T Lymphocyte Subsets, Cytokines, and Effector Functions T. R. MOSMANN Department of Immunology University of Alberta Edmonton, Alberta T6G 2H7, Canada

INTRODUCTION During strong immune responses there are often characteristic patterns of effector functions associated with particular infectious agents. Antibody and delayed type hypersensitivity (DTH) response patterns are often mutually exclusive, suggesting that the cells mediating these responses are cross-inhibitory. Many effector and regulatory functions of the immune system are regulated by cytokines secreted by several cell types, particularly helper T cells. The discovery of different types of T helper cells'+2offers at least a partial explanation for this reciprocal regulation. Two types of T helper cell, defined by their cytokine secretion patterns, were originally identified amongst mouse T cell clones, and have more recently been confirmed in normal mouse and humad T cell populations, particularly during chronic anti-parasite or anti-bacterial responses (reviewed in415). Cytokine Secretion Phenotypes of TH Cells

THI cells secrete IL2, IFNy and lymphotoxin, whereas TH2 cells secrete IL4, IL5, IL6, ILlO and express mRNA for the P600 Several other cytokines are secreted by both types of cell. Recent suggests that the THI and TH2 patterns may represent only two out of several possible cytokine secretion phenotypes. Naive T cells, or long-term resting memory T cells, secrete predominantly or exclusively IL2 when first stimulated, and then appear to differentiate into one of several effector phenotypes secreting the TH1, TH2 or other cytokine patterns. These effector cell phenotypes are present in both large and small cell populations (T. R. Mosmann, J. H. Schumacher and R. C. Budd, submitted), but may have only a short life span of days or weeks. Although the THI and TH2 phenotypes are very distinct among T cell clones and also in normal populations during strong (e.g., anti-parasite) responses, the full extent of T cell diversity is not known. Several additional cytokine secretion phenotypes have been identified amongst T cell clones grown in tissue culture, but the significance of some of the minor phenotypes is not yet known. Future data from normal individual T cells will be necessary to reveal the full range of patterns. Functions and Cross-Regulation of TH Subtypes

Although each of the different T cell cytokine secretion phenotypes is expected to lead to significantly different functions, the THl and TH2 cell types have been most thoroughly characterized. TH2 cells are excellent helpers for antibody 89

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production and, in the absence of TH1 cells, will induce high IgE levels due to IL4 production.I0 Although TH1 cells can provide help for antibody production, excess T H l activation inhibits B cell activation, and IgE production is inhibited at all levels of T H l activation due to production of IFNy. A strong DTH-like reaction is induced by THI clones," and the T H l cytokine pattern is associated with DTH reactions in vivo. Although the two major patterns of antibody and DTH may be due to TH2 and T H l responses, respectively, the immune system can clearly induce several other patterns of effector functions, and so the THI /TH2 regulation is only part of the total immune response regulation. The cross-regulation of T H l and TH2 responses can be partially explained by cytokine effects leading to reciprocal inhibition: IFNy, produced by TH1 cells, inhibits proliferation of TH2 cells.'* Since strong TH2 responses lead to inhibition of THl-like responses, we searched for a cross-inhibitory cytokine that would inhibit some aspect of TH1 function. This led to the discovery of IL10, which is produced by TH2 cells, and which inhibits T H l cytokine p r o d ~ c t i o nThis . ~ effect is due to an inhibition of the ability of macrophage-like cells to present antigen to T H l cells.'3 In addition to these cross-regulatory effects on the functions of T H l and TH2 mature effector cells, there are also cytokine effects on the differentiation of precursor T cells into various effectors such as THl and TH2 cells. For example, activation of precursor T cells that secrete only IL2 results in the development of TH2-like cells if the culture is carried out in the presence of IL4.I4 Isolation of the cDNA clone for ILlO allowed the production of recombinant IL10,'5 and this led to the discovery of several additional functions of ILlO, including costimulation of proliferation of mast cell lines16 and thymocytes, l 7 induction of MHC class I1 antigens on B cells,18and inhibition of cytokine production by macrophages.I9 These functions are generally consistent with the overall functions of TH2 cells and several are similar to those of IL4.*' Regulation of Allergic Responses

Although the regulation of many facets of the immune response is likely to be extremely complex, the control of allergic responses appears to be one of the less complex regulatory systems, and many of the major regulatory cytokines are THlor TH2-specific. Several features of the allergic response are enhanced by TH2 cytokines. IL4 induces B cells to switch to IgE production.2' IL5 is the major eosinophil growth and differentiation factor,22and IL3, IL4,23and IL10l6 are all TH2 products that synergize to induce proliferation of mast cells. Mast cells and eosinophils bind IgE and use this as a receptor for recognizing antigen and mediating many allergic functions. Thus TH2 activation leads to a concerted enhancement of the allergic response. In contrast, THl cells produce IFNy which inhibits mast cell activation, B cell switching to IgE production, and TH2 cell proliferation. These inhibitory effects of TH1 cells are in turn inhibited by ILlO so that either TH1 or TH2 cells can establish dominance, depending on the nature of the initial response. Many of these cross-regulatory functions have been verified in vivo, but the mechanisms that initially induce preferentially TH1 or TH2 responses are largely unknown. Regulation of EffectorFunctions during Parasite Infections

During anti-parasite responses, there are often strong antibody responses accompanied by suppression of DTH, or strong DTH responses with suppression of

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antibody production. A striking example of this reciprocal regulation is the immune response to Leishmania infection, which is either strongly TH1- or TH2-like, depending on the strain of Only the THI-like response is able to eradicate the infection. Anti-IFNy treatment of mice that normally show the TH1 healing response results in a conversion to a TH2 response that does not cure the infection, while treatment of nonhealer TH2 mice with anti-IL4 leads to the THI response and a cure. In two other parasite systems, infection by Nippostrongylus brasiliensis and Schistosoma mansoni, ILlO is responsible for the inhibition of IFNy synthesis observed in in uitro culture systems in response to lectin or antigen stimulation,26and thus ILlO is likely to be another of the mediators that regulates the reciprocal nature of antibody and DTH responses.

CONCLUSION Although several cytokine regulatory effects are now known, it is likely that many additional regulatory loops remain to be discovered. The recent discovery of the function of another TH2-specific cytokine, P600 (J-M. Heslan, L. Guilbert and T. R. Mosmann, unpublished) suggests that there are additional regulators to be characterized, and the existence of additional T cell cytokine secretion phenotypes may also help to explain further complexities of the regulation of the class of immune response. REFERENCES

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ANNALS NEW YORK ACADEMY OF SCIENCES J. A. LEVERAH & T. R. MOSMANN. 1990. Heterogeneity of mouse helper T cells: evidence from bulk cultures and limiting dilution cloning for precursors of Thl and Th2 cells. J. Immunol. 144: 1629-1639. COFFMAN, R. L., B. W. SEYMOUR, D. A. LEBMAN, D. D. HIRAKI, J. A. CHRISTIANSEN, B. SHRADER, H. M. CHERWINSKI, H. F. SAVELKOUL, F. D. FINKELMAN, M. W. BOND& T. R. MOSMANN.1988. The role of helper T cell products in mouse B cell differentiation and isotype regulation. Immunol. Rev. 102: 5-28. CHER,D. J. & T. R. MOSMANN. 1987. Two types of murine helper T cell clone. 11. Delayed-type hypersensitivity is mediated by THI clones. J. Immunol. 138: 3688-3694. GAJEWSKI, T. F. & F. W. FITCH.1988. Anti-proliferative effect of IFN-gamma in immune regulation. I. IFN-gamma inhibits the proliferation ofTh2 but not Thl murine helper T lymphocyte clones. J. Immunol. 140: 4245-4252. P. VIEIRA,T. R. MOSMANN, M. HOWARD, K. W. FIORENTINO, D. F., A. ZLOTNIK, MOORE& A. O’GARRA.1991. IL-I0 acts on the antigen-presenting cell to inhibit cytokine production by Thl cells. J. Immunol. 146: 3444-3451. SWAIN,S. L., A. D. WEINBERG, M. ENGLISH & G. HUSTON.1990. IL-4 directs the development of Th2-like helper effectors. J. Immunol. 145: 3796-3806. MOORE,K. W., P. VIEIRA,D. F. FIORENTINO, M. L. TROUNSTINE, T. A. KHAN& T. R. MOSMANN.1990. Homology of cytokine synthesis inhibitory factor (IL-10) to the Epstein-Barr virus gene BCRFI. Science 248: 1230-1234. K. W. MOORE& D. THOMPSON-SNIPES, L., V. DHAR,M. W. BOND,T. R. MOSMANN, RENNICK. 1991. Interleukin 10: a novel stimulatory factor for mast cells and their progenitors. J. Exp. Med. 173: 507-510. I., T. SUDA,K. W. MOORE,T. R. MOSMANN&A. ZLOTNIK. 1990. InterleuMACNEIL, kin 10: a novel cytokine growth cofactor for mature and immature T cells. J. Immunol. 145: 4167-4173. Go, N. F., B. E. CASTLE,R. BARRETT, P. VIEIRA,R. KASTELEIN, W. DANG,T. R. MOSMANN, K. W. MOORE& M. HOWARD.1990. Interleukin 10 (IL-10) a novel B cell stimulatory factor: unresponsiveness of “XID” B cells. J. Exp. Med. 172: 1625- 1631. FIORENTINO, D. F., A. ZLOTNIK, T. R. MOSMANN, M. HOWARD & A. 0. O’GARRA. 1991. ILlO inhibits cytokine production by activated macrophages. J. Immunol. 147: 3815-3822. MOSMANN, T. R. & A. ZLOTNIK. 1989. Multiple functions of IL4 and its role in immune regulation. In Growth Factors, Differentiation Factors, and Cytokines. A. Habenicht, Ed. 129. Springer-Verlag. Heidelberg-New York. COFFMAN, R. L., J. OHARA,M. W. BOND, J. CARTY,A. ZLOTNIK& W. E. PAUL. 1986. B cell stimulatory factor 1 enhances the IgE response of lipopolysaccharideactivated B cells. J. Immunol. 136: 4538-4541. D. J. WARREN & G. G. KLAUS.1986. Eosinophil SANDERSON, C. J., A. 0. O’GARRA, differentiation factor also has B-cellgrowth factor activity: proposed name interleukin 4. Proc. Natl. Acad. Sci. USA 83: 437-440. MOSMANN, T. R., M. W. BOND,R. L. COFFMAN, J. OHARA& W. E. PAUL.1986. Tcell and mast cell lines respond to B-cell stimulatory factor I . Roc. Natl. Acad. Sci. USA 83: 5654-5658. B. J. HOLADAY, R. L. COFFMAN & R. M. LOCKSLEY. HEINZEL, F. P., M. D. SADICK, 1989. Reciprocal expression of interferon gamma or IL4 during the resolution or progression of murine leishmaniasis. Evidence for expansion of distinct helper T cell subsets. J. Exp. Med. 169 59-72. SCOTT,P. 1989. The role ofTHl and TH2 cells in experimental cutaneous leishmaniasis. Exp. Parasitol. 68: 369-372. 1991. SHER,F. A., D. F. FIORENTINO, P. CASPAR,E. PEARCE& T. R. MOSMANN. Production of IL-I0 by CD4+ T lymphocytes correlates with down-regulation of THI cytokine synthesis in helminth infection. J. Immunol. 147: 2713-2716.