J.-P. de Villartay:

T~ T C E L L S

characteristics, and another more closely resembling ~[3 T cells (Kyes and Hayday; Bluestone and Matis). Thus, the whole gamut of T cell specificities would be observed, such as antiviral (Carding), anti-allo MHC (numerous) and anti-heat shock proteins. Selection and repertoire o f ~ T cells. - - From reading the papers presented in this Forum, and the literature, it is apparent that the nature of the ligands for "r~ T ceils is also an open question. Confusion about this issue might also be compounded by superantigen expansion of a subset of "rBT cells in the periphery. In addition, a degree of heterogeneity in the "r8 T-cell syster could indicate that the possible ligan~ might be diverse. To me, the expansion of a subset of T cells in the periphery: be they 0t[3 or "rS, does not constitute "peripheral positive selection", but rather simple reactivity to antigen or superantigen. TB T cells and disease association. - The involvement of yB T cells in certain diseases may provide a clue as to their specificities or functional significance (see Holoshitz, and Rust et al.). In humans, yB T cell are few in number, even at epithelial surfaces, and it is difficult to imagine that a totally y~i-deficient individual would be compromised at all with regard to immune capabilities. Yet the preferential accumulation of ~'~ T cells in the gut lesions of coeliac disease patients indicates that y8 T cells in humans probably also conform, at least partially, to the features of TB T cells described in other species. It might just take more of an insult to the integrity of the tissue to evoke a yB response, and the specificity and biological response of y8 T cells in coeliac disease probably resembles that of TB T cells in the gut of other species, despite the reported non-restricted usage of V~ and Vy gene segments by y6 T cells in these human gut lesions. With regard to yB T-cell expansion in autoimmunity or imm u n o d e f i c i e n c y , a n u m b e r of possibilities exist. "t'B T-cell numbers may be normal in such patients, but appear elevated because of a decline in 0t[3 T-cell numbers. Moreover, an impaired 0t[3 system may lead to many infected

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and stressed cells, or numerous mycobacterial infections, which might expand the y8 T-cell system. I believe that "r8 T-cell expansion in rheumatoid arthritis is most likely a consequence of the disease, i.e. reactivity to heat shock protein in the lesions, rather than a mechanism that initiates the disease. Evolution/relation o f y~ T cells to 0t[3. - - y~ T cells might be remarkably similar to 0t[3T ceils, although perhaps less sophisticated. Like 0t[3 T cells, "t'~ T cells show dichotomy in function by mediating target cell lysis, and secreting interleukins and providing B-cell help (Gakrill et al., Vidovic and Dembic). Bluestone and Matis and Spits et al. point out the fact that ~'~ T cells may be more skewed to MHC recognition than is currently appreciated, suggesting that ~[3 and (at least a subset of) ~'~ T-cell recognition of antigen with an MHC product is similar. Moreover, the unrestricted NK-like killing by y~ T cells is probably artifactual (Spits et al.). Our studies in sheep on mixed lymphocyte reactivity indicate that T~ T cells can show a vigorous response (Mackay and Matzinger, unpublished), and this is unrelated to a heat shock or stress response. The various ligands identified for y~ T cells, such as classical MHC, MHC-like, and others, and the different distributions of ~8 T cells coupled with different -~,8gene usage and specificities, has led to a plausible notion that a[3 and 2"8T cells may have evolved similarly, with a subset of ~ T cells retaining a more primitive-type profile, and another resembling 0ti3T cells (Kyes and Hayday, Bluestone and Matis). The relative levels of two different y~ populations in different species might provide an explanation for the apparent heterogeneity m ~'~ T-cell features among spectes. J.-P. de Villartay: After having read all the contributions made to this Forum on y/8 cells, I must say that I felt a little lonely with my attempt to bring some elements on the regulation of TCR-8 and TCR-0t gene usage. On the other hand, it has

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33 r~ F O R U M

IN IMMUNOLOGY

been, for me, a very useful review of the current views regarding the biological role of the TCR-y/8 subset of T cells. I will try to summarize what I learned and ask the questions raised by the reading of these interesting contributions. Until now, one immune system composed of the T cells expressing the 0t/13 TCR had seemed to be sufficient to accomplish defence against foreign pathogens. ~/~3 T cells seemed to be a fairly homogeneous lineage of T lymphocytes that differentiate within the thymus where they acquire, as a result of both positive and negative selection, the capacity of discriminating self from nonself while generating a broad repertoire of antigenic specificities. On the contrary, the TCR-y/~ already appears as a much more disparate system. The first comment that comes to mind from this series of paper is that y/~ T cells constitute a very heterogeneous population. Although y/3 T cells clearly show, at least in the mouse, a tropism for epithelia, each epithelium seems to have a preferential set of y/8 T cells, utilising specific V'r genes. These cells utilize various V~ genes and show a large junctional diversity in the gut, but have very little diversity in the skin and are almost monoclonal in the lung (BID sequences). These lymphocytes are either lhvmnden~nclont

I~.l¢~n]

nr

thxrmr~n_

depe~dent (gut and lung). In addition to the "epithelium-specific" subset, y/8 T cells are aL ~ found in the periphery (blood, spleen) where they utilise yet another set of specific V'r and V8 sequences and show extensive junctional diversity. Peripheral y/8 cells are thymoindependent and they seem to be positively selected in the periphery upon continuous challenge by their ligand. One conclusion that can be drawn from this Forum is that the Iigand of y/~ T lymphocytes, and therefore the precise biological function of these cells, remain a black box. I would like to come back to one feature of y/8 lymphocytes which is brought out by many contributors: the vast majority of the y/8 T cells, whether "peripherical" or "epithelial" (gut and lung) are thymoindependent, as they are found in nude mice as well as in human DiGeorge patients. This property is im-

portant because it cancels the dogma according to which the thymus is the main differentiating organ for T lymphocytes. Indeed, data are now accumulating which present evidence that T cells can fully differentiate outside of the thymus. S. Carding et al. (19~0) have recently reported the existence of fully rearranged and expressed TCR-y- and -g-chain genes within the foetal liver and the gut (two endoderm derivatives like the thymus) before the first colonization of the thymic lobes. D. Guy-Grand et al. (1990, in press) have, in the same way~ presented strong evidence indicating that the y/~ T lymphocytes of the gut epithelium could arise from progenitors of bone marrow origin that have migrated in the gut, where they further differentiate in the context of the gut microenvironment. This model is sustained by the finding of recombinase activity (expression of the RAG-1 gene) among T lymphocytes of the gut. Further evidence for a possible gut-induced differentiation of T lymphocytes is the presence of cells having the phenotype of pro-T (CD7*CD2CD3-) among human intestinal intraepithelial lymphocytes (Jarry et al., 1990). Then another question comes up which is whether y/~ T cells can undergo negative selection and by which mechanism does it occur ? Many papers in this Forum have shown that y/8 T cells can indeed respond to MHC molecules or MHC-like molecules, implying the necessity, as for 0t/13 T cells, of eliminating potentially self-reactive y/8 lymphocytes. S. Hedrick and A. Dent present evidence that, in a transgenic model, potentially selfreactive y/8 T cells are indeed negatively selected. This selection seems to be the result of clonal deletion rather than clonal anergy, a situation similar to the negative selection of the ~/[3 T cells. However, this mechanism has been described for the negative selection of the y/8 within the thymus and may not be transposable to the negative selection of the thymoindependent subset of y/~ T cells. M. Bonneville et al. (1990) have shown, using a similar model of transgenic mice, that clonal anergy could be mainly responsible for negative selection

T~ T C E L L S

of ~,/8 T cells both in the thymus and in the periphery. The last point I would like to mention is the interesting observation that the -f/8 T-cell system varies considerably among species. Whereas ruminants have about half of their circulating T cells composed of the -f/8 subset, humans seem to have lost the very res-

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tricted pattern of epithelial y/5 T lymphocytes. The situation of the mouse would be somewhere in-between. C. Mackay and W. Hein have clearly pointed out that different species may to some extent rely on the ~'/8 T cells for their immune defence. In this respect, ethological studies might be of great interest in trying to define the precise role of the T/8 T-cell system.

References. BONNEVILt.E,M. et aL (1990), Self tolerance to transgenic "r/8 T cells by intrathymic inactivation. Nature (Lond.), 344, 163. CARD,NO,S.R. et aL (1990), Developmentally regulated fetal thymic and extrathymic T-cell receptor "r/8 gene expression. Gene and Development, 4, 1304. GtJY-G~AND, D. et al., Two gut intraepithelial CD8 + lymphocyte population with different T-cell receptor: a role for the gut epithelium in T-cell differentiation. J. exp. Med. (in press). JARRY,A. et ai. (1990), Subsets of CD3 + (T-cell receptor cx/~or "r,/8) and CD3- lymphocytes isolated from normal human gut epithelium display phenotypical features different from their counterparts in peripheral blood. Europ. J. lmmunoi., 20, 1097.

H. Spits, H. Yssel and J.E. de Vries: Selection o f V~9V82 using TCR'(8 + cells in the human foetal thymus.

It is intriguing that the V-f9V82 pair is expressed predominantly in early foetal development. As indicated by Triebel, it is very well possible that these cells are positively selected in the foetal thymus, although it cannot be formally excluded that cells that rearrange V~9 to J's other than JP or that join V-fl0 or 11 are negatively selected. If the cells are positively selected, the "selecting" antigen is perhaps expressed in the thymus only in early foetal development and not after birth, because V~,9V82-positive cells are rare in the postnatal thymus. As indicated by Bolhuis and colleagues, most if not all V'r9V82-expressing cells recognize an antigen on Daudi cells. We have found that foetal TCR3,8 + thymocyte clones expressing V'r9 and V82 respond to Daudi by producing cytokines, but do not lyse Daudi cells. It is suggested that Daudi cells express a 58-kDa stress pro-

tein, which may be the target antigen recognized by the V-f9V~2-positive cells. It ma) also be possible that Daudi .ells express the ligand molecule that selects ellS. the V~,9V~,2-expressing C..... It should also be noted that foetal TCR~'8 + thymocyte clones produce significant levels of IL-4 and IL-5, while postnatal thymocyte clones fail to produce those cytokines (Krangel et aL, 1990, J. exp. Med., 172, 847). IL-4 is known to have profound effects on T-cell growth and probably also on differentiation. It is possible that V-f9V82* foetal thymocytes produce IL-4 in vivo after activation with antigens expressed in certain parts of the foetal thymus and thereby influencing growth a n d / o r differentiation of TCR0t[3+ T cells. However, we have yet to demonstrate that foetal V-r9V82 cells produce IL-4 or other cytokines in vivo. One should realize that using the terminology "positive selection" does not imply that TCR~,8+ are selected in the same way as TCR0~13÷ T cells, In a recent workshop about TCR'f8 + T cells held in Schloss Elmau, Germany, it was