The CCR6/CCL20 axis plays an important role in intestinal immunity. During normal development and immune homeostasis, CCR6-mediated signals help to organize lymphoid tissues such as Peyer’s patches (PPs), mesenteric lymph nodes (MLNs) and gut-associated lymphoid tissue (GALT) by recruiting lymphoid and myeloid cells, including DCs and macrophages. In addition, CCR6-mediated signals are central to innate immune responses to normal intestinal flora, and modulations in CCR6 signals can have a significant impact on gut inflammatory responses to tissue damage and trauma. The relative CCR6-dependent chemotactic response of DCs and macrophages, and subsequent activation and effector function of these cell populations, plays an important role in intestinal immune responses.
As with other tissues, CCR6-mediated signals are critical for the organization of lymphoid tissues and the maintenance of leukocytes at sites critical for immune surveillance. In the gut, areas of secondary lymphoid organogenesis, such as PPs, isolated lymphoid follicles (ILFs), MLNs, and GALT show constitutive expression of CCL20, important for the chemotaxis of immature DCs [45
]. In addition, expression of CCL20 (both mRNA and protein) can be induced in the follicle-associated epithelium (FAE) common to ILFs and PPs by organogenesis signals (such as lymphotoxin-beta signaling) [46
]. CCL20 can also be induced in other intestinal epithelial cells in response to infection, in particular through LPS stimulation [47
]; in this way, CCR6/CCL20 mediated signals can induce chemotaxis of CCR6-expressing dendritic cells and macrophages to sites of infection to help participate in the immune response.
Loss of CCR6/CCL20 signals can have a profound impact on innate immune cells in both the intestine and the peritoneal cavity. For example, CCR6−/−
mice exhibit significant reductions in both DC and macrophage populations (both of which are myeloid lineage cells) in the peritoneal cavity, with no significant modulation in other lymphoid populations [28
]. These results suggest that CCR6-mediated signals may play a more critical role in myeloid recruitment to the intestine (as compared to lymphoid recruitment) during homeostasis. The role of CCR6 in the organization of lymphoid structures in the intestinal mucosa may extend past the myeloid compartment as well; recent studies indicate that lineage-negative lymphoid tissue inducer cells in gut cryptopatches (CPs) express CCR6, and CCR6−/−
mice exhibit inhibition of cryptopatch formation [48
]. The CCR6/CCL20 axis is not the only chemotactic pathway for DCs in the intestine; for example, CCL9 can also recruit DCs to the subepithelial dome [49
]. However, it is clear that CCR6-mediated signals can play a role in the maintenance of DC and macrophage populations throughout the intestinal mucosa.
In addition to its role in gut homeostasis, CCR6-mediated signals are also essential for immune responses to microbes and microbial products in the intestinal mucosa. For example, CCR6−/−
mice have impaired antibody responses to both oral immunizations and mucosal virus infections; interestingly, this reduction in antibody production appears localized to the gut, as systemic antibody levels are not perturbed in CCR6−/−
mice in these models [50
DCs in the subepithelial dome (SED) also appear to be critical for the activation and proliferation of CD4+
T cells at the site of infection in murine models of enteric pathogen infection, as CCR6-expressing T cells were reduced in number in LNs and PPs during infection when transferred into CCR6-deficient hosts [51
]. In addition, CCR6+
DCs are recruited to the FAE of PPs in response to bacterial infection, suggesting that CCR6/CCL20 signals are critical both for homeostasis and active inflammation [51
]. CCR6 can also modulate harmful inflammatory processes in the gut, as shown in the CCR6−/−
mouse model with inflammatory bowel disease (IBD) [52
]. Interestingly, the observed pathology of CCR6-deficient IBD models depends on the agent used; while dextran sodium sulfate (DSS)-induced IBD is less severe in CCR6−/−
mice, trinitrobenzene sulfonic acid (TNBS)-induced IBD is more severe, as compared to wild-type mice [52
]. The observed differences in disease severity may be a reflection of the modulation of leukocyte populations in the intestine as a result of CCR6 deficiency, as DSS- and TNBS-induced colitis is thought to occur via differing mechanisms (myeloid non-specific inflammation vs. lymphoid antigen-driven inflammation, respectively) [52
CCR6 also plays an important role in the modulation of inflammatory responses initiated by tissue insult and trauma. For example, CCL20 is induced by bateria-induced peritonitis, and CCR6−/−
mice are resistant to peritonitis-induced mortality in a surgical model of severe sepsis [28
]. Interestingly, this protection appears to be due to mechanisms beyond leukocyte migration to the intestinal mucosa. While CCR6−/−
mice do exhibit reductions in both macrophages and DCs in the peritoneal cavity, CCR6−/−
macrophages exhibit reduced inflammatory responses to LPS stimulation in vitro
, as evidenced by cytokine production [28
]. Additionally, CCR6−/−
mice exhibit reduced cytokine and chemokine levels in the peritoneal cavity during acute peritonitis, suggesting that the reduced sepsis-induced mortality may be due to the impaired proinflammatory capacity of immune cells lacking CCR6−/−
. These results suggest that CCR6-mediated signals in macrophages and DCs may be important for cell activation during exposure to microbes and/or microbial products; however, the specific mechanistic link governing the connection between microbial products and CCR6-mediated signals, for example, remains unknown.