IL-1α Signaling Initiates the Inflammatory Response to Virulent Legionella pneumophila In Vivo

Legionella pneumophila is an intracellular bacterial pathogen that is the cause of a severe pneumonia in humans called Legionnaires' disease. A key feature of L. pneumophila pathogenesis is the rapid influx of neutrophils into the lungs, which occurs in response to signaling via the IL-1R. Two distinct cytokines, IL-1 alpha and IL-1 beta, can stimulate the type I IL-1R. IL-1 beta is produced upon activation of cytosolic sensors called inflammasomes that detect L. pneumophila in vitro and in vivo. Surprisingly, we find no essential role for IL-1 beta in neutrophil recruitment to the lungs in response to L. pneumophila. Instead, we show that IL-1 alpha is a critical initiator of neutrophil recruitment to the lungs of L. pneumophila-infected mice. We find that neutrophil recruitment in response to virulent L. pneumophila requires the production of IL-1 alpha specifically by hematopoietic cells. In contrast to IL-1 beta, the innate signaling pathways that lead to the production of IL-1 alpha in response to L. pneumophila remain poorly defined. In particular, although we confirm a role for inflammasomes for initiation of IL-1 beta signaling in vivo, we find no essential role for inflammasomes in production of IL-1 alpha. Instead, we propose that a novel host pathway, perhaps involving inhibition of host protein synthesis, is responsible for IL-1 alpha production in response to virulent L. pneumophila. Our results establish IL-1 alpha as a critical initiator of the inflammatory response to L. pneumophila in vivo and point to an important role for IL-1 alpha in providing an alternative to inflammasome-mediated immune responses in vivo.