Cellular and Molecular Immunology (2017) 14, 1017–1019 & 2017 CSI and USTC All rights reserved 2042-0226/17 www.nature.com/cmi LETTER TO THE EDITOR TLR5: beyond the recognition of flagellin Jingyi Yang and Huimin Yan Cellular and Molecular Immunology (2017) 14, 1017–1019; doi:10.1038/cmi.2017.122; published online 20 November 2017 he innate immune system plays an fundamental functions of TLR5 beyond dissemination to extraintestinal organs, Tessential role in the host defense the recognition of flagellin are only the overgrowth of toxigenic members or against infections by initially sensing beginning to be appreciated. Studies on opportunistic pathogenic bacterial inva- and recognizing diverse microbial patho- TLR5 in recent years have revealed sion and infection need to be constantly gens and directing adaptive immune different scenarios after the recognition monitored and prevented. Accordingly, responses to their infections. How does of flagellinbyTLR5intherespiratory TLR5 is used by the mucosal immune the innate immune system sense and tract, gastrointestinal tract, liver or even system of the gut to detect flagellin for recognize pathogens? The identification in the dorsal root ganglion neurons. the surveillance of gastrointestinal micro- of pattern recognition receptors (PRRs) TLR5 is expressed constitutively in biota. Thus, intestinal epithelial cell during the past decades has brought us epithelial cells and immune cells, such TLR5 senses the composition and loca- into a new era to explore this funda- as monocytes and immature DCs. TLR5 lization of the intestinal microbiota to mental question. PRRs are an array of is preferentially expressed on the apical prevent diseases associated with intestinal germline-encoded multifunctional pro- side of respiratory epithelia in both mice inflammation.7 Aberrantly elevated TLR5 teins that do not recognize specific and humans.3,4 Thus, TLR5 can induce activation in response to bacterial flagel- pathogens but rather recognize con- early signaling dedicated to protective lin might result in an impairment of the served molecular patterns associated with innate immune responses against epithelial barrier integrity or the exacer- various classes of pathogens, leading to respiratory infection. For example, air- bation and perpetuation of chronic gut fl 8 fl pathogen-associated molecular patterns way epithelium TLR5 can sense Pseudo- in ammation. Improper bacterial agel- (PAMPs).1 Among the PRRs, there is a monas aeruginosa and initiate an early lin recognition by TLR5 is also linked to fl family of Toll-like receptors (TLRs), host in ammatory reaction to clear the changes in the gut microbiota composi- 5 including TLR5, that recognize a wide invading pathogen. Moreover, the early tion, distorted adipose tissue metabolism fl 9 variety of PAMPs and elicit innate clearance of P. aeruginosa from the air- and in ammation. As an ancient innate fl immune responses. TLR5 is known to ways depends on agellin-TLR5-MyD88- immune receptor, TLR5 is crucial for specifically sense and recognize flagellin, dependent signaling in respiratory both immune homeostasis and protec- 6 the major structural protein of bacterial epithelial cells. tion against bacterial infection in mam- fi flagella.2 It is conceivable that the In contrast with the respiratory tract, mals, birds, amphibians, sh and 10 immune system is equipped with TLR5 TLR5 is expressed mostly on the baso- reptiles. Further investigation of the for the detection of flagellin in different lateralsideofintestinalepithelialcells most primitive function of TLR5 will and recognizes the bacterial locomotion cells, tissues and organs. However, the shed new light on the role of TLR5 in component flagellin for detecting both sustaining gut immune homeostasis Mucosal Immunity Research Group, State Key whether bacteria have crossed the gut and the clearance of intestinal pathogenic Laboratory of Virology, Wuhan Institute of Virol- epithelia1 (Figure 1a). The expression bacteria. ogy, Chinese Academy of Sciences, Wuhan, profile and subcellular distribution fea- The liver is a unique anatomical and Hubei 430071, China Correspondence: Professor Y Han, PhD, Mucosal tures of TLR5 in the gastrointestinal tract immunological site because of its phy- Immunity Research Group, State Key Laboratory are in line with the prominent location siological roles in the metabolism of of Virology, Wuhan Institute of Virology, Chinese of bacteria in the gastrointestinal tract nutrients and the confluence of hepatic Academy of Sciences, Wuhan 430071, China. E-mail: [email protected] and the requirements of the gut immune arterial and venous blood (Figure 1b). Received: 6 August 2017; Accepted: 8 October system to address gastrointestinal micro- The liver is continually exposed to var- 2017 biota, in which commensal ious pathogens with both systemic and Known and unknown function of TLR5 J Yang and H Yan 1018 Figure 1 TLR5 is in the intestine and liver for the recognition of flagellin and beyond. (a) TLR5 is expressed on the basolateral side of intestinal epithelial cells and functions in the normal surveillance of flagellated bacteria. (b) The liver receives the intestinal venous blood circulation and arterial supply and acts as a functional vascular firewall that clears commensals that have penetrated either the intestinal or systemic vascular circuits. (c) TLR5 is expressed at a low level by hepatocytes, which can be activated by flagellin specifically in the liver. TLR5 signaling in the liver modulates local immunity by secreting cytokines and chemo-attracting immunocytes, such as neutrophils, macrophages, NK cells, dendritic cells and T cells. These cells may help to eliminate bacteria directly and regulate specificimmune responses. gut origins. In light of this situation, the expressed on LSECs.12 The role of TLR5 systemically administered flagellin analog liver plays a role in immune surveillance in the liver has been largely neglected generates protective antitumor T-cell for pathogen detection and host because of its low expression in this immunity in the liver through coordina- defense.11 The ability of liver immune organ, but in recent years, increasingly tion with CXCR3-expressing NK cells.15 sentinels to detect pathogens is also important activities of TLR5 in the liver These studies reveal that the effects of dependent on the array of PRRs have been observed (Figure 1c). For flagellin-TLR5 activation in the liver go expressed by liver sinusoidal endothelial example, hepatocyte TLR5 was found to far beyond the recognition of flagellin. In cells (LSECs), hepatocytes, Kupffer cells detect flagellin in the liver, promote light of this, the protective effects and (KCs), DCs and liver-resident lympho- bacterial clearance from the liver and possible side effects of the activation of cytes. The liver expresses a wide array of protect the liver against chronic inflam- the TLR5-signaling pathway in the liver PRRs, including TLRs. Given the anato- matory diseases, although its precise by flagellin should be evaluated more mical location and functional features of mechanism has not yet been defined. carefully, especially in regard to its pro- the liver, the response to a number of In this detection and clearance process, posed systemic use for tumor patients. At TLR ligands in the liver often differs the liver may act as a functional vascular the least, the over-activation of TLR5 from the response observed in other firewall that clears commensals that have signaling in the liver should be avoided tissues. This is represented mostly as penetrated either the intestinal or sys- because liver injury induced by high the immunosuppressive response, which temic vascular circuits.13 Another study doses of flagellin was observed in is speculated to be a mechanism used to showed that the liver is the major organ mice.16 avoid a constant state of liver inflamma- that responds to the TLR5 agonist.14 In addition to the roles played by tion induced by continuous exposure to Moreover, hepatocytes were found to TLR5 in the respiratory tract, gastroin- bacteria-derived molecules, such as LPS, be the key liver cell type that responds testinal tract and liver, a recent study on flagellin and so on. Although TLRs are rapidly and directly to a flagellin analog mechanical allodynia in neuropathic pain expressed by most cells in the liver, but not to LPS, and the activation of liver showed that the effect of TLR5 activation including hepatocytes, LSECs, HSCs, TLR5 has been shown to play a central in neurons even works beyond its recog- KCs and lymphocytes, TLR5 is only role in flagellin’s radioprotective and nition of flagellin.17 This study found expressed at low levels in hepatocytes, anticancer properties.14 The following that TLR5 is expressed by peripheral KCs and hepatic DCs; however, it is not study further demonstrated that the sensory neurons, in which TLR5 Cellular and Molecular Immunology Known and unknown function of TLR5 J Yang and H Yan 1019 mediates touch sensation and permits and the Grants of Deutsche barrier dysfunction in experimental ileitis. fl – the rapid transport of certain small Forschungsgemeinschaft (TRR60 and In amm Bowel Dis 2017; 23: 392 403. 9 Pekkala S, Munukka E, Kong L, Pollanen E, molecules across the neuronal mem- GK1949). Autio R, Roos C et al. Toll-like receptor 5 in brane through an unknown obesity: the role of gut microbiota and mechanism.17 This introduces a promis- adipose tissue inflammation. Obesity 2015; 23:581–590. ing new therapeutic strategy to treat 10 Voogdt CG, Bouwman LI, Kik MJ, Wagenaar JA, 1 Akira S, Uematsu S, Takeuchi O. Pathogen persistent pain by targeting TLR5. On van Putten JP. Reptile Toll-like receptor 5 recognition and innate immunity. Cell 2006; unveils adaptive evolution of bacterial fla- the other hand, it also suggests a more 124:783–801. gellin recognition. Sci Rep 2016; 6: 2 Hayashi F, Smith KD, Ozinsky A, Hawn TR, intriguing role for TLR5 as a gate-keeper 19046. Yi EC, Goodlett DR et al. The innate immune for cellular ion channels or as a regulator 11 Jenne CN, Kubes P. Immune surveillance by response to bacterial flagellin is mediated by the liver.