The Roles of IRF-3 and IRF-7 in Innate Antiviral Immunity against Dengue Virus

We investigated the roles of IFN regulatory factor (IRF)-3 and IRF-7 in innate antiviral immunity against dengue virus (DENV). Double-deficient Irf-3(-/-)7(-/-) mice infected with the DENV2 strain S221 possessed 1,000-150,000 fold higher levels of viral RNA than wild-type and single-deficient mice 24 h postinfection (hpi); however, they remained resistant to lethal infection. IFN-alpha/beta was induced similarly in wild-type and Irf-3(-/-) mice post-DENV infection, whereas in the Irf-7(-/-) and Irf-3(-/-)7(-/-) mice, significantly low levels of IFN-alpha/beta expression was observed within 24 hpi. IFN-stimulated gene induction was also delayed in Irf-3(-/-)7(-/-) mice relative to wild-type and single-deficient mice. In particular, Cxcl10 and Ifn alpha 2 were rapidly induced independently of both IRF-3 and IRF-7 in the Irf-3(-/-)7(-/-) mice with DENV infection. Higher levels of serum IFN-gamma, IL-6, CXCL10, IL-8, IL-12 p70, and TNF were also observed in Irf-3(-/-)7(-/-) mice 24 hpi, at which time point viral titers peaked and started to be cleared. Ab-mediated blockade experiments revealed that IFN-gamma, CXCL10, and CXCR3 function to restrict DENV replication in Irf-3(-/-)7(-/-) mice. Additionally, the IFN-stimulated genes Cxcl10, Ifit1, Ifit3, and Mx2 can be induced via an IRF-3- and IRF-7-independent pathway that does not involve IFN-gamma signaling for protection against DENV. Collectively, these results demonstrate that IRF-3 and IRF-7 are redundant, albeit IRF-7 plays a more important role than IRF-3 in inducing the initial IFN-alpha/beta response; only the combined actions of IRF-3 and IRF-7 are necessary for efficient control of early DENV infection; and the late, IRF-3- and IRF-7-independent pathway contributes to anti-DENV immunity.