Studies on the role of interleukin-12 in acute murine toxoplasmosis.

Interleukin-12 (IL-12) is important in the regulation of resistance to Toxoplasma gondii in mice with severe combined immunodeficiency (SCID). The protective ability of IL-12 in SCID mice appears to be through its activity on natural killer (NK) cells to induce production of interferon-gamma (IFN-gamma). In this study we assessed the role of IL-12 in the acute stage of toxoplasmosis in immunocompetent mice. Administration of IL-12 to BALB/c mice infected with the virulent C56 strain of T. gondii remarkably delayed time to death. The protective activity of IL-12 was abrogated by administration of monoclonal antibodies to IFN-gamma or tumour necrosis factor-alpha (TNF-alpha), and by depletion of NK cells using an antisera against asialoGM1. Whereas BALB/c mice infected with the ME49 strain of T. gondii survived infection, administration of anti-IL-12 to infected mice resulted in 100% mortality accompanied by decreased serum levels of IFN-gamma. Furthermore, this treatment significantly reversed the suppression of spleen cell proliferation to concanavalin A (Con A), which is associated with the acute stage of infection, and resulted in decreased ex vivo production of IFN-gamma, IL-2, IL-4 and IL-10 in response to Con A. Our results indicate an important role for IL-12 in mediating resistance to T. gondii during acute infection in immunocompetent mice, that NK cells are required for this protective activity, and that IL-12 is involved in the immunosuppression which accompanies this infection.     

Interleukin-18 (IL-18) enhances innate IL-12-mediated resistance to Toxoplasma gondii

Innate resistance to Toxoplasma gondii is dependent on the ability of interleukin-12 (IL-12) to stimulate natural killer (NK) cell production of gamma interferon (IFN-gamma). Since IL-18 is a potent enhancer of IL-12-induced production of IFN-gamma by NK cells, SCID mice (which lack an adaptive immune response) were used to assess the role of IL-18 in innate resistance to T. gondii. Administration of anti-IL-18 to SCID mice infected with T. gondii resulted in an early reduction in serum levels of IFN-gamma but did not significantly decrease resistance to this infection. In contrast, administration of exogenous IL-18 to infected SCID mice resulted in increased production of IFN-gamma, reduced parasite burden, and a delay in time to death. The protective effects of IL-18 treatment correlated with increased NK cell numbers and cytotoxic activity at the local site of administration and with elevated levels of inducible nitrous oxide synthose in the spleens of treated mice. In addition, in vivo depletion studies demonstrated that the ability of exogenous IL-18 to enhance resistance to T. gondii was dependent on IL-12, IFN-gamma, and NK cells. Together, these studies demonstrate that although endogenous IL-18 appears to have a limited role in innate resistance to T. gondii, treatment with IL-18 can augment NK cell-mediated immunity to this pathogen.     

Suppression of interleukin-2 production by macrophages in genetically susceptible mice infected with Leishmania major.

Spleen cells from BALB/c mice infected with 2 X 10(7) L. major promastigotes and developing progressive disease produced significantly lower levels of interleukin-2 (IL-2) in response to concanavalin A stimulation than did spleen cells from uninfected mice. In contrast, spleen cells from sublethally irradiated and infected mice, which were able to contain lesion development, produced significantly higher levels of IL-2. The increase in IL-2 production closely paralleled lesion regression. Mice protectively immunized by four intravenous injections with lethally irradiated promastigotes also produced enhanced levels of IL-2, which were sustained after challenge infection. In contrast, spleen cells from BALB/c mice given four s.c. injections of irradiated promastigotes produced high levels of IL-2 before but not after infection. These mice eventually produced levels of IL-2 indistinguishable from those of unimmunized mice with progressive disease. There is thus an inverse relation between disease progression and the ability of spleen cells to produce IL-2. Spleen cells from mice with uncontrolled disease not only produced lower levels of IL-2 but also impaired IL-2 production by normal spleen cells. The ability to inhibit IL-2 was abrogated by passing the cells through a Sephadex G-10 column, removal of plastic adherent cells, and removal of carbonyl iron-ingesting cells. Furthermore, Sephadex G-10 column-treated and plastic adherent, nonspecific esterase-positive spleen cells from mice with progressive disease were able to suppress IL-2 production by normal splenic T cells. The suppressive activity of the adherent cells was not affected by treatment with anti-Thy-1.2 antibody and complement. In contrast, adherent spleen cells from uninfected mice were devoid of such suppressor activity. The depressed IL-2 production by spleen cells from progressively infected mice could be restored to that of normal spleen cells by the addition of indomethacin to the culture. There was however, no correlation between IL-2 production and IL-1 activity in infected or immunized BALB/c mice. Thus, it appears that the suppression of IL-2 production is mediated by prostaglandins elaborated by macrophages from chronically infected mice.     

Role of interleukin-10 in regulation of T-cell-dependent and T-cell-independent mechanisms of resistance to Toxoplasma gondii.

Interleukin-10 (IL-10) is a cytokine which can inhibit T-cell and natural killer (NK) cell functions associated with cell-mediated immunity to intracellular infections. The production of IL-10 by mice infected with Toxoplasma gondii has been implicated in the suppression of lymphocyte proliferation observed during acute toxoplasmosis, as well as susceptibility to infection with this parasite. We have used C57BL/6 mice which lack a functional IL-10 gene (IL-10(-/-) mice) to investigate the role of IL-10 in acute toxoplasmosis. Intraperitoneal infection of IL-10(-/-) mice with T. gondii resulted in 100% mortality by day 13, whereas wild-type C57BL/6 (WT) mice survived acute infection. IL-10(-/-) mice infected with T. gondii had significantly higher serum levels of IL-12 and gamma interferon (IFN-gamma) than WT mice. Early mortality of infected IL-10(-/-) mice was prevented by treatment with IL-10 and significantly delayed by neutralizing antibodies to IL-12 and IFN-gamma. Further studies revealed that SCID/IL-10(-/-) mice infected with T. gondii had delayed time to death compared to IL-10(-/-) mice, indicating that lymphocytes contributed to death of IL-10(-/-) mice. In addition, infected SCID/IL-10(-/-) mice survived longer than infected SCID mice. These latter data indicate that in mice lacking lymphocytes, endogenous IL-10 is associated with increased susceptibility to T. gondii. However, the lack of IL-10 does not alter the infection-induced suppression of T cell and NK cell functions. Our experiments reveal that IL-10 is associated with protection or increased susceptibility to infection with T. gondii, depending on whether mice possess lymphocytes, and demonstrate the important roles of IL-12 and IFN-gamma in the early infection-induced mortality observed in the IL-10(-/-) mice.     

Susceptibility of interleukin-2-deficient mice to Toxoplasma gondii is associated with a defect in the production of gamma interferon

Costimulation through the B7-CD28 interaction is an important second signal for T-cell activation, and previous studies have shown that CD28(-/-) mice infected with Toxoplasma gondii generate suboptimal CD4(+) T-cell responses, associated with a defect in production of the T-cell growth factor interleukin-2 (IL-2). To address the role of IL-2 in the expansion of T cells during toxoplasmosis, IL-2(-/-) mice were infected with T. gondii and their ability to generate a protective T-cell response was assessed. Although IL-2(-/-) mice produced normal levels of IL-12p40, they had reduced levels of gamma interferon (IFN-gamma) in serum, had an increased parasite burden, and succumbed to infection with T. gondii within 20 days. Fluorescence-activated cell sorter analysis revealed that, although uninfected IL-2(-/-) mice had an increased number of activated T cells compared with uninfected IL-2(+/+) mice, following infection they were unable to further upregulate this population. Examination of the ability of splenocytes from uninfected and infected mice to produce IFN-gamma revealed that IL-2(-/-) mice were hyporesponsive to stimulation with anti-CD3 or parasite antigen compared with wild-type mice, and the addition of IL-2 alone or in combination with IL-12 or stimulation with phorbol myristate acetate and ionomycin did not restore the production of IFN-gamma. Together, these studies reveal that IL-2(-/-) mice are unable to generate a protective IFN-gamma response following infection with T. gondii and suggest that IL-2(-/-) mice have an intrinsic defect in their ability to activate and expand IFN-gamma-producing T cells required for resistance to T. gondii.     

Roles of gamma interferon and other cytokines in suppression of the spleen cell proliferative response to concanavalin A and toxoplasma antigen during acute toxoplasmosis.

Suppressed splenocyte proliferation in response to mitogen and toxoplasma lysate antigen (TLA) is observed in mice acutely infected with Toxoplasma gondii. Recently, we reported that NG-monomethyl-L-arginine (NMMA), an inhibitor of reactive nitrogen intermediate (RNI) production, partially restored proliferative responses of splenocytes from infected mice. In the present study we have examined the effect of NMMA on production of cytokines by splenocytes from mice acutely infected with T. gondii and assessed the role of gamma interferon (IFN-gamma) and interleukin-10 (IL-10) in the RNI-mediated suppression. Stimulation with concanavalin A (ConA) or TLA of splenocytes from CBA/Ca mice infected for 7 days resulted in increased production of IFN-gamma, IL-4, and IL-10 but reduced levels of IL-2 when compared with cultures of splenocytes from uninfected mice. Whereas addition of NMMA did not alter levels of cytokines produced by splenocytes from uninfected mice, splenocytes from infected mice stimulated with ConA produced significantly higher levels of IL-10 and reduced levels of IL-2 and IL-4. Addition of anti-IFN-gamma monoclonal antibodies to cultures of spleen cells from mice infected for 7 or 14 days remarkably decreased the levels of nitrite and resulted in a 47- and 4-fold increase in proliferation induced by stimulation with ConA or TLA, respectively. Anti-IL-10 did not reduce levels of nitrite produced in culture but did result in a fourfold increase in the proliferative response of splenocytes from mice infected for 14 days. In vivo administration of anti-IFN-gamma or anti-IL-10 monoclonal antibodies to infected mice partially restored ex vivo spleen cell proliferative responses by approximately 40 and 15%, respectively. Our data indicate that IFN-gamma is important in inducing the RNI-mediated immunosuppression, which, in turn, affects production of cytokines by splenocytes. Our data also demonstrate that IL-10 is involved in the suppression observed but that this activity is independent of RNI.     

Requirement of non-T cells that produce gamma interferon for prevention of reactivation of Toxoplasma gondii infection in the brain

We examined the mechanism of resistance against reactivation of infection with Toxoplasma gondii in the brain. BALB/c-background gamma interferon (IFN-gamma)-knockout (IFN-gamma(-/-)) and control mice were infected and treated with sulfadiazine beginning 4 days after infection for 3 weeks. After discontinuation of treatment, IFN-gamma(-/-) mice succumbed to toxoplasmic encephalitis (TE) and died, whereas control animals did not develop TE and survived. Adoptive transfer of immune spleen cells from infected control mice did not prevent development of TE or mortality in the IFN-gamma(-/-) mice. To examine whether the failure of the cell transfer to protect against TE is unique to IFN-gamma(-/-) mice, athymic nude and SCID mice that lack T cells were infected and injected with the immune spleen or T cells in the same manner as IFN-gamma(-/-) mice. Whereas control nude and SCID mice that had not received the immune cells developed severe TE and died after discontinuation of sulfadiazine, those that had received the cells did not develop TE and survived. Before cell transfer, IFN-gamma mRNA was detected in brains of infected nude and SCID but not in brains of IFN-gamma(-/-) mice. IFN-gamma mRNA was also detected in brains of infected SCID mice depleted of NK cells by treatment with anti-asialo GM1 antibody, and such animals did not develop TE after receiving immune T cells. Thus, IFN-gamma production by non-T cells, in addition to T cells, is required for prevention of reactivation of T. gondii infection in the brain. The IFN-gamma-producing non-T cells do not appear to be NK cells.     

T-cell-independent granuloma formation in response to Mycobacterium avium: role of tumour necrosis factor-alpha and interferon-gamma.

We used Mycobacterium avium infection in severe combined immunodeficiency (SCID) mice to examine T-cell-independent mechanisms of inflammatory cell recruitment. SCID mice infected with a virulent strain of M. avium (TMC724) were able to recruit macrophages to sites of mycobacterial replication and formed organized and coherent granulomas in the absence of functional T cells. Phagocyte recruitment was almost totally ablated by neutralization of either tumour necrosis factor-alpha (TNF-alpha) or interferon-gamma (IFN-gamma) in vivo demonstrating that granuloma formation was dependent on the presence of these cytokines. This was concomitant with a reduction in the in situ cytokine mRNA levels otherwise induced in infected mice, for chemokines, pro-inflammatory and regulatory cytokines, including TNF-alpha, IFN-gamma, macrophage inflammatory protein-1 alpha, interleukin-1 beta (IL-1 beta) and IL-10. Furthermore, in vivo treatment of infected mice with anti-asialo GM-1 antisera, which depletes natural killer (NK) cells, prevented recruitment of inflammatory cells. In vitro studies confirmed that M. avium was able to elicit IFN-gamma from SCID spleen in a dose-dependent manner. These data show for the first time that secretion of IFN-gamma from NK cells can mediate a T-cell-independent pathway of granuloma formation and cellular infiltration in response to mycobacteria.     

Tumor necrosis factor alpha and interleukin-12 contribute to resistance to the intracellular bacterium Brucella abortus by different mechanisms.

Both interleukin-12 (IL-12) and tumor necrosis factor alpha (TNF-alpha) are produced early in intracellular bacterial infection. Depletion of either IL-12 or TNF-alpha by a single injection of specific antibody 4 h before the injection of Brucella abortus 19 led to the exacerbation of infection 2 weeks later. Whereas the effect of IL-12 depletion on resistance was persistent and exacerbation was still significant 6 weeks later, the bacterial numbers in mice depleted of TNF-alpha were similar to the bacterial numbers in control infected mice by 6 weeks postinfection. Massive splenomegaly, which is often seen in 2-week Brucella-infected mice, was not observed in IL-12- or TNF-alpha-depleted mice. Both IL-12- and TNF-alpha-depleted mice showed reduced cell accumulation in the spleen compared with the massive cell accumulation in control infected mice. Granuloma formation in livers was much reduced in IL-12-depleted mice but not in TNF-alpha-depleted mice. Gamma interferon (IFN-gamma) production by cells from TNF-alpha-depleted mice was not significantly different from that of cells from control infected mice. In contrast, the production of IFN-gamma by both CD4+ and CD8+ T cells from IL-12-depleted mice was greatly reduced, compared with that from control infected mice. This effect was still observed when the antibody injection was delayed for up to 7 days postinfection, but injections of anti-IL-12 antibody into mice with established Brucella infection had no significant effect on IFN-gamma production by T cells. Taken together, these results suggested that IL-12 contributed to resistance mainly via an IFN-gamma-dependent pathway and had a profound effect on the induction of acquired cellular resistance. In contrast, TNF-alpha was involved in resistance possibly via direct action on effector cells and may not be essential for the induction of acquired cellular resistance.     

Role of endogenous IFN-gamma in macrophage programming induced by IL-12 and IL-18.

Besides the established role of interleukin-12 (IL-12) and IL-18 on interferon-gamma (IFN-gamma) production by natural killer (NK), T, and B cells, the effects of these cytokines on macrophages are largely unknown. Here, we investigated the role of IL-12/IL-18 on nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha) production by CD11b(+) adherent peritoneal cells, focusing on the involvement of endogenously produced IFN-gamma. C57BL/6 cells released substantial amounts of NO when stimulated with IFN-gamma or lipopolysaccharide (LPS), but failed to respond to IL-12 or IL-18 or both. However, IL-12/IL-18 pretreatment was able to program these cells to release 6-8-fold more NO and TNF-alpha in response to LPS or Trypanosoma cruzi stimulation, with NO levels directly correlating with macrophage resistance to intracellular parasite growth. Analysis of IL-12/IL-18-primed cells from mice deficient in IFN-gamma, IFNGR, and IFN regulatory factor-1 (IRF-1) revealed that these molecules were essential for LPS-induced NO release, but TNF-alpha production was IFN-gamma independent. Conversely, the myeloid differentiation factor 88 (MyD88)-dependent pathway was indispensable for IL-12/IL-18-programmed LPS-induced TNF-alpha production, but not for NO release. Contaminant T and NK cells largely modulated the IL-12/IL-18 programming of LPS-induced NO response through IFN-gamma secretion. Nevertheless, a small population of IFN-gamma(+) cells with a macrophage phenotype was also identified, particularly in the peritoneum of chronically T. cruzi-infected mice, reinforcing the notion that macrophages can be an alternative source of IFN-gamma. Taken together, our data contribute to elucidate the molecular basis of the IL-12/IL-18 autocrine pathway of macrophage activation, showing that endogenous IFN-gamma plays an important role in programming the NO response, whereas the TNF-alpha response occurs through an IFN-gamma-independent pathway.     

IL-12 and IFN-gamma production, and NK cell activity, in acute and chronic experimental Trypanosoma cruzi infections

Resistance to acute Trypanosoma cruzi infection is mainly associated with a Th1 immune response, characterized by gamma-interferon (IFN-gamma) production and activation of macrophages. The outcome of the Th1 response in the spleen and serum of BALB/c and C3H mice infected with T. cruzi, Tulahuén strain was studied. The levels of interleukin-12 p40 (IL-12 p40) and IFN-gamma, as well as natural killer (NK) cell cytotoxicity were determined at different time-points during the acute phase, and the production of cytokines was also studied in the chronic infection. At 2 days post-infection (pi), spleen cells from C3H mice increased their NK cell activity and the ex vivo spontaneous release of both IL-12 p40 and IFN-gamma. On the other hand, BALB/c mice reached low levels of NK cell cytotoxicity and no IFN-gamma production was detected at this time pi, but the cytokine was released at high amounts in the second week of the infection. Seric IL-12 p40 concentrations showed a 3-fold increase in both mouse strains on the second day pi and remained high throughout the acute phase. However, seric IFN-gamma levels increased during the late acute infection and were higher in BALB/c than in C3H mice. In chronically infected mice IL-12 p40 was as high as in the acute phase in the serum of both strains, but only BALB/c mice still produced IFN-gamma. To the authors' knowledge this is the first report showing the protein levels of IL-12 p40 determined in vivo in acute and chronic T. cruzi infections. The results reveal differences between both mouse strains in the mechanisms controlling the onset and fate of the Th1 response triggered by the parasite and a long lasting pro-inflammatory stimuli.     

The CD40/CD40 ligand interaction is required for resistance to toxoplasmic encephalitis

Since the CD40/CD40 ligand (CD40L) interaction is involved in the regulation of macrophage production of interleukin 12 (IL-12) and T-cell production of gamma interferon (IFN-gamma), effector cell functions associated with resistance to Toxoplasma gondii, the role of CD40L in immunity to this parasite was assessed. Infection of C57BL/6 mice with T. gondii results in an upregulation of CD40 expression on accessory cell populations at local sites of infection as well as in lymphoid tissues. Splenocytes from C57BL/6 mice infected with T. gondii for 5 days produced high levels of IL-12 and IFN-gamma when stimulated with toxoplasma lysate antigen, and blocking CD40L did not significantly alter the production of IFN-gamma or IL-12 by these cells. Similar results were observed with splenocytes and mononuclear cells isolated from the brains of chronically infected mice. Interestingly, although CD40L(-/-) mice infected with T. gondii produced less IL-12 than wild-type mice, they produced comparable levels of IFN-gamma but succumbed to toxoplasmic encephalitis 4 to 5 weeks after infection. The inability of CD40L(-/-) mice to control parasite replication in the brain correlated with the ability of soluble CD40L, in combination with IFN-gamma, to activate macrophages in vitro to control replication of T. gondii. Together, these results identify an important role for the CD40/CD40L interaction in resistance to T. gondii. However, this interaction may be more important in the control of parasite replication in the brain rather than the generation of protective T-cell responses during toxoplasmosis.     

Host resistance and immune deviation in pigeon cytochrome c T-cell receptor transgenic mice infected with Toxoplasma gondii

Resistance to Toxoplasma gondii has been shown to be mediated by gamma interferon (IFN-gamma) produced by NK, CD4(+), and CD8(+) T cells. While studies of SCID mice have implicated NK cells as the source of the cytokine in acute infection, several lines of evidence suggest that IFN-gamma production by CD4(+) T lymphocytes also plays an important role in controlling early parasite growth. To evaluate whether this function is due to nonspecific as opposed to T-cell receptor (TCR)-dependent stimulation by the parasite, we have examined the resistance to T. gondii infection of pigeon cytochrome c transgenic (PCC-Tg) Rag-2(-/-) mice in which all CD4(+) T lymphocytes are unreactive with the protozoan. When inoculated with the ME49 strain, PCC-Tg animals exhibited only temporary control of acute infection and succumbed by day 17. Intracellular cytokine staining by flow cytometry revealed that, in contrast to infected nontransgenic controls, infected PCC-Tg animals failed to develop IFN-gamma-producing CD4(+) T cells. Moreover, the CD4(+) lymphocytes from these mice showed no evidence of activation as judged by lack of upregulated expression of CD44 or CD69. Nevertheless, when acutely infected transgenic mice were primed by PCC injection, the lymphokine responses measured after in vitro antigen restimulation displayed a strong Th1 bias which was shown to be dependent on endogenous interleukin 12 (IL-12). The above findings argue that, while T. gondii-induced IL-12 cannot trigger IFN-gamma production by CD4(+) T cells in the absence of TCR ligation, the pathogen is able to nonspecifically promote Th1 responses against nonparasite antigens, an effect that may explain the immunostimulatory properties of T. gondii infection.     

Transient control of interleukin-4-producing natural killer T cells in the livers of Listeria monocytogenes-infected mice by interleukin-12.

Unconstrained development of gamma interferon (IFN-gamma)-secreting natural killer (NK) cells and T helper (Th) 1 cells is central to protection against Listeria monocytogenes. In contrast, interleukin 4 (IL-4) is considered harmful. IL-12 produced by infected macrophages promotes, and IL-4 interferes with, protective antilisterial immunity. The liver NK T lymphocytes, which are a potent source of IL-4, are downregulated at an intermediate stage of listeriosis. Here we demonstrate that endogenous IL-12 participates in the control of IL-4-producing liver NK T lymphocytes during listeriosis. The effects of L. monocytogenes infection on IL-4-producing liver NK T lymphocytes were reversed by antibody neutralization of IL-12 but not of IFN-gamma or tumor necrosis factor alpha (TNF-alpha). IL-4 production by liver NK T lymphocytes was virtually unaffected by heat-killed L. monocytogenes (HKL). Viable L. monocytogenes markedly increased the numbers of IL-12 producers in livers in parallel with an increase in macrophage numbers, whereas HKL failed to do so with similar efficiency. These results indicate that in the liver endogenous IL-12 improves protective immunity against listeriosis by downregulating IL-4-producing NK T lymphocytes. Moreover, our findings that HKL have a low level of IL-12-inducing activity and fail to control IL-4-producing NK T lymphocytes in the liver are consistent with the lesser protective capacity of HKL compared to that of live listeriae.     

Critical role of the liver CD8+ CD122+ T cells in the generalized Shwartzman reaction of mice

We examined the role of mouse CD8+ CD122+ T cells, which increase in number with age, in the generalized Shwartzman reaction. This reaction was induced by IL-12 priming and subsequent LPS challenge (after 24 h) in mice of various ages (4-50 weeks of age). Although most young mice (4 or 6 weeks of age) survived, mortality essentially increased with increasing age of the mice, and all mice of 20 weeks of age or older died within 48 h. Serum TNF-alpha levels after LPS challenge also increased age dependently. The neutralization of either IL-12-induced IFN-gamma or LPS-induced TNF-alpha improved the survival of middle-aged (25-week-old) mice. Both IFN-gamma production after IL-12 priming and TNF-alpha production from the liver mononuclear cells after LPS challenge were also prominent in the middle-aged mice. CD8+CD122+ T cells cultured with IL-12 produced a much larger amount of IFN-gamma than CD8+CD122- T cells. Although the depletion of NK/NK T cells did not decrease the IFN-gamma or TNF-alpha production in the Shwartzman reaction of the middle-aged mice, an additional depletion of CD8+CD122+ T cells did decrease such production and also improved mouse survival. Furthermore, young mice transferred with CD8+CD122+ T cells from aged B6 nude mice showed an enhanced Shwartzman reaction.     

Blockade of costimulation prevents infection-induced immunopathology in interleukin-10-deficient mice

Interleukin-10 (IL-10) is associated with inhibition of cell-mediated immunity and downregulation of the expression of costimulatory molecules required for T-cell activation. When IL-10-deficient (IL-10KO) mice are infected with Toxoplasma gondii, they succumb to a T-cell-mediated shock-like reaction characterized by the overproduction of IL-12 and gamma interferon (IFN-gamma) associated with widespread necrosis of the liver. Since costimulation is critical for T-cell activation, we investigated the role of the CD28-B7 and CD40-CD40 ligand (CD40L) interactions in this infection-induced immunopathology. Our studies show that infection of mice with T. gondii resulted in increased expression of B7 and CD40 that was similar in wild-type and IL-10KO mice. In vivo blockade of the CD28-B7 or CD40-CD40L interactions following infection of IL-10KO mice with T. gondii did not affect serum levels of IFN-gamma or IL-12, nor did it prevent death in these mice. However, when both pathways were blocked, the IL-10KO mice survived the acute phase of infection and had reduced serum levels of IFN-gamma and alanine transaminase as well as decreased expression of inducible nitric oxide synthase in the liver and spleen. Analysis of parasite-specific recall responses from infected IL-10KO mice revealed that blockade of the CD40-CD40L interaction had minimal effects on cytokine production, whereas blockade of the CD28-B7 interaction resulted in decreased production of IFN-gamma but not IL-12. Further reduction of IFN-gamma production was observed when both costimulatory pathways were blocked. Together, these results demonstrate that the CD28-B7 and CD40-CD40L interactions are involved in the development of infection-induced immunopathology in the absence of IL-10.     

Endogenous interleukin-12 is involved in resistance to Brucella abortus infection.

Protective immunity against Brucella abortus is mediated by acquired cellular resistance, with gamma interferon (IFN-gamma)-producing T cells playing a key role. Interleukin-12 (IL-12) is a cytokine that has a profound effect on the induction of IFN-gamma-producing Th1 and NK cells. Here we report that depletion of endogenous IL-12 before infection of mice significantly exacerbated brucella infection. IL-12-depleted mice also had reduced splenomegaly resulting from infection and showed a decrease in percentage and absolute numbers of macrophages compared with those in control infected mice. Furthermore, spleen cells from IL-12-depleted mice had a reduced ability to produce nitrite, a product of activated macrophages. This could be the result of the low production of IFN-gamma by splenic T cells observed in the IL-12-depleted mice. The mechanism whereby IL-12 controls antibacterial resistance is discussed.     

T cells are essential for bacterial clearance, and gamma interferon, tumor necrosis factor alpha, and B cells are crucial for disease development in Coxiella burnetii infection in mice

Coxiella burnetii, the etiological agent of Q fever, has two phase variants. Phase I has a complete lipopolysaccharide (LPS), is highly virulent, and causes Q fever in humans and pathology in experimental animals. Phase II lacks an LPS O side chain, is avirulent, and does not grow well in immunocompetent animals. To understand the pathogenicity of Q fever, we investigated the roles of immune components in animals infected with Nine Mile phase I (NM I) or Nine Mile phase II (NM II) bacteria. Immunodeficient mice, including SCID mice (deficient in T and B cells), SCIDbg mice (deficient in T, B, and NK cells), nude mice (deficient in T cells), muMT mice (deficient in B cells), bg mice (deficient in NK cells), mice deficient in tumor necrosis factor alpha (TNF-alpha(-/-) mice), and mice deficient in gamma interferon (IFN-gamma(-/-) mice), were compared for their responses to infection. SCID, SCIDbg, nude, and IFN-gamma(-/-) mice showed high susceptibility to NM I, and TNF-alpha(-/-) mice showed modest susceptibility. Disease caused by NM I in SCID, SCIDbg, and nude mice progressed slowly, while disease in IFN-gamma(-/-) and TNF-alpha(-/-) mice advanced rapidly. B- and NK-cell deficiencies did not enhance clinical disease development or alter bacterial clearance but did increase the severity of histopathological changes, particularly in the absence of B cells. Mice infected with NM II showed no apparent clinical disease, but T-cell-deficient mice had histopathological changes. These results suggest that T cells are critical for clearance of C. burnetii, either NM I or NM II, that IFN-gamma and TNF-alpha are essential for the early control of infection, and that B cells are important for the prevention of tissue damage.     

Suppressor cells induced by influenza virus inhibit interleukin-2 production in mice.

PR8 virus depressed interleukin-2 (IL-2) and natural killer (NK) cell activity in BALB/c infected mice. IL-2 production was not dependent on (i) a decreased number of T cells or (ii) a primary defect in IL-1 production, but on a T-suppressor cell subpopulation. In fact, when T suppressor cells were removed from infected spleen cells, we observed normal levels of IL-2 activity.