A reporter mouse model to map RIG-I expression and function in vivo
The cytosolic RNA sensor RIG-I is a critical component of the innate immune response to RNA viruses, yet few tools exist to track RIG-I in vivo. Herein, we report the development of a RIG-I reporter mouse, termed RIG-ImScar, that enables fluorescence imaging and tractable immunoprecipitation of RIG-I in vivo. These mice revealed enrichment of RIG-I expression in circulatory, lymphoid, and mucosal tissues and broad expression across immune cell types. RIG-I itself is upregulated in response to IFN. To distinguish RIG-I sentinel cells, which are programmed to express RIG-I at steady state, from IFN-responder cells, we crossed RIG-ImScar mice with Mx1GFP mice. Blocking IFNAR with a monoclonal antibody showed that basal RIG-I expression in immune cells was largely independent of type I IFN signaling, although Ly6Chi monocytes required IFNAR for RIG-I expression. To probe functional responses in vivo, we intranasally administered a RIG-I-specific agonist, SLR14. Mapping RIG-I expression across the lung identified alveolar macrophages as a critical RIG-I sensor cell population, and depletion of alveolar macrophages, but not neutrophils, slightly impaired lung IFN responses. Together, these findings support the use of the RIG-ImScar model to probe RIG-I dynamics in vivo and predict functional sensor cell populations across tissues.