Recent developments in the human maternal-fetal immune interaction

Recent developments

in the human maternal-fetal

immune Yung Wai University

This

review

likely

to

discusses

have

reproductive

the

of the

Lake and Ashley King

of Cambridge, Cambridge, UK

the

greatest

immunology:

histocompatibility

interaction

complex

three

important

impact the

on

establishment

class

I antigen

trophoblast-lymphocyte-crossreactive

regulatory

protein;

and elucidation

of decidual

Current

Opinion

in Immunology

During 1990, the nature of two important trophoblast antigens became clearer. The class I antigen expressed by the population of extravillous trophoblast was established as a product of the HLA-G locus although its function is still unknown. The trophoblast-lymphocyte-crossreactive (TIK) antigen was shown to be identical to a member of the family of complement regulatoty proteins and it is possible that this molecule may protect the placenta against complement-mediated attack. Another area of human reproductive immunology that has advanced signiiicantly during the past year is the clarification of the lineage of the unusual large granular lym phocytes (LGLs) that constitute the major population of bone marrow-derived cells in decidua. It is becoming increasingly clear that although these lymphocytes are related to natural killer (NK) cells, they appear to represent a uterine-speciiic cell population. Their possible interaction with trophoblast via the HLA-G molecule is commanding much research interest. These recent developments in trophoblast antigens and decidual LGLSwill form the basis of this review. MHC class I antigen

Immunohistochemical studies have defined two subpopulations of trophoblast on the basis of HLA class I antigen expression: villous trophoblast (cytotrophoblast and syncytiotrophoblast) is class I negative and extravillous trophoblast is class I positive. Antigen localizaton has now progressed to detection of mRNA. Using different full-length class I cDNA probes and in situ hybridization, a combined study from the University of Kansas,

understanding of the

are

human

trophoblast

major

as HLA-G; lineage

that of

protein

of the

large granular

Introduction

developmemts

our

the

identification

as a complement

and possible functions

lymphocytes.

1991, 3:762-766

USA and the University of Cambridge, UK made the interesting observation that villous cytotrophoblast contains class I mRNA although these cells do not express the antigen [ 1.1. Thus, trophoblast can now be divided into three subpopulations on the basis of class I expression: villous syncytiotrophoblast (mRNA- , protein- >;villous cytotrophoblast (mRNA+, protein ->; and extravillous trophoblast (mKNA+ , protein+). The control of trophoblast class I expression, therefore, appears to occur at different cellular levels, depending on the population. The nature of the unusual major histocompatibility complex (MHC) class I antigen expressed by extravillous trophoblast has been established as a product of the nonclassical class I locus, HLA-G.Ellis and her colleagues [2*] screened a series of cDNA libraries from the BeWo chonocarclnoma cell line. One of the clones they obtained had a nucleotide sequence that did not correspond to HLA-A, B or C. Instead, its sequence had a very high degree of homology with a genomic clone HIA6.0 identified from a B-lymphoblastoid cell line that is now designated HLAG. Two characteristic features of HLA-Gwhen compared to HLAA,B or C are: firstly, its truncated cytoplasmic region is 19 amino acids shorter, which could account for its observed smaller size (heavy chain of 39 kD); and secondly, there is a specific sequence in its transmembrane region. Using the polymerase chain reaction for ampliIication, Ellis et al [2*] demonstrated that similar HLA-G sequences can be found in normal term chorionic plate trophoblast cDNA. Kovats and her colleagues [3**] were the first to present evidence that HLA-G actually codes for an expressed protein. An HI-&negative parental cell line transfected with HLA-G was found to express surface molecules that react with the anti-monomorphic class I monoclonal antibody, ~6/32, and with anti-

Abbreviations &m-p2 microglobulin; IFN-interferon; IL-interleukin; IAK-lymphokine-activated killer; LCL-large granular lymphocyte; mAbmonoclonal antibody; MCP-membrane cofactor protein; MHC-major histocompatibility complex; NK-natural killer; TCR-T-cell receptor; TLX-trophoblast-lymphocyte-crossreactive.

762

@ Current

Biology

Ltd ISSN 0952-7915

Recentdevelopmentsin the human maternal-fetal immune interactionLoke and King

g2 microglobulin, but not with polymorphic class I antisera. Immunoprecipitation of metabolically labelled proteins from these transfectants with W6/32 revealed heavy chains of live isoforms exhibiting molecular weights of 37-39kD and isoelectric points of 4.55.5PI on two-dimensional gels. Immunoprecipitation of normal first trimester extravilIous trophoblast with W6/32 showed a similar array of molecules, which suggests that they represent the same proteins. There are several interesting features of trophoblast HLA-G. Firstly, antigens analysed from a total of 20 individuals show no variations in the positions of the isoforms, indicating that the heavy chain is not polymorphic. Secondly, the 37k~ acidic isoform appears to be secreted and not expressed on the cell membrane, because it can be immunoprecipitated from culture supernatant but cannot be identified by ~2Uactoperoxidase surface labelling. Thirdly, the amount of HLA-G is greatly reduced on third trimester trophoblast compared with first trimester, so its expression is in some way linked to early placental development. Among choriocarcinoma cell lines derived from trophoblast tumours, Kovats et al. [3**] did not lind the HIAG a-chain in BeWo and JAR, but the JEG line (a derivative of BeWo) did express a cell-associated HLAG as well as a secreted form. In contrast, Ellis et al [4] originaIly described an electrophoretically non-polymorphic glycoprotein of 40kD molecular weight associated with f12microglobulin ( P2m> in BeWo. Their subsequent cloning of an HIA-G cDNA [2*] together with the northem blot analysis of I-ILA-G on these cells by Risk and Johnson [ 51 further conlirms the expression of this class I antigen by BeWo. In our own experience (YW Loke, A King, unpublished data), we have noticed that the BeWo cell line is particularly variable in HIA expression, and it is advisable to use JEG, which appears to be more stable in this respect for experimental purposes. Wei and Orr [6] have analysed a variety of human tissues for the presence of HLA-E,F and G transcripts, and found that HLA-Gis present only in extraembtyonic tissues such as the placenta and placental membranes. This apparent tissue restriction suggests that HI&G may have a role to play in pregnancy, but what this role is remains to be determined. We ourselves believe that HLA-G may influence trophoblast-decidual LGL interaction (see later>. In the evolutionary context, recent cloning and sequencing of the MHC class I genes from the cotton-top tamarin monkey by Watkins et al. [7] revealed these molecules to have limited polymorphism and to be more closely related to human HLA-G than to classical HLA-A, B and C. There is, however, one major difference: the tamarin HIA-G has a normal cytoplasmic tail and is the size of a classical class I molecule. If the tamarin G-like antigen is indeed the ancestor of the human analogue, then evolutionary pressures must have adapted it for a role in placentation by inducing a loss in polymorphism. The class I products that are expressed by the tamarin trophoblast have not yet been established. Although the accumulated evidence now reveals the unusual trophoblast class I antigen with a-chain of

39kD to be HLA-G, another question remains unanswered. Grabowska et al. [8*] observed that an additional band of 45 kD molecular weight was co-precipitated, using W6/32, with the 39kD protein from %-methionine metabolically-labelled first trimester trophoblast immunoprecipitate. This is unlikely to be from contaminating non-trophoblast cells because an identical pattern was seen in JEG choriocarcinoma cell line [9]. Does trophoblast express another class I molecule with HLA-G and if so, what is this other antigen? HLA-C is a possibility. Ellis and colleagues [lo*] have isolated another clone from a BeWo cDNA library whose sequence shows a high level of homology with the published sequence of HIA-C. There are, however, several unique amino-acid substitutions, particularly at the a1 and a2 domains. Because these are the areas where many of the antibody-binding epitopes reside, these substitutions could explain the frequently observed selective reactivity of extravillous trophoblast to many class I antibodies. The predicted size of the BeWo protein, from the nucleotide sequence, is in the region of 45 kD. If trophoblast does indeed express two class I molecules (HLA-G and possibly HLA-C), any hypothesis regarding the immunological survival of these cells must take this expression into account. How MHC class I expression is regulated in trophoblast is not clear. Recently, Boucraut and colleagues [ 1l*] transfected cloned HIA-A or HIA-B a-chain genes into the MHC-negative JAR choriocarcinoma cell line. They observed expression of the exogenous product in association with endogenous g2rn after transfection but the endogenous class I gene was not activated. These results show that the factors responsible for the extinction of endogenous class I genes in JAR do not affect similar transfected genes and are, therefore, consistent with the idea of &acting suppressive mechanisms, perhaps mediated through DNA methylation of appropriate areas of class I genes such as CpG islands. The possibility that the exogenous genes are escaping a transacting suppressive factor cannot be entirely excluded. Interestingly, interferon (IFN)-y is observed to increase cell-surface expression of the transfected class I-endogenous B2rn heterodimers without concomitant upregulation of the endogenous class I gene product, This cytokine has also been observed to enhance both mRNA and surface expression of class I antigen on normal extravillous trophoblast and on the class I positiveJEG choriocarcinoma cell line using northern blotting and flow cytometry analysis respectively [12]. However, because the probe used in this study was a full length class I cDNA and the monoclonal antibody (mAb) is W6/32, it is not possible to determine which of the two trophoblast class I antigens is upregulated by IFN-y. We have recently shown that IFN-a, in contrast to IFN-y, has no effect on trophoblast class I expression [ 131.

TLX antigen Purcell and his colleagues [14*] have produced evidence to show that the TLX antigen, described by McIntyre and Faulk [15] several years ago, shares physical, immunological and biochemical properties with the com-

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plement regulatory protein, membrane cofactor protein (MCP) or CD46. This suggests that these are the same molecule. MCP belongs to a gene family of proteins that regulate complement activation by preventing ampliiication of C3 deposition. Its biological role, therefore, could be to prevent complement-mediated lysis of trophoblast. The existing hypothesis that maternal immune recognition of allogeneic TLX is required for successful pregnancy [l6] now becomes increasingly diflicult to justify. However, before we completely dismiss the above hypothesis, it should be noted that TLK/CD46 does exhibit a degre of polymorphism, and so it is pertinent to ask why this heterogeneity has evolved because polymorphism is clearly not required as a complement regulatory protein. MCP has been demonstrated by immunohistology to be present in all trophoblast populations, including syncytiotrophoblast, villous cytotrophoblast, cells of the cy totrophoblast columns, all extravillous trophoblast and cytotrophoblast of the amniochorion. Both first trimester and term material are reactive [ 171 so MCP appears to be a rather ubiquitous molecule. In a recent restriction fragment length polymorphism analysis of the human CD46 gene of a number of individuals, Risk and her colleagues [18] detected that sev eral of the polymorphic bands occurred less frequently in women who suffered from three or more consecutive lirst trimester spontaneous abortions compared with normal controls. In particular, the 4.5 kb band from Hind III digest of genomic DNA was completely absent in women with spontaneous abortion compared with a 75% frequency in controls. If these findings are confirmed in larger samples, then it is possible that certain specific sequence variations influence the transcription and expression of CD46 which, in turn, may result in the production of an aberrant form of the protein that is unable to fullil its complement regulating function. In this situation, placental trophoblast will not be adequately protected from maternal complement-mediated attack, thus providing a possible etiological mechanism for spontaneous recurrent abortion. It should be pointed out that potentially cytolytic anti-trophoblast antibodies have not been convincingly demonstrated in pregnant women (YW Lake, A King, unpublished data), so that MCP is likely to guard against C3 deposition generated via the alternative rather than the classical pathway. Uterine large granular lymphocytes

by the majority of uterine leukocytes. Although immunohistology has failed to demonstrate any CD16+ cells in the uterus, lO--15% C~16+ cells were detectable with flow cytometry, albeit at low antigenic density. However, the relative surface density of CD56 (now known to be identical to the embryonic form of the neural cell adhesion molecule, E-NCAM) [22] is greatly increased on decidual LGL to 22 times that found on peripheral blood classical NK cells. These findings indicate that, although phenotypically very similar to the tiny agranular CDSGbright,CD16-, CD3- subset in peripheral blood ( N l%>, decidual CD56 + cells differ because they are mainly granulated and express an increased density of CD56. Tabibzadeh [ 231 has shown that CD45 + lymphoid cells proliferate in the secretoty (luteal) phase of the menstrual cycle when the glandular and stromal elements cease proliferating and undergo progressive pre-decidual changes. This lymphoid proliferation has now been attributed to CD56+ cells using double immunohistology for CD56 and the proliferation marker Ki-67 [21**].

Table 1. Phenotype of the main population of decidual large granular lymphocytes.

Markers

Haematopoietic

Distribution/Function

CD45 +

(Leukocyte common

CDcjf,bright+ +

(E-N CAM)

cell marker NK

antigen)

CD16dimf

(FcR Ill)

CD57-

(NK subset)

Early T-cell

CD2+

(E-rosette receptor)

markers

CD7+

fT cells NK/cells)

Late T-cell

CD4 -

m-helper/inducer)

markers

CD8 -

(T-cytotoxic/suppressor)

CD5-

(Peripheral blood T cells)

CD3-

(Associated with T cell

markers

antigen receptor) lntegrins

Interleukin-2R

Activation molecules

It is now firmly established that the major population of leukocytes present in the placental bed during the time that the extrmillous trophoblast infiltrates de-

Phenotype

CDlla+

(LFA-1)

CD18+

(g-chain to LFA-I)

CD25-

fp55)

p75+

ip75)

CD69 +

?

CD45RA ?? HML-1 *

? ?

LFA, lymphocyte function-associated antigen; NK, natural killer,

macrophages ( - 20%), and a few CD3 + T cells ( - 10%) [19,20]. An extensive phenotypic analysis by King et al. [21-] using flow cytometry has confirmed that although the majority of CD56+ cells express the early T-lineage markers, CD2 and CD7, they do not express the mature Tcell antigens such as CD3, CD5 and CD4/8 (Table 1).

Of the LGI,NK markers used, only CD56 was expressed

Functionally, it is clear that decidual CD56+ cells are capable of cytolytic activity against K562, although at lower levels than seen with PBL [24,25]. Uterine LGLSalso re-

spond to interleukin (IL)-2 by proliferating and becoming potent lymphokine-activated killer @AK>cells capable of greatly Increased killing of K562 [26*,27*]. We have demonstrated killing of first trimester cultured tro-

Recent developments

in the human

phoblast cells (which resemble extravillous trophoblast phenotypically in viva) by these decidual LAK cells [26*] although these trophoblast cells cannot be killed by naive decidual CD56+ cells [24]. In contrast, a separate study has reported that freshly isolated term chorionic plate trophoblast were not killed by decidual LAK cells. This contradictory finding may indicate different susceptibility of term and first trimester trophoblast. It is the behaviour of the latter that would be most relevant, as these are the cells that invade early decidua in viva. Alternatively, this findiig may be a result of technical differences in experimental protocols such as whether the target cells are labelled in suspension or as a monolayer. In general the latter is to be preferred w-hen dealing with an adherent cell population. When choriocarcinoma cell lines are used as targets, JEG cells are seen to be more susceptible than BeWo to killing by IL-2-stimulated decidual LAKeffecters [ 26*,27’]. This may suggest that JEG and BeWo represent different stages of trophoblast differentiation. As mentioned earlier, we favour the use of JEG as a more stable representative of the extravillous trophoblast population. A different approach to analyzing the phenotype and function of decidual LGLS has been used by Christmas et al. [28**] who have generated CD3- clones from decidual leukocytes. Although the proliferative frequency was low, 36 clones were obtained that all had T-cell receptor (TCR) y- and &genes in the germ-line configuration. These clones did not appear entirely representative of the CD56b@ht, CD3 - cells in vivo, as half of them expressed CD16 and it was only these CD16+ clones that exhibited NK activity. This raises the possibility that only the CD56+, CD16+ cells identified by flow cytometry [21**] are responsible for the NK activity in decidua. We have also found that the number of CDl6+, CD56+ cells increase after culture of decidual leukocytes with IL-2 for 4 days, indicating that such culture conditions are selective for a subpopulation (YW Lake, unpublished data). Also, although proliferation in vivo is clearly a progesterone-dependent phenomenon, neither progesterone alone, nor decidual stromal cell or trophoblast conditioned media induce a proliferative response in vitro (YW Loke, A King, unpublished data) indicating that present culture conditions do not reflect the decidual environment. These recent phenotypic and functional studies have led to the hypothesis that uterine CDSbbfi@ LGLS are uterine-specific leukocytes that migrate from the bone marrow to the uterus, proliferate there under the influence of progesterone, and acquire cytoplasmic granules and increased surface expression of CD56. Although some CD16 + cells are present, CDIGbriRhtcells (classical NK cells) are not seen in the uterus in vivo and conversely CDSbb%@ cells have not been identified in any other organ or mucosal surfaces. The observation that these cells are related spatially and temporally to the establishment of the human placenta suggests they may have a role to play in pregnancy. We believe that this role is to control implantation by limiting trophoblast migration into maternal decidua and spiral arteries. TroE

maternal-fetal

immune

interaction

Loke and King

phoblast HLA-G could act as a regulatory molecule. The effects of decidual LGL on HLA-G transfectants are now being studied. The results are awaited with much interest. In view of the unusual natures of both the trophoblast class I antigen and the decidual LGLs, the interactions in the early placental bed should no longer be viewed in terms of transplantation immunology. The control of human placentation is likely to involve a more primitive defence system that is as yet poorly defined.

References and recommended reading Papers of have been . of .. of

special interest, published highlighted as: interest outstanding interest

within the annual period of review,

1. .

HUNT JS, FISHBACKJL, CHUMBLEYG, LOKE YW: Identification of Class I MHC mRNA in Human First Trimester Trophoblast Cells by In Situ Hybridization. J Immunol 1990, 144:442&4425. This paper reports the distinction of three trophoblast subpopulations on the basis of HL4 class I mRNA and protein expression. 2. .

ELLISSA, PALMERMS, MCMICHAELAJ: Human Trophoblast and the Choriocarcinoma Cell Line BeWo Express a Truncated HLA Class I Molecule. J Immunol 1990, 144:731-735. Documents that the cDNA sequence from BeWo choriocarcinoma cell line and from normal term chorionic plate trophoblast showed a high degree of homology with a genomic clone HIA6.0 (now designated HIAG). KOVATS S, MAIN EL, LIBRACHC, STUBBLEBINEM, FISHER SJ, DER: A Class I Antigen, HLA-G, Expressed in Human Trophoblasts. Science 1990, 248:22(&223. The most comprehensive paper to date on the trophoblast HIAG molecule and the first to present aidence that HlA-G codes for an expressed protein. Describes live &forms of the antigen with molecular weights of 37-39 kD and isoelectric points of 4.5-5.5 PI. 3. ..

4.

ELLISSA, SARGENTIL, REDMANCWG, MCMICHAELAJ: Evidence for a Novel HLA Antigen Found on Human Extravillous Trophoblast and a Choriocarcinoma Cell Line. Immunology 1986, 59:59-l.

5.

RISKJM, JOHNSON PM: Northern Blot Analysis of HLA-G Expression by BeWo Human Choriocarcinoma Cells. J Reprod Immunol 1990, 18:19’+203.

6.

WEI X, ORR HT: Differential Expression HLA-G Transcripts in Human Tissue. 1990, 29:131-142.

7.

of HL4-E, HLA-F and

Human Immunology

WATKINS DI, CHEN Zw, HUGHES AL, EVANSMG, TEDDER TF, LE~VINNL: Evolution of the MHC Class I Genes of a New World Primate from Ancestral Homologues of Human NonClassical Genes. Nature 1990, 346:6M3.

8. .

GRAROWSKA A, CARTERN, IQKE YW: Human Trophoblast Cells in Culture Express an Unusual Major Histocompatibility Complex Class I-like Antigen. Am J Reprod Immunol 1990, 23:1&l% Reports that normal first trimester extravillous trophoblast expresses two class 1 antigens with heavy chains of 45 kD and 40 kD.

9.

DE WIT TFR, VLOEMANSS, VAN DEN EISEN PJ, HAWORTH 4

STERNPL: Differential Expression of the HLA Class 1 Multigene Family by Human Embtyonal Carcinoma and Choriocarcinoma Cell Lines. J Immunol 1990, 144:108&1087. ELLISSA, STRACHANT, PALMERMS, MCMICHAELAJ: Complete Nucleotide Sequence of a Unique HLA Class I C Locus Product Expressed on the Human Choriocarcinoma Cell Line BeWo. J Immunol 1990, 142:3281-3285. Reports the identification of HLA-C in BeWo choriocarcinoma cell line. 10. .

765

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11.

BOUCRAT J, HAKEM R, GAUT’HIER A, FAUCHET R, LE BOUTEILLER P:

.

Transfected Trophoblast-derived Human Cells Can Express a Single HLA Class I Allelic Product. Tissue Antigens 1991, 37z84-83. Provides an insight into possible mechanisms for the control of class I expression in trophoblast. 12.

13.

GRABOWSKA A, CHUMBLEY G, CARTER N, IDK!Z YW: Interferongamma Enhances mRNA and Surface Expression of Class I Antigen on Human Extravillous Trophoblast. Pkxentu 1990, 11:301-308. CHUMELEYG, HAWI.EYS, CARTER NP, LOKE YW: Human Extravillous Trophoblast MHC Class I Expression is Resistant to Regulation by Interferon-a. J Reprod Immunol, in press.

14. .

PURCELL DFJ, MCKENZIE IFC, LUBLIN DM, JOHNSON PM, ATKINSONJP, OGLESBY TJ, DEACON NJ: The Human Cellsurface Glycoproteins HuLy-m5, Membrane Co-factor Protein (MCP) of the Complement System, and Trophoblast Leucocyte-common (TLX) Antigen are, CD46. Immunology 1990, 70:155-161. Documents evidence that TLX and MCP are the same.

15.

MCIN’IYRE JA, Cross-reactive 1982, 427-35.

16.

MCIN’IYRE JA: In Search of Trophoblast-lymphocyte Crossreactive (TLX) Antigens. Am J R@vd Immunol Microbial 1988, 17:10&110.

FAULK WP:

Auotypic (TLX) Cell Surface

Trophoblast-lymphocytes Antigens. Hum Immunol

17.

Expression of Hsr B-L, HUNT JS, ATKINSONJP: Differential Complement Regulatory Proteins on Sub-populations of Human Trophoblast Cells. J Reprod Immunoll991, 19:2of)-223.

18.

RISK JM, FLANAGANBF, JOHNSON PM: Polymorphism of the Human CD46 Gene in Normal Individuals and in Recurrent Spontaneous Abortion. Hum Immunol 1991, 30:162-167.

19.

BUIMER JN: Decidual Cellular munol 1989, 1:1141-1147.

20.

KING A, WELLINGSV, GARDNER L, LOKE Yw: Immunocytochemical Characterization of the Unusual Large Granular Lymphocytes in Human Endometrium Throughout the Menstrual Cycle. Hum Immunol 1989, 24:195-205.

21. ..

KING A, BALENDRAN N, WOODING P, CARTERNP, IQKE yw: Phenotypic and Morphologic Characterization of Novel CD3Y, CD56bright+ lymphocytes in the Pregnant Human Uterus. Developmental Immunology 1991, 1:16%190.

Responses. ’

Curr

@in

Im-

A detailed phenotypic and morphologic analysis of CDSbb@t cells in both blood and decidua using flow cytometry and electron microscopy. The lineage and relationships of these cells fo classic NK cells is discussed. 22.

LWER LL, TESTI R, BINAL J, PHILLIPSJH: Identity of Leu-19 (CD56) Leukocyte Differentiation Antigen and Neural Cell Adhesion Molecule. J Eap Med 1989, 169:2233-2238.

23.

TABIBZADEHS: Proliferative Activity of Lymphoid Cells in the Human Endometrium Throughout the Menstrual Cycle. J Clin Endo Metab 1330, 70:437443.

24.

KING A, BIRKBY C, IQKE YW: Early Human Dccidual Cells Exhibit NK Activity Against the K562 Cell Line but not Against First Trimester Trophoblast. Cell Immunol 1989, 118:337-344.

25.

FERRY BL, STARKEYPM, SARGENT IL, WATT GMO, JA%SON M, REDMANCWG: Cell Populations in the Human Early Pregnancy Deciduz Natural Killer Activity and Response fo Interleukin-2 of CDSGPositive Large Granular Lymphocytes. Immunolo~ 1990, 70:446452.

KING A, LUKE YW: Human Trophoblast and JEG Choriocarcinema Cells are Sensitive to Lysis by IL-2 Stimulated De cidual NK Cells. Cell Immunol 1990, 129343H48. Reports that normal first trimester trophoblast is susceptible to lysis by IL-2-stimulated decidual LGL.

26 .

27. .

FERRYBL, SARGENTIL, STARKEYPM, REDMANCWG: Cytotoxic Activity Against Trophoblast and Choriocarcinoma Cells of Large Granular Lymphocytes from Human Early Pregnancy Decidua. Cell Immunol 1991, 132:140-149. A report which contradicts that of King et al. [26*] and suggests that term chorionic plate trophoblast is not susceptible to IL-2-stimulated decidual LGL. CHRISTMASSE, BULMERJN, MEAGERA, JOHNSON PM: Phenotypic and Functional Analysis of Human CD3Decidual Leucocyte Clones. Immunology 1990, 71:182-189. The first description of successful cloning of CD3 decidual leukocytes. Thirty six clones were obtained with T-ceil receptor y or 6 genes in germ-line configuration. Half the clones were CDl6+ and these exhibited NK activity. 28. ..

YW Lake and A King, Division of Cellular and Genetic Pathology, De partment of Pathology, University of Cambridge, Tennis Court Road, Cambridge CB2 lQP, UK