Cell-Intrinsic Type I Interferon Signaling as a Pleiotropic Orchestrator of CD4 T Cell Immunity.

Type I interferons (IFN-I) are pleiotropic cytokines best known for their antiviral impacts. However, they are known to also impact immune responses outside of viral infection through directly signaling many populations of innate and adaptive immune cells. Here, we focus on the complex body of findings from viral, bacterial, and parasitic infection models, cancer and autoimmunity studies, as well as in vitro experiments using human and murine T cells, demonstrating that IFN-I can be directly sensed by CD4 T cells. Such signaling has been shown to influence many central aspects of antigen-specific CD4 T cell responses, including proliferation, apoptosis, effector subset differentiation, and memory formation. These effects are frequently divergent and sometimes opposing, likely reflecting how differences in variables related to the IFN-I signal, overall inflammatory milieu, and the CD4 T cell integrate to shape outcomes. Indeed, we discuss findings supporting a framework in which dynamic engagement of canonical and non-canonical signaling pathways downstream of IFN-I, which are contingent on a cell's activation state, play a key role in determining whether and how IFN-I promotes, restrains, or otherwise reprograms CD4 T cell fates. Together, these observations highlight the impressive scope of regulation that IFN-I signals to CD4 T cells can exert, parallel to its actions on other immune and non-immune cell types. They also suggest that harnessing such signaling could offer powerful therapeutic strategies to shape CD4 T cell immunity in diverse context-dependent situations.