Project Summary Chikungunya virus (CHIKV) is a single-stranded RNA arthritogenic alphavirus and an emerging mosquito- transmitted pathogen. Acute CHIKV symptoms include rash, fever, arthritis, arthralgias, and myalgias. Since 2013, more than 1.9 million cases have been reported throughout the Caribbean, Central, and South America. Following recovery from acute CHIKV disease, chronic muscle and joint symptoms can persists 60% of patients. Replicating CHIKV is not detected during chronic time points. Yet, histological changes and noninfectious CHIKV RNA has been detected in patients and animal models. Unfortunately, tools to study chronic CHIKV have been inadequate to elucidate the cause of disease and are too insensitive to visualize the cells harboring persistent CHIKV RNA. In addition, the host factors that contribute to this chronic disease are largely unknown. As no specific antiviral therapies or vaccines for CHIKV exist, further understanding of host and viral factors that mediate acute and chronic CHIKV is crucial to combat this virus. Type I interferons (IFNs) are major effector cytokines of the innate immune system and include 14 IFN subtypes, IFN, and others. All type I IFNs signal through the shared heterodimeric IFN/ receptor to elicit antiviral, anti-proliferative, pro-apoptotic, or immunomodulatory effects. Previous work demonstrated that type I IFNs are robustly induced during both acute and chronic CHIKV infection in vivo and are necessary for protection. Despite signaling through a single, shared receptor, IFN subtypes have distinct potencies in vitro and differential functions in vivo. We have found that during acute disease, mice lacking IFN or IFN are more susceptible to acute CHIKV-induced disease. IFN limits viral replication and dissemination of CHIKV while IFN limits inflammation mediated by neutrophils. In both cases, these actions are mediated by signaling on nonhematopoietic cells. Yet, the cell types upon which type I IFNs act, the mechanistic correlates of protection, and whether IFN and IFN play a role during chronic disease remain uncertain. The proposed aims will determine the mechanistic basis of differential type I IFN protection or pathogenicity during acute CHIKV and the actions of the various subtypes during chronic disease. Therefore, we propose to use various Cre expressing mice and a mouse model of CHIKV to delete the type I IFN receptor on muscle cells, muscle satellite cells, or fibroblasts and examine acute pathogenesis and attempt to elucidate how the type I IFN subtypes mediate their differential effects during acute disease. In addition, we propose to study the impact of the type I IFN subtypes on chronic CHIKV inflammation, persistent RNA, and the cells that survive acute infection using our novel reporter system. Overall, the proposed work will address numerous gaps in current knowledge: which nonhematopoietic cells are important for type I IFN-mediated protection from CHIKV, how cells that...