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Toxoplasma gondii Genotype Determines MyD88-Dependent Signaling in Infected Macrophages¹

Leesun Kim^*, Barbara A. Butcher^*, Chiang W. Lee^*, Satoshi Uematsu, Shizuo Akira and Eric Y. Denkers^2,*

^* Department of Microbiology and Immunology, College of Veterinary Medicine, Cornell University, Ithaca, NY; and Department of Host Defense, Research Institute for Microbial Diseases, Osaka University, Osaka, Japan

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.

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