ABSTRACT Influenza virus is a serious public health threat causing high levels of morbidity and mortality. The annual disease burden from influenza places a significant strain on our healthcare infrastructure and economy. This is despite many efforts to control disease with yearly vaccination, antiviral drug therapies, and other medical and public health interventions. To address the continuing health and economic costs, there is a clear need to better understand the molecular mechanisms of influenza virus replication and how these can be manipulated for therapeutic benefit. Influenza virus exploits, and in some cases subverts, cellular factors and pathways to promote replication. We identified RNA:protein interactions between viral and host partners that impact innate immune responses during infection. We showed IFIT2 is a critical cellular protein that binds viral mRNAs to enhance replication. This was surprising, as IFIT2 is one of the first proteins expressed in response to viral infection and displays broad-spectrum antiviral activity. The mechanisms underlying the antiviral activity of IFIT2, and how this is co-opted into a proviral effector by influenza virus, are not yet known. We also showed that influenza nucleoprotein (NP) is a key viral protein that binds host RNAs. This assigns a new activity to NP that our data suggest is part of a previously unappreciated strategy to dampen innate immune responses. The overall goal of this application is to determine how these RNA:protein complexes composed of both viral and host components manipulate innate immune responses to support viral replication. In Aim 1, we investigate how IFIT2 functions as a front-line defender in a broadly acting antiviral response. We hypothesize that IFIT2 enhances translation of antiviral proteins, an event that is repurposed by influenza virus to promote production of viral proteins. We test this using experiments that investigate the processes by which IFIT2 engages target RNAs, affects translation of its bound mRNAs, and alters infection. Aim 2 interrogates the impact of interactions between viral NP and host RNAs. This aim proposes that NP:RNA complexes moderate innate immune responses. We investigate this by studying RNAs and infection-induced events that activate innate immune pathways, and discern the impact on viral replication. The results from this proposal will establish a mechanistic understanding of the viral and host factors regulating innate immune responses and how influenza virus tips the balance to favor replication. Moreover, while our studies focus on influenza virus, the underlying mechanisms we discover will have broad impacts on the general understanding of host antiviral responses and unexpected strategies used by viruses to counteract them. Completion of this proposal will provide fundamental knowledge that can contribute to new therapeutic approaches or cellular targets that can be exploited for the rational development of anti-influenza vi...