Induction of type I interferons by bacteria

Type I interferons (IFNs) are secreted cytokines that orchestrate diverse immune responses to infection. Although typically considered to be most important in the response to viruses, type I IFNs are also induced by most, if not all, bacterial pathogens. Although diverse mechanisms have been described, bacterial induction of type I IFNs occurs upon stimulation of two main pathways: (i) Toll-like receptor (TLR) recognition of bacterial molecules such as lipopolysaccharide (LPS); (ii) TLR-independent recognition of molecules delivered to the host cell cytosol. Cytosolic responses can be activated by two general mechanisms. First, viable bacteria can secrete stimulatory ligands into the cytosol via specialized bacterial secretion systems. Second, ligands can be released from bacteria that lyse or are degraded. The bacterial ligands that induce the cytosolic pathways remain uncertain in many cases, but appear to include various nucleic acids. In this review, we discuss recent advances in our understanding of how bacteria induce type I interferons and the roles type I IFNs play in host immunity.