The cytosolic DNA- and RNA-sensing pathways play important and non-redundant roles in host defense against vaccinia infection

Abstract

Abstract Vaccinia virus is a prototypic poxvirus, and during the life cycle of the virus, viral DNA and RNA can be detected by the nucleic acid-sensing pathways to induce the expression of type I IFN and IFN-stimulating genes, which leads to the establishment of antiviral state. Poxviruses have evolved multiple strategies to evade host immunity. Vaccinia E3, which consists of two distinct Z-DNA-binding and dsRNA-binding domains, is a critical virulence factor. Virus lacks E3L gene (ΔE3L) is non-pathogenic in wild-type mice in an intranasal infection model. We have previously reported that ΔE3L infection of murine primary keratinocytes (KCs) induces Ifnb, Il6, Ccl5 and Ccl4 gene expression and protein secretion via a MAVS/IRF3-dependent mechanism. Here we show that MDA5, a cytosolic dsRNA sensor, is essential for the induction of innate immunity by ΔE3L in KCs, whereas PKR, another cytosolic dsRNA sensor, is not. ΔE3L gains virulence in MDA5, MAVS, or IRF3-deficient mice, but not in STING-or MyD88-deficient mice, in an intranasal infection model. The MDA5-, MAVS-, or IRF3-deficient mice lost 20% of weight but all recovered in response to ΔE3L infection. By contrast, both the MDA5 and STING-mediated sensing pathways contribute to host defense against WT vaccinia virus infection. Lastly, whereas ΔE3L virus fails to replicate in wild-type murine primary fibroblasts, it gains replication capacity in cGAS or STING-deficient murine primary fibroblasts. Taken together, these results indicate that vaccinia viral nucleic acids are detected via the cytosolic DNA- and/or RNA-sensing pathways in a cell-type dependent manner, and both pathways play important and non-redundant roles in host defense against vaccinia infection.