Functional analyses on roles of intetferon-IRF-family transcriptional factor

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In this study, we analyzed the molecular mechanism for antiviral defense triggered by interferon-α / (IFN-α/β) and the signalling pathways which lead to type I IFN gene induction by TLR activation. In particular, we focused on IFN-regulatory factors (IRFs), which are known to be essential factors upon viral infection, and have elucidated several novel roles of the IFN-IRF system in activation of innate immune response. First, we found that the level of p53 gene, which is known to encode a tumor suppressor, is upregulated by IFN treatment through activation of ISGF3 transcriptional complex, contributing to enhancement of p53 response to stress signals. We showed that p53 is phosphorylated and activated upon viral infection, and that p53 is essentially involved in the induction of apoptosis in virally infected cells. This result demonstrated an interrelationship between IFN-α/β signaling and p53-mediated response, which provided a novel aspect in terms of a linkage between tumor suppression and immunity. Second, we analyzed various hideficient mice to show that IRF-7 mediates a novel MyD88-dependent pathway for the induction of type I IFN gene expression by TLR9 subfamily members in plasmacytoid dendritic cells (pDCs). In addition, we found a spaciotemporal regulation, by which pDCs are capable to produce high levels of IFN- α/β. On the other hand, IRF-5 is found to be essentially involved in TLR-mediated production of proinflammatory cytokines. Recently, we also found that IRF-5 is activated upon viral infection and plays a role in virus-induced apoptosis in a distinct manner from p53. Another IRF-family member, IRF-4, is shown to be a negative regulator of IRF-5 by competitively inhibiting the interaction of IRF-5 with MyD88. Thus, we demonstrated that IRF-family transcriptional factors are critical downstream mediators of TLRs to regulate the activation of innate immune response.