RIG-I-like receptors (RLRs): viral sensors that recognize Coxsackieviruses

The innate immune system is a vital part of the body‟s defences against viral pathogens. RIG-I and MDA5 belong to the retinoic acid inducible gene-I (RIG-I)-like receptors (RLRs) family and function as cytoplasmic PRRs that are involved in the elimination of actively replicating RNA viruses. Their location and their differential responses to RNA viruses emphasises the complexity of the innate detection system. RIG-I and MDA5 contribute to antiviral signalling in different ways depending on the virus involved. Coxsackieviruses are positive sense, single-stranded RNA viruses belonging to the Enterovirus genus of the Picornaviridae family. They cause many serious diseases, including viral myocarditis (which can lead on to dilated cardiomyopathy), aseptic meningitis, and pancreatitis. In order to identify which RLR recognises these viruses and which RNA species triggers RLR activation during Coxsackievirus infection, viral ssRNA and replicative intermediates of Coxsackievirus RNA as well as synthetic dsRNA were used in this study. The results revealed that MDA5 recognises not the genomic ssRNA but the dsRNA generated by the replication of these viruses. Confocal microscopy provided unique evidence between the relationship of viral dsRNA and MDA5 while cytokine assays using HEK-MDA5 cells showed a strong immune response to Coxsackievirus and the dsRNA intermediates. This shows very strong evidence that MDA5 is a key sensor of the dsRNA intermediate of Coxsackieviruses. As RIG-Is role in Coxsackie recognition still needs to be verified Huh7 and Huh7.5.1 cells were used and showed no difference in immune response in the absence of RIG-I to Coxsackievirus infection, as well as the isolated ssRNA, suggesting that the VPg group present on the RNA blocks recognition. Furthermore immunoprecipitation experiments showed that in response to Coxsackievirus stimulation, RLRs homodimerise as well as heterodimerise with LGP2, potentially upregulating their activity as a possible mechanism for viral detection. The data presented here show a much clearer role for RLRs in Coxsackievirus infection, while opening new questions as to MDA5s role in the diseases caused by Coxsackieviruses, as well as the specifics behind dimerisation of the RLR