Type I IFN Signaling Constrains IL-17A/F Secretion by γδ T Cells during Bacterial Infections

Recognition of intracellular bacteria by macrophages leads to secretion of type I IFNs. However, the role of type I IFN during bacterial infection is still poorly understood. Francisella tularensis, the causative agent of tularemia, is a pathogenic bacterium that replicates in the cytosol of macrophages leading to secretion of type I IFN. In this study, we investigated the role of type I IFNs in a mouse model of tularemia. Mice deficient for type I IFN receptor (IFNAR1(-/-)) are more resistant to intradermal infection with E tularensis subspecies novicida (E novicida). Increased resistance to infection was associated with a specific increase in IL-17A/F and a corresponding expansion of an IL-17A(+) gamma delta T cell population, indicating that type I YFNs negatively regulate the number of IL-17A(+) gamma delta T cells during infection. Furthermore, IL-17A-deficient mice contained fewer neutrophils compared with wild-type mice during infection, indicating that IL-17A contributes to neutrophil expansion during F novicida infection. Accordingly, an increase in IL-17A in IFNAR1(-/-) mice correlated with an increase in splenic neutrophil numbers. Similar results were obtained in a mouse model of pneumonic tularemia using the highly virulent E tularensis subspecies tularensis SchuS4 strain and in a mouse model of systemic Listeria monocytogenes infection. Our results indicate that the type I IFN-mediated negative regulation of IL-17A(+) gamma delta T cell expansion is conserved during bacterial infections. We propose that this newly described activity of type I IFN signaling might participate in the resistance of the IFNAR1(-/-) mice to infection with E novicida and other intracellular bacteria. The Journal of Immunology, 2010, 184: 3755-3767.