The Effects of IFN-λ on Epithelial Barrier Function Contribute to K. pneumoniae ST258 Pneumonia.

IFN-λ and IL-22, cytokines that share the co-receptor IL-10RB, are both induced over the course of Klebsiella pneumoniae ST258 (KP35) pneumonia. IL-22 is known to protect mucosal barriers whereas the effects of IFN-λ on the mucosa are not established. We postulated that IFN-λ plays a role in regulating the airway epithelial barrier to facilitate cellular trafficking to the site of infection. In response to IFN-λ the transmigration of neutrophils across a polarized monolayer of airway epithelial cells was increased, consistent with diminished epithelial integrity. KP35 infection increased epithelial permeability and pretreatment with IFN-λ amplified this effect and facilitated bacterial transmigration. These effects of IFN-λ were confirmed in vivo, in that mice lacking the receptor for IFN-λ (Ifnlr1-/-), were protected from bacteremia in a murine model of KP35 pneumonia. Conversely, the integrity of the epithelial barrier was protected by IL-22 with subsequent impairment of neutrophil and bacterial transmigration in vitro. Maximal expression of IL-22 in vivo was later in the course of infection than IFN-λ production with high levels of IL-22 produced by recruited immune cells at 48 hours consistent with a role in epithelial barrier recovery. The divergent and opposing expression of these two related cytokines suggest a regulated interaction in the host response to KP35 infection. A major physiological effect of IFN-λ signaling is in decreasing epithelial barrier integrity, which facilitates immune cell recruitment but also enables K. pneumoniae invasion.