Middle East respiratory syndrome coronavirus-encoded accessory proteins impair MDA5-and TBK1-mediated activation of NF-κB

Middle East respiratory syndrome coronavirus (MERS-CoV) is a newly emerging coronavirus which is zoonotic from bats and camels. Its infection in humans can be fatal especially in patients with preexisting conditions due to smoking and chronic obstructive pulmonary disease (COPD). Among the 25 proteins encoded by MERS-CoV, 5 accessory proteins seem to be involved in viral evasion of the host immune responses. Here we report that ORF4a, ORF4b, and ORF8b proteins, alone or in combination, effectively antagonize nuclear factor kappa B (NF-κB) activation. Interestingly, the inhibition of NF-κB by MERS-CoV accessory proteins was mostly at the level of pattern recognition receptors: melanoma differentiation-associated gene 5 (MDA5). ORF4a and ORF4b additively inhibit MDA5-mediated activation of NF-κB while that of retinoic acid-inducible gene 1 (RIG-I) is largely not perturbed. Of note, ORF8b was found to be a novel antagonist of MDA5-mediated NF-κB activation. In addition, ORF8b also strongly inhibits Tank-binding kinase 1 (TBK1)-mediated induction of NF-κB signaling. Taken together, MERS-CoV accessory proteins are involved in viral escape of NF-κB-mediated antiviral immune responses.