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Signaling1
* Institute of Medical Microbiology and Hygiene, Philipps University, Marburg, Germany;
Department of Infectious Diseases, St. Jude Children’s Research Hospital, Memphis, TN 38105; and
Department of Hygiene and Medical Microbiology, University of Heidelberg, Heidelberg, Germany
Toxoplasma gondii is an intracellular parasite that survives and multiplies in professional phagocytes such as macrophages. Therefore, T. gondii has to cope with the panel of antimicrobial host immune mechanisms, among which IFN-
plays a crucial role. We report in this study that in vitro infection of murine macrophages with viable, but not with inactivated, parasites results in inhibition of IFN- signaling within the infected cells. Thus, infection of RAW264.7 macrophages with tachyzoites inhibited IFN--induced STAT-1 tyrosine phosphorylation, mRNA expression of target genes, and secretion of NO. These effects were dependent on direct contact of the host cells with living parasites and were not due to secreted intermediates. In parallel, we report the induction of suppressor of cytokine signaling-1 (SOCS-1), which is a known feedback inhibitor of IFN- receptor signaling. SOCS-1 was induced directly by viable parasites. SOCS overexpression in macrophages did not affect tachyzoite proliferation per se, yet abolished the inhibitory effects of IFN- on parasite replication. The inhibitory effects of T. gondii on IFN- were diminished in macrophages from SOCS-1–/– mice. The results suggest that induction of SOCS proteins within phagocytes due to infection with T. gondii contributes to the parasite’s immune evasion strategies.
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