Decreased resistance of B cell-deficient mice to infection with Toxoplasma gondii despite unimpaired expression of IFN-gamma, TNF-alpha, and inducible nitric oxide synthase

The role of B cells in resistance against Toxoplasma gondii was studied using B cell-deficient (muMT) mice. Following peroral infection with 10 cysts of the ME49 strain, all muMT mice survived the acute stage of the infection but died between 3 and 4 wk after infection. In contrast, all control mice were alive at 8 wk after infection. At the stage during which muMT animals succumbed to the infection, parasite replication and pathology were most evident in their brains; small numbers of tachyzoites were also detectable in their lungs. Significantly greater numbers of T. gondii cysts and areas of inflammation associated with tachyzoites were observed in brains of muMT than in control mice. Large areas of necrosis associated with numerous tachyzoites were observed only in brains of muMT mice. Anti-T. gondii IgG Abs were detected only in sera of control mice, whereas similar levels of IFN-gamma were detected in sera of both strains of mice. Amounts of mRNA for IFN-gamma, IL-10, and inducible NO synthase in the brain did not differ between infected muMT and control mice. Expression of mRNA for TNF-alpha was increased in brains of muMT mice. Administration of polyclonal rabbit anti-T. gondii IgG Ab prevented early mortality and pathology associated with tachyzoites in the brain in the infected muMT mice. These results indicate that B cells play an important role, most likely through their production of specific Abs, in resistance to persistent active (tachyzoite) infection with T. gondii in mice, especially in the brain and lung.     

Simultaneous depletion of CD4+ and CD8+ T lymphocytes is required to reactivate chronic infection with Toxoplasma gondii.

C57BL/6 mice chronically infected with an avirulent strain (ME-49) of Toxoplasma gondii were used to study the mechanisms by which T lymphocytes and IFN-gamma prevent reactivation of latent infection. Infected animals were treated with mAb, either anti-CD8, anti-CD4, anti-CD4 plus anti-CD8, anti-IFN-gamma, or anti-CD4 plus anti-IFN-gamma and the mice followed for survival, histopathology, cyst numbers, and spleen cell cytokine responses. In agreement with previously published findings, treatment with anti-IFN-gamma antibodies fully reactivated the asymptomatic infection, inducing massive necrotic areas in the brain with the appearance of free tachyzoites and death of all animals within 2 wk. Mice treated with the combination of anti-CD4 plus anti-CD8 antibodies showed augmented pathology and mortality nearly identical to the anti-IFN-gamma- treated animals. In contrast, treatment with anti-CD4 or anti-CD8 mAb alone failed to result in significantly enhanced brain pathology or mortality. In additional experiments, full reactivation of infection was observed in mice treated with anti-CD4 plus anti-IFN-gamma indicating that CD4+ lymphocytes are not required for the pathology resulting from IFN-gamma neutralization. Cytokine measurements on parasite Ag-stimulated spleen cells from mAb-treated mice indicated that both CD4+ and CD8+ cells produce IFN-gamma whereas only CD4+ cells contribute to parasite Ag-induced IL-2 synthesis. Together, these results suggest that CD4+ and CD8+ lymphocytes act additively or synergistically to prevent reactivation of chronic T. gondii infection probably through the production of IFN-gamma.     

IL-10 is required for prevention of necrosis in the small intestine and mortality in both genetically resistant BALB/c and susceptible C57BL/6 mice following peroral infection with Toxoplasma gondii

The role for IL-10 in the immunopathogenesis of acute toxoplasmosis following peroral infection was examined in both genetically susceptible C57BL/6 and resistant BALB/c mice. C57BL/6-background IL-10-targeted mutant (IL-10-/-) mice all died in 2 wk after infection with 20 cysts of the ME49 strain, whereas only 20% of control mice succumbed. Histological studies revealed necrosis in the small and large intestines and livers of infected IL-10-/- mice. The necrosis in the small intestine was the most severe pathologic response and was not observed in control mice. Treatment of infected IL-10-/- mice with either anti-CD4 or anti-IFN-gamma mAb prevented intestinal pathology and significantly prolonged time to death. Treatment of these animals with anti-IL-12 mAb also prevented the pathology. Significantly greater amounts of IFN-gamma mRNA were detected in the lamina propria lymphocytes obtained from the small intestine of infected IL-10-/- mice than those from infected control mice. In common with C57BL/6-background IL-10-/- mice, BALB/c-background IL-10-/- mice all died developing intestinal pathology after infection. Control BALB/c mice all survived even after infection with 100 cysts and did not develop the intestinal lesions. Treatment with anti-IFN-gamma mAb prevented the pathology and prolonged time to death of the infected IL-10-/- mice. These results strongly suggest that IL-10 plays a critical role in down-regulating IFN-gamma production in the small intestine following sublethal peroral infection with Toxoplasma gondii and that this down-regulatory effect of IL-10 is required for prevention of development of IFN-gamma-mediated intestinal pathology and mortality in both genetically resistant BALB/c and susceptible C57BL/6 mice.     

Induction of tumor necrosis factor-alpha and inducible nitric oxide synthase fails to prevent toxoplasmic encephalitis in the absence of interferon-gamma in genetically resistant BALB/c mice

Following infection with Toxoplasma gondii, certain strains of mice, such as BALB/c, are genetically resistant to development of toxoplasmic encephalitis (TE) and establish a latent chronic infection as do humans. Thus, these animals appear to be a suitable model to analyze the mechanism of resistance to TE. Since the mechanism for their genetic resistance is unknown, we examined the role of interferon-gamma (IFN-gamma) tumor necrosis factor-alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS) in the resistance using BALB/c-background IFN-gamma-deficient (IFN-gamma(-/-)) mice. IFN-gamma(-/-) and control mice were infected with the ME49 strain of T. gondii and treated with sulfadiazine to establish chronic infection. After discontinuing sulfadiazine, the IFN-gamma(-/-) mice all died, whereas the control mice all survived. Histological studies revealed remarkable inflammatory changes associated with large numbers of tachyzoites in brains of the IFN-gamma(-/-) mice but not in the control mice after discontinuation of sulfadiazine. Large amounts of mRNA for tachyzoite-specific SAG1 were detected in brains of only the IFN-gamma(-/-) mice. IFN-gamma mRNA was detected in brains of only the control mice, whereas mRNA for TNF-alpha and iNOS were detected in brains of both strains of mice. The amounts of the mRNA for TNF-alpha and iNOS did not differ between these mice. Treatment of IFN-gamma(-/-) mice with recombinant IFN-gamma prevented development of TE. These results demonstrate that IFN-gamma is crucial for genetic resistance of BALB/c mice against TE and that TNF-alpha and iNOS are insufficient to prevent TE in the absence of IFN-gamma.     

IFN-gamma overproduction and high level apoptosis are associated with high but not low virulence Toxoplasma gondii infection.

Toxoplasma gondii is an opportunistic intracellular parasite which induces a highly strong type 1 cytokine response. The present study focuses on defining the factors influencing the outcome of infection with tachyzoites of the type I, highly lethal RH strain, relative to the type II, low virulence strain ME49. Infection with the RH strain led to widespread parasite dissemination and rapid death of mice; in contrast, mice survived low virulence strain ME49 infection, and tachyzoite dissemination was much less extensive. Furthermore, massive apoptosis and disintegration of the splenic architecture was characteristic of RH, but not ME49, infection. In addition, hyperinduction of IFN-gamma and lack of NO production were found during RH, in contrast to ME49 infection. These data demonstrate that Toxoplasma strain characteristics exert a profound effect on the host immune response and that the latter itself is a crucial determinant in parasite virulence.     

Cold stress-induced modulation of inflammatory responses and intracerebral cytokine mRNA expression in acute murine toxoplasmosis

The effects of a physical stressor, cold water stress (CWS), within the central nervous system were investigated in the acute phase of infection with Toxoplasma gondii. Female BALB/c mice were subjected to CWS for 5 min each day for 8 days prior to oral infection with 20 cysts of the low virulent ME 49 strain. Animals were killed at 10-day intervals to detect inflammation, gliosis, and expression of intracerebral cytokine mRNAs. Zones of inflammation were detected by Nissl staining and gliosis by immunoreactivity to glial fibrillary acidic protein. Larger zones of inflammation and reactive astrogliosis were consistently observed in mice subjected to CWS and infected (CWS +INF) compared to control infected (INF) mice. Expression of interleukin (IL)-1beta, IL-2, IL-12, interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha, and inducible nitric oxide synthase (iNOS) were decreased in CWS+INF mice at 10 days postinoculation (PI), followed by a gradual increase after day 20 PI. This was in contrast to increased expression of these cytokines at 10 days PI in INF mice with a gradual decline thereafter. Inflammation and astrogliosis in CWS+INF mice were associated with an increased expression of IL-1beta, IL-6, IL-12, and TNF-alpha between 20 and 30 days PI. These findings correlated with the continuous gene expression of tachyzoite surface antigen (SAG)-1 mRNA in CWS+INF mice compared to its sharp decline in INF mice after 20 days PI. These results suggest that CWS delays regulation and control of intracerebral Toxoplasma gondii during acute infection in BALB/c mice by decreasing the early expression of IFN-gamma, IL-2, TNF-alpha, iNOS, IL-1beta, and IL-12, while increasing the expression of IL-6, a counterregulatory cytokine.     

Nitric oxide and the Th2 response combine to prevent severe hepatic damage during Schistosoma mansoni infection.

During infection with Schistosoma mansoni, NO production increases following the deposition of parasite eggs in the liver. In wild-type C57BL/6 mice, NO levels peak during the sixth week of infection and are subsequently down-regulated. Inducible NO synthase (iNOS) mRNA was found in diseased liver tissue along with TNF-alpha and IFN-gamma, which are known promoters of iNOS expression. Mice treated with aminoguanidine, a selective inhibitor of iNOS, exhibited cachexia and exacerbated liver pathology, suggesting that NO limits hepatocyte damage when the liver is first exposed to eggs. Hepatic iNOS is up-regulated in SCID mice, indicating that NO production is part of an innate response. Studies with infected highly susceptible IL-4-/- mice revealed that prolonged NO production is in itself deleterious and that a major function of the Th2 response, which is severely compromised in the absence of IL-4, is to regulate NO production. In these animals, plasma NO levels are high compared with those in infected wild-type mice and remain elevated until death. Nevertheless, the underlying importance of NO is illustrated by the finding that aminoguanidine treatment leads to more severe liver disease and reduced time to death in infected IL-4-/- mice.     

Prooxidant diet provides protection during murine infection with Toxoplasma gondii

Toxoplasmosis, particularly toxoplasmic encephalitis, has emerged as a major cause of morbidity and mortality in patients with acquired immunodeficiency syndrome. Patients infected with human immunodeficiency virus typically experience chronic oxidative stress, and concurrent infection with the intracellular parasite Toxoplasma gondii would be expected to further exacerbate this condition. The present study was conducted to determine whether vitamin E and selenium supplementation might be beneficial in a murine model of toxoplasmosis. To investigate the effect of these antioxidants on the severity of parasitic infection. Swiss Webster (SW) or C57Bl/6J mice infected with oocysts of the ME49 strain of T. gondii were maintained on diets containing no vitamin E or selenium, no vitamin E and 8 ppm selenium, 400 IU/kg vitamin E plus 8 ppm selenium, or vitamin E and selenium at the levels present in standard rodent chow (16 IU/kg and 0.2 ppm, respectively). The results of the study showed that increased dietary supplementation with vitamin E and selenium resulted in trends toward increased tissue cyst number, tissue pathology, and weight loss during infection. In contrast, both resistant SW and susceptible C57Bl/6J mice fed a deficient diet (complete absence of vitamin E and selenium) showed the lowest mean numbers of tissue cysts and very little evidence of tissue pathology during chronic infection.     

Demonstration of antigenic similarities and variations in excretory/secretory antigens of Toxoplasma gondii.

C57BL/6 mice were orally infected with different doses of cysts of ME49 strain of Toxoplasma gondii to produce groups of acutely and chronically infected mice. Sera were obtained at different periods post-infection. SDS-PAGE was ran with excretory/secretory antigens of ME49 and RH strains of T. gondii, followed by Western blot analyses using the above sera and anti- IgA, IgM, IgG as conjugates. The SDS-PAGE profiles of the two antigens were similar. However the antigenic bands showed variations in all blots, most evidently in IgA blots of chronic sera. IgG blots showed greatest similarities in reactive bands. In IgM blots, more common bands were shown in chronic sera than in acute sera. Variations and similarities in prominence of some bands and time of their appearance were also noted, especially in IgM and IgG blots of chronic sera. Thus antigenic variations and similarities are present in excretory/secretory products of different strains of T. gondii.     

A human origin type II strain of Toxoplasma gondii causing severe encephalitis in mice

Despite its capacity for sexual reproduction and global distribution, Toxoplasma gondii has a highly clonal structure, with the majority of isolates belonging to three distinct clonal types. Congenital toxoplasmosis has been associated with type I and type II strains. We here present the first characterization of a T. gondii strain (BGD1) from South-East Europe, isolated from the umbilical blood of a 24-week-old fetus in Serbia. Genotyping, performed by PCR-RFLP using a set of nested PCR markers (5'SAG2, 3'SAG2, BTUB, SAG3, and GRA6), showed that the BGD1 strain possessed a type II genotype. The cytokine patterns in Swiss-Webster mice inoculated with brain cysts of BGD1 and the prototype type II ME49 strain were similar until 180 days post-infection, with highly elevated IFN-gamma, IL-12 and IL-10 by d7 and decreasing thereafter. While both strains induced pneumonia and hepatitis in acute infection (d14), chronic infection (d56) was characterized, in addition to hepatitis, by severe meningoencephalitis, associated with numerous brain cysts. Thus, the BGD1 strain of T. gondii has type II genotypic and immunologic characteristics, but unlike other type II strains of human origin, induces severe encephalitis, making it an alternative to the sheep-derived ME49 strain for experimental models of infection.     

Changes in blastogenic responses and antibody titers of mice infected with Toxoplasma gondii]

This study was performed to observe the cell-mediated and humoral immune responses in mice which were infected with Beverley, Fukaya and ME49 strain of Toxoplasma gondii, respectively. The blastogenic responses of splenocytes using [3H]-thymidine and serum antibody titers were measured weekly up to 10 weeks after infection. The blastogenic responses of splenocytes treated with concanavalin A and Toxoplasma lysate were significantly declined in the 3 strain groups as compared with the non-infected group (p less than 0.05), however lipopolysaccharide-treated blastogenic responses were not significantly different between infected and non-infected groups. The serum IgG antibody titers in the three infected groups increased from 2 weeks after infection, and the serum IgM antibody titers increased until 4 weeks after infection. No significant differences were revealed in blastogenic responses and serum antibody titers among the 3 groups. The present study suggested that cell-mediated immune responses were involved in T. gondii infected mice and blastogenic responses of T lymphocytes were inhibited in acute T. gondii infection.     

Exacerbated susceptibility to infection-stimulated immunopathology in CD1d-deficient mice

Mice lacking functional CD1d genes were used to study mechanisms of resistance to the protozoan parasite Toxoplasma gondii. Wild-type (WT) BALB/c mice, CD1d-deficient BALB/c mice, and WT C57BL/6 mice all survived an acute oral infection with a low dose of mildly virulent strain ME49 T. gondii cysts. In contrast, most CD1d-deficient C57BL/6 mice died within 2 wk of infection. Despite having parasite burdens that were only slightly higher than WT mice, CD1d-deficient C57BL/6 mice displayed greater weight loss and intestinal pathology. In C57BL/6 mice, CD4(+) cells can cause intestinal pathology during T. gondii infection. Compared with WT mice, infected CD1d-deficient C57BL/6 mice had higher frequencies and numbers of activated (CD44(high)) CD4(+) cells in mesenteric lymph nodes. Depletion of CD4(+) cells from CD1d-deficient mice reduced weight loss and prolonged survival, demonstrating a functional role for CD4(+) cells in their increased susceptibility to T. gondii infection. CD1d-deficient mice are deficient in Valpha14(+) T cells, a major population of NKT cells. Involvement of these cells in resistance to T. gondii was investigated using gene-targeted Jalpha18-deficient C57BL/6 mice, which are deficient in Valpha14(+) T cells. These mice did not succumb to acute infection, but experienced greater weight loss and more deaths than B6 mice during chronic infection, indicating that Valpha14(+) cells contribute to resistance to T. gondii. The data identify CD4(+) cells as a significant component of the marked susceptibility to T. gondii infection observed in CD1d-deficient C57BL/6 mice, and establish T. gondii as a valuable tool for deciphering CD1d-dependent protective mechanisms.     

Protection against lethal toxoplasmosis in mice by an avirulent strain of Toxoplasma gondii: stimulation of IFN-gamma and TNF-alpha response

In this study, we examined whether the PTN strain (isolated from an AIDS patient) of Toxoplasma gondii could induce cross-protection in mice against infection with a lethal dose of the PLK strain. Mice were first infected with tachyzoites (5 x 10(5)) of PTN and 5 days later challenged with PLK (1 x 10(5), LD(90)) parasites. None of these mice succumbed to infection until day 21 after infection, whereas 100% of the mice given the same dose of PLK infection alone died between 5 and 11 days after infection. The protection was accompanied by an increased expansion of NK cells and CD4 + T cells. This condition was associated by increased production of IFN-gamma and an augmented number of IFN-gamma-producing cells in the spleen. Further, PTN + PLK-infected mice showed higher production of TNF-alpha and nitrite compared to PLK-infected mice. Mice infected with the PTN strain had an enhanced capacity to activate the immune system early in infection since they produced higher levels of IFN-gamma, TNF-alpha, and NO than PLK-infected mice. Administration of anti-IFN-gamma mAb or anti-asialo GM1 antibody resulted in 100 and 20% mortality, respectively, in PTN-infected mice but no death in PTN + PLK-infected mice. Together, these results suggest that early production of IFN-gamma and NK-cell activity is important in protection against PTN infection, whereas in PTN + PLK infection components of adaptive immunity rapidly developed following elaboration of an effective early innate immune response.     

Role of CD28 in the generation of effector and memory responses required for resistance to Toxoplasma gondii.

CD28 deficient (CD28-/-) mice were used to study the role of costimulation in the T cell-mediated, IFN-gamma-dependent mechanism of resistance to Toxoplasma gondii. These mice were resistant to infection with the ME49 strain of T. gondii. Analysis of the immune response of acutely infected CD28-/- mice revealed that IL-12 was required for T cell production of IFN-gamma and this was independent of the CD40/CD40 ligand interaction. A similar mechanism of IL-12-dependent, CD28/B7 independent production of IFN-gamma by T cells was also observed in wild-type mice. Interestingly, although chronically infected wild-type mice were resistant to rechallenge with the virulent RH strain of T. gondii, chronically infected CD28-/- mice were susceptible to rechallenge with the RH strain. This deficiency in the protective memory response by CD28-/- mice correlated with a lack of IL-2 and IFN-gamma in recall responses and reduced numbers of CD4+ T cells expressing a memory phenotype. Together, our findings demonstrate that CD28 is not required for the development of a protective T cell response to T. gondii, but CD28 is required for an optimal secondary immune response.     

An opposite role is exerted by the acarian Myocoptes musculinus in the outcome of Toxoplasma gondii infection according to the route of the protozoa inoculation

Infection with Toxoplasma gondii leads to a Th1 immune response. Alternatively, the acarian Myocoptes musculinus induces a disease in BALB/c mice that involves Th2 immune mechanisms. In this study, we investigated whether infestation by M. musculinus induces Th2 immune response in C57BL/6 mice and if this response influences the T. gondii-induced Th1 response when mice are inoculated by intraperitoneal or oral route. The animals were infected with M. musculinus and one month later with T. gondii ME-49 strain and the survival and immune response were monitored. The co-infected animals displayed higher mortality rate and the spleen cells showed a decreased IFN-gamma and elevated IL-4 and IL-5 production. These changes were associated with severe pneumonia and wasting condition. On the other hand, when mice were orally infected with 100 T. gondii cysts, co-infection prolonged the survival rates and ameliorated intestinal lesions in association with a significant drop in IFN-gamma levels in sera. These results indicate the interference of Th2 response induced by M. musculinus in a T. gondii-induced Th1 response. Altogether, these data demonstrate the profound interactions between the immune response induced against unrelated organisms T. gondii and M. musculinus, and suggest that this type of interactions may impact clinical disease.     

Horizontal transmission of Toxoplasma gondii in squirrel monkeys (Saimiri sciureus).

The possibility of horizontal transmission of T. gondii was examined in squirrel monkeys. After three monkeys were inoculated perorally with 1.1-2.1 x 10(3) cysts of the T. gondii ME49, the animals were divided into two cages and maintained with one normal monkey for each cage as a cagemate. Two out of the three T. gondii-inoculated monkeys died, and the remaining one monkey was sacrificed in a moribund state one week after infection because of acute toxoplasmosis. Many T. gondii tachyzoites were recovered from broncho-alveolar lavages and were also found histopathologically in the lung, liver, spleen, kidney and lymph nodes and impression smears of tissues from the three T. gondii-inoculated monkeys by Giemsa staining. Anti-T. gondii antibody was examined by immunoblot assay in these animals, and the antibody to T. gondii major surface membrane protein (p30) could be detected after the start of experiment. Furthermore, a specific band of T. gondii NTPase gene was observed by PCR in the liver and lung of infected and cagemate monkeys, and the sequence of the second PCR products obtained from the cagemates, which were clinically normal but gave a positive result in immunoblotting assay, was exactly the same as the sequence of the NTPase gene of T. gondii ME49. These findings suggested that transmission of T. gondii from the infected monkeys to cagemates occurred easily, and since many T. gondii tachyzoites were recovered from the bronchoalveolar lavages of the three T. gondii-inoculated monkeys, we suggest that aerosol infection plays an important role for the enzootic toxoplasmosis in colonies of squirrel monkeys.     

Immunosuppression with cyclophosphamide favors reinfection with recombinant Toxoplasma gondii strains

The aim of this study was to verify the effect of immunosuppression by cyclophosphamide (Cy) on susceptibility of BALB/c mice subjected to challenge with recombinant strains of Toxoplasma gondii. Animals were prime infected with the D8 (recombinant I/III) or the ME49 (type II) non-virulent strains, weekly immunosuppressed with Cy and challenged with the CH3 or EGS virulent strains (I/III). Parasites recovered from surviving mice were submitted to PCR-RFLP analysis to confirm co-infection. Prime-infection with the D8 strain conferred more protection against challenge with the CH3 and EGS strains when compared with ME49 prime infection. Cy treatment caused significant leukopenia in the infected mice, what probably favors reinfection after challenge. Reinfection was associated with increased levels of IgA. Otherwise, Cy-treated mice presented significantly lower IgA levels after challenge, suggesting involvement of this immunoglobulin on protection against reinfection. In conclusion, BALB/c mice susceptibility to reinfection by T. gondii is related to genetic differences among the strains used for primary and challenge infections. Alteration of the host's immune integrity by Cy probably compromises the protection previously established by primary infection.     

Schistosome-infected IL-4 receptor knockout (KO) mice, in contrast to IL-4 KO mice, fail to develop granulomatous pathology while maintaining the same lymphokine expression profile.

Th2 lymphocytes have been postulated to play a major role in the immunopathology induced by Schistosoma mansoni infection. Nevertheless, infected IL-4 knockout (KO) and wild-type (wt) mice develop egg granulomas comparable in size. To further investigate the function of the Th2 response in egg pathology we studied IL-4Ralpha-deficient mice, which are nonresponsive to both IL-4 and IL-13. In striking contrast to IL-4 KO animals, infected IL-4Ralpha KO mice developed only minimal hepatic granulomas and fibrosis despite the presence of CD3+ T cells in the residual egg lesions. Moreover, liver lymphokine mRNA levels in these animals and IL-4 KO mice were equivalent. In addition, infected IL-4Ralpha-deficient, IL-4-deficient, and wt animals developed similar egg Ag-specific IgG Ab titers, arguing that CD4-dependent Th activity is intact in KO mice. As expected, IFN-gamma secretion was strongly up-regulated in mesenteric lymph node cultures from both groups of deficient animals, a change reflected in increased serum IgG2a and IgG2b Ab levels. Surprisingly, Th2 cytokine production in infected IL-4Ralpha KO mice was not abolished but was only reduced and resembled that previously documented in IL-4 KO animals. This residual Th2 response is likely to explain the ability of IL-4 KO mice to generate egg granulomas, which cannot be formed in IL-4Ralpha-deficient animals because of their lack of responsiveness to the same cytokine ligands. Taken together, these findings argue that tissue pathology in schistosomiasis requires, in addition to egg-specific CD4+ lymphocytes, a previously unrecognized IL-4Ralpha+ non-T cell effector population.     

Acute toxoplasmosis leads to lethal overproduction of Th1 cytokines

Virulence in Toxoplasma gondii is strongly influenced by the genotype of the parasite. Type I strains uniformly cause rapid death in mice regardless of the host genotype or the challenge dose. In contrast, the outcome of infections with type II strains is highly dependent on the challenge dose and the genotype of the host. To understand the basis of acute virulence in toxoplasmosis, we compared low and high doses of the RH strain (type I) and the ME49/PTG strain (type II) of T. gondii in outbred mice. Differences in virulence were reflected in only modestly different growth rates in vivo, and both strains disseminated widely to different tissues. The key difference in the virulent RH strain was the ability to reach high tissue burdens rapidly following a low dose challenge. Lethal infections caused by type I (RH) or type II (PTG) strain infections were accompanied by extremely elevated levels of Th1 cytokines in the serum, including IFN-gamma, TNF-alpha, IL-12, and IL-18. Extensive liver damage and lymphoid degeneration accompanied the elevated levels of cytokines produced during lethal infection. Increased time of survival following lethal infection with the RH strain was provided by neutralization of IL-18, but not TNF-alpha or IFN-gamma. Nonlethal infections with a low dose of type II PTG strain parasites were characterized by a modest induction of Th1 cytokines that led to control of infection and minimal damage to host tissues. Our findings establish that overstimulation of immune responses that are normally necessary for protection is an important feature of acute toxoplasmosis.     

Toxoplasma gondii genotype determines MyD88-dependent signaling in infected macrophages

Infection of mouse macrophages with Toxoplasma gondii elicits MAPK activation and IL-12 production, but host cell signaling pathways have not been clearly delineated. Here, we compared macrophage signaling in response to high virulence type I (RH) vs low virulence type II (ME49) strain infection. Tachyzoites of both strains induced p38 MAPK-dependent macrophage IL-12 release, although ME49 elicited 2- to 3-fold more cytokine than RH. IL-12 production was largely restricted to infected cells in each case. RH-induced IL-12 release did not require MyD88, whereas ME49-triggered IL-12 production was substantially dependent on this TLR/IL-1R adaptor molecule. MyD88 was also not required for RH-stimulated p38 MAPK activation, which occurred in the absence of detectable upstream p38 MAPK kinase activity. In contrast, ME49-driven p38 MAPK activation displayed an MyD88-dependent component. This parasite strain also induced MyD88-dependent activation of MKK4, an upstream activator of p38 MAPK. The results suggest that RH triggers MAPK activation and IL-12 production using MyD88-independent signaling, whereas ME49 uses these pathways as well as MyD88-dependent signaling cascades. Differences in host signaling pathways triggered by RH vs ME49 may contribute to the high and low virulence characteristics displayed by these parasite strains.