EXPERIMENTAL PARASITOLOGY 73, 106-l@ (1991)
MINIREVIEW Regulation
of Immunity RICK
Department
of Zoology,
University
in Trypanosoma
cruzi Infection
L. TARLETON of Georgia,
Athens, Georgia 30602, U.S.A.
TARLETON, R. L. 1991. Regulation of immunity in Trypanosoma imental Parasitology 73, 106-109. Q 1~91Academicpress,~nc.
Infection with Trypanosoma cruzi is accompanied by suppression of immune responses of a variety of types and antigen specificities. The specific mechanisms which account for suppression of immune responses in T. cruzi infection are not agreed upon and although the significance of immunosuppression in this infection is not known, it is likely that both immunity to T. cruzi and pathogenesis resulting from the infection are influenced by the downregulation of immune responses.
cruzi infection. Exper-
clear. At the cellular level, CD8+ T cells have a major influence on IL-2 production in this system as shown by in vitro cell depletion experiments (Tarleton 1990) and more recently in studies using mice deficient in CD8+ T cells due to deletion of the P-2-microglobulin gene (Tarleton and Kohler, unpublished results). Immune exhaustion or prior stimulation may also contribute to the defect in IL-2 production since an in vitro “rest” period of 48-72 hr restores the ability of lymphocytes from T. cruzi-infected mice to produce IL-2 (Tarleton 1988). IL-2 production in general is INTERLEUKIN-2 PRODUCTION known to be regulated primarily at the tranThe increased understanding of the role scriptional level and the correlation of deof cytokines in regulating immunity has pressed IL-2 production with low to absent provided new insights into and approaches IL-2 mRNA in mitogen-stimulated T cells to the study of immunity to parasites. With from T. cruzi-infected mice suggests that respect to T. cruzi infection, interleukin 2 this is the probable level of regulation in T. (IL-2) has been focused on, both because of cruzi infection (Nabors and Tarleton 1991). its central role as the primary T cell growth This transcriptional regulation in T cells factor and because its production is se- from T. cruzi-infected mice seems to be verely suppressed during the acute phase of highly selective because neither “houseinfection (Harel-Bellan et al. 1983; Tarleton keeping” genes such as actin nor genes and Kuhn 1984). Exogenous IL-2 can alter which are normally transcriptionally active the immune responsiveness of lymphocytes in activated T cells (e.g., IL-2 receptor) are from T. cruzi-infected mice as well as the affected. However, the transcription of ccourse of infection, but cannot completely myc, a gene whose activation normally proovercome immunosuppression or result in ceeds that of IL-2 in the activated T cell, parasitological cure in the mouse model of seems to be regulated in T cells from T. T. cruzi infection (Tarleton and Kuhn 1984; cruzi-infected mice very similarly to IL-2 Choromanski and Kuhn 1985, 1987). The (Soong and Tarleton, unpublished results). cellular and molecular mechanisms of IL-2 Investigation of the nuclear factors imporsuppression in T. cruzi infection are not yet tant in IL-2 gene transcription may reveal 106 0014-4894191$3.00 Copyright0 1991by AcademicPress,Inc. All rights of reproductionin any form reserved.
MINIREVIEW:
T.
CrUZi
additional details on the regulation of IL-2 production in T. cruzi infection. OTHER
LYMPHOKINES
Despite the rather profound suppression of IL-2 production, the production of other T cell products is not as drastically affected by T. cruzi infection. IL-3 production is suppressed, although not to the same degree as IL-2, but the production of GMCSF, IL-4, IL-5, and IL-6 is either unaffected or marginally higher in mitogenstimulated spleen cells from T. cruziinfected mice (Nabors and Tarleton, unpublished results). However, besides the suppression of IL-2 production the most dramatic change in lymphokine production in T. cruzi-infected mice is the elevated levels of IFN-y (Nabors and Tarleton 1991). Throughout T. cruzi infection, the ability of splenic T cells to make IFN--y remains at or above that of spleen cells from normal mice. In addition, spleen cells from T. cruzi-infected mice produce IFN-y in response to parasite antigens as well as mitogens. Serum levels of IFN-y are also high as are IFN-y mRNA levels and spontaneous IFN-y production in unstimulated spleen cells from acutely infected mice. However, serum levels of IFN-y drop significantly after 18-20 days of infection, at the time when IL-2 production is minimal. IL-2 treatment of spleen cells from infected mice enhances both spontaneous and parasite antigeninduced IFN-y production. Thus, although in vitro mitogen-induced production of IFN-y stays relatively intact in T. cruziinfected mice, in vivo production may be affected by the inability to produce sufficient amounts of IL-2. Infusion of IFN-y has been shown to significantly decrease parasite load in vitro and in vivo (Reed et al. 1987; Wirth et al. 1985); however, the endogenous production seems to be insufficient to mediate resistance in vivo. Relatively resistant and more susceptible mouse strains show comparable levels of IFN-y
IMMUNOREGULATION
107
production in vitro and in vivo (Nabors and Tarleton 1991). T CELL SUBPOPULATIONS Based upon lymphokine secretion patterns of murine T cell clones, two T helper cell subsets have been described, the THl which secrete IL-2 and IFN-y, and the TH2 which secretes IL-4, IL-5, IL-6, and IL-IO (Mosmann et al. 1986; Street and Mosmann 1991). Based on this pattern of lymphokine production, our observations on IL-2 and IFN-v production suggest that THl cells were suppressed for IL-2 production but not for IFN-y production. However, other T cell susbets as well as NK cells are also reported to produce IFN-7. Therefore antibody depletion protocols were used to identify the relative contributions of these cell types to IFN-y production. Surprisingly, these experiments demonstrated that the major producers of IFN-y in the spleens of T. cruzi-infected mice are Thy-l bearing CD4- CD8 - cells (Nabors and Tarleton 1991). In terms of T helper subpopulations, it appears that THl cells in T. cruzi-infected mice are suppressed with respect to IL-2 production and do not contribute (because of suppression?) to the abundant production of IFN-r. TH2 cells, although not necessarily suppressed, are also not markedly stimulated by T. cruzi infection. IL-5 and IL-6 are detectable in supernatants from mitogen-stimulated cells but mRNA for these and other TH2 lymphokines, including IL-IO, are absent in either freshly isolated or mitogen/antigen stimulated spleen cells (Nabors and Tarleton, unpublished results). Thus, although TH2 cells can regulate lymphokine production by THl cells, it seems unlikely that it is a strong Th2 response in T. cruzi-infected mice which suppresses production of the THl lymphokine IL-2. This conclusion is supported by the retention of IFN-y production (a THl-like response which can itself inhibit TH2 activity) and by the pattern of antibody production in T. cruzi-infected mice (i.e., a pre-
108
MINIREVIEW:
T.
Cruzi
dominance of IgG2a and relatively lower IgGl, IgA and IgE; Minoprio et al. 1989; D’Imperio Lima et al. 1985, 1986). The relative “inactivity” of the CD4+ T cell population in the acute phase of T. crud infection is confirmed by experiments using antibody treatment to deplete T cell subsets in vivo in infected mice (Tarleton and Shields, unpublished results). CD4+ T cells are required to initiate the response to T. cruzi and control the infection in resistant mouse strains. However, depletion of CD4+ T cells after 2 weeks of infection does not affect the ability of the mouse to survive the acute infection. Thus by the midpoint of the acute infection in mice, CD4+ T cells are not necessary for control of T. cruzi, either because other responses are relatively more important (e.g., humoral immunity) or because the CD4+ T cell population is not capable of contributing (because of suppression?) to the antiparasite response. The same is true of the CD8+ T cell population in mice acutely infected with T. cruzi. Although CD8+ T cells are required at the initiation of infection for survival, they can be depleted after approximately 30 days of infection without effect on the survival of relatively resistant mouse strains. One subset of T cells which is just beginning to be studied in this and other infectious diseases is CD4- CDK (double negative) T cell. As noted above, this population appears to be the major producer of IFN-y in mice acutely infected with T. cruzi. In particular, it is of interest to know if these are o-p or y-S TCR-bearing cells. Expansion of y-6 TCR-bearing cells has been noted in T. cruzi-infected mice although the role of these cells in immunity to the parasite has not been investigated (Minoprio et al. 1989; Nabors and Tarleton, unpublished results). SUMMARY
Immunity to T. cruzi is complex, involving among other components, antibody pro-
IMMUNOREGULATION
duction, CD4+ helper cells, CD8+ T cells as both regulators and effecters of immunity, and possibly, double-negative T cells. In addition, several of these components have been implicated in pathogenesis in the chronic infection. Although the immunosuppression observed in the infection seems quite severe, it also appears to provide for a sufficient level of immune responsiveness to control the infection in most hosts. At the same time, immunosuppression may provide the regulatory control necessary to prevent massive chronic pathogenesis in all hosts. Continued study of the relative roles of lymphocyte populations and the products they secrete in immunity and pathogenesis may provide the understanding necessary to enhance immunity to T. cruzi without the feared consequence of increased pathogenesis. ACKNOWLEDGMENTS This work was supported by grants from the American Heart Association, the UNDP/WORLD BANK/ WHO Special Programme for Research and Training in Tropical Diseases, and Grant AI-22070 from the National Institutes of Health. REFERENCES CHOROMANSKI, L., AND KUHN, R. E. 1985. Interleukin 2 enhances specific and nonspecific immune responses in experimental Chagas’ disease. Infection and Immunity 50, 354-357. CHOROMANSKI, L., AND KUHN, R. E. 1987. Use of parasite antigens and interleukin-2 to enhance suppressed immune responses during Trypanosoma cruzi infection in mice. Znfection and Zmmunity 55, 403408. D’IMPEIUO LIMA, M. R., JOSKOWICZ, M., COUTINHO, A., KIPNIS, A., AND EISEN, H. 1985. Very large and isotypically atypical polyclonal plaque-forming cell responses in mice infected with Trypanosoma cruzi. European Journal of Immunology 15, 201-203. D’IMPERIO LIMA, M. R., EISEN, H., MINOPRIO, P., JOSKOWICZ, M., AND COUTINHO, A. 1986. Persistence of polyclonal B cell activation with undetectable parasitemia in late stages of experimental Chagas’ disease. Journal of Immunology 137, 353-356. HAREL-BELLAN, A., JOSKOWICZ, M., FRADELIZI, D., AND EISEN, H. 1983. Modification of T-cell proliferation and interleukin 2 production in mice infected with Trypanosoma cruzi. Proceedings of the National Academy of Sciences USA 80, 3466-3469.
MINIREVIEW:
T. cruzi
MINOPRIO, P., BANDEIRA, A., PERREIRA, P., MOTA SANTOS, T., AND COUTINHO, A. 1989. Preferential expansion of Ly-1 and CD4- CD8- T cells in the polyclonal lymphocyte responses to murine T. cruzi infection. International Immunology 1, 176-184. MINOPRIO, P., ITOHAIU, S., HEUSSER, C., TONEGAWA, S., AND COUTINHO, A. 1989. Immunobiology of murine T. cruzi infection: The predominance of parasite nonspecific responses and the activation of TCRI T cells. Immunology Review 112, 183-207. MOSMANN, T. R., CHERWINSKI, H., BOND, M. W., GIEDLIN, M. A., AND COFFMAN, R. L. 1986. Two types of murine helper T cell clone. I. Definition according to profiles of lymphokine activities and secreted proteins. Journal of Immunology 136, 2348-2357. NABORS,G. S., ANDTARLETON,R. L. 1991. Differential control of interferon-gamma and IL-2 production during Trypanosoma cruzi infection. Journal of Immunology, in press. REED, S. G., NATHAN, C. F., PIHL, D. L., RODRICKS, P., SHANEBECK, K., CONLON, P. J., AND GRABSTEIN,K. H. 1987. Recombinant granulocytel macrophage colony stimulating factor activates macrophages to inhibit Trypanosoma cruzi and release hydrogen peroxide: Comparison with inter-
IMMUNOREGULATION
109
feron gamma. Journal of Experimental Medicine 166, 1734-1746. STREET, N. E., AND MOSMANN, T. R. 1991. Functional diversity of T lymphocytes due to secretion of different cytokine patterns. FASEB Journal 5, 171177. TARLETON,R. L., AND KUHN, R. E. 1984. Restoration of in vitro immune responses of spleen cells from mice infected with Trypanosoma cruzi by supematants containing interleukin 2. Journal of Immunology 133, 1570-1575. TARLETON, R. L. 1988. Ttypanosoma cruzi-induced suppression of IL-2 production. I. Evidence for the presence of IL-2 producing cells. Journal of Immunology 140, 2763-2768. TARLETON, R. L. 1990. Depletion of CD8+ T cells increases susceptibility and reverses vaccineinduced immunity in mice infected with Trypanosoma cruzi. Journal of Immunology 144, 717-724. WIRTH, J. J., KIERSZENBAUM,F., SONNENFELD,G., AND ZLOTNIK,A. 1985. Enhancing effects of gamma interferon on phagocytic cell association with and killing of Trypanosoma cruzi. Infection and Immunity 49, 61-66. Received 2 April 1991; accepted 5 April 1991
MINIREVIEW Regulation
of Immunity RICK
Department
of Zoology,
University
in Trypanosoma
cruzi Infection
L. TARLETON of Georgia,
Athens, Georgia 30602, U.S.A.
TARLETON, R. L. 1991. Regulation of immunity in Trypanosoma imental Parasitology 73, 106-109. Q 1~91Academicpress,~nc.
Infection with Trypanosoma cruzi is accompanied by suppression of immune responses of a variety of types and antigen specificities. The specific mechanisms which account for suppression of immune responses in T. cruzi infection are not agreed upon and although the significance of immunosuppression in this infection is not known, it is likely that both immunity to T. cruzi and pathogenesis resulting from the infection are influenced by the downregulation of immune responses.
cruzi infection. Exper-
clear. At the cellular level, CD8+ T cells have a major influence on IL-2 production in this system as shown by in vitro cell depletion experiments (Tarleton 1990) and more recently in studies using mice deficient in CD8+ T cells due to deletion of the P-2-microglobulin gene (Tarleton and Kohler, unpublished results). Immune exhaustion or prior stimulation may also contribute to the defect in IL-2 production since an in vitro “rest” period of 48-72 hr restores the ability of lymphocytes from T. cruzi-infected mice to produce IL-2 (Tarleton 1988). IL-2 production in general is INTERLEUKIN-2 PRODUCTION known to be regulated primarily at the tranThe increased understanding of the role scriptional level and the correlation of deof cytokines in regulating immunity has pressed IL-2 production with low to absent provided new insights into and approaches IL-2 mRNA in mitogen-stimulated T cells to the study of immunity to parasites. With from T. cruzi-infected mice suggests that respect to T. cruzi infection, interleukin 2 this is the probable level of regulation in T. (IL-2) has been focused on, both because of cruzi infection (Nabors and Tarleton 1991). its central role as the primary T cell growth This transcriptional regulation in T cells factor and because its production is se- from T. cruzi-infected mice seems to be verely suppressed during the acute phase of highly selective because neither “houseinfection (Harel-Bellan et al. 1983; Tarleton keeping” genes such as actin nor genes and Kuhn 1984). Exogenous IL-2 can alter which are normally transcriptionally active the immune responsiveness of lymphocytes in activated T cells (e.g., IL-2 receptor) are from T. cruzi-infected mice as well as the affected. However, the transcription of ccourse of infection, but cannot completely myc, a gene whose activation normally proovercome immunosuppression or result in ceeds that of IL-2 in the activated T cell, parasitological cure in the mouse model of seems to be regulated in T cells from T. T. cruzi infection (Tarleton and Kuhn 1984; cruzi-infected mice very similarly to IL-2 Choromanski and Kuhn 1985, 1987). The (Soong and Tarleton, unpublished results). cellular and molecular mechanisms of IL-2 Investigation of the nuclear factors imporsuppression in T. cruzi infection are not yet tant in IL-2 gene transcription may reveal 106 0014-4894191$3.00 Copyright0 1991by AcademicPress,Inc. All rights of reproductionin any form reserved.
MINIREVIEW:
T.
CrUZi
additional details on the regulation of IL-2 production in T. cruzi infection. OTHER
LYMPHOKINES
Despite the rather profound suppression of IL-2 production, the production of other T cell products is not as drastically affected by T. cruzi infection. IL-3 production is suppressed, although not to the same degree as IL-2, but the production of GMCSF, IL-4, IL-5, and IL-6 is either unaffected or marginally higher in mitogenstimulated spleen cells from T. cruziinfected mice (Nabors and Tarleton, unpublished results). However, besides the suppression of IL-2 production the most dramatic change in lymphokine production in T. cruzi-infected mice is the elevated levels of IFN-y (Nabors and Tarleton 1991). Throughout T. cruzi infection, the ability of splenic T cells to make IFN--y remains at or above that of spleen cells from normal mice. In addition, spleen cells from T. cruzi-infected mice produce IFN-y in response to parasite antigens as well as mitogens. Serum levels of IFN-y are also high as are IFN-y mRNA levels and spontaneous IFN-y production in unstimulated spleen cells from acutely infected mice. However, serum levels of IFN-y drop significantly after 18-20 days of infection, at the time when IL-2 production is minimal. IL-2 treatment of spleen cells from infected mice enhances both spontaneous and parasite antigeninduced IFN-y production. Thus, although in vitro mitogen-induced production of IFN-y stays relatively intact in T. cruziinfected mice, in vivo production may be affected by the inability to produce sufficient amounts of IL-2. Infusion of IFN-y has been shown to significantly decrease parasite load in vitro and in vivo (Reed et al. 1987; Wirth et al. 1985); however, the endogenous production seems to be insufficient to mediate resistance in vivo. Relatively resistant and more susceptible mouse strains show comparable levels of IFN-y
IMMUNOREGULATION
107
production in vitro and in vivo (Nabors and Tarleton 1991). T CELL SUBPOPULATIONS Based upon lymphokine secretion patterns of murine T cell clones, two T helper cell subsets have been described, the THl which secrete IL-2 and IFN-y, and the TH2 which secretes IL-4, IL-5, IL-6, and IL-IO (Mosmann et al. 1986; Street and Mosmann 1991). Based on this pattern of lymphokine production, our observations on IL-2 and IFN-v production suggest that THl cells were suppressed for IL-2 production but not for IFN-y production. However, other T cell susbets as well as NK cells are also reported to produce IFN-7. Therefore antibody depletion protocols were used to identify the relative contributions of these cell types to IFN-y production. Surprisingly, these experiments demonstrated that the major producers of IFN-y in the spleens of T. cruzi-infected mice are Thy-l bearing CD4- CD8 - cells (Nabors and Tarleton 1991). In terms of T helper subpopulations, it appears that THl cells in T. cruzi-infected mice are suppressed with respect to IL-2 production and do not contribute (because of suppression?) to the abundant production of IFN-r. TH2 cells, although not necessarily suppressed, are also not markedly stimulated by T. cruzi infection. IL-5 and IL-6 are detectable in supernatants from mitogen-stimulated cells but mRNA for these and other TH2 lymphokines, including IL-IO, are absent in either freshly isolated or mitogen/antigen stimulated spleen cells (Nabors and Tarleton, unpublished results). Thus, although TH2 cells can regulate lymphokine production by THl cells, it seems unlikely that it is a strong Th2 response in T. cruzi-infected mice which suppresses production of the THl lymphokine IL-2. This conclusion is supported by the retention of IFN-y production (a THl-like response which can itself inhibit TH2 activity) and by the pattern of antibody production in T. cruzi-infected mice (i.e., a pre-
108
MINIREVIEW:
T.
Cruzi
dominance of IgG2a and relatively lower IgGl, IgA and IgE; Minoprio et al. 1989; D’Imperio Lima et al. 1985, 1986). The relative “inactivity” of the CD4+ T cell population in the acute phase of T. crud infection is confirmed by experiments using antibody treatment to deplete T cell subsets in vivo in infected mice (Tarleton and Shields, unpublished results). CD4+ T cells are required to initiate the response to T. cruzi and control the infection in resistant mouse strains. However, depletion of CD4+ T cells after 2 weeks of infection does not affect the ability of the mouse to survive the acute infection. Thus by the midpoint of the acute infection in mice, CD4+ T cells are not necessary for control of T. cruzi, either because other responses are relatively more important (e.g., humoral immunity) or because the CD4+ T cell population is not capable of contributing (because of suppression?) to the antiparasite response. The same is true of the CD8+ T cell population in mice acutely infected with T. cruzi. Although CD8+ T cells are required at the initiation of infection for survival, they can be depleted after approximately 30 days of infection without effect on the survival of relatively resistant mouse strains. One subset of T cells which is just beginning to be studied in this and other infectious diseases is CD4- CDK (double negative) T cell. As noted above, this population appears to be the major producer of IFN-y in mice acutely infected with T. cruzi. In particular, it is of interest to know if these are o-p or y-S TCR-bearing cells. Expansion of y-6 TCR-bearing cells has been noted in T. cruzi-infected mice although the role of these cells in immunity to the parasite has not been investigated (Minoprio et al. 1989; Nabors and Tarleton, unpublished results). SUMMARY
Immunity to T. cruzi is complex, involving among other components, antibody pro-
IMMUNOREGULATION
duction, CD4+ helper cells, CD8+ T cells as both regulators and effecters of immunity, and possibly, double-negative T cells. In addition, several of these components have been implicated in pathogenesis in the chronic infection. Although the immunosuppression observed in the infection seems quite severe, it also appears to provide for a sufficient level of immune responsiveness to control the infection in most hosts. At the same time, immunosuppression may provide the regulatory control necessary to prevent massive chronic pathogenesis in all hosts. Continued study of the relative roles of lymphocyte populations and the products they secrete in immunity and pathogenesis may provide the understanding necessary to enhance immunity to T. cruzi without the feared consequence of increased pathogenesis. ACKNOWLEDGMENTS This work was supported by grants from the American Heart Association, the UNDP/WORLD BANK/ WHO Special Programme for Research and Training in Tropical Diseases, and Grant AI-22070 from the National Institutes of Health. REFERENCES CHOROMANSKI, L., AND KUHN, R. E. 1985. Interleukin 2 enhances specific and nonspecific immune responses in experimental Chagas’ disease. Infection and Immunity 50, 354-357. CHOROMANSKI, L., AND KUHN, R. E. 1987. Use of parasite antigens and interleukin-2 to enhance suppressed immune responses during Trypanosoma cruzi infection in mice. Znfection and Zmmunity 55, 403408. D’IMPEIUO LIMA, M. R., JOSKOWICZ, M., COUTINHO, A., KIPNIS, A., AND EISEN, H. 1985. Very large and isotypically atypical polyclonal plaque-forming cell responses in mice infected with Trypanosoma cruzi. European Journal of Immunology 15, 201-203. D’IMPERIO LIMA, M. R., EISEN, H., MINOPRIO, P., JOSKOWICZ, M., AND COUTINHO, A. 1986. Persistence of polyclonal B cell activation with undetectable parasitemia in late stages of experimental Chagas’ disease. Journal of Immunology 137, 353-356. HAREL-BELLAN, A., JOSKOWICZ, M., FRADELIZI, D., AND EISEN, H. 1983. Modification of T-cell proliferation and interleukin 2 production in mice infected with Trypanosoma cruzi. Proceedings of the National Academy of Sciences USA 80, 3466-3469.
MINIREVIEW:
T. cruzi
MINOPRIO, P., BANDEIRA, A., PERREIRA, P., MOTA SANTOS, T., AND COUTINHO, A. 1989. Preferential expansion of Ly-1 and CD4- CD8- T cells in the polyclonal lymphocyte responses to murine T. cruzi infection. International Immunology 1, 176-184. MINOPRIO, P., ITOHAIU, S., HEUSSER, C., TONEGAWA, S., AND COUTINHO, A. 1989. Immunobiology of murine T. cruzi infection: The predominance of parasite nonspecific responses and the activation of TCRI T cells. Immunology Review 112, 183-207. MOSMANN, T. R., CHERWINSKI, H., BOND, M. W., GIEDLIN, M. A., AND COFFMAN, R. L. 1986. Two types of murine helper T cell clone. I. Definition according to profiles of lymphokine activities and secreted proteins. Journal of Immunology 136, 2348-2357. NABORS,G. S., ANDTARLETON,R. L. 1991. Differential control of interferon-gamma and IL-2 production during Trypanosoma cruzi infection. Journal of Immunology, in press. REED, S. G., NATHAN, C. F., PIHL, D. L., RODRICKS, P., SHANEBECK, K., CONLON, P. J., AND GRABSTEIN,K. H. 1987. Recombinant granulocytel macrophage colony stimulating factor activates macrophages to inhibit Trypanosoma cruzi and release hydrogen peroxide: Comparison with inter-
IMMUNOREGULATION
109
feron gamma. Journal of Experimental Medicine 166, 1734-1746. STREET, N. E., AND MOSMANN, T. R. 1991. Functional diversity of T lymphocytes due to secretion of different cytokine patterns. FASEB Journal 5, 171177. TARLETON,R. L., AND KUHN, R. E. 1984. Restoration of in vitro immune responses of spleen cells from mice infected with Trypanosoma cruzi by supematants containing interleukin 2. Journal of Immunology 133, 1570-1575. TARLETON, R. L. 1988. Ttypanosoma cruzi-induced suppression of IL-2 production. I. Evidence for the presence of IL-2 producing cells. Journal of Immunology 140, 2763-2768. TARLETON, R. L. 1990. Depletion of CD8+ T cells increases susceptibility and reverses vaccineinduced immunity in mice infected with Trypanosoma cruzi. Journal of Immunology 144, 717-724. WIRTH, J. J., KIERSZENBAUM,F., SONNENFELD,G., AND ZLOTNIK,A. 1985. Enhancing effects of gamma interferon on phagocytic cell association with and killing of Trypanosoma cruzi. Infection and Immunity 49, 61-66. Received 2 April 1991; accepted 5 April 1991
