The role of TLRs, NLRs, and RLRs in mucosal innate immunity and homeostasis

Abstract

The mucosal surfaces of the gastrointestinal tract are continually exposed to an enormous antigenic load of microbial and dietary origin, yet homeostasis is maintained. Pattern recognition molecules (PRMs) have a key role in maintaining the integrity of the epithelial barrier and in promoting maturation of the mucosal immune system. Commensal bacteria modulate the expression of a broad range of genes involved in maintaining epithelial integrity, inflammatory responses, and production of antimicrobial peptides. Mice deficient in PRMs can develop intestinal inflammation, which is dependent on the microbiota, and in humans, PRM polymorphisms are associated with exacerbated inflammatory bowel disease. Innate immune responses and epithelial barrier function are regulated by PRM-induced signaling at multiple levels, from the selective expression of receptors on mucosal cells or compartments to the expression of negative regulators. Here, we describe recent advances in our understanding of innate signaling pathways, particularly by Toll-like receptors and nucleotide-binding domain and leucine-rich repeat containing receptors at mucosal surfaces.

Figure 1

Nucleotide-binding domain and Leucine-rich repeat containing receptor (NLR)-and Toll-like receptor (TLR)-mediated signaling cascades in the gut epithelia. NLRs are composed of N-terminal effector domains that mediate protein–protein interactions with their binding partners. NOD1 and NLRP4 (IPAF) are composed of one caspase recruitment domain (CARD), whereas NOD2 is composed of two. All NLRs have a central oligomerization domain and C-terminal LRR regions. Cytosolic L. monocytogenes are recognized by NOD1 and NOD2, H. pylori is identified by NOD1, and S. typhimurium is recognized by NOD2. Extracellular flagellated S. typhimurium is recognized by TLR5, whereas NLRP4 identifies flagellin, which reaches the cytosol. NOD1 and NOD2 interact with the inhibitor of apoptosis proteins (cIAP1 and cIAP2), which have been found to possess C-terminal RING-finger domains with E3 ligase activity. cIAP1 or cIAP2 bind and ubiquitinate RIP2, adding K63-linked ubiquitin units to the kinase protein. Ubiquitinated RIP2 binds and activates TAK1. The kinase activity of TAK1 activates both IKKγ and MKK, which culminates in the translocation of NF-κB and mitogen-activated protein kinase into the nucleus, upregulating the transcription of proinflammatory genes and mediating the antibacterial effects by the upregulation of defensins. The bacterial translocation complex SipB mediates the translocation of flagellin across the plasma membrane, which is recognized by NLRP4, which in turn recruits apoptosis speck-like protein (ASC) through its N-terminal CARD domain. ASC then recruits and activates caspase-1, forming the NLRP4 inflammasome. TLRs contain an N-terminal LRR domain, a transmembrane domain, and a C-terminal cytoplasmic TIR domain. TLR5 is activated on ligation to extracellular flagellin, and TLR5 recruits adaptor MyD88 to upregulate a signaling cascade that activates the proinflammatory transcription factors NF-κB and AP1. Basolaterally polarized TLR5 is found in the colon and is upregulated by invasive bacteria that have breached the transepithelial barrier. TLR4 and TLR2 are found on the epithelial surface at a relatively low level; however, the expression of TLR4 and TLR2 is increased by proinflammatory cytokines. Basolateral TLR9 has been found to activate NF-κB, whereas apical TLR9 has been shown to cause the accumulation of ubiquitinated IκB preventing the activation of NF-κB.

Figure 2

Hypothetical model for ATP-driven NLRP3 activation in promoting Th17 responses in the intestine. ATP produced by commensal bacteria (red boxes) acts on CD70(high)CD11c(low) cells to enhance the Toll-like receptor (TLR)-independent secretion of the Th17 cell-polarizing cytokines, IL-6, IL-23, and TGF-β. In parallel, ATP may act on intestinal dendritic cells (DCs) and macrophages through P2X7 receptors to enhance NLRP3 inflammasome activation, promoting caspase-1 activity and secretion of IL-1β. These Th17-polarizing cytokines act in synergy to drive Th17 responses in the lamina propria.