Role of the gut-associated and secondary lymphoid tissue in the induction of chronic colitis

The intestinal tract is colonized by more than 100 trillion (1014) microorganisms, with the vast majority residing within the colonic lumen.1,2 The gut-associated lymphoid tissue (GALT; Peyer’s patches [PPs], isolated lymphoid follicles) as well as the gut-draining mesenteric lymph nodes (MLNs) play important roles in mounting mucosal immune responses to invading pathogenic and/or commensal microorganisms while minimizing the tissue inflammation and systemic immune activation that may arise from these protective responses.3–11 Studies from several different laboratories suggest that commensal bacterial antigens continuously gain access to the intestinal interstitium where they are endocytosed by intestinal dendritic cells (DCs) and then transported from the lamina propria and PPs to the draining MLNs.6–8,12 Using carefully regulated, low-level cellular immune responses to these nonpathogenic bacteria, the MLNs provide for both anatomical and functional compartmentalization of the adaptive immune responses, thereby generating a “firewall” to the systemic dissemination of these microorganisms.6,7 The mechanisms by which the cellular elements of the GALT and MLNs mediate these distinct and seemingly opposing functions are only now beginning to be revealed. Although there is good evidence demonstrating that MLNs play a critical role mounting and regulating mucosal immune responses to enteric bacteria, the evidence demonstrating a role for PPs as a major inductive site for adaptive immune responses is somewhat vague and ill-defined.6–8,13 Recent work by Worbs et al13 demonstrate that the MLNs (but not PPs) are required for the induction of systemic tolerance to orally administered luminal antigens. Those investigators convincingly showed that ovalbumin-loaded DCs are transported from the gut interstitium and PPs to the MLNs by the afferent lymphatics, where tolerance to these luminal antigens is induced. There is an accumulating literature suggesting that a break in tolerance to enteric antigens may be responsible for the induction of the chronic intestinal inflammation observed in patients who suffer from the inflammatory bowel diseases (IBDs; Crohn’s disease; ulcerative colitis).14–17 Indeed, the role of commensal bacteria in the pathogenesis of experimental IBD has been well characterized in number of different animal models of chronic gut inflammation.14,16,18 Although the etiology of these chronic inflammatory disorders has not been definitively elucidated, there is accumulating evidence to suggest that chronic gut inflammation arises from a complex interaction among genetic, immune, and environmental factors.16,18,19 A growing body of experimental and clinical data suggest that chronic gut inflammation arises from a dysregulated immune response to enteric bacterial antigens. An immunologically important yet undefined aspect of disease pathogenesis in animal models of IBD is the anatomic location(s) where naive T cells encounter enteric antigens to yield colitogenic effector cells. It has been assumed that naive T cells migrate to the GALT/MLNs, where they are primed and polarized to yield T helper 1 (Th1) and/or Th17 effector cells. These colitogenic effector cells then exit the GALT and/or MLNs by way of the efferent lymphatic vessels, enter into the systemic circulation, and home to the gut where they initiate intestinal inflammation.15–18,20 In reality, there have been few studies that have assessed the role of the GALT and/or MLNs in the induction of chronic gut inflammation. Spahn et al21 demonstrated that dextran sulfate sodium (DSS)-induced colitis is more severe in mice devoid of MLNs, suggesting an overall suppressive function of these gut-draining lymph nodes, whereas Dohi et al22 showed an exacerbated inflammatory infiltrate and Th1 response in PP- and colonic patch-deficient mice exposed to intrarectal trinitrobenzene sulfonic acid (TNBS) dissolved in ethanol. In contrast to these studies, work by Makita et al23 suggests that neither the GALT, MLNs, spleen, nor peripheral lymph nodes were required for the induction (or suppression) of chronic colitis in mice. These investigators intercrossed lymphotoxin-α deficient (LTα−/−) mice with recombinase activating gene-2 deficient (RAG-2−/−) animals to generate LTα−/− × RAG−/− double-deficient progeny that are devoid of all organized lymphoid tissue.23 They found that adoptive transfer of naive or antigen-experienced (activated/memory) CD4+ T cells obtained from the lamina propria of mice with active colitis to LTα−/− × RAG-2−/− recipients induced colonic inflammation; however, the onset and severity of disease was delayed when compared to the disease induced by T-cell transfer to littermate controls (i.e., LTα+/+ × RAG-2−/− recipients). Although compelling, unanswered questions remain. For example, it is unclear how a global defect in LTα during development of lymphopenic RAG−/− mice affects lymphotoxin receptor-β expression and subsequent lymphotoxin signaling events associated with T-cell trafficking, priming, and polarization following adoptive transfer of naive T-cells. Therefore, the objectives of this study were to evaluate, using both surgical as well as genetic approaches, the role of gut-associated and secondary lymphoid tissue in the conversion of naive T-cells to colitogenic Th1/Th17 effector cells and their induction of chronic colitis. We present data demonstrating that neither the spleen, GALT, MLNs nor peripheral lymph nodes (PLNs) are required for the induction of chronic gut inflammation.