ABSTRACT
Innate immune recognition of intracellular pathogens involves both extracellular and cytosolic surveillance mechanisms. The intracellular protozoan parasite Trypanosoma cruzi triggers a robust type I IFN response in both immune and nonimmune cell types. In this study, we report that signaling through TBK1 and IFN regulatory factor 3 is required for T. cruzi-mediated expression of IFN-beta. The TLR adaptors MyD88 and TRIF, as well as TLR4 and TLR3, were found to be dispensable, demonstrating that T. cruzi induces IFN-beta expression in a TLR-independent manner. The potential role for cytosolic dsRNA sensing pathways acting through RIG-I and MDA5 was ruled out because T. cruzi was shown to trigger robust expression of IFN-beta in macrophages lacking the MAVS/IPS1/VISA/CARDif adaptor protein. The failure of T. cruzi to activate HEK293-IFN-beta-luciferase cells, which are highly sensitive to cytosolic triggers of IFN-beta expression including Listeria, Sendai virus, and transfected dsRNA and dsDNA, further indicates that the parasite does not engage currently recognized cytosolic surveillance pathways. Together, these findings identify the existence of a novel TLR-independent pathogen-sensing mechanism in immune and nonimmune cells that converges on TBK1 and IFN regulatory factor 3 for activation of IFN-beta gene expression.