Redundant and Cooperative Interactions between TLR5 and NLRC4 in Protective Lung Mucosal Immunity against <b><i>Pseudomonas aeruginosa</i></b>

Abstract

Flagellin is the major structural component of flagella expressed by Pseudomonas aeruginosa (PA) and other bacteria. This protein has been shown to activate the Toll-like receptor TLR5 and the Nod-like receptor Nlrc4/Ipaf, culminating in the expression of innate cytokines and antimicrobial molecules. In this study, we tested the hypothesis that TLR5 and Nlrc4 in combination are required for maximal protective lung innate mucosal immunity against PA. To test this hypothesis, we compared innate immune responses in wild-type (WT) C57B6 mice challenged with PA intratracheally to those observed in mice genetically deficient in TLR5 (TLR5^-/-) or Nlrc4 (Nlrc4^-/-) alone or in combination (TLR5/Nlrc4^-/-). As compared to WT, TLR5^-/- and Nlrc4^-/- mice, we observed a significant increase in mortality in TLR5/Nlrc4^-/- mice, which was associated with a >5,000-fold increase in lung PA colony-forming units and systemic bacterial dissemination. The increased mortality observed in double-deficient mice was not attributable to differences in lung leukocyte influx or lung injury responses. Levels of biologically active IL-1β and IL-18 were reduced in the bronchoalveolar lavage fluid from PA-infected Nlrc4^-/- and TLR5/Nlrc4^-/- but not TLR5^-/- mice, indicating the requirement for Nlrc4-dependent caspase-1 activation. Similarly, decreased production of biologically active IL-1β and activation of caspase-1 was observed in PA-stimulated pulmonary macrophages isolated from Nlrc4^-/- and TLR5/Nlrc4^-/- but not TLR5^-/- mice, whereas the expression of iNOS and the production of NO were significantly reduced in cells from double-mutant but not single-mutant mice. Collectively, our findings indicate that TLR5 and Nlrc4 have both unique and redundant roles in lung antibacterial mucosal immunity, and the absence of both pathogen recognition receptors results in an increase in susceptibility to invasive lung infection.

Inflammasome gene profile is modulated in septic patients, with a greater magnitude in non-survivors

“…Pattern recognition receptors (PRRs) co-operate to recognize a variety of microbial infections by sensing microbial or danger molecules [4]. The same is true for the diverse NLR-inflammasomes that play redundant and complementary roles, as shown for NLRC4 and TLR-5 [31] and NLRP3 and NLRC4 [43]. Thus, one of the most relevant contributions of the present work derives from the network analysis showing gene interactions and the predicted activation and inhibition of cellular functions.…”

Emerging Roles of Inflammasomes in Acute Pneumonia

“…Several studies have documented the activation of NLRC4 in the presence of type III and type IV secretions involving the injection of needles and formation of pores in host cell membranes causing translocation of virulence factors such as flagellin and rods. These effects are observed in P. aeruginosa, Legionella pneumophila, Burkholderia pseudomallei, and Escherichia coli infections in mice (28,(35)(36)(37)(38)(39). Furthermore, the mechanism of recognition and activation appears to be different for different pathogens.…”

IL-24 Promotes Pseudomonas aeruginosa Keratitis in C57BL/6 Mouse Corneas

“…Our previous studies showed that mouse corneas pretreated with flagellin, which signals through TLR5, are protective against PA infection (7–9). Flagellin pretreatment has also been demonstrated to be protective in localized infections of the lung (10, 11) and gut (12), radiation pneumonitis (13), and systematically against biological or physical insults (14). We found that this flagellin-induced protection is due to the reprograming of gene expression in epithelial cells to enhance the innate defense against invasive pathogens.…”