Mechanism of pathogen‐specific TLR4 activation in the mucosa: Fimbriae, recognition receptors and adaptor protein selection

Abstract

The mucosal host defence discriminates pathogens from commensals, and prevents infection while allowing the normal flora to persist. Paradoxically, Toll-like receptors (TLR) control the mucosal defence against pathogens, even though the TLR recognise conserved molecules like LPS, which are shared between pathogens and commensals. This study proposes a mechanism of pathogen-specific mucosal TLR4 activation, involving adhesive ligands and their host cell receptors. TLR4 signalling was activated in CD14-negative, LPS-unresponsive epithelial cells by P fimbriated, uropathogenic Escherichia coli but not by a mutant lacking fimbriae. Epithelial TLR4 signalling in vivo involved the glycosphingolipid receptors for P fimbriae and the adaptor proteins Toll/IL-1R (TIR) domain-containing adaptor inducing IFN-b (TRIF)/TRIF-related adaptor molecule (TRAM), but myeloid differentiation protein 88 (MyD88)/TIR domain-containing adaptor protein were not required for the epithelial response. Substituting the P fimbriae with type 1 fimbriae changed TLR4 signalling from the TRIF to the MyD88 adaptor pathway. In addition, the adaptor proteins and the fimbrial type were found to influence bacterial clearance. Trif -/-and Tram -/-mice remained infected with P fimbriated E. coli but cleared thetype 1 fimbriated strain, while Myd88 -/-mice became carriers of both the P and the type 1 fimbriated bacteria. Thus, TLR4 may be engaged specifically by pathogens, when the proper cell surface receptors are engaged by virulence ligands. IntroductionToll-like receptors (TLR) control the innate host defence at mucosal surfaces and yet commensals do not induce an inflammatory response at those sites. The relative inertia to the commensals is paradoxical, as the TLR recognise conserved pathogen-associated molecular patterns (PAMP), which are present on both pathogenic and commensal bacteria. Lipolysaccharide (LPS), flagellin, peptidoglycan or DNA may bind directly to the TLR but in addition, co-receptors are needed to enhance the TLR response to these conserved ligands [1][2][3]. Myeloid cells use CD14 and MD-2 as co-receptors for LPS in TLR4 signalling, and minute amounts of LPS may elicit a strong systemic inflammatory response [4,5]. However, the TLR response to pathogen attack at mucosal surfaces is controlled by different interactions. Epithelial cells must be protected from constant TLR activation by commensals and their PAMP, in order for mucosal integrity to be maintained. The epithelial cells thus lack co-receptors like CD14 [6][7][8][9][10][11], and in addition, commensals have been suggested to actively suppress epithelia responses through NF-jB [12,13]. Yet, mucosal pathogens evoke rapid TLR4-dependent responses at mucosal sites, suggesting that alternative ligands and receptors might be involved in mucosal TLR activation. Pathogens use adhesive surface ligands to break the inertia of the mucosal barrier [14][15][16]. The innate defence is activated as a direct result of these interactions and epithelial cells produce the first wave of me...

A Role of Toll-IL-1 Receptor Domain-Containing Adaptor-Inducing IFN-β in the Host Response to Pseudomonas aeruginosa Lung Infection in Mice

“…Thus, effective immunity against bacterial infection can be developed through MyD88-independent mechanisms. This notion is supported by a recent study demonstrating that the TRIF pathway but not the MyD88 pathway is needed for P fimbriated Escherichia coli-induced signaling in epithelial cells (22).…”

Renal Collecting Duct Epithelial Cells React to Pyelonephritis-Associated Escherichia coli by Activating Distinct TLR4-Dependent and -Independent Inflammatory Pathways

“…In C3H/HeJ Lps d mice the renal inflammatory response to P fimbriated E. coli is TLR4 dependent (55,56), and this activation signaling in epithelial A498 cells lacking CD14 was independent of LPS and lipid A myristoylation (56). More recently, Fischer et al (15), using murine models of ascending UTI, have shown that P and type 1 fimbriated E. coli may use different adaptor molecules to influence neutrophil activation and bacterial clearance, but that in both cases MyD88 is required for efficient bacterial clearance. These results suggested that P and type 1 fimbriae, through their recognition by their respective epithelial cell receptors, may engage specific TLR4-associated proteins for the induction of innate immune response in the urinary tract (15).…”

“…Fisher et al (15) also showed that the binding of type 1 or P fimbriae to their respective uroepithelial cell receptors activates a TLR4-mediated mucosal response, as assessed by analyses of neutrophil recruitment to the urinary tract of infected mice, through the requirement of distinct adaptor proteins. These findings suggested that the recognition of fimbrial adhesins by cell surface mucosal receptors may directly activate, independently of LPS, the TLR4 signaling pathway.…”

“…Although MyD88 is required for antibacterial effector function, urine neutrophil recruitment still occurred in MyD88 Ϫ/Ϫ mice infected with P fimbriated E. coli (15). These results raised the question of whether the stimulated secretion of MIP-2 caused by UPECs in Lps n MCDs occurs through MyD88-dependent and/or MyD88-independent pathways.…”

The PorB porin from commensal Neisseria lactamica induces Th1 and Th2 immune responses to ovalbumin in mice and is a potential immune adjuvant

“…One could speculate that purified bacterial components from both pathogenic and non-pathogenic species could equally signal via TLRs in vitro but they might require different adaptor molecules (i.e. MyD88 or TRIF-TRAM [72]) or even initiate distinct intracellular cascades of events when they are integral part of live bacteria [73]. Furthermore, pathogenic signals might require different co-receptors for TLR signaling: for example, porins from pathogenic N. meningitidis signal via TLR2/TLR1 [65].…”