Tumor necrosis factor alpha mediates resistance to Trypanosoma cruzi infection in mice by inducing nitric oxide production in infected gamma interferon-activated macrophages.

Cell invasion by Trypanosoma cruzi and its intracellular replication are essential for continuation of the parasite life cycle and for production of Chagas' disease. T. cruzi is able to replicate in nucleated cells and can be killed by activated macrophages. Gamma interferon (IFN-gamma) is one of the major stimuli for the activation of macrophages and has been shown to be a key activation factor for the killing of intracellular parasites through a mechanism dependent upon nitric oxide (NO) biosynthesis. We show that although the addition of exogenous tumor necrosis factor alpha (TNF-alpha) does not potentiate the trypanocidal activity of IFN-gamma in vitro, treatment of resistant C57BI/6 mice with an anti-TNF-alpha monoclonal antibody increased parasitemia and mortality. In addition, the anti-TNF-alpha-treated animals had decreased NO production, both in vivo and in vitro, suggesting an important role for TNF-alpha in controlling infection. In order to better understand the role of TNF-alpha in the macrophage-mediating killing of parasites, cultures of T. cruzi-infected macrophages were treated with an anti-TNF-alpha monoclonal antibody. IFN-gamma-activated macrophages failed to kill intracellular parasites following treatment with 100 micrograms of anti-TNF-alpha. In these cultures, the number of parasites released at various time points after infection was significantly increased while NO production was significantly reduced. We conclude that IFN-gamma-activated macrophages produce TNF-alpha after infection by T. cruzi and suggest that this cytokine plays a role in amplifying NO production and parasite killing.     

During Trypanosoma cruzi Infection CD1d-Restricted NK T Cells Limit Parasitemia and Augment the Antibody Response to a Glycophosphoinositol-Modified Surface Protein

Trypanosoma cruzi is a protozoan parasite that chronically infects many mammalian species and in humans causes Chagas' disease, a chronic inflammatory disease. The parasite expresses glycophosphoinositol (GPI), which potently stimulates interleukin 12 (IL-12) production. During T. cruzi infection IL-12, and possibly GPI, might stimulate NK T cells to affect the protective and chronic inflammatory responses. Here we report that during T. cruzi infection CD1d-restricted NK T cells are stimulated as NK T-cell-deficient mice have greater parasitemia. Furthermore, during T. cruzi infection the percentages of NK T cells in the liver and spleen become decreased for prolonged periods of time, and in vitro stimulation of NK T cells derived from livers of chronically infected mice, compared to uninfected mice, results in increased gamma interferon and IL-4 secretion. Moreover, in NK T-cell-deficient mice the chronic-phase antibody response to a GPI-modified surface protein is decreased. These results indicate that, during the acute infection, NK T cells limit parasitemia and that, during the chronic phase, NK T cells augment the antibody response. Thus, during T. cruzi infection the quality of an individual's NK T-cell response can affect the level of parasitemia and parasite tissue burden, the intensity of the chronic inflammatory responses, and possibly the outcome of Chagas' disease.     

Resistant mice lacking interleukin-12 become susceptible to Trypanosoma cruzi infection but fail to mount a T helper type 2 response

Interleukin-12 (IL-12) is essential to resistance to Trypanosoma cruzi infection because it stimulates the synthesis of interferon-gamma (IFN-gamma) that activates macrophages to a parasiticidal effect. Investigation of mice deprived of IL-12 genes (IL-12 knockout mice) has confirmed the important role of IL-12 and IFN-gamma in controlling parasitism in T. cruzi infection. However, it has not yet been addressed whether a shift towards a T helper type 2 (Th2) pattern of cytokine response occurred in these mice that might have contributed to the aggravation of the infection caused by IL-12 deprivation. We examined the course of T. cruzi (Y strain) infection and the regulation of cytokine responses and nitric oxide production in C57BL/6 IL-12 p40-knockout mice. The mutant mice were extremely susceptible to the infection as evidenced by increased parasitaemia, tissue parasitism and mortality in comparison with the control C57BL/6 mouse strain (wild-type) that is resistant to T. cruzi. A severe depletion of parasite-antigen-specific IFN-gamma response, without an increase in IL-4 or IL-10 production, accompanied by reduced levels of nitric oxide production was observed in IL-12 knockout mice. We found no evidence of a shift towards a Th2-type cytokine response. In IL-12 knockout mice, the residual IFN-gamma production is down-regulated by IL-10 but not by IL-4 and nitric oxide production is stimulated by tumour necrosis factor-alpha. Parasite-specific immunoglobulin G1 antibody levels were similar in IL-12 knockout and wild-type mice, whereas IL-12 knockout mice had much higher levels of immunoglobulin G2b.     

Effects of interleukin-4 deprivation and treatment on resistance to Trypanosoma cruzi

Trypanosoma cruzi (Y strain)-infected interleukin-4(-/-) (IL-4(-/-)) mice of strains 129/J, BALB/c, and C57BL/6 showed no significant difference in parasitemia levels or end point mortality rates compared to wild-type (WT) mice. Higher production of gamma interferon (IFN-gamma) by parasite antigen (Ag)-stimulated splenocytes was observed only for C57BL/6 IL-4(-/-) mice. Treatment of 129/J WT mice with recombinant IL-4 (rIL-4), rIL-10, anti-IL-4, and/or anti-IL-10 monoclonal antibodies (MAbs) did not modify parasitism. However, WT mice treated with rIL-4 and rIL-10 had markedly increased parasitism and suppressed IFN-gamma synthesis by spleen cells stimulated with parasite Ag, concanavalin A, or anti-CD3. Addition of anti-IL-4 MAbs to splenocyte cultures from infected WT 129/J, BALB/c, or C57BL/6 mice failed to modify IFN-gamma synthesis levels; in contrast, IL-10 neutralization increased IFN-gamma production and addition of rIL-4 and/or rIL-10 diminished IFN-gamma synthesis. We conclude that endogenous IL-4 is not a major determinant of susceptibility to Y strain T. cruzi infection but that IL-4 can, in association with IL-10, modulate IFN-gamma production and resistance.     

Regulation of Trypanosoma cruzi infection in mice by gamma interferon and interleukin 10: role of NK cells.

Gamma interferon (IFN-gamma) plays an important role in experimental Trypanosoma cruzi infections, presumably by controlling the early replication of parasites in host macrophages. In this work, we show that NK cells represent an important cell type responsible for the production of most of the IFN-gamma in the early stage of T. cruzi infection and that the in vivo treatment of mice with anti-NK1.1 monoclonal antibody made resistant animals susceptible to the infection. Through in vitro experiments, we demonstrate that normal splenocytes from euthymic or athymic nude mice cultivated for 48 h with live T. cruzi trypomastigotes produced elevated levels of IFN-gamma. In addition, NK-depleted splenocytes show a drastic reduction of IFN-gamma production in response to live T. cruzi trypomastigotes. We also demonstrated that IFN-gamma production is dependent on a factor secreted by adherent cells. Supernatants of spleen cells from athymic nude mice are able to induce IFN-gamma production by normal splenocytes when cultured with trypomastigotes. The addition of anti-interleukin-10 to these cultures resulted in a marked increase in IFN-gamma production. On the other hand, the absence of NK cells led to an increased secretion of interleukin-10 upon in vitro stimulation with T. cruzi. Taken together, these results suggest that NK cells are the major source of IFN-gamma that could be involved in limiting the replication of T. cruzi in host macrophages during the early acute phase of the infection.     

Interleukin-12-mediated resistance to Trypanosoma cruzi is dependent on tumor necrosis factor alpha and gamma interferon.

The aim of this study was to determine if interleukin-12 (IL-12) has a role in the immune response to Trypanosoma cruzi. Infection of BALB/c mice with the virulent Tulahuen strain of T. cruzi is characterized by a high-level parasitemia, pathology in the heart associated with the presence of amastigotes, and death during the acute phase of the disease. Administration of IL-12 to BALB/c mice infected with T. cruzi resulted in a reduced parasitemia and a significant delay in the time to death compared with those for infected controls. This protective effect was correlated with increased levels of gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) in serum. To determine if these cytokines were involved in the protective effects of IL-12, we treated infected mice with IL-12 alone or in combination with monoclonal antibodies specific for IFN-gamma or TNF-alpha. These antibodies antagonized the protective effect of exogenous IL-12. Treatment of infected mice with a polygonal antibody specific for IL-12 resulted in a significant increase in parasitemia but did not affect the time to death. These latter studies demonstrate a role for endogenous IL-12 in resistance to T. cruzi. Together, our data identify an IL-12-mediated mechanism of resistance to T. cruzi, which is dependent on IFN-gamma and TNF-alpha.     

Pivotal role of interleukin-12 and interferon-gamma axis in controlling tissue parasitism and inflammation in the heart and central nervous system during Trypanosoma cruzi infection.

The role of cytokines in the control of tissue parasitism and pathogenesis of experimental Chagas' disease was investigated. Wild-type and different cytokine as well as inducible nitric oxide synthase (iNOS) knockout mice were infected with the Colombian strain of Trypanosoma cruzi, and the kinetics of tissue parasitism, inflammatory reaction, parasitemia, and mortality were determined. We demonstrate the pivotal role of the interleukin (IL)-12/interferon (IFN)-gamma/iNOS axis and the antagonistic effect of IL-4 in controlling heart tissue parasitism, inflammation, and host resistance to acute infection with T. cruzi. Further, the heart and central nervous system were shown the main sites of reactivation of T. cruzi infection in mice lacking functional genes for IFN-gamma and IL-12, respectively. Our results also show that in contrast to IFN-gamma knockout (KO) mice, splenocytes from IL-12 KO mice infected with T. cruzi produced low levels of IFN-gamma upon stimulation with antigen. Consistently, high levels of anti-T. cruzi IgG2a antibodies were detected in the sera from IL-12 KO, but not from IFN-gamma KO mice, infected with the Colombian strain of T. cruzi. Thus, our results suggest that the level of IFN-gamma deficiency is a major determinant of the site of reactivation of T. cruzi infection in immunocompromised host.     

CD40 ligation prevents Trypanosoma cruzi infection through interleukin-12 upregulation

Because of the critical role of the CD40-CD40 ligand (CD40L) pathway in the induction and effector phases of immune responses, we investigated the effects of CD40 ligation on the control of Trypanosoma cruzi infection. First, we observed that supernatants of murine spleen cells stimulated by CD40L-transfected 3T3 fibroblasts (3T3-CD40L transfectants) prevent the infection of mouse peritoneal macrophages (MPM) by T. cruzi. This phenomenon depends on de novo production of nitric oxide (NO) as it is prevented by the addition of N-nitro-L-arginine methyl ester, a NO synthase inhibitor. NO production requires interleukin (IL)-12-mediated gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) synthesis as demonstrated by inhibition experiments using neutralizing anti-IL-12, anti-IFN-gamma, and anti-TNF-alpha monoclonal antibodies (MAb). We found that an activating anti-CD40 MAb also directly stimulates IFN-gamma-activated MPM to produce NO and thereby to control T. cruzi infection. To determine the in vivo relevance of these in vitro findings, mice were injected with 3T3-CD40L transfectants or 3T3 control fibroblasts at the time of T. cruzi inoculation. We observed that in vivo CD40 ligation dramatically reduced both parasitemia and the mortality rate of T. cruzi-infected mice. A reduced parasitemia was still observed when the injection of 3T3-CD40L transfectants was delayed 8 days postinfection. It was abolished by injection of anti-IL-12 MAb. Taken together, these data establish that CD40 ligation facilitates the control of T. cruzi infection through a cascade involving IL-12, IFN-gamma, and NO.     

Role of IFN-alpha/beta and IL-12 in the activation of natural killer cells and interferon-gamma production during experimental infection with Trypanosoma cruzi

Control of Trypanosoma cruzi infection depends largely upon the production of interferon (IFN)-gamma. During experimental infection this cytokine is produced early, mainly by natural killer (NK) cells and later by T cells. As NK cells have been reported to participate in defence against T. cruzi, it is of importance to study the regulation of NK cell functions during infection with the parasite. Several innate cytokines regulate NK cell activity, among them being interferon (IFN)-alpha and IFN-beta (type 1 IFNs) and interleukin (IL)-12, which have all been reported to be involved in protection against T. cruzi. The role of these cytokines in regulation of NK cell functions and disease outcome were studied by infection of mutant mice lacking the IFN-alpha/beta receptor (IFNalpha/betaR-/-) or IL-12 (IL-12-/-) with T. cruzi. IFNalpha/betaR-/- mice were unable to activate the cytotoxic response but produced IFN-gamma, and were not more susceptible than controls. IL-12-/- mice were extremely susceptible and failed to produce T cell-derived IFN-gamma and nitric oxide (NO), although NK cytotoxicity was induced. The results indicate that IL-12 protects against T. cruzi by initiating T cell-mediated production of IFN-gamma, but that endogenous IFN-alpha/beta and NK cell cytotoxicity are not of major importance in defence.     

Gene gun-mediated delivery of an interleukin-12 expression plasmid protects against infections with the intracellular protozoan parasites Leishmania major and Trypanosoma cruzi in mice

An interleukin-12 (IL-12) expression plasmid was transferred, using a gene gun, to mice infected with Leishmania major or Trypanosoma cruzi. Transfer of the IL-12 gene to susceptible BALB/c mice resulted in regression of lesion size and reduced the number of parasites in draining lymph nodes (LN) at the site of L. major infection. Coincident with these protective effects, the T-helper type (Th) response shifted towards Th1, as evaluated by cytokine production in vitro and L. major-specific antibody responses. Protective effects of the IL-12 gene were also observed in T. cruzi infection. Treatment of BALB/c mice infected with T. cruzi enhanced the production of interferon-gamma (IFN-gamma) by spleen cells, while suppressed production of interleukin-10 (IL-10) compared with control mice. Administration of anti-CD4 or anti-CD8 monoclonal antibody (mAb) abolished the protective immunity against T. cruzi infection, and treatment with the IL-12 gene could not restore the resistance in these mice. Mice depleted of natural killer (NK) cells with anti-asialo GM1 also became susceptible to infection, while the resistance was restored when these mice were treated with the IL-12 gene. Thus, target cells for the treatment appear to be CD4+ and CD8+ T cells, which are ordinarily activated by NK cells. These results suggest that the transfer of cytokine genes using a gene gun is an effective method for investigating the roles of cytokines and gene therapy in infectious diseases.     

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.     

Interleukin-12 stimulation of lymphoproliferative responses in Trypanosoma cruzi infection.

The cytokine interleukin-12 (IL-12) is essential for resistance to Trypanosoma cruzi infection because it stimulates the synthesis of interferon-gamma (IFN-gamma), a major activator of the parasiticidal effect of macrophages. A less studied property of IL-12 is its ability to amplify the proliferation of T or natural killer (NK) lymphocytes. We investigated the role of endogenously produced IL-12 in the maintenance of parasite antigen (T-Ag)-specific lymphoproliferative responses during the acute phase of T. cruzi infection. We also studied whether treatment with recombinant IL-12 (rIL-12) would stimulate T-Ag-specific or concanavalin A (Con A)-stimulated lymphoproliferation and abrogate the suppression that is characteristic of the acute phase of infection. Production of IL-12 by spleen-cell cultures during T. cruzi infection increased in the first days of infection (days 3-5) and decreased as infection progressed beyond day 7. The growth-promoting activity of endogenous IL-12 on T-Ag-specific proliferation was observed on day 5 of infection. Treatment of cultures with rIL-12 promoted a significant increase in Con A-stimulated proliferation by spleen cells from normal or infected mice. Enhanced T-Ag-specific proliferation by rIL-12 was seen in spleen cell cultures from infected mice providing that nitric oxide production was inhibited by treatment with the competitive inhibitor NG-monomethyl-L-arginine (NMMA). Enhancement of proliferation promoted by IL-12 occurred in the presence of neutralizing anti-interleukin-2 (IL-2) antibody, suggesting that this activity of IL-12 was partly independent of endogenous IL-2. Thymidine incorporation levels achieved with rIL-12 treatment of the cultures were approximately 50% of those stimulated by rIL-2 in the same cultures.     

WSX-1 is required for resistance to Trypanosoma cruzi infection by regulation of proinflammatory cytokine production

WSX-1 is a class I cytokine receptor with homology to the IL-12 receptors and is essential for resistance to Leishmania major infection. In the present study, we demonstrated that WSX-1 was also required for resistance to Trypanosoma cruzi. WSX-1-/- mice exhibited prolonged parasitemia, severe liver injury, and increased mortality over wild-type mice. WSX-1-/- splenocytes produced enhanced levels of Th2 cytokines, which were responsible for the prolonged parasitemia. Massive necroinflammatory lesions were observed in the liver of infected WSX-1-/- mice, and IFN-gamma that was overproduced in WSX-1-/- mice compared with wild-type mice was responsible for the lesions. In addition, vast amounts of various proinflammatory cytokines, including IL-6 and TNF-alpha, were produced by liver mononuclear cells in WSX-1-/- mice. Thus, during T. cruzi infection, WSX-1 suppresses liver injury by regulating production of proinflammatory cytokines, while controlling parasitemia by suppression of Th2 responses, demonstrating its novel role as an inhibitory regulator of cytokine production.     

The Fas death pathway controls coordinated expansions of type 1 CD8 and type 2 CD4 T cells in Trypanosoma cruzi infection

We investigated the role of the Fas ligand (FasL)/Fas death pathway on apoptosis and cytokine production by T cells in Trypanosoma cruzi infection. Anti-FasL, but not anti-TNF-alpha or anti-TRAIL, blocked activation-induced cell death of CD8 T cells and increased secretion of IL-10 and IL-4 by CD4 T cells from T. cruzi-infected mice. CD4 and CD8 T cells up-regulated Fas/FasL expression during T. cruzi infection. However, Fas expression increased earlier in CD8 T cells, and a higher proportion of CD8 T cells was activated and expressed IFN-gamma compared with CD4 T cells. Injection of anti-FasL in infected mice reduced parasitemia and CD8 T cell apoptosis and increased the ratio of CD8:CD4 T cells recovered from spleen and peritoneum. FasL blockade increased the number of activated T cells, enhanced NO production, and reduced parasite loads in peritoneal macrophages. Injection of anti-FasL increased IFN-gamma secretion by splenocytes responding to T. cruzi antigens but also exacerbated production of type 2 cytokines IL-10 and IL-4 at a late stage of acute infection. These results indicate that the FasL/Fas death pathway regulates apoptosis and coordinated cytokine responses by type 1 CD8 and type 2 CD4 T cells in T. cruzi infection.     

Resistance to acute babesiosis is associated with interleukin-12- and gamma interferon-mediated responses and requires macrophages and natural killer cells

We examined the role of the cytokines gamma interferon (IFN-gamma) and interleukin-12 (IL-12) in the model of acute babesiosis with the WA1 Babesia. Mice genetically deficient in IFN-gamma-mediated responses (IFNGR2KO mice) and IL-12-mediated responses (Stat4KO mice) were infected with the WA1 Babesia, and observations were made on the course of infection and cytokine responses. Levels of IFN-gamma and IL-12 in serum increased 24 h after parasite inoculation. The augmented susceptibility observed in IFNGR2KO and Stat-4KO mice suggests that the early IL-12- and IFN-gamma-mediated responses are involved in protection against acute babesiosis. Resistance appears to correlate with an increase in nitric oxide (NO) production. In order to assess the contribution of different cell subsets to resistance against the parasite, we also studied mice lacking B cells, CD4+ T cells, NK cells, and macrophages. Mice genetically deficient in B lymphocytes or CD4+ T lymphocytes were able to mount protective responses comparable to those of immunosufficient mice. In contrast, in vivo depletion of macrophages or NK cells resulted in elevated susceptibility to the infection. Our observations suggest that a crucial part of the response that protects from the pathogenic Babesia WA1 is mediated by macrophages and NK cells, probably through early production of IL-12 and IFN-gamma, and induction of macrophage-derived effector molecules like NO.     

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.     

Gamma interferon production is critical for protective immunity to infection with blood-stage Plasmodium berghei XAT but neither NO production nor NK cell activation is critical

We have examined the roles of gamma interferon (IFN-gamma), nitric oxide (NO), and natural killer (NK) cells in the host resistance to infection with the blood-stage malarial parasite Plasmodium berghei XAT, an irradiation-induced attenuated variant of the lethal strain P. berghei NK65. Although the infection with P. berghei XAT enhanced NK cell lytic activity of splenocytes, depletion of NK1.1(+) cells caused by the treatment of mice with anti-NK1.1 antibody affected neither parasitemia nor IFN-gamma production by their splenocytes. The P. berghei XAT infection induced a large amount of NO production by splenocytes during the first peak of parasitemia, while P. berghei NK65 infection induced a small amount. Unexpectedly, however, mice deficient in inducible nitric oxide synthase (iNOS-/-) cleared P. berghei XAT after two peaks of parasitemia were observed, as occurred for wild-type control mice. Although the infected iNOS-/- mouse splenocytes did not produce a detectable level of NO, they produced an amount of IFN-gamma comparable to that produced by wild-type control mouse splenocytes, and treatment of these mice with neutralizing anti-IFN-gamma antibody led to the progression of parasitemia and fatal outcome. CD4(-/-) mice infected with P. berghei XAT could not clear the parasite, and all these mice died with apparently reduced IFN-gamma production. Furthermore, treatment with carrageenan increased the susceptibility of mice to P. berghei XAT infection. These results suggest that neither NO production nor NK cell activation is critical for the resistance to P. berghei XAT infection and that IFN-gamma plays an important role in the elimination of malarial parasites, possibly by the enhancement of phagocytic activity of macrophages.     

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.     

NK1.1+ cells and T-cell activation in euthymic and thymectomized C57Bl/6 mice during acute Trypanosoma cruzi infection

Natural killer (NK) cells may provide the basis for resistance to Trypanosoma cruzi infection, because the depletion of NK1.1 cells causes high levels of parasitemia in young C57Bl/6 mice infected with T. cruzi. Indeed, NK1.1 cells have been implicated in the early production of large amounts of interferon (IFN)-gamma, an important cytokine in host resistance. The NK1.1 marker is also expressed on special subpopulations of T cells. Most NK1.1+ T cells are of thymic origin, and their constant generation may be prevented by thymectomy. This procedure, by itself, decreased parasitemia and increased resistance in young mice. However, the depletion of NK1.1+ cells by the chronic administration of a monoclonal antibody (MoAb) (PK-136) did not increase the parasitemia or mortality in thymectomized C57Bl/6 mice infected with T. cruzi (Tulahuen strain). To study the cross-talk between NK1.1+ cells and conventional T cells in this model, we examined the expression of activation/memory markers (CD45RB) on splenic CD4+ and CD8+ T cells from young euthymic or thymectomized mice with or without depletion of NK1.1+ cells and also in aged mice during acute infection. Resistance to infection correlated with the amount of CD4+ T cells that are already activated at the moment of infection, as judged by the number of splenic CD4+ T cells expressing CD45RB(-). In addition, the specific antibody response to T. cruzi antigens was precocious and an accumulation of immunoglobulin (Ig)M with little isotype switch occurred in euthymic mice depleted of NK1.1+ cells. The data presented here suggest that NK1.1+ cells have important regulatory functions in euthymic, but not in thymectomized mice infected with T. cruzi. These regulatory functions include a helper activity in the generation of effector or activated/memory T cells.