The MIs system

Immunology Today, Vol. 9, Nos 7andS, 1988

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strong T-cell stimulatory determinants In general, only products of MHC genes strongly stimulate naive T cells. However, in mice a set of determinants encoded by MIs loci provide even stronger stimulator]/signals for T cells. It is reasonable to suppose that these determinants, recognized by T cells at extremely high precursor frequency, may serve important functions in the immune system. For this reason, MIs-specific T-cell activation has generated a great deal of interest: here, Ryo Abe and Richard Hodes review recent advances in the understanding of the MIs system and discuss the influence of MIs expressionon T-cell repertoire selection. One of the striking characteristics of the immune system is its ability to recognize an extremely wide repertoire of foreign antigens. This diversity is mediated by the clonally expressed antigen-specific receptors used by T cells (e.g. the T-cell receptor ~13 dimer) and by B cells (immunoglobulin). As a corollary of this wide diversity, the frequency of T cells or B cells specific for any single antigenic determinant is in general extremely low. In fact, the precursor frequency of T cells specific for any given foreign antigen is undetectable in the proliferative response of naive T-cell populations. One prominent exception to this generalization is the repertoire of T cells specific for antigens encoded by genes of the major histocompatibility complex (MHC). The high precursor frequency of T cells specific for MHC antigens results in a strong primary mixed lymphocyte reaction (MLR) among MHC-disparate lymphocytes 1. Although the significance of the high frequency of T cells recognizing foreign histocompatibility antigens remains a significant puzzle, it has been well established that 'self' MHC gene products do play a central role in the immune system. In the mouse, another set of determinants exists that can function as strong stimuli for the proliferative responses of naive T cells. These determinants, initially described by Festenstein2,3, are known as minor lymphocyte-stimulating (MIs) antigens and are defined by their ability to stimulate strong primary proliferative responses in MLRs involving MHC-identical strains. MIs determinants have also been reported to mediate a number of other T cell-dependent responses4-6. Figure 1 shows the responses of BIO.BR (H-2 k) T cells to allogeneic cells from different inbred mice strains. If inactivated (irradiated or mitomycin C treated) BIO.D2 (H-2 d) lymphocytes are cocultured with MHC-congenic B10.BR (H-2 k) normal T cells, the B10.BR T cells recognize and respond to allogeneic H-2 d gene products, resulting an strong proliferation and 3H-thymidine uptake by T cells. Although the same BIO.BR T cells are not stimulated by syngeneic BIO.BR cells or by H-2-identical CBA/CaH cells, they do respond strongly to another H-2-compatible strain, AKR/J. At any number of responding T cells, the magnitude of 3H-thymidine uptake by BIO.BR T cells reacting to H-2-Jdentical AKR/J stimulators 230

ExperimentalImmunologyBranch,NationalCancerInstitute, National Institutesof Health,Bethesda,MD 20892, USA.

RyoAbe and RichardJ. Hodes is in fact much greater than that of the same T cells reacting with H-2-different BIO.D2. Festenstein originally described a similar strong proliferative response between the H-2d-compatible inbred strains, BALB/c and DBA/2 (Ref. 2). This strong stimulation was induced by determinants encoded by a single locus, designated the MIs locus, located on chromosome 1 (Ref. 7). The MIs locus was originally described as having four alleles, MIsa, MIsb, MIsc and MIsd, which encode polymorphic determinants2, 3. These determinants have widely differing stimulatory capacities: MIsa and MIsd are strongly stimulatory, MIsc is intermediate and MIs b is generally nonstimulatory. The existence of another strongly stimulatory MIs type, MIsx, has recently been reported 8. BIO.BR T cells and AKR/J stimulators that caused strong T-cell proliferative responses (Fig. 1) form an MHC-identical combination that differs at MIs, since AKR/J and BIO.BR have been typed as MIsa and MIs b, respectively. Using limiting dilution analysis, it has been shown that the strong primary proliferative responses of heterogeneous T cells to MIs differences reflect a high precursor frequency of T cells recognizing MIs determinants9, lo. The precursor frequency of T cells recognizing strongly stimulatory MIsa or MIsd is at least five times the frequency of T cells recognizing allogeneic MHC gene products lo. In addition to such extraordinarily high precursor frequencies of MIs-reactive T cells, the response pattern among MIs-different strains is another unique characteristic of the MIs system. In general, MLRs among allogeneic lymphocytes from MHC-disparate strains are bidirectional: T cells from one strain can respond to those from any other strain, and inactivated lymphocytes from one strain can stimulate those from any other strain. However, MIs-specific proliferation in MLRs is strikingly different in this regard. The schematic response pattern of T cells to MHC-compatible, MIs-disparate stimulators among the H-2 k strains AKR/J (Mlsa), BIO.BR !Mlsb), C3H/HeJ (MIs c) and CBA/J (MIsd) is shown in Fig. 2. MIs b T cells respond to each of the three other MIs types, MIsa, MIsc and MIs d but do not stimulate any of these cells. In contrast, MIs d cells stimulate T cells of each of the other MIs types but do not respond to any other MIs type. Ordy one combination of mutual stimulation is seen, between MIsa and MIsc. Another striking characteristic of the MIs-specific MLR is the quantitative variation in T-cell responses to different MIs types. For example, responses of MIsb T cells to MIsa and MIsd are much stronger than responses to MIsc, and MIsc T cells respond much more strongly to MIs d stimulators than do MIsa T cells2,3,11,12 The observed pattern of responses among MIsdisparate strains has led several investigators to challenge the original idea that MIs is a truly polymorphic (~ 1988, Elsevier Publications, Cambridge 0167 - 4919/88/$02.00

Immunology Today, Vol. 9, Nos 7andS, 1988

system. For example, a high degree of cross-reactivity between MIsa and MIsd determinants has been shown by several groups in primary MLR13 as well as in cloned T-cell responses14.15. In addition, the anti-MIsc response is marginal or even undetectable in certain experimental conditions 13. Based upon these observations, some investigators have suggested that MIs is not polymorphic but is composed of only two alleles, the aid allele, encoding a strongly stimulatory molecule, and the null b allele13; alternatively, MIs b and MIsc strains may simply express quantitatively less of the nonpolymorphic determinants encoded by MIs a'd (Ref. 16). It has been further proposed that these nonpolymorphic determinants may function as 'lectin-like' stimuli to T cells13 or might represent cell interaction molecules that play a general role in T-cell recognition 16. In contrast, others have reported that Mls b cells could stimulate Mlsa- and Mlsd-responding T cells, and have proposed that the products of the different Mls alleles express determinants that are specific for each allele 17. To clarify the nature of Mls determinants and of the genes that encode them, a number of laboratories have recently used cloned populations of Mls-specific T cells in combination with primary MLR to assess the critical issues of polymorphism and allelism in the Mls system 11.14.15.18-20.

Polymorphismand allelismin the MIssystem Polymorphismin MIs determinants

Despite significant effort_, antibodies specific for MIs gene products have not been identified. Therefore, the analysis of MIs has proceeded solely by assessing the responses of T cells in MHC-compatible strain combinations. Recently, T-cell clones have proved to be powerful tools in the identification of determinants recognized by T lymphocytes that may not be detectable by seroI,-,,-,;,--.I AAIPh --All. ,v~,,.o, means. c':--.., ~,,,~.: ,v,,~T ~.=,,~ ca,, ,=Luy,,i~u dll-"three conventionally stimulatory Mls types (Fig. 2), one approach has been to generate T-cell clones from BIO.BR (H-2 k, Mls b) by positive selection with Mls-different lymphocytes derived from AKRIJ (H-2 k, Mlsa), C3HIHeJ (H-2 k, M/s c) and CBA/J (H-2 k, MIs d) strains 11.19 In order to use cloned T-cell populations for this purpose, it was critical to establish the specificities of these clones, just as the rigorous identification of reactivities is essential in serological analysis. The specificity of potentially Mlsa-specific clones (such as B10. BR anti-AKR/ J clones) has been established by testing their patterns of response to BXD recombinant inbred strains ~1.1s,18.19. Unlike MIs a, the MIsc gene has not yet been formally mapped: therefore, the only available means to determine whether a T-cell clone is MIsc specific is to compare its pattern of proliferative responses with the primary anti-MIs c responses of unprimed T cells to different stimulators 19. AKR/J (H-2 k, MIs ~) T cells can detect MIsc (see Fig. 2) determinants on MHC-compatible splenocytes. Therefore, the response patterns of unprimed AKR/J T cells and BIO.BR anti-C3H/HeJ cloned T cells were compared for responses to stimulators derived from a number of genetically different mice including standard inbred strains, BXH recombinant inbred strains [originated from B6 (MIs b) and C3H/HeJ (Mist)], and (AKR/JxC3H/HeJ)F1 x AKR/J [(MIs a x MIs c) F1 x MIs a] backcross mice. The response patterns of unprimed heterogeneous AKR/J T cells and anti-C3H/HeJ cloned T

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cells were identical, indicating that these clones recognize the determinants that define the MIs c response. Moreover these Mlsa-specific and Mist-specific clones showed a reciprocal pattern of specificity, in that MIsaspecific clones responded to MIsa but not MIsc stimulators, whereas Mlsc-specific clones responded to MIsc but not to MIsa (Table 1). Compared with MIsa- or Mist-specific clones, the •response pattern of cloned BIO.BR T cells positively selected with CBA/J (MIs d) was interesting. All clones responded to CBA/J as expected. In addition, however, each of these clones had one of two patterns of cross-reactivity (Table 1): seven of nine clones responded to AKR/J (MIs a) and two clones responded to C3H/HeJ (Mist) 11.19,2°. All reacted with one stimulator, but none reacted with both MIsa and MIsc stimulators. Moreover, -II

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specific clones were reactive with CBAIJ in addition to their original specificities. These findings showed that Mlsa and Mlsc determinants were distinct, as evidenced by the reciprocal specificities of Mlsa-specific and Mlsc-specific T-cell clones; hence there is polymorphism within the Mls system. This was consistent with bidirectional responsiveness between MIs a and Mlsc strains in primary MLR.

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231

Immunology Today, Vol. 9, Nos 7and8, 1988

Table 1. Reactivitiesof cloned BIO.BRT cellsto MIs-different stimulatorcells Responsepattern of cloned T cellsto stimulatorsfrom

ClonedT cell

MIsa

(AKR/J)

BIO.BRanti-AKPCJ BIO.BRanti-C3H/HeJ i310.BRanti-CBA/Jno. 1 no. 2

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+ or - indicatesthe presenceor absenceof responsesbasedon the comparison with cultureswith syngeneicstimulators.

These results also revealed that MIs ~ cells coexpress MIs a and MIsc determinants and that Mlsd-specific T-cell responses are in fact composed of the sum of clonally distributed anti-MIs a and anti-MIs c T-cell responses. Additional data consistent with this conclusion have also been generated recently in studies of primary proliferative responses to MIs (Refs 1 1, 12, 21). Based upon this model, the complicated and often unidirectional stimulatory pattern of primary MLR (Fig. 2) can be explained. Since CBA/J mice express both MIs a and MIsc determinants, T cells from CBA/J mice are genetically tolerant to MIs a and MIsc. Therefore, CBA/J cells can stimulate either T cells from MIs a (anti-MIs c responses) or MIsc (anti-MIs a responses), but neither MIs a nor MIsc is stimulatory for MIs d. Allelism in the MIs system The model of MIs polymorphism described above

presented an interesting logical dilemma. Since no inbred mouse can express the product of more than one allele encoded by a single locus, CBA/J cells cannot coexpress two allelic gene products, MIs a and MIsc. In fact, the gene(s) encoding MIsc determinants has not ~-~-,,

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of the genes encoding MIs a and MIsc was recently carried out using both primary MLR and MIs-specific cloned T cells (Table 2) 2o. The stimulatory capacities of spleen cells from progeny of (AKR/J x C3H/HeJ)F1 x BIO.BR [(M/s a x M/Sc) F1 x MIs b] breedings were tested in primary responses as well as for M/s-specific clones. If the MIs a and MIs c determinants expressed by AKR/J and C3H/HeJ, respectively, were encoded by allelic genes as originally proposed, all the offspring from this breeding would be either MIs ab or MIs :d genotypically; and,

because MIs b is not stimulatory, the offspring would be phenotypically MIs a or MIsc. However, the results obtained from this analysis were not consistent with allelism of the genes encoding these two determinants 2°. MIs typing of offspring resulted in four different patterns, corresponding phenotypically to MIs a, MIs c, MIs a'c and MIs b. Based upon these results, it was proposed 11,20.22 that in contrast to the original description of a polymorphic, multi-allelic, single gene system, the MIs system is composed of at least two independent genes, one of which maps to the Mis locus on chromosome 1 and encodes strongly stimulatory MIs a determinants, and the other of which has not yet been mapped but is unlinked to the first locus and encodes more weakly stimu!atory MIsc determinants (Table 3). Strains can independently express the products of these two loci, and their pattern of expression will produce the conventional MIs a, MIs b, MIs c and MIs d phenotypes. To date, there is no clear evidence indicating that structural polymorphism exists within either the MIs a or the MIsc system.

The identity of MIs stimulatorydeterminants Tissuedistribution of stimulatory MIs determinants

Since MIs determinants are not serologically defined, the tissue distribution of MIs has been investigated by testing the ability of various populations to elicit an MIs-specific response. Von Boehmer and Sprent first reported that B cells but not T cells can stimulate MIs responses 23. Since that time there has been general agreement that, under the conditions studied, only MHC class II-positive cells can stimulate MIs-specific T cells 24-26. This finding is consistent with the apparently critical role of MHC class II in recognition of MIs (see below). There has been some disagreement, however, about the precise identity of the MHC class II-positive cells that function effectively in presenting Mis. Macrophages 24, splenic adherent cells 2s, dendritic cells 26 and B cells 23,27 have all been reported to be stimulators. The role of B cells has received particular attention since Webb et al. 27 showed that mitomycin-treated, but not heavily irradiated, B cells are competent stimulators. The loss of MIs a stimulatory ability in chronically B-cell-depleted (anti-p, treated) mice 28 and the enhancement of stimulatory ability by treatment with anti-lgD (8) antibody 29 are also consistent with the functional importance of B cells in MIs stimulation.

Table 2. Patternof distributionof MIsaand MIscdeterminantson stimulatorsfrom (AKR/JxC3H/HeJ) x BIO.BRoffspring Patternof MIs expressionin offspring

Pattern 1 Pattern 2 Pattern 3 Pattern4

Responsepattern of T cells rVIIsa-specificresponse*

Mist-specific responset

Primary MLR ClonedT cell ÷ + _ + + _ _

Primary MLR ClonedT cell _ _ + + + +

No. of mice

33 37 Total 70

232

*Primary responseswere generatedfrom C3H/HeJrespondingcells.Cloneswere Mlsa-specificas describedin the text. ~Primaryresponseswere generatedfrom AKR/Jrespondingcells.Cloneswere Mlsc-specificas describedin the text. + or - indicatesthe presenceor absenceof responsesbasedon the comparisonwith cultureswith syngeneicstimulators.

Immunology Today, Vol. 9, Nos 7andS, 1988

0

Attemptsto identifyMIsgene(s)andproctuct(s) Table 3. ProposedMIsgeneticsof prototypicstrains Neither MIs genes nor MIs gene products have yet Strain ConventionalMlstype MIs productexpressed been structurally identified. In direct attempts to identify Locus 1 Locus2 MIs gene product molecules, a number of investigators (Chromosome1) (Unknown) have examined antibodies made in strain combinations such that one might expect to find anti-MIs activity. The BIO.BR b Nonstimulatory Nonstimulatory greatest attention has been focused on a family of AKR/J a a Nonstimulatory heterogeneous3o.31 and more recently monoclona132 C3H/HeJ c Nonstimulatory c antibodies specific for cell surface antigens encoded by CBA/J d a c clenes linked to the MIs locus on chromosome 1. RecentLy, it was shown that several such antibodies, including specific 8.9.16.43-4s and Mist-specific 38 T-cell responses those specific for LyM-1 (Ref. 30), Ly-17 (Ref. 31) and also indicate that MHC class II antigens play an important Ly-m20 (Ref. 32), recognize polymorphic determinants of role in T-cell recognition of MIs determinants. the Fc~/ receptor (Fc~/R) of mouse B cells and DeKruyff et al. 42 have shown that, although both MIsa monocytes33. Since the tissue distribution of Fc~/R and a permissive H-2 type are required for activation of expression is similar to that of MIs stimulatory ability, it Mlsa-specific T cells, these two requirements need not be has been suggested that the MIs gene product is in fact the Fc3tR33. However, this now appears to be unlikely since provided by the same stimulator cell. That is, neither MIsa recombinations between MIs and the LyM-1 gene34 as cells of a nonpermissive H-2 type nor MIsb cells of a well as between the Fc~/R eL chain gene and the MIsa permissive H-2 type will stimulate Mlsa-reactive T cells, but a mixture of these two stimulator populations will gene have been detected by restriction fragment length induce a T-cell response. These findings suggest that polymorphism analysis (Seldin et al., submitted). In adMIsa determinants contributed by one cell can be prodition, recent sequence analysis indicates that MIsa and cessed and presented in association with the appropriate non-MIs a strains have identical Fc,yR (Ly-17) 13 chain H-2 determinants of another cell, and that the observed genes (J. Ravetch, pers. commun.). The complete seMHC influence on MIs recognition may therefore be quencing of the Fc~R eL chain gene, now in progress, related to conventional MHC-restricted recognition of may provide a definitive answer to this question. At foreign antigens by T cells. present, no additional information concerning the identity of MIs is available. TheT-cellreceptorof MIs-respondingTcells A number of studies have attempted to assess directly T-cell recognitionof MIs determinants the role of the T-cell receptor eL13dimer in MIs recogniThe mechanism by which T cells recognize MIs detertion. Antibodies specific for the T-cell receptor [[3 chain minants is a subject of interest and controversy. The (KJ 16), as well as antibodies specific for CD4, can inhibit extraordinarily high precursor frequency of T cells recogMIs-specific responses8,16. These data suggest that T-cell nizing MIs determinants has led some investigators to recognition of MIs determinants involves the conventionsuggest that this recognition is not likely to be mediated --I "It" ----II . . . . L... L l l d L ,,-_ Lll~ d, ,-L~,, receptor, c_l.l.IlILIIUUyll it remains .~IU:~:~IUIU . . . :,_,_ ,L_, ~,Ul~ly via LII~ LUIIVUilLIUIIr'Sl LIUIIrally U,~I,,,,IIU:~U~,,,I I ' L U l l luobserved inhibition was the result of direct inhibition of ceptor eL13heterodimer, and a unique receptor structure T-cell activation by these antibodies42. Recently, two for MIs has been proposed 14.3s. This question has been groups have shown that strong associations exist beaddressed by comparing the nature of T-cell recognition tween the T-cell receptor Vp genes expressed by T cells of MIs gene products with T-cell recognition of convenand the ability of these cells to recognize MIsa. Kappler et tional antigens, and by directly examining T-cell receptor aL47 found that approximately 80% of the T-cell hybrigene usage in MIs-reactive T cells. domas that express V~8.1 are specific for MIsa, whereas MacDonald et al. 48 reported that Mlsa-reactive T cells Theinfluenceof MHCgeneproductson MIsrecognition express predominantly Vp6. In these two reports, the Since T cells are generally stimulated by nominal strong relationship between T-cell receptor gene usage antigens or non-MHC alloantigens in the context of self and MIsa specificity was reinforced by the finding that MHC gene products, the role of MHC gene products in strains expressing MIsa have largely eliminated (presumT-cell responses to MIsa and MIsc has been extensively ably in the process of maintaining self tolerance) those T investigated. The magnitude of responses of cells expressing V~8 1 or V~6. heterogeneous36.37and cloned 19.38T-cell populations to Several aspects of these T-cell receptor deletions in MIsc determinants is dependent on the H-2 haplotype of MIsa strains are of particular interest. Although it has stimulator cells. Using linear regression analysis of primary responses to MIsc determinants, Gress et aL 39 generally been reported that only CD4 ÷ T cells are responsive to MIsa, both CD4 ÷ and CD8 + T cells expressdiscovered that T cells appear to recognize MIsc in conjunction with MHC gene products. In MIsa responses, ing Vp81 or Vp6 are deleted in MIsa strains47.48. There are stimulator cell MHC type also appears to influence at least two possible explanations for this observation. First, CD8 ÷ as well as CD4 ÷ T cells may be able to responsesls,~8.4°.4~. This MHC effect in MIsa recognition recognize MIsa under appropriate conditions, either is unusual, however, in that most - although not a,~luniquely duriny the induction of self tolerance, or more H-2 types are permissive, even for the Mlsa-specific response of a given cloned T cell. Similarly, using a generally. The report of MIsa reactivity in a CD8 + cytosystem of negative selection, Molnar-Kimber and Sprent toxic T-cell clone is consistent with this possibility49. If this were the case, then self tolerance to MIsa could found (hat anti-MIsa responses do not show typical H-2 independently delete both CD4+CD8 - and CD4-CD8 + restriction 42. The observations from several laboratories that anti-class II antibodies can inhibit MIsa- Mlsa-reactive T cells. Alternatively, if only CD4 + cells are

233

ImmunologyToday,Vol. 9, Nos 7 and 8, 1988

rgy'l w

234

Mlsa-reactive, then the deletion of CD8 ÷ as well as CD4 ÷ cells suggests that negative selection of the T-cell repertoire during the generation of self tolerance might occur at the level of a common precursor of both CD4 ÷ and CD8 ÷ cells, such as CD4+CD8 ÷ thymic precursors48. It should also be noted that not all V~81- or Vl~6-expressing T cells are reactive to MIsa (Refs 47,3,8). This implies that T-cell structures other than V~-encoded sequences alone (i.e. other 13 chain sequences, permissive oL chain sequences or non-(x13structures) may be important to MIsa reactivity. All these studies imply that the conventional (xf3 heterodimer is critically involved in MIsa recognition. In addition, the demonstration that MI.;a strains effectively delete T cells expressing certain VI} genes dramatically illustrates the biologic significance of MIs in overall T-cell repertoire selection. A number of investigators have suggested that MIs products are not cell surface antigens recognized in a conventional manner by clonotypic (x13 T-cell receptors 14.16.3s.44.s°. Webb et aL have provided evidence for the existence of a unique T-cell recognition structure for MIsa. They generated T-cell hybridomas that have dual reactivities to self plus X and to MIsa (Ref. 35) or triple reactivities to self plus X, allogeneic H-2 and MIsa (S. Webb et al., submitted). By taking advantage of the chromosomal instability of T-cell hybridomas, they monitored changes in the functional specificities of subclones, and correlated these changes with T-cell receptor expression. While some hybrids selectively lost MIsa reactivity but retained their other reactivities, others showed the converse pattern. In contrast, no hybrids showed any segregation between their self plus X and allogeneic H-2 reactivities. These findings suggested that MIsa recognition may involve a receptor distinct from that used in conventional antigen recognition. However, Webb et al. also found that hybridoma variants that were unresponc ; i v ~ t n l ~ l c a h = r i I ~ e ÷ + N ~ ; . ,-..;,-.;..~1 T . ~ " -,- -: ..................,,,,,~,,v, '5" ,a, ,-~.~,,receptor (x~rlc]in. Based on these results,they proposed that two receptors are involved in T-cell recognition of Mlsa: one is the conventional T-cell receptor oL13 heterodimer, and the other isa unique Mlsa receptor. Janeway et al.44 have reported that antigenpresenting cells(APCs) from M/s o mice present antigen more efficientlythan do APCs from Mls b mice in the activationof antigen-specificT cells.This observation has been confirmed by some investigators5o but not by others (B. Needleman et al., submitted). On the basis of theirfindings,Janeway and his colleagues proposed that the M/s a gene regulates in some manner the overall efficiencyof interactionbetween an M H C classll-positive APE and an M H C class ll-specificT cell,for example by influencing the expression of a potentially nonpolymorphic cellinteractionmolecule 16 The recent finding of preferentialVI}8 1 and Vi~6 gene usage in Mlsa-reactive T cells implies tFlat clonally expressed T-cellreceptors play a criticalrole in Mlsa-specific responses. However, it is not clear whether an Mis s molecule exists that is the specific antigenic ligand recognized by the oL13receptor,or whether the M/s a gene somehow facilitatescellularinteractionsin which the oL13 receptor interactswith, for example, relativelynonpolymorphic class IIM H C determinants. Kappler et al.47 have proposed that Mlsa-like products may be peptides that are preferentiallyassociated with M H C molecules on the

cells involved in positive selection during T-cell development (e.g. thymic epithelium), and that this Mlsa/MHC ligand might have enhanced affinity for T-cell receptors that otherwise have only a weak affinity for unmodified self MHC. Positive selection on the basis of this enhanced affinity could account for the skewing of the receptor repertoire toward recognition of antigen in association with self MHC. Concludingremarks Although the biochemical indentity of MIs gene products and the precise mechanism of their recognition by T cells have not been determined, the past few years have seen a significant advance in the understanding of this system. A revised notion of the genetics of MIs expression has been proposed that suggests that the independent products of at least two gene loci are involved in the MIs system. Polymorphism exists between the products (MIs a and MIs ¢) of these two loci, but structural polymorphism may not exist in allelic products of either one locus. MHC gene expression plays an important part in T-cell recognition of both MIsa and MIsc determinants, but the nature of this role is unclear. Recent evidence strongly supports a role for polymorphic T-cell receptor 13 chain structures in ~ilsa recognition, while other evidence suggests that a unique MIsa receptor may also be important. Perhaps most dramatically, self tolerance to MIs results in profound deletions from the T-cell repertoire of cells expressing selected V~ genes, indicating the importance of MIs products in the overall process of T-cell repertoire selection. These recent findings have broadened and intensified interest in MIs, and have focused attention on the possibility that understanding MIs will be important to elucidating the general mechanisms involved in T-cell activation and repertoire generation. The authors thank R.E. Gress and J.R. Wunderlich for their critical reviewsof this manuscriptand M.E. Dorf, J.W. Kappler, J. Sprent and S.R. Webb for their willingness to make experimental findings availableto us before publication References 1 Wilson, D.B., Blyth,J.L.and Nowell, P.C.(1968)J. Exp. Med. i28, 1i57-ii8i 2 Festenstein,H. (1973) Transplant. Rev. 15, 62-88 3 Festenstein,H. (1974) Transplantation 18, 555-557 4 Wolters, E.A.J.,Brons, N.H.S.,van der Kwast, T.H. and Benner, R. (1980) Cell. Immunol. 51, 215-225 5 Macphail, 5., Yron, I. and Stutman, O. (1982)J. Exp. Med. 156, 610-621 6 Berumen,L., Halle-Pannenko, O. and Festenstein,H. (1983) Eur. J. Imrnunol. 13, 292-300 1 Festenstein,H., Bishop,C. and Taylor, B.A. (1977) Immunogenetics 5, 357-361 8 Janeway, C.A., Jrand Katz, M.E. (1985)J. Immunol. 134, 2057-2068 9 Janeway, C.A., Jr, Lerner, E.A.,Jason,J.M and Jones,B. (1980) Immunogenetics 10, 481-497 10 Miller, R.A.and Stutman, O. (1982)J. ImmunoL 128, 2258-2264 11 Abe, R., Ryan,J.J. and Hodes, R.J.(1987)./. Exp. Med. 166, 1113-1129 12 Ryan,J.J., Mond, J.J.and Finkelman,F.D.(1987)J. Immunol. 138, 4085-4092 13 Molnar-Kimber,K.L. and Sprent,J. (1981) Transplantation 31,376-378

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T[ ]r. =. , n, . ,c, ,m Dmhr:mo ', n = i ¢ un t, .h. ,o :. ...'.t, .,."u. .:. i. .t , 'n n , . . , . , , , . . . . . . . . , , : , . . .c, .m

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Besides being activated throughthe T-cell receptorinresponse to antigen presented in the context of self MHC molecules, T lymphocytes can also be activated by mitogens such as plant lectins. In this case two separate signals - one from the mitogen the other from interleukin 2 - are required to complete the cell cycle. Here, John Hadden details the biochemical events associated with mitogenic activation and emphasizes the importance of calcium and arachidonic acid metabolism in these processes. T lymphocytes are triggered to proliferate by at least two signals. Lectin mitogens induce 1" cells to enter a 'GI' phase of the cell cycle from a resting 'GO' phase of the cell cycle, to enlarge and to synthesize protein and RNA (blastogenesis). The lymphocyte enters DNA synthesis ('S' phase) only when a second signal is presented. This second process is initiated by interleukin 1 (IL-1) pro-

Programof Immunopharmacology,Universityof South FloridaMedical College, 12901BruceB. DownsBlvd, Tampa,FL33612, USA. (~) 1 9 8 8 , Elsevier Publications, C a m b r i d g e

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of T lymphocytes by mitogenicantigens JohnW. Hadden duced by adherent accessory cells. The IL-1 then acts on primed 1 lymphocytes to induce the production of interleukin 2 (IL-2) and the appearance of cell surface receptors for IL-2. As a result of the second IL-2 signal, T cells transit a G1-S boundary and complete the cycle. Subsequent divisions require only the presence of IL-2. The purpose of this review is to summarize the current information on the transmembrane signaling events involved in T-lymphocyte activation (see Ref. 1). The mitogen does not have to enter the cell, since insolubilized lectins such as phytohemagglutinin (PHA) and concanavalin A (Con A) will induce transformation in the absence of ligand or receptor internalization. For both Con A and PHA, binding is virtually complete within 30 minutes and a pulse as short as three hours is

235