Project Summary/Abstract (Project 3, Vance) Intracellular bacterial pathogens remain a major cause of human mortality and morbidity. By working collaboratively with other P01 Projects, we focus in Project 3 on the role of type I interferons (IFNs) in the innate immune response to three intracellular bacterial pathogens: Listeria monocytogenes, Mycobacterium tuberculosis, and Legionella pneumophila. As we and others have shown previously, all three pathogens induce the expression of type I IFNs via the STING pathway. However, while type I IFNs are generally accepted to play a crucial role in orchestrating anti-viral immunity, the roles of type I IFNs in the responses to bacteria are complex. Indeed, our work and that of other laboratories has shown that type I IFNs tend to exacerbate infection by bacterial pathogens, including M. tuberculosis, L. pneumophila and L. monocytogenes. Our preliminary data suggest that type I IFN-dependent induction of the interleukin-1 receptor antagonist (IL-1Ra) is an important mechanism by which type I IFNs exacerbate bacterial infections. Our preliminary data also suggest the existence of regulatory pathways to prevent the adverse induction of type I IFNs during bacterial infections. We identify SP140 and MORC3 as two novel negative regulators of type I IFN expression. We propose three Aims to explain why type I IFNs hinder anti-bacterial immunity and to explain how interferons are normally properly regulated during bacterial infections, in both mice and humans. In Aim 1, we test the hypothesis that type I IFNs antagonize anti-bacterial immunity via induction of IL-1 receptor antagonist, and determine the underlying mechanism. In Aim 2, we dissect the mechanism by which SP140 negatively regulates type I IFN transcription during bacterial infections. In Aim 3, we identify and characterize MORC3 as a novel negative regulator of type I IFN in human cells, and determine its function during bacterial infection.