The Regulation Of Innate Immunity Response To Influenza Virus B Infection And The Nucleocytoplasmic Shuttle Of M1 Protein During Viral Replication

Host innate immunity response is initiated upon virus invasion. Here, we report that robust production of type I and III interferons (IFNs), IFN-stimulated genes(ISGs), and preinflammatory factors can be induced by influenza B virus infection rather than influenza A virus in alveolar epithelial cells (A549) during early infection. This response is mainly dependent on RIG-I-mediated signal pathway. The infection by influenza B virus promotes intense Lys 63-linked ubiquitination of RIG-I, resulting in cytokine eruption. It is known that influenza A virus NS1 protein (NS1-A) interacts with RIG-I and TRIM25 respectively to suppress the activation of RIG-I-mediated signaling. However, the present results indicate that influenza B virus NS1 protein (NS1-B) is unable to interact with RIG-I but engages in the formation of a RIG-I/TRIM25/NS1-B complex. Furthermore, we demonstrate that the N-terminal RNA-binding domain (RBD) of NS1-B is responsible for interaction with TRIM25, and this interaction blocks the inhibitory effect on RIG-I ubiquitination from NS1-B C-terminal effector domain (TED). Our findings reveal a novel mechanism of the host cytokine response to influenza B virus infection through regulatory interplay between host and viral proteins.Influenza B virus is an enveloped negative-strand RNA virus, that contributes considerably to annual influenza illnesses in human. The matrix protein of influenza B virus (BM1) acts as a cytoplasmic-nuclear shuttling protein during the early and late stages of infection. The mechanism of this intracellular transport of BM1 was revealed through the identification of two leucine-rich CRM1-dependent nuclear export signals (NESs) (3-14aa and 124-133aa), one bipartite nuclear localization signal (NLS) (76-94aa), and two phosphorylation sites (80T and 84S) in BM1. The biological function of the NLS and NES regions were determined through the observation of the intracellular distribution of EGFP-tagged signal peptides, and WT, NES-mutant and NLS-mutant EGFP-BM1. Furthermore, the NLS phosphorylation sites 80T and 84S, were found to be required for the nuclear accumulation of EGFP-NLS and for the efficient binding of EGFP-BM1 to human importin-al. Moreover, all of these regions/sites were required for the generation of viable influenza B virus in a 12-plasmid virus rescue system.