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Host and Parasite Requirements for CD8 T cell IFNγ Responses to Toxoplasma gondii Infections


Host resistance to Toxoplasma gondii infections relies on CD8 T cell IFNγ responses. Since the manipulation of CD8 T cells may influence T. gondii’s ability to achieve chronic infection, we investigated host and parasite requirements for eliciting this response. Naïve CD8 T cell IFNγ responses to the endogenous T. gondii vacuolar resident antigen, TGD057 (T57), were analyzed. T57-specific CD8 T cells, isolated from naïve transnuclear mice, responded to T. gondii-infected bone marrow-derived macrophages in an antigen-dependent manner. T57 IFNγ responses to TGD057 were dependent on host immunity pathways downstream of regulatory Immunity-Related GTPases (IRGs), including partial dependence on Guanylate-Binding Proteins (GBPs), but not the parasite’s protein export machinery ASP5 and MYR1. Multiple T. gondii ROP5 isoforms and allele types, including ‘avirulent’ ROP5A from clade A and D T. gondii strains, were able to suppress CD8 T cell IFNγ responses. However, removal of ROP5 was not enough to elicit maximal CD8 T cell IFNγ production to T. gondii. Instead macrophage expression of the pathogen sensor NLRP3 was an absolute requirement while other members of the inflammasome cascade, including ASC, Caspase 1/11 and Gasdermin D, were only partially required. Hence, a novel NLRP3-dependent but inflammasome-independent pathway controls IFNγ differentiation of naïve CD8 T cells to T. gondii. Furthermore, a unique phenotypic pattern emerged where CD8 T cells responded vigorously to all T. gondii strains except those from clade A, suggesting the presence of a Regulator Of CD8 T cell Response (ROCTR). Genetic mapping implicates multiple ROCTR candidates may be at play, including two encoded on chromosomes X and XII and a weak association with GRA35 as a potential ROCTR candidate. GRA35 has been shown to localize to the parasitophorous vacuole membrane (PVM) with the aid of two dense granules, GRA42 and GRA43. Our studies suggest that GRA35, GRA42, and GRA43 are able to modulate the CD8 T cell response and postulate that ROCTRs may be targeted to the PVM by ASP5 and GRA43. Altogether, our data suggest that T. gondii utilize several strategies, perhaps multilayered, to regulate the CD8 T cell response, which is highly sensitive to NLRP3 and PVM integrity.

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