Sensing of Picornavirus Infections

The sensing of viral infection through pattern recognition receptors is necessary to trigger an immune response through the expression of interferons (IFNs) and cytokines. For RNA viruses, which replicate in the cytoplasm, the cytoplasmic RIG-I like receptors are responsible for this sensing. The contributions of these receptors to sensing viruses of the Picornaviridae family were investigated. Using bone marrow derived macrophages from MDA-5 and RIG-I null mice we showed that encephalomyocarditis virus (EMCV) and coxsackievirus B3 (CVB3), picornaviruses of the cardiovirus and the enterovirus genus respectively, are sensed by both MDA-5 and RIG-I. Sensing of these viruses in macrophages leads to the expression of type I IFN early after infection with IFNβ expression reduced in the absence of each sensor, and IFNα; expression reduced in the absence of MDA-5. However, in macrophages EMCV and CVB3 do not grow and we find that the sensing of viruses differs in murine embryonic fibroblasts (MEFs) and HeLa cells in which there is efficient replication. In MEFs RIG-I was found to be essential for the expression of type I IFNs but contribute to increases in viral titers in response to CVB3 infections. MDA-5 inhibited CVB3 replication but in an IFN independent manner. In HeLa cells MDA-5 was found to have no effect on the sensing of EMCV but to be important for the sensing of poliovirus. However mutants of these viruses changed the involvement of MDA-5 in sensing infection, indicating picornaviruses can evade sensing through different pathways.