The MyD88-Dependent, but Not the MyD88-Independent, Pathway of TLR4 Signaling Is Important in Clearing Nontypeable Haemophilus influenzae from the Mouse Lung

Abstract

TLRs are important for the recognition of conserved motifs expressed by invading bacteria. TLR4 is the signaling receptor for LPS, the major proinflammatory component of the Gram-negative cell wall, whereas CD14 serves as the ligand-binding part of the LPS receptor complex. Triggering of TLR4 results in the activation of two distinct intracellular pathways, one that relies on the common TLR adaptor MyD88 and one that is mediated by Toll/IL-1R domain-containing adaptor-inducing IFN-β (TRIF). Nontypeable Haemophilus influenzae (NTHi) is a common Gram-negative respiratory pathogen that expresses both TLR4 (LPS and lipooligosaccharide) and TLR2 (lipoproteins) ligands. To determine the roles of CD14, TLR4, and TLR2 during NTHi pneumonia, the following studies were performed: 1) Alveolar macrophages from CD14 and TLR4 knockout (KO) mice were virtually unresponsive to NTHi in vitro, whereas TLR2 KO macrophages displayed a reduced NTHi responsiveness. 2) After intranasal infection with NTHi, CD14 and TLR4 KO mice showed an attenuated early inflammatory response in their lungs, which was associated with a strongly reduced clearance of NTHi from the respiratory tract; in contrast, in TLR2 KO mice, lung inflammation was unchanged, and the number of NTHi CFU was only modestly increased at the end of the 10-day observation period. 3) MyD88 KO, but not TRIF mutant mice showed an increased bacterial load in their lungs upon infection with NTHi. These data suggest that the MyD88-dependent pathway of TLR4 is important for an effective innate immune response to respiratory tract infection caused by NTHi.

Toll/IL-1 Receptor Domain-Containing Adaptor Inducing IFN-β (TRIF)-Mediated Signaling Contributes to Innate Immune Responses in the Lung during Escherichia coli Pneumonia

“…4 and 5). Although our investigation demonstrates a role for TRIF in E. coli-induced lung inflammation, a recent study using H. influenzae indicates that TRIF does not play a role in cytokine and chemokine expression in the lungs (19). The discrepancy between our findings and their observations (19) could be explained by the nature of the pathogens and time points used to measure chemokines/cytokines in the lungs.…”

“…In our model of murine pneumonia, we have highlighted the importance of TRIF for antibacterial defense in the lung against E. coli. Although a previous study investigated the role of TRIF in H. influenzae infection, TRIF does not contribute to antibacterial defense in the lung in their model (19). To our knowledge, we have demonstrated for the first time that TRIF is capable of limiting bacterial growth in the lungs.…”

“…The MyD88-dependent cascade involves TIRAP and MyD88 (11-13), whereas the MyD88-independent cascades involve TRIF and TRAM (14,15). Although there is abundant evidence from in vivo studies that MyD88-dependent signaling cascades contribute to lung defense against bacterial pathogens (17)(18)(19)(20)(21)(22)(23), the role of the MyD88-independent cascade in the induction of host defense in the lung has not been well studied. We attempted to define the role of TRIF, an adaptor in the MyD88-independent cascade, in host defense in the lung against a Gram-negative bacterium E. coli.…”

“…In this regard, the sole previous investigation determined the role of TRIF to a bacterial pathogen and concluded that TLR4-mediated TRIF-dependent cascade does not play a role in neutrophil recruitment in the lungs after nontypeable H. influenzae infection. However, two earlier investigations (19,37) determined the role of TLR4 in pulmonary defense against H. influenzae in vivo using TLR4-mutant mice and reported an enhanced susceptibility in mutant mice. Therefore, our study is the first one demonstrating the role of TRIF associated with TLR4 in the induction of neutrophil recruitment in the lung against a bacterial pathogen.…”

“…For example, MyD88 is shown to be important for host response in the lung against Pseudomonas aeruginosa (17,18), nontypeable Haemophilus influenzae (19), and Klebsiella pneumoniae (20,21). We recently reported that TIRAP plays a critical role in host response in the lung during pneumonia induced by Escherichia coli (22) and K. pneumoniae (23), but not Pseudomonas aeruginosa (23).…”