Suppression of the IFN-α and -β Induction through Sequestering IRF7 into Viral Inclusion Bodies by Nonstructural Protein NSs in Severe Fever with Thrombocytopenia Syndrome Bunyavirus Infection

Induction of type I IFNs during viral infection is crucial for host defense. IRF 3 and IRF7 play a critical role as key transcription factors in the activation of the IFN induction. Viruses have evolved a variety of strategies to evade innate immunity. Our previous studies have shown that the nonstructural protein (NSs) of the severe fever with thrombocytopenia syndrome virus (SFTSV) can suppress the IFN-beta induction through its interaction with tank-binding kinase-1 and sequestering the inhibitor of nuclear factor kappa B kinase (IKK) complex into the inclusion bodies formed by NSs. In this study, we characterized the unique function of IRF7 in innate immunity and its role in inducing IFN-alpha in particular, regulated by NSs during the SFTSV infection in several cell types of human origin. Whereas IRF3 is constitutively expressed, IRF7 was significantly induced differentially in various cell types in response to SFTSV infection, promoted the induction of IFN-alpha 2 and -alpha 4, and further induced IFN-beta, thus contributing to suppressing the viral replication. Our data indicate that NSs directly interacted with and sequestered IRF7 into the inclusion bodies, which is different from IRF3 indirectly interacting with NSs. Although interaction of NSs with IRF7 did not inhibit IRF7 phosphorylation, p-IRF7 was trapped in the inclusion bodies, resulting in a significant reduction of the IFN-alpha 2 and -alpha 4 induction and therefore enhanced viral replication. Interaction of the viral NSs with both IRF7 and IRF3 and subsequent sequestration of these transcription factors into viral inclusion bodies, a unique strategy used by this phlebovirus, may ensure effective evasion and suppression of host innate immunity.