NLRC4 Inflammasome-Mediated Production of IL-1β Modulates Mucosal Immunity in the Lung against Gram-Negative Bacterial Infection

Bacterial flagellin is critical to mediate NLRC4 inflammasome-dependent caspase-1 activation. However, Shigella flexneri, a nonflagellated bacterium, and a flagellin (fliC) knockout strain of Pseudomonas aeruginosa are known to activate NLRC4 in bone marrow-derived macrophages. Furthermore, the flagellin-deficient fliC strain of P aeruginosa was used in a mouse model of peritonitis to show the requirement of NLRC4. In a model of pulmonary P aeruginosa infection, flagellin was shown to be essential for the induction of NLRC4-dependent caspase-1 activation. Moreover, in all P aeruginosa studies, IL-1 beta production was attenuated in NLRC4(-/-) mice; however, the role of IL-1 beta in NLRC4-mediated innate immunity in the lungs against a nonflagellated bacterium was not explored. In this article, we report that NLRC4 is important for host survival and bacterial clearance, as well as neutrophil-mediated inflammation in the lungs following Klebsiella pneumoniae infection. NLRC4 is essential for K. pneumoniae-induced production of IL-1 beta, IL-17A, and neutrophil chemoattractants (keratinocyte cell-derived chemokines, MIP-2, and LPS-induced CXC chemokines) in the lungs. NLRC4 signaling in hematopoietic cells contributes to K. pneumoniae-induced lung inflammation. Furthermore, exogenous IL-1 beta, but not IL-18 or IL-17A, partially rescued survival, neutrophil accumulation, and cytokine/chemokine expression in the lungs of NLRC4(-/-) mice following infectious challenge. Furthermore, IL-1R1(-/-) mice displayed a decrease in neutrophilic inflammation in the lungs postinfection. Taken together, these findings provide novel insights into the role of NLRC4 in host defense against K. pneumoniae infection. The Journal of Immunology, 2012, 188: 5623-5635.