Demonstration of Functional Heterogeneity of T lymphocytes and Identification of Their Two Major Subsets

A commentary on

Ly antigens as markers for functionally distinct subpopulations of thymus-derived lymphocytes of the mouse
by Kisielow P, Hirst JA, Shiku H, Beverley PCL, Hoffmann MK, Boyse EA, et al. Nature (1975) 253:219–20. doi: 10.1038/253219a0

Cells responsible for the specificity of the immune response and thymus function remained unknown until mid 1960s when lymphocytes were identified as mediators of cellular and humoral immunity (1–4). Soon afterward in vivo reconstitution experiments showed that cellular and humoral immunity are mediated by two different types of lymphocytes: thymus-derived T cells and thymus-independent, bone marrow derived antibody producing B cells, respectively (5). This finding created a need for a simple and rapid method to distinguish these two classes of lymphocytes, and the immune system itself provided the best tool for this task. In 1969, Martin Raff (6) showed that antibodies to Thy1 antigen [cell surface molecule expressed chiefly in the thymus and brain (7)] react with all thymus-dependent but not with thymus-independent lymphocytes. This allowed the distinguishing and separation of functionally different but morphologically identical lymphocytes allowing study of their specific activities. Meanwhile Edward Boyse, Lloyd Old, and colleagues identified two other cell surface antigens expressed mainly in the mouse thymus, Ly1 and Ly2 (8), which were thought to characterize all T cells, like Thy1.

By that time T cells were known to have several functions and an important question emerged: do different functions reflect their functional heterogeneity or are manifestations of homogenous population of multifunctional cells?

Late in 1972, I obtained a postdoctoral WHO fellowship. Being interested in lymphocyte biology and cell surface immunogenetics I contacted Dr. Lloyd Old, who thanks to the recommendation of my PhD supervisor, Professor Czesław Radzikowski invited me to his laboratory at the Sloan-Kettering Institute in New York. During our first conversation Dr. Old suggested that I join Hiroshi Shiku, another postdoc, to study the mechanism of target cell killing by T cells. Dr. Old proposed to use a panel of antisera known to react with different surface antigens on thymocytes, to find if they could block cytotoxicity and in this way learn about possible involvement in this process of thus identified molecules. Dr. Shiku had just applied to the mouse system a new in vitro method, developed to evaluate cytotoxicity of lymphocytes by measuring the remaining radioactivity of adherent target cells labeled with tritiated proline (9). The specific antisera were provided by Dr. Boyse, among them anti Ly1 and anti-Ly2. The only information we had at that time about their reactivity with lymphocytes in different lymphoid organs was derived from in vitro absorption assays, which indicated that thymus had higher absorption capacity than lymph nodes and lymph nodes higher than spleen (8) correlating with the number of Thy1 positive cells in these tissues. My task was then to test the antisera against the effector cell population to determine their optimal titers using a complement dependent, trypan blue exclusion cytotoxicity assay.

Our repeated attempts to block cytotoxicity by pre-incubation of allo-reactive effector cells from immunized mice were unsuccessful¹ but because I noticed that the proportions of lymphocytes lysed by Thy1, Ly1, and Ly2 antisera in lymph nodes, spleen, and thymus were slightly different, we decided to test the effect of elimination of the cells sensitive to these antisera on the ability of surviving cells to kill target cells. The results showed that elimination of cells with Ly2 antisera (which lysed less lymphocytes than Ly1 antisera) inhibited cytotoxicity significantly stronger than elimination of lymphocytes with Ly1 antisera, suggesting that cytotoxic T cells can be distinguished from other T cells by their Ly1^low(−)Ly2^high(+) surface phenotype. Some doubts, however, still remained because the methods used were not sufficiently precise to be sure that the relatively small differences we observed were real. Moreover, since we only tested cytotoxicity (10), the proof of functional heterogeneity of T cells was missing. In order to find whether Ly1⁻2⁺ and Ly1⁺2⁻ lymphocytes may have different functions, I approached John Hirst (who in the Herbert Oettgen laboratory studied the helper activity using the Mishell-Dutton cell culture method) and proposed to test the effect of elimination of lymphocytes with Ly antisera on this regulatory function. Already, the first experiment produced clear-cut results: we observed practically no inhibition by treatment with Ly2 antisera and almost complete inhibition with Ly1 antisera, indicating that helper and killer T cells have different Ly phenotypes: Ly1⁺2⁻ and Ly1⁻2⁺, respectively. Importantly, I found that lymphocytes with different sensitivity to Thy1, Ly1, and Ly2 antisera are also present in the thymus of non-immunized normal mice, strongly suggesting that lymphocytes with different Ly phenotypes are generated independently of antigenic stimulation. These findings (11) represented the first unequivocal demonstration of functional heterogeneity of T cells and identified – as subsequent studies showed – their two major subsets. Moreover, identification of cells with three different Ly phenotypes (Ly1⁺2⁺, Ly1⁺2⁻, and Ly1⁻2⁺) in the thymus (11, 12) opened new possibilities to study T cell development and intra-thymic mechanisms of selection. Years later monoclonal antibodies ousted the use of antisera. CD4, the newly discovered T cell specific molecule (13, 14) and functional counterpart of Ly2 (CD8) on Ly2⁻ T cells turned out to better define T cell subsets than Ly1(CD5), which was later shown to be also expressed at a low level on Ly2⁺ T cells and on some B cells (15). Consequently, subsets originally defined by Ly antisera as Ly1⁺2⁻ and Ly1⁻2⁺ became re-defined and re-named as CD4⁺8⁻ and CD4⁻8⁺.

Our results, which I publicly reported for the first time in summer 1974 at the workshop chaired by Martin Raff during the International Congress of Immunology in Brighton, UK, were obtained between December 1972 and August 1973. Although immediately recognized as very important (Dr. Boyse in his letter to WHO, dated August 7, 1973 wrote: “Dr. Kisielow together with two other members of our group here has begun what may be a very important finding in the immunology of lymphocytes. Identification of different lines of T lymphocytes that may perform different function is a crucial step in our understanding of immunological responses and hence is of relevance to cancer no less than to several other fields of medicine”) the publication in Nature (11) was delayed by non-scientific reasons.

After my return to Poland, Harvey Cantor repeated, confirmed, and extended our results by providing firm evidence for antigen independent generation of Ly1⁺2⁻ and Ly1⁻2⁺ subsets (12). In addition, he showed that Ly1⁺2⁻ lymphocytes cooperate not only with B cells but also with Ly1⁻2⁺ lymphocytes in the generation of killer activity and obtained the first suggestive evidence that antigen recognition by Ly1⁺2⁻ and Ly1⁻2⁺ may be restricted by different classes of MHC antigens (16). This was around the time of discovering the MHC restriction phenomenon (17) and some time later it was firmly established that antigen recognition by CD4⁺8⁻ and by CD4⁻8⁺ T cells is restricted by class II and class I MHC molecules respectively, and that CD4 and CD8 molecules specifically interact with restricting MHC molecules on antigen-presenting cells (18). Puzzling, for a long time unanswered questions concerned the identity of the immediate precursor cells of CD4⁺8⁻ and CD4⁻8⁺ lineages and the mechanism of their intra-thymic selection. Many researchers, including myself (19), tried to decipher the developmental potential of the obvious candidate cell, i.e., CD4⁺8⁺ (Ly1⁺2⁺) thymocyte. My desire to study this problem started my long time collaboration with Harald von Boehmer, who generously kept inviting me, and my students, to the Basel Institute for Immunology. There, we were fortunate to participate in the project aimed at studying T cell development and selection in TCR transgenic mice, which led to the identification of CD4⁺8⁺ thymocytes as a target of positive (20–22) and negative (23, 24) selection allowing us to gain insight into the mechanisms of these processes. Learning how T cell subsets, which we had identified 15 years earlier (11) are born in the thymus was one of the happiest experience in my scientific life, despite being aware that efforts to understand the immune system represent a continuous, never-ending asymptotic process where the answer to the given question is as good as the number of new questions it generates.

Conflict of Interest Statement

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

I would like to dedicate this article to Professor Czesław Radzikowski and to Dr. Lloyd Old. I would also like to thank Harald von Boehmer and the Basel Institute for Immunology for long lasting support.

Footnote

1. ^Our failure to block cytotoxicity by Ly2 antisera was probably due to the idiosyncratic properties of our method since some years later E. Nakayama and colleagues achieved blocking by measuring chromium release from labeled non- adherent target cells. (Nakayama, E., Shiku, H., Stockert, E., Oettgen H. F. and Old L. J. Cytotoxic T cells: Lyt phenotype and blocking of killing activity by Lyt antisera. (1979) Proc. Natl. Acad. Sci. 76: 1977–81).

References

1. Gowans JL, Knight EJ. The route of re-circulation of lymphocytes in the rat. Proc R Soc Lond B Biol Sci (1964) 159:257–82. doi:10.1098/rspb.1964.0001

CrossRef Full Text | Google Scholar

2. Gowans JL, Uhr JW. The carriage of immunological memory by small lymphocytes in the rat. J Exp Med (1966) 124:1017–30. doi:10.1084/jem.124.5.1017

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

3. Parrot DM, East J. The role of the thymus in neonatal life. Nature (1962) 195:347–8. doi:10.1038/195347a0

CrossRef Full Text | Google Scholar

4. Miller JFAP, Mitchell GF. The thymus and the precursors of antigen reactive cells. Nature (1967) 216:659–63. doi:10.1038/216659a0

CrossRef Full Text | Google Scholar

5. Mitchell GF, Miller JFAP. Cell to cell interaction in the immune response. II. The source of hemolysin-forming cells in irradiated mice given bone marrow and thymus or thoracic duct lymphocytes. J Exp Med (1968) 128:821–37. doi:10.1084/jem.128.4.821

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

6. Raff M. Theta isoantigen as a marker of thymus derived lymphocytes in mice. Nature (1969) 224:378–9. doi:10.1038/224378a0

CrossRef Full Text | Google Scholar

7. Reif AE, Allen JMV. The AKR thymic antigen and its distribution in leukemias and nervous tissues. J Exp Med (1964) 120:413–33. doi:10.1084/jem.120.3.413

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

8. Boyse EA, Miyazawa M, Aoki T, Old LJ. La-A and Ly-B: two systems of lymphocyte isoantigens in the mouse. Proc R Soc Lond B Biol Sci (1968) 178:175–93. doi:10.1098/rspb.1968.0032

CrossRef Full Text | Google Scholar

9. Shiku H, Bean MA, Old LJ, Oettgen HF. Cytotoxic reactions of murine lymphoid cells studied with a [³H] proline microcytotoxicity test. J Natl Cancer Inst (1975) 54:415–25.

Pubmed Abstract | Pubmed Full Text | Google Scholar

10. Shiku H, Kisielow P, Bean MA, Takahashi T, Boyse EA, Oettgen HF, et al. Expression of T-cell differentiation antigens on effector cells in cell-mediated cytotoxicity in vitro: evidence for functional heterogeneity related to the surface phenotype of T cells. J Exp Med (1975) 141:227–41. doi:10.1084/jem.141.1.227

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

11. Kisielow P, Hirst JA, Shiku H, Beverley PCL, Hoffmann MK, Boyse EA, et al. F. Ly antigens as markers for functionally distinct subpopulations of thymus-derived lymphocytes of the mouse. Nature (1975) 253:219–20. doi:10.1038/253219a0

CrossRef Full Text | Google Scholar

12. Cantor EA, Boyse EA. Functional subclasses of T lymphocytes bearing different Ly antigens. I. The generation of functionally distinct T-cell subclasses is a differentiative process independent of antigen. J Exp Med (1975) 141:1376–89. doi:10.1084/jem.141.6.1376

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

13. Reinhertz EL, Kung PC, Goldstein G, Schlossman SF. Separation of functional subsets of human T cells by a monoclonal antibody. Proc Natl Acad Sci U S A (1979) 76:4061–5. doi:10.1073/pnas.76.8.4061

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

14. Dialynas DP, Quan ZS, Wall KA, Pierres A, Quantans J, Loken MR, et al. Characterization of the murine T cell surface molecule, designated L3T4, identified by monoclonal antibody GK 1.5: similarity of L3T4 to the human Leu-3/T4 molecule. J Immunol (1983) 131:2445–51.

Pubmed Abstract | Pubmed Full Text | Google Scholar

15. Ledbetter JA, Rouse RV, Micklem HS, Herzenberg LA. T cell subsets defined by expression of Lyt-1,2,3 and Thy-1 antigens. Two-parameter immunofluorescence and cytotoxicity analysis with monoclonal antibodies modifies current views. J Exp Med (1980) 152:280–95. doi:10.1084/jem.152.2.280

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

16. Cantor EA, Boyse EA. Functional subclasses of T lymphocytes bearing different Ly antigens. II. Coopertion between subclasses of Ly+ cells in the generation of killer activity. J Exp Med (1975) 141:1390–9. doi:10.1084/jem.141.6.1376

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

17. Zinkernagel RM, Doherty PC. Restriction of in vitro T-cell mediated cytotoxicity in lymphocytic choriomeningitis within a syngeneic or semiallogeneic system. Nature (1974) 248:701–2. doi:10.1038/248701a0

CrossRef Full Text | Google Scholar

18. Meuer SC, Schlossman SF, Reinherz EL. Clonal analysis of human cytotoxic T lymphocytes: T4⁺ and T8⁺ effector T cells recognize products of different major histocompatibility complex regions. Proc Natl Acad Sci U S A (1982) 79:4395–9. doi:10.1073/pnas.79.14.4395

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

19. Draber P, Kisielow P. Identification and characterization of immature thymocytes responsive to T cell growth factor. Eur J Immunol (1981) 11:1–7. doi:10.1002/eji.1830110102

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

20. Teh HS, Kisielow P, Scott B, Kishi H, Uematsu Y, Bluthmann H, et al. Thymic major histocompatibility complex antigens and the αβ T-cell receptor determine the CD4/CD8 phenotype of T cells. Nature (1988) 335:229–33. doi:10.1038/335229a0

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

21. Kisielow P, Teh HS, Bluthmann H, von Boehmer H. Positive selection of antigen-specific T cells in thymus by restricting MHC molecules. Nature (1988) 335:730–3. doi:10.1038/335730a0

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

22. Kisielow P, Miazek A. Positive selection of T cells: rescue from programmed cell death and differentiation require continual engagement of the T cell receptor. J Exp Med (1995) 181:1975–84. doi:10.1084/jem.181.6.1975

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

23. Kisielow P, Bluthmann H, Staerz UD, Steinmetz M, von Boehmer H. Tolerance in T-cell receptor transgenic mice involves deletion of nonmature CD4⁺8⁺ thymocytes. Nature (1988) 333:742–6. doi:10.1038/333742a0

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

24. Swat W, Ignatowicz L, von Boehmer H, Kisielow P. Clonal deletion of immature CD4⁺8⁺ thymocytes in suspension culture by extrathymic antigen-presenting cells. Nature (1991) 351:150–3. doi:10.1038/351150a0

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar