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

Abstract

Bacterial flagellin is critical to mediate NLRC4 inflammasome-dependent caspase-1 activation. However, Shigella flexneri, a non-flagellated bacterium, and a flagellin (fliC) knockout strain of Pseudomonas aeruginosa (Pa) are known to activate NLRC4 in bone marrow-derived macrophages. Furthermore, the fliC knockout strain of Pa was used in a mouse model of peritonitis to show the requirement of NLRC4. In a model of pulmonary Pa infection, flagellin was shown to be essential for the induction of NLRC4-dependent caspase-1 activation. Moreover, in all Pa studies, IL-1β production was attenuated in NLRC4−/− mice; however, the role of IL-1β in NLRC4-mediated innate immunity in the lungs against a non-flagellated bacterium was not explored. Here, we report that NLRC4 is important for host survival and bacterial clearance as well as neutrophil-mediated inflammation in the lungs following Klebsiella pneumoniae (Kp) infection. NLRC4 is essential for Kp-induced production of IL-1β, IL-17A, and neutrophil chemoattractants (KC, MIP-2, and LIX) in the lungs. NLRC4 signaling in hematopoietic cells contributes to Kp-induced lung inflammation. Furthermore, exogenous IL-1β, 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 after infection. Taken together, these findings provide novel insights into the role of NLRC4 in Kp-induced host defense.

Activation of inflammasome signaling mediates pathology of acute P. aeruginosa pneumonia

“…The contribution of the NLRC4 inflammasome in stromal cells as well as AMs was previously shown to be important for P. aeruginosa clearance, although this was in mice with a different genetic background (19,60). Clearance of another Gram-negative bacteria, Klebsiella pneumoniae, was also positively influenced by the presence of the NLRC4 inflammasome, although significant differences in bacterial clearance were only observed after a 48-hour infection (14,42,43). Thus, additional effectors of inflammasome signaling are likely to contribute to bacterial clearance through other mechanisms.…”

“…Flagella, in addition to activating NF-κB via TLR5, function as ligands for nonopsonic phagocytosis, providing an efficient mechanism for internalization of bacteria and delivery of flagellin to the inflammasome (8)(9)(10)(11)(12)(13)34). We compared the consequences of infection with the P. aeruginosa strain PAK (an isolate whose WT form is motile and expresses several type III-secreted toxins), fliC PAK mutants (which do not express flagella), and motAB PAK mutants (which express flagella, but do not swim) (14)(15)(16)(17)(18)(19)(20)(21)(22)35). Although fliC PAK was not as effectively cleared from the airway as motAB PAK and WT PAK (P < 0.05 and P < 0.01, respectively), equivalent numbers of each strain were found in the lung tissue (Figure 1, A and B).…”

“…Shed components of P. aeruginosa, including flagella (TLR5), LPS (TLR4), and cell wall lipoproteins (TLR2), are immunostimulatory to varying extents, even if few intact bacteria gain direct access to the epithelial surface (8)(9)(10)(11)(12)(13). After priming by LPS or other PAMPs, activation of the NLR family CARD domain-containing protein 4 (NLRC4) inflammasome by internalized flagellin stimulates caspase-1, resulting in pyroptosis -a highly proinflammatory mode of cell death associated with tissue damage -and release of inflammatory cytokines, such as IL-1β and IL-18 (14)(15)(16)(17)(18)(19)(20)(21)(22). Both IL-1β and IL-18 have been identified as mediators of local inflammation in the lung and tissue remodeling systemically (1, 2, [23][24][25][26][27][28][29].…”

Deciphering tissue-induced Klebsiella pneumoniae lipid A structure

“…To extract lipid A from organs, they were homogenized in 1 mL PBS, an aliquot was used for bacterial load determination, and the rest of the homogenate was lyophilized. BALF, obtained as previously described (55), was also lyophilized. Dry material was then used for the lipid A extraction using the aforementioned protocols.…”

Critical Role of Aquaporins in Interleukin 1β (IL-1β)-induced Inflammation

“…In particular, the macrophage-derived cytokine IL-1␤ drives inflammation by enhancing granulocyte differentiation, migration, and accumulation to inflammatory sites. This cytokine has been implicated in lung inflammatory reactions in response to bacteria, viruses, inorganic particles, and drugs (1)(2)(3)(4).…”