus, the transgenic expression of the p58.2 NKR prevented the rejection of transgenic, H-2 mismatched, bone marrow grafts which express the HLA-Cw3 allele (recognized by p58.2). This study demonstrates, for the first time, an in vivo effect of human inhibitory NKR expression [62°']. As the NKR transgene is expressed in T
of special interest of outstanding interest
5.
Moretta A, Bottino C, Pande D, Tripodi G, Tambuasi G, Viale O, Orengo A, Barbaresi M, Merli A, Ciccone E, Moretta L: Identification of four subsets of human CD3-CD16+ NK cells by the expression of clonally distributed functional surface molecules. Correlation between subset assignment of NK clones end ability to mediate specific elloantigen recognition. J Exp Med 1990, 172:1589-1598.
6.
Ciccone E, Pende D, Vials O, Di Donato C, Tripodi G, Orengo AM, Guardiola J, Moretta A, Moretta L: Evidence of a natural killer (NIO cell repertoire for (silo)antigen recognition: definition of five distinct NK-determined allospecificities in humans. J Exp Med 1992, 175:709-718.
7.
Litwin V, Gumperz J, Parham P, Phillips JH, Lanier LL: Specificity of HLA class I antigen recognition by human NK clones: evidence for cionel heterogeneity, protection by self end nonself alleles, and influence of the target cell type, J Exp Mad 1993, 178:1321-1336.
8.
s. 10. •
Wagtmann N, Biassoni R, Cantoni C, Verdiani S, Malnati MS, Vitale M, Bottino C, Moretta L, Moretta A, Long EO: Molecular clones of the p58 NK cell receptor reveal immunoglobulin related molecules with diversity in both the extra- end intracellular domains. Immunity 1995, 2:439-449. Golonna M, Samaridis J: Cloning of immunoglobulin-superfamily members associated with HLA-C and HLA-B recognition by human natural killer cells. Science 1995, 268:405-408.
Valiante NM, Uenert K, Shilling HG, Smits BJ, Parham P: Killer cell receptors: keeping pace with MHC class I evolution. /mmunol Rev 1997, 155:155-164. An interesting analysis of the co-evolution of the Ig superfamily natural killer cell receptors (NKRs) and the MHC class I epitopes they recognize. NKRs are of relatively recent origin in primate evolution.
HLA class I specific inhibitory receptors Moretta and Moretta
699
11.
Moretta A, Bottino C, Vitale M, Pende D, Biassoni R, Mingari MC, Moretta L: Receptors for HI.A-class I-molecules in human Natural Killer cells. Annu Ray Immuno11996, 14:619-648.
22.
Yokoyama WM, Seaman WE: The Ly49 and NKR-P1 gene families encoding lectin-like receptors on natural killer cells: the NK gene complex. Annu Rev Immunol 1993, 11:613-635.
12.
Moretta A, Vital• M, Bottino C, Orengo AM, Morelli L, Augugliaro R, Barbaresi M, Ciccone E, Moretta L: I)58 molecules as putaUve receptors for MHC class I molecules in human Natural Killer (NK) cells. Anti-p58 antibodies reconstitute lysis of MHC class I-protected cells in NK clones displaying different speclficitles. J Exp Med 1993, 178:597-604.
23.
Aramburu J, Balboa MA, Ramirez A, Silva A, Acevedo A, SanchezMadrid F, de Land~zuri MO, Lbpez-Botet M: A novel functional cell surface dlmer (K1>43) expressed by natural killer cells and T cell receptor-y/6-1- T lymphocytes. Inhibition of IL-2 dependent proliferation by enU-Kp43 monodonal antibody. J Immuno11990, 44:3238-3247.
13.
Moretta A, Sivori S, Vitale M, Pende D, Morelli L, Augugliaro R, Bottino C, Moretta L: Existence of both inhibitory (p58) and activatory (I)50) receptors for HLA-C molecules in human Natural Killer cells. J Exp Med 1995, 182:875-884.
24.
Perez-Vitlar JJ, Melero I, Rodr;guez A, Carretaro M, Aramburu J, Sivori S, Orengo AM, Moretta A, Lbpez-Botet M: Functional ambivalence of the Kp43 (CD94) NK cell-associated surface antigen. J Immuno11995, 154:5779-5788.
25.
Moretta A, Vitale M, Sivod S, Bottino C, Mar•Ill L, Augugliaro R, Barbaresi M, Pende D, Ciccone E, Lbpez-Botet M, Moretta L: Human Natural Killer cell receptors for HLA-class I molecules. Evidence that the Kp43 (CD94) molecule functions as a receptor for HLA-B alleles. J Exp Med 1994, 180:545-555.
14. =e
Biassoni R, Cantoni C, Falco M, Verdiani S, Bottino C, Vitale M, Conte R, Poggi A, Moretta A, Moretta L: The HLA-C-specific 'activatory' or 'inhibitory' Natural Killer cell receptors display highly homologous extracellular domains but differ in their transmembrane and intrecytoplasmic portions. J Exp Mad 1996, 183:645-650. A follow up to the previous discovery [13] of the activating forms of HLAC-specific natural killer cell receptors (NKRs). The molecular cloning of the genes encoding the activating receptors shows an amino acid sequence identity of the activating receptors with the inhibitory receptors in the extracellular domains, but there are important differences in the transmembrane and intracytoplasmic regions. 15.
Bottino C, Sivori S, Vitale M, Cantoni C, Falco M, Panda D, Morelli L, Augugliaro R, Semenzato GP, Biassoni Ret al.: A novel surface molecule homologous to the p58/p50 family of receptors is selectively expressed on a subset of human Natural Killer cells and induces both tdggedng of cell functions and proliferation. Eur J Immunol 1996, 26:1816-1824.
16.
Litwin V, Gumperz JE, Parham P, Phillips JH, Lanier LL: NKB1 : • Natural Killer cell receptor involved in the recognition of polymorphic HLA-B molecules. J Exp Med 1994, 180:537-543.
17.
Lanier LL, Gumperz JE, Parham P, Melero I, Lbpez-Botet M, Phillips JH: The NKB1 and HP-3E4 NK cell receptors are structurally distinct glycoproteins and independently recognize polymorphic HLA-B and HLA-C molecules. J Immunol 1995, 154:3320-3327.
18. •
Vitale M, Sivori S, Pende D, Augugliaro R, Di Donato C, Amoroso A, Malnati M, Bottino C, Moretta L, Moretta A: Physical and functional independency of p70 and p58 NK cell receptors for HI.A-class I. Their role in the definition of different groups of alloreimtive NK cell clones. Proc Natl Acad Sci USA 1996, 93:1453-1457. The HLA-Bw4 specific natural killer cell receptor (NKR) pTO corresponds to the previously descdbed NKB1 molecule [15]. The authors of this paper show that when p70 is co-expressed with another inhibitory NKR, p58, both receptors function independently. Furthermore, expression of pT0 either alone or in combination with other NKRs is shown to determine the ability of the NKR-bearing cell to lyse allogeneic cells. This study reaches similar conclusions as those in [17]. 19.
Gumperz JE, Litwin V, Phillips JH, Lanier LL, Parham R The Bw4 public epitope of HLA-B molecules confers reactivity with Natural Killer cell clones that express NKB1, a putaUve HLA receptor. J Exp Med 1995, 181:1133-1144.
20. •
Pende D, Biassoni R, Cantoni C, Vardiani S, Falco M, Di Donato C, Accame L, Bottino C, Moretta A, Moretta L. The natural killer cell receptor specific for HLA-A ellotypes: • novel member of the p58/pTO family of inhibitory receptors which is charactodzed by three Igolike domains and is expressed as a 140 kD disulphide-linked dimer. J Exp Mad 1996, 184:505-518. A novel three Ig-like dorr~n natural killer cell receptor (NKR) is described that is specific for a defined group of HLA-A alleles. Unlike pTO, this NKR can be cell-surface expressed as both a monomer and homodimer. 21. •
26. ••
Sivori S, Vital• M, Bottino C, Marcenaro E, Sansevsrino L, Parolini S, Moretta L, Moretta A: CD94 functions as a natural killer cell inhibitory receptor for different HLA-dase-I alleles. IdenUficaUon of the inhibitory form of CD94 by the use of novel monodonal antibodies. Eur J Immunol 1996, 26:24872492. The authors demonstrate that the CD94 natural killer cell receptor (NKR) has a broad specificity for different HLA class I alleles and they cleerly distinguish the inhibitory form from the activating form by the aid of mAbs specific for a molecule associated with the inhibitory form. 27. •,
Phillips JH, Chang C, Mattson J, Gumperz JE, Parham P, Lanier LL: CD94 and a novel associated protein (94AP) form of • NK cell receptor involved in the recogniUon of HLA-A, HLA-B, and HLA-C allotypes. Immunity 1996, 5:163-172. Using a slightly different approach to that of [26"°], the authors reach a similar conclusion, namely that the CDg4 natural killer cell receptor (NKR) has a broad specificity for different HLA class I alleles. Again, the inhibitory form of CD94 is clearly distinguished from the activating form. 28. •-
Carretero M, Cantoni C, Belln T, Bottino C, Biassoni R, Rodriguez A, Perez-Villar JJ, Moretta L, Moretta A, Lbpez-Botet M: The CD94 and NKG2-A C-type lecUns covalenUy assemble to form a NK cell inhibitory receptor for HLA class I molecules. Eur J Immunol 1997, 27:563-567. Thanks to mAbs specific for the inhibitory form of the CD94 natural killer cell receptor (NKR) [26"], this study shows that the 'inhibitory form' is actually composed of CD94 associated with NKG2A. Although the corresponding cDNA of NKG2A, a type II molecule, has bQen known for several years, its molecular product and function had remained elusive. See also [29°°]. 29. **
Lazetic S, Chang C, Houchins JP, Lanier LL, Philips JH: Human natural killer cell receptors involved in MHC class I recognition are disulphide-linked heterodimers of CD94 and NKG2 subunits. J Immuno/1996, 157:4741-4745. The authors reach the same conclusion as that of [28"'], that is, that CD94 and NKG2A assemble to form heterodimers that are an actual HLA class I specific inhibitory receptor. 30.
Mingari MC, Vital• C, Cantoni C, Bellomo R, Pont• M, Schiavetti F, Bertone S, Moretta A, Moretta L: Interleukin-15-induced maturaUon of human Natural Killer cells from early thymic precursors. Selective expression of CD94/NKG2A as the only HI.A-class I specific inhibitory receptor. Eur J Immunol 1997, 27:1374-1380. In an attempt to identify the mechanisms involved in the de nova expression of inhibitory natural killer cell receptors (NKRs) by CD7 + CD34 + precursors undergoing maturation towards mature NK cells, the authors show that IL* 15 is capable of inducing the expression of CD94-NKG2A in parallel with the acquisition of cytolytic function. This mechanism prevents the lysis of autologous cells. •e
31.
Rbnard V, Cambiaggi A, Vely F, Blery M, Olcese L, Olivero S, Bouchet M, Vivier E: Trensduction of cytotoxic signals in Natural Killer cells: a general model of fine tuning between ectivatory end inhibitory pathways in lymphocytes. Immunol Ray 1997, 155:205-221.
32. ,o
Olcese L, Lang P, V61y F, Cambiaggi A, Marguet D, Blery M, HiDDenKL. Biassoni R. Moretta A. Moretta L e t al.: Human and
D6hring C, Scheidegger D, Samaridis J, Celia M, Colonna M: A human killer inhibitory receptor specific for HLA-A. J Immunol 1996, 156:3098-3101.
Describes the same HLA-A specific natural killer cell receptor (NKR) as that of [20"]. The authors in this paper describe only the monomeric form of the receptor.
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mouse Natural Killer cell inhibitory receptors recruit the PTPIC and PTP1 D protein tyrosine phosphatases. J Immuno/1996, 156:4531-4534.
This study allows us to explain the molecular mechanism by which the crosslinking of inhibitory natural killer cell receptors (NKRs) leads to the downregulation of NKR+ NK cell or T lymphocyte cell killing activity. The mechanism involves the recruitment of the Src homology 2 domain containing protein tyrosine phosphatase (SHP)-I hemopoietic phosphstase to the NKR immunoreceptor tyrosine based inhibition motifs. See also [ 3 3 " - 3 5 " ] . 33. •o
Burshtyn DN, Scharenberg AM, Wagtmann N, Rajagopalan S, Berrada K, Yi T, Kinet JP, Long EO. Recruitment of tyrosine phosphatase HCP by the killer cell inhibitory receptor. Immunity 1996, 4:77-85. This study allows us to explain the molecular mechanism by which the crosslinking of inhibitory natural killer cell receptors (NKRs) leads to the downregulation of NKR + NK cell or T lymphocyte cell killing activity. The mechanism involves the recruitment of the Src homology 2 domain containing protein tyrosine phosphatase (SHP)-I hemopoietic phosphatase to the NKR immunoreceptor tyrosine based inhibition motifs. See also [32°',34°°,35"°]. 34. ••
Fry AM, Lanier LL, Weiss A. Phosphotyrosines in the killer cell inhibitory receptor motif of NKB1 are required for negative signalling and for association with protein tyrosine phosphatase 1C. J Exp Med 1996, 184:295-300. This study allows us to explain the molecular mechanism by which the crosslinking of inhibitory natural killer cell receptors (NKRs) leads to the downregulation of NKR + NK cell or T lymphocyte cell killing activity. The mechanism involves the recruitment of the Src homology 2 domain containing protein tyrosine phosphatase (SHP)-I hemopoietic phosphatase to the NKR immunoreceptor tyrosine based inhibition motifs. See also [32",33"',35"']. 35. •=
Campbell KS, Dessing M, Lbpez-Botet, M, Celia M, Colonna M: Tyrosine phosphorylation of a human killer inhibitory receptor recruits protein phosphatase 1C. J Exp Med 1996, 184:93-100. This study allows us to explain the molecular mechanism by which the crosslinking of inhibitory natural killer cell receptors (NKRs) leads to the downregulation of NKR + NK cell or T lymphocyte cell killing activity. The mechanism involves the recruitment of the Src homology 2 domain containing protein tyrosine phosphatase (SHP)-I hemopoietic phosphatase to the NKR immunoreceptor tyrosine based inhibition motifs. See also [32"°-34"']. 36. •
Blery M, Delon J, Trautmann A, Cambiaggi A, Olcese L, Biassoni R, Moretta L, Chavrier P, Moretta A, Daron M, Vivier E: Reconstituted killer-cell inhibitory receptors for MHC class I molecules control mast cell activation induced via immunoroceptor tyrosine-based activation motifs. J Biol Chem 1997, 272:8989-8996. The authors show that inhibitory natural killer cell receptors (NKRs) need to be cross-linked with activating receptors involved in the activation of kinases. A likely explanation for this requirement is that Tyr phospho~lation of ITIMs in the cytoplasmic tail of NKRs is required for the recruitment of Src homology 2 domain containing protein tyrosine phosphatases. 37. •
Binstadt BA, Brumbaugh KM, Leibson PJ: Signal transduction by human NK cell MHC-recognizing receptors. Immuno/Rev 1997, 155:197-203. An interesting review that also contains new data and a model for natural killer cell receptor (NKR) signalling. 38.
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39.
Parham P: Keeping mother at bay. Curt Biol 1996, 6:638-641.
40.
Le Bouteiller P, Lenfant E Antigen-presenting function(s) of the non-classical HLA-E, -F and -G class I molecules: the beginning of • story. Res Immunol 1996, 147:301-313.
41.
Geraghty DE. Structure of the HLA class I region and expression of its resident genes. Curr Opin Immunol 1993, 5:3-7.
42.
Perez-Villar JJ, Melero I, Navarro F, Carretero M, Bellon T, Llano M, Colonna M, Geraghty DE, Lbpez-Botet M: The CD94/NKG2A inhibitory receptor complex is involved in the Natural Killer cell-mediated recognition of cells expressing HLA-G1. J /mmuno/1997, in press.
According to these authors, the recognition of HLA-G molecules is mediated by the CD94-NKG2A complex and not by other known natural killer cell receptors (NKRs). See also [43°-45°]. 43. •
Pende D, Sivori S, Accame L, Pareti L, Falco M, Geraghty D, Le Bouteiller P, Moretta L, Moretta A: HLA-G recognition by human natural killer ceils. Involvement of CD94 both as inhibitory and as activating receptor complex. Eur J Immunol 1997, 27:1875-1880. As in [42"], recognition of HLA-G molecules is shown to be mediated by the CD94-NKG2A coomplex. In addition, the authors show that the activating form of the CD94 receptor can recognize HLA-G. See also [44",45"]. 44.
Pazmany L, Mandelboim O, Val6s-Gbmez M, Davis DM, Reyburn HT, Strominger JL: Protection from Natural killer cellsmediated lysis by HLA-G expression on target cells. Science 1996, 274:792-795. The authors identify the p58 molecules p58.1 and p58.2 as the putative HLA-G-specific receptors. See also [42",43",45"]. •
45.
Munz C, Holmes N, King A, Loke WY, Colonna M, Schild H, Rammensee HG: Human histocompatibility leukocyte antigen (HLA-G) molecules inhibit NKAT3 expressing natural killer cells. J Exp Mad 1997, 185:385-391. The authors suggest a role for the HLA-Bw4 specific p70 natural killer cell receptor (NKR) in the recognition of HLA-G. See also [42"-44"]. •
46.
Kagi D, Hengartner H: Different roles for cytotoxic T cells in the control of infections with cytopathic versus noncytopathic viruses. Curr Opin Immuno11996, 8:472-477.
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Beck S, Barrell BG. Human cytomegalovirus encodes a glycoprotein homologous to MHC class I-antigens. Nature 1988, 331:269-272.
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Reybum HT, Mandelboim O, Val~s-Gomez M, Davis DM, Pazmani L, Strominger JL: The class I MHC homologue of human cytomegalovirus inhibits attack by natural killer cells. Nature 1997, 386:514-516. The authors show that an MHC class I homologous, HCMV-encoded, protein protects HCMV-infected cells from NK cell mediated lysis. 51. ••
Farrell HE, Hally H, Lynch DM, Reming P, She,am GR, Scalzo hA, Davis-Poynter NJ. Inhibition of natural killer cells by a cytomegalovirus MHC class I homologoue in vivo. Nature 1997, 386:510-514. The authors show that an MHC class I homologous, HCMV-ancoded, protein protects HCMV-infected cells from NK cell mediated lysis. 52.
Ferrini S, Cambiaggi A, Meazza R, Sforzini S, Marciano S, Mingari MC, Moretta L: T cell clones expressing the NK-related p58 receptor molecule display heterogeneity in phenotypic properties and p58 function. Eur J Immuno11994, 24:22942298.
53.
Mingari MC, Vitale C, Cambiaggi A, Schiavetti F, Melioli G, Ferrini S, Poggi A: Cytolytic T lymphocytes displaying natural killer (NK)-Iike activity: expression of NK-relatad functional receptors for HLA class I molecules (p58 and CD94) and inhibitory effect on the TCR-mediated target cell lysis or lymphokine production. Int Immunol 1995, 4:697-703.
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Phillips JH, Gumperz JE, Parham P, Lanier LL: Superantigendependent, cell-mediated cytotoxicity inhibited by MHC class I receptor on T lymphocytes. Science 1995, 128:403-405.
55. •
D'Andrea A, Chang C, Phillips JH, Lanier LL: Regulation of T cell lymphokine production by killer cell inhibitory receptor recognition of self HLA class I alleles. J Exp Med 1996, 184:789-794. This study confirms and extends the previous data by Mingari et el. showing that cross-linking of natural killer cell receptors (NKRs) leads to inhibition of lymphokine production by CD8 + NKR+ cells [53].
HE& class I specific inhibitory receptors Moretta and Moretta
56. •
Mingari MC, Pont• M, Cantoni C, Vitale C, Schiavetti F, Bertone S, Bellomo R, Tradori Cappai A, Biassoni R: HI.A-class I-specific inhibitory receptors in human cytolytic T lymphocytes: molecular characterization, distribution in lymphoid tissues and co-expression by individual T cells. Int Immunol 1997, 9:485-491. This paper shows that natural killer cell receptor (NKR) + T lymphocytes are present in tonsils, lymph nodes whereas they are absent in thymus and umbilical cord blood. In addition, polymerase chain reaction amplified T lymo phocyte NKR cDNAs are shown to be identical to those of NK cells. 57. •,
Mingari MC, Schiavetti F, Pont• M, Vital• M, Maggi E, Romagnani S, Demarest J, Pantaleo G, Fauci AS, Moretta L: Human CD8 + T lymphocyte subsets that express HLA-class I specific inhibitory receptors represent oligoclonally or monoclonally expanded cell populations. Proc Nat/Acad Sci USA 1996, 93:12433-12438. In this study the authors show that natural killer cell receptor (NKR) + T cells isolated from healthy donors display a memory phenotype and are oligoclonal or monoclonal in nature. 58. =.
Ikeda H, Lath6 B, Lehmann F, Van Baren N, Baurain JF, De Smet C, Chambost H, Vitaie M, Moretta A, Boon T, Coulie PG: Characterization of an antigen that is recognized on a melanoma showing partial HLA loss by CTL expressing an NK inhibitory receptor./mmunity 1997, 6:199-208. This study demonstrates that tumor antigen specific CTLs may express inhibitory natural killer cell receptors (NKRs) and that this may lead to impaired antitumor responses. 59. ••
De Maria A, Ferraris A, Guastella M, Pilia S, Cantoni C, Polero L, Mingari MC, Bassetti D, Fauci AS, Moretta L: Expression of
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HI.A-class I-specific inhibitory Natural Killer cell receptors in HIV-specific cytolytic T lymphocytes. Impairment of specific cytolytic functions. Proc Natl Acad Sci USA 1997, in press. This study shows that different inhibitory natural killer cell receptor (NKRs) are highly expressed on CD8 + T cells from HIV patients. In addition, the authors show that these NKRs are expressed on HIV-specific CTLs and prevent lysis of autologous HIV antigen expressing target cells. 60.
Mandelboim O, Davis DM, Reybum HT, Vales-Gomez M, Sheu EG, Pazmany L, Strominger JL: Enhancement of class II-restdcted T cell responses by costimulatory NK receptors for d a n I MHC proteins. Science 1996, 274:2097-2100. The activating p50 molecules, when expressed in T lymphocytes, may allow responses to lower doses of superantigens, provided that target cells express the appropriate HLA-C ligand. •
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Sivori S, Vital• M, Morelli L, Sanasevarino L, Augugliaro R, Bottino C, Moretta L, Moretta A: p46, a novel Natural Killer cellspecific surface molecule which mediates cell activation. J Exp Mad 1997, in press. This study describes p46, a novel surface molecule which is strictly NKspecific. More importantly, p46 induces potent NK cell triggering and may be involved in the process of HLA class I independent NK cell triggering. 62. ••
Cambiaggi A, Verthuy C, Naquet P, Romagn6 F, Farrier P, Biassoni R, Moretta A, Moretta L, Vivier E: NK-cell acceptance of H-2 mismatch bone-marrow grafts in transgenic mice expressing HLA-Cw3 specific killer-cell inhibitory receptor (CD158b). Proc Natl Acad Sci USA 1997, 94:8088-8092. This study provides the first evidence for an in vivo role for human natural killer cell receptors (NKRs) in the immune response against mistmatched bone marrow grafts in a transgenic mice model.