The protozoan parasite Toxoplasma gondii infects all nucleated cells and establishes life-long chronic infections in virtually any warm-blooded vertebrate. Eliminating the ability of this parasite to establish chronic infections in humans and animals is central to controlling its pathogenesis, however, there is currently no human vaccine or drug capable of doing this. Our lab has identified a series of highly polymorphic parasite secreted effector proteins, including a large superfamily of >182 SRS protein adhesins, that are essential for 1) entry into host cells and 2) regulating host immunity to promote chronic infections. SRS proteins are expressed in a development-specific manner, and we have shown by gene-knockout studies that four of these antigens expressed by the tachyzoite stage are critical virulence factors: SRS29B, SRS29C, SRS34A, and SRS57. SRS57 is a pivotal adhesin required for establishing infection, whereas SRS29B, SRS29C, and SRS34A are primarily immunomodulating factors that elicit strong immunity in all infected hosts. We recently produced parasite strains deficient in the expression of SRS29B, SRS34A, SRS29C, and SRS57. Srs29B- strains were less pathogenic, infected mice had lower parasite burdens, produced substantially less IFN-gamma, and failed to induce a lethal ileitis, in contrast to wild type parasite infections. Our results suggest that SRS29B is a critical virulence factor and that mice die from parasite burden and severe immune pathology dependent on SRS29B expression. Current investigations are testing whether collapse of regulatory T and B cells during acute ileitis is dependent on SRS29B. Srs34A- strains possessed an invasion defect, and are less virulent in murine infections. Previously, we showed that mouse virulent Toxoplasma strains weakly express SRS29C, whereas all avirulent strains are high expressors. We therefore tested whether SRS29C expression level is sufficient to alter mouse virulence. When expressed transgenically in a virulent strain at levels equivalent to those found in avirulent strains, the SRS29C transgenic strains were highly attenuated. These data suggest that SRS29C is a pivotal virulence factor and that its expression level is a critical determinant governing the outcome of infection. Importantly, we identified that SRS29C expression altered the levels of bioactive IL-1 beta and IL-18 during infection of murine BMDMs and mice, respectively. Our unpublished work has established that SRS29C specifically interacts with heparan sulfate, a glycosaminoglycan (GAG), and our crystal structure has identified two residues in the SRS29C binding cleft that are essential for this interaction. Overexpression of the mutant SRS29C that fails to bind host GAGs restores murine virulence. Our current work is exploring the precise molecular details by which the SRS29C-host GAG interaction impacts host immunity to alter parasite pathogenesis. Specifically, we have shown that Toxoplasma activates the inflammasome sensors NLRP1 and NLRP3, and that caspase 1/11 dependent induction of IL-1 beta and IL-18 are required to regulate parasite proliferation and promote murine survival. Understanding the structural basis and the type of immunity induced during these natural infections should lay the foundation for therapeutic interventions, either prophylactic or vaccine-based, to limit infectivity and induce sterilizing immunity against this widespread zoonotic pathogen.
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