Suppression of type I IFN production by Middle-East respiratory syndrome coronavirus ORF8b accessory protein through inhibition of HSP70-dependent activation of IKKε

Abstract

Middle-East respiratory syndrome coronavirus (MERS-CoV) is another human coronavirus causing severe disease in addition to severe acute respiratory syndrome coronavirus. A robust IFN response is not induced by MERS-CoV, indicating that it has developed countermeasures to evade host innate immunity. Here we report on the type I IFN antagonism of MERS-CoV ORF8b accessory protein. AntiORF8b antibodies were raised to show the abundant expression of ORF8b in MERS-CoV-infected cells. ORF8b-deficient MERS-CoV was constructed and compared with wild-type virus for replication dynamics and type I IFN induction patterns. This mutant virus was capable of inducing IFN-β more robustly. Enforced expression of ORF8b resulted in the suppression of IRF3-dependent induction of IFN-β production by Sendai virus and poly(I:C). Mechanistically, ORF8b operated prior to the phosphorylation and activation of IRF3. It interacted preferentially with chaperone protein HSP70 and prevented it from interacting with and activating IKKε that phosphorylates IRF3. In other words, ORF8b competed with IKKε for interaction with HSP70. Overexpression of HSP70 reversed the suppression of IFN-β production by ORF8b and this effect required full activity of IKKε. Our findings suggest an essential role for HSP70 chaperon in the activation of IKKε activation, which is suppressed by MERSCoV accessory protein ORF8b. Supported by Hong Kong Research Grants Council (T11-707/15-R).