Viral skin infections are a significant cause of morbidity and mortality, with millions of individuals infected worldwide with the poxvirus molluscum contagiosum virus (MCV). The related poxvirus Vaccinia virus (VACV) was used to immunize hundreds of millions of people during the smallpox eradication program, and remains a backbone of the many widely used viral vaccine vectors. The goal of the immune system is to restrict virus spread until an adaptive immune response can be mounted that will clear the virus. Interferons (IFN) are a family of protein cytokines that upregulate IFN stimulated genes (ISGs) to restrict virus replication and spread and also activate the immune response via recruitment and activation of innate and effector immune cell populations. The IFN family consists of Type I, II and III, each using separate receptors. All IFN receptors signal through the Janus kinase (JAK)/ Signal Transducer and Activator of Transcription (STAT) pathway. STAT1 signals via interactions with a number of other STAT proteins to transduce signals after receptor binding of IFNs. After IFN receptor signaling, STAT1 is phosphorylated and subsequent nuclear translocation leads to activation of a large number of ISGs. Deletion of STAT1 can dramatically reduce the immune response to IFN in vitro and in vivo, and mice lacking STAT1 often succumb to systemic challenge with virus, including VACV. Because of the tight linkage of IFN signaling and STAT1, the lethal phenotype of STAT1-deficient mice upon virus challenge is most often assumed to be due to lack of the action of IFN. Our preliminary data demonstrate that peripheral dermal infection with VACV, which is normally restricted to the site of infection, causes death of mice lacking STAT1. Importantly, our preliminary data also indicate that ablation or depletion of all three IFNs did not confer lethal susceptibility to dermal VACV, indicating that the lethal phenotype of STAT1-deficient mice is likely a product of IFN-independent STAT1 signaling. A role for STAT1 has been implicated in other signaling pathways, such as those mediated by CD117 (c-kit), IL-27R, IL-21R and a number of Toll-Like Receptors (TLR). Our system, where we have a non-lethal infection in IFN-deficient animals, but a lethal phenotype in STAT1-deficient mice, offers us a unique opportunity to probe the IFN-independent role of STAT1 in protective antiviral immunity. We will examine the IFN-independent role of STAT1 in 2 Specific Aims. In Aim 1, we will compare the host range of VACV, along with the innate and adaptive anti-VACV immune responses, in mice lacking STAT1 or all three IFN receptors. Using this strategy we can identify IFN- independent STAT1-mediated events that may be responsible for protection against lethal VACV challenge. In Aim 2, we will examine STAT1 phosphorylation in mice lacking all three IFN receptors, as well as target genes where expression in induced in a STAT1-dependent, IFN-independent manner. At the conclusion of the proposed investigation we will have identified cells, immune processes and target genes that can provide the basis for a future RO1 proposal investigating IFN-independent STAT1-mediated antiviral immunity.