Suppression of antibacterial Th17 responses by preexisting antiviral responses. (a) Under normal conditions, infection by a bacterial pathogen induces IL-23 production by dendritic cells (DCs) and leads to the generation and expansion of Th17 cells, which are essential for bacterial clearance. (b) In a coinfection condition, viral infection induces type I interferon (IFN) production by DCs, which can inhibit IL-23 production by these cells. This viral-induced reduction in IL-23 results in compromised IL-17 responses due to insufficient expansion of IL-17-producing cells, leading to reduced host defenses against bacterial infection.