Chemokine receptor CCR6 transduces signals that activate p130Cas and alter cAMP-stimulated ion transport in human intestinal epithelial cells

Human colon epithelial cells express the G protein-coupled receptor CCR6, the sole receptor for the chemokine CCL20 (also termed MIP-3). CCL20 produced by intestinal epithelial cells is upregulated in response to proinflammatory stimuli and microbial infection, and it chemoattracts leukocytes, including CCR6-expressing immature myeloid dendritic cells, into sites of inflammation. The aim of this study was to determine whether CCR6 expressed by intestinal epithelial cells acts as a functional receptor for CCL20 and whether stimulation with CCL20 alters intestinal epithelial cell functions. The human colon epithelial cell lines T84, Caco-2, HT-29, and HCA-7 were used to model colonic epithelium. Polarized intestinal epithelial cells constitutively expressed CCR6, predominantly on the apical side. Consistent with this, apical stimulation of polarized intestinal epithelial cells resulted in tyrosine phosphorylation of the p130 Crk-associated substrate (Cas), an adaptor/scaffolding protein that localizes in focal adhesions and has a role in regulating cytoskeletal elements important for cell attachment and migration. In addition, CCL20 stimulation inhibited agonist-stimulated production of the second messenger cAMP and cAMP-mediated chloride secretory responses by intestinal epithelial cells. Inhibition was abrogated by pertussis toxin, consistent with signaling through G_i proteins that negatively regulate adenylyl cyclases and cAMP production. These data indicate that signaling events, occurring via the activation of the apically expressed chemokine receptor CCR6 on polarized intestinal epithelial cells, alter specialized intestinal epithelial cell functions, including electrogenic ion secretion and possibly epithelial cell adhesion and migration. CCL20; macrophage inflammatory protein-3; forskolin; G protein-coupled receptors; tyrosine phosphorylation     

CCL28 has dual roles in mucosal immunity as a chemokine with broad-spectrum antimicrobial activity

CCL28 is a CC chemokine signaling via CCR10 and CCR3 that is selectively expressed in certain mucosal tissues such as exocrine glands, trachea, and colon. Notably, these tissues commonly secrete low-salt fluids. RT-PCR analysis demonstrated that salivary glands expressed CCL28 mRNA at the highest levels among various mouse tissues. Single cells prepared from mouse parotid glands indeed contained a major fraction of CD3(-)B220(low) cells that expressed CCR10 at high levels and CCR3 at low levels and responded to CCL28 in chemotaxis assays. Morphologically, these cells are typical plasma cells. By immunohistochemistry, acinar epithelial cells in human and mouse salivary glands were strongly positive for CCL28. Furthermore, human saliva and milk were found to contain CCL28 at high concentrations. Moreover, the C terminus of human CCL28 has a significant sequence similarity to histatin-5, a histidine-rich candidacidal peptide in human saliva. Subsequently, we demonstrated that human and mouse CCL28 had a potent antimicrobial activity against Candida albicans, Gram-negative bacteria, and Gram-positive bacteria. The C-terminal 28-aa peptide of human CCL28 also displayed a selective candidacidal activity. In contrast, CCL27, which is most similar to CCL28 and shares CCR10, showed no such potent antimicrobial activity. Like most other antimicrobial peptides, CCL28 exerted its antimicrobial activity in low-salt conditions and rapidly induced membrane permeability in target microbes. Collectively, CCL28 may play dual roles in mucosal immunity as a chemoattractant for cells expressing CCR10 and/or CCR3 such as plasma cells and also as a broad-spectrum antimicrobial protein secreted into low-salt body fluids.     

An indispensable role for the chemokine receptor CCR10 in IgA antibody-secreting cell accumulation

The differential expression of chemokines and chemokine receptors, by tissues and leukocytes, respectively, contributes to the specific accumulation of leukocyte subsets to different tissues. CCR10/CCL28 interactions are thought to contribute to the accumulation of IgA Ab-secreting cells (ASC) to mucosal surfaces, such as the gastrointestinal tract and the lactating mammary gland. Although the role of CCL28 in lymphocyte homing is well established, direct in vivo evidence for CCR10 involvement in this process has not been previously shown. In this study, we describe the generation of a CCR10-deficient mouse model. Using this model, we demonstrate that CCR10 is critical for efficient localization and accumulation of IgA ASC to the lactating mammary gland. Surprisingly, IgA ASC accumulation to the gastrointestinal tract is minimally impacted in CCR10-deficient mice. These results provide the first direct evidence of CCR10 involvement in lymphocyte homing and accumulation in vivo, and demonstrate that reliance on CCR10-mediated recruitment of IgA ASC varies dramatically within mucosal tissues.     

Expression of the C-C chemokine receptor CCR3 in human airway epithelial cells

Chemokine-induced eosinophil chemotaxis is mediated primarily through the C-C chemokine receptor, CCR3. We have now detected CCR3 immunoreactivity on epithelial cells in biopsies of patients with asthma and other respiratory diseases. CCR3 mRNA was detected by Northern blot analysis after TNF-alpha stimulation of the human primary bronchial epithelial cells as well as the epithelial cell line, BEAS-2B; IFN-gamma potentiated the TNF-alpha-induced expression. Western blots and flow cytometry confirmed the expression of CCR3 protein. This receptor is functional based on studies demonstrating eotaxin-induced intracellular Ca(2+) flux and tyrosine phosphorylation of cellular proteins. The specificity of this functional response was confirmed by blocking these signaling events with anti-CCR3 mAb (7B11) or pertussis toxin. Furthermore, (125)I-eotaxin binding assay confirmed that CCR3 expressed on epithelial cells have the expected ligand specificity. These studies indicate that airway epithelial cells express CCR3 and suggest that CCR3 ligands may influence epithelial cell functions.     

CCR6 as a mediator of immunity in the lung and gut

Chemokines are key mediators of leukocyte recruitment during pathogenic insult and also play a prominent role in homeostasis. While most chemokine receptors bind to multiple chemokines, CCR6 is unique in that this receptor is one of only a few that can bind only a single chemokine ligand, CCL20. CCR6 is an important receptor that is involved in regulating several aspects of mucosal immunity, including the ability to mediate the recruitment of immature dendritic cells (DCs) and mature DCs, and professional antigen presenting cells (APCs) to the sites of epithelial inflammation. Further, CCR6 mediates the homing of both CD4⁺ T (T-helper; Th) cells and DCs to the gut mucosal lymphoid tissue. DCs, which are known to be essential immune cells in innate immunity and in the initiation of adaptive immunity, play a central role in initiating a primary immune response. Herein, we summarize the role of CCR6 in immune responses at epithelial and mucosal sites in both the lung and gut based on a review of the current literature.     

CCR10 and its ligands in regulation of epithelial immunity and diseases

Epithelial tissues covering the external and internal surface of a body are constantly under physical, chemical or biological assaults. To protect the epithelial tissues and maintain their homeostasis, multiple layers of immune defense mechanisms are required. Besides the epithelial tissue-resident immune cells that provide the first line of defense, circulating immune cells are also recruited into the local tissues in response to challenges. Chemokines and chemokine receptors regulate tissue-specific migration, maintenance and functions of immune cells. Among them, chemokine receptor CCR10 and its ligands chemokines CCL27 and CCL28 are uniquely involved in the epithelial immunity. CCL27 is expressed predominantly in the skin by keratinocytes while CCL28 is expressed by epithelial cells of various mucosal tissues. CCR10 is expressed by various subsets of innate-like T cells that are programmed to localize to the skin during their developmental processes in the thymus. Circulating T cells might be imprinted by skin-associated antigen- presenting cells to express CCR10 for their recruitment to the skin during the local immune response. On the other hand, IgA antibody-producing B cells generated in mucosa-associated lymphoid tissues express CCR10 for their migration and maintenance at mucosal sites. Increasing evidence also found that CCR10/ligands are involved in regulation of other immune cells in epithelial immunity and are frequently exploited by epithelium-localizing or-originated cancer cells for their survival, proliferation and evasion from immune surveillance. Herein, we review current knowledge on roles of CCR10/ligands in regulation of epithelial immunity and diseases and speculate on related important questions worth further investigation.     

Regulated production of the chemokine CCL28 in human colon epithelium

The chemokine CCL28 is constitutively expressed by epithelial cells at several mucosal sites and is thought to function as a homeostatic chemoattractant of subpopulations of T cells and IgA B cells and to mediate antimicrobial activity. We report herein on the regulation of CCL28 in human colon epithelium by the proinflammatory cytokine IL-1, bacterial flagellin, and n-butyrate, a product of microbial metabolism. In vivo, CCL28 was markedly increased in the epithelium of pathologically inflamed compared with normal human colon. Human colon and small intestinal xenografts were used to model human intestinal epithelium in vivo. Xenografts constitutively expressed little, if any, CCL28 mRNA or protein. After stimulation with the proinflammatory cytokine IL-1, CCL28 mRNA and protein were significantly increased in the epithelium of colon but not small intestinal xenografts, although both upregulated the expression of another prototypic chemokine, CXCL8, in response to the identical stimulus. In studies of CCL28 regulation using human colon epithelial cell lines, proinflammatory stimuli, including IL-1, bacterial flagellin, and bacterial infection, significantly upregulated CCL28 mRNA expression and protein production. In addition, CCL28 mRNA expression and protein secretion by those cells were significantly increased by the short-chain fatty acid n-butyrate, and IL-1- or flagellin-stimulated upregulation of CCL28 by colon epithelial cells was synergistically increased by pretreatment of cells with n-butyrate. Consistent with its upregulated expression by proinflammatory stimuli, CCL28 mRNA expression was attenuated by pharmacological inhibitors of NF-kappaB activation. These findings indicate that CCL28 functions as an "inflammatory" chemokine in human colon epithelium and suggest the notion that CCL28 may act to counterregulate colonic inflammation.     

CCR9A and CCR9B: two receptors for the chemokine CCL25/TECK/Ck beta-15 that differ in their sensitivities to ligand

We isolated cDNAs for a chemokine receptor-related protein having the database designation GPR-9-6. Two classes of cDNAs were identified from mRNAs that arose by alternative splicing and that encode receptors that we refer to as CCR9A and CCR9B. CCR9A is predicted to contain 12 additional amino acids at its N terminus as compared with CCR9B. Cells transfected with cDNAs for CCR9A and CCR9B responded to the chemokine CC chemokine ligand 25 (CCL25)/thymus-expressed chemokine (TECK)/chemokine beta-15 (CK beta-15) in assays for both calcium flux and chemotaxis. No other chemokines tested produced responses specific for the cDNA-transfected cells. mRNA for CCR9A/B is expressed predominantly in the thymus, coincident with the expression of CCL25, and highest expression for CCR9A/B among thymocyte subsets was found in CD4+CD8+ cells. mRNAs encoding the A and B forms of the receptor were expressed at a ratio of approximately 10:1 in immortalized T cell lines, in PBMC, and in diverse populations of thymocytes. The EC50 of CCL25 for CCR9A was lower than that for CCR9B, and CCR9A was desensitized by doses of CCL25 that failed to silence CCR9B. CCR9 is the first example of a chemokine receptor in which alternative mRNA splicing leads to proteins of differing activities, providing a mechanism for extending the range of concentrations over which a cell can respond to increments in the concentration of ligand. The study of CCR9A and CCR9B should enhance our understanding of the role of the chemokine system in T cell biology, particularly during the stages of thymocyte development.     

Regulated MIP-3alpha/CCL20 production by human intestinal epithelium: mechanism for modulating mucosal immunity

Human intestinal epithelial cells secrete an array of chemokines known to signal the trafficking of neutrophils and monocytes important in innate mucosal immunity. We hypothesized that intestinal epithelium may also have the capacity to play a role in signaling host adaptive immunity. The CC chemokine macrophage inflammatory protein (MIP)-3alpha/CCL20 is chemotactic for immature dendritic cells and CD45RO(+) T cells that are important components of the host adaptive immune system. In these studies, we demonstrate the widespread production and regulated expression of MIP-3alpha by human intestinal epithelium. Several intestinal epithelial cell lines were shown to constitutively express MIP-3alpha mRNA. Moreover, MIP-3alpha mRNA expression and protein production were upregulated by stimulation of intestinal epithelial cells with the proinflammatory cytokines tumor necrosis factor-alpha or interleukin-1alpha or in response to infection with the enteric bacterial pathogens Salmonella or enteroinvasive Escherichia coli. In addition, MIP-3alpha was shown to function as a nuclear factor-kappaB target gene. In vitro findings were paralleled in vivo by increased expression of MIP-3alpha in the epithelium of cytokine-stimulated or bacteria-infected human intestinal xenografts and in the epithelium of inflamed human colon. Mucosal T cells, other mucosal mononuclear cells, and intestinal epithelial cells expressed CCR6, the cognate receptor for MIP-3alpha. The constitutive and regulated expression of MIP-3alpha by human intestinal epithelium is consistent with a role for epithelial cell-produced MIP-3alpha in modulating mucosal adaptive immune responses.     

Cutting edge: the orphan chemokine receptor G protein-coupled receptor-2 (GPR-2, CCR10) binds the skin-associated chemokine CCL27 (CTACK/ALP/ILC)

We recently reported the identification of a chemokine (CTACK), which has been renamed CCL27 according to a new systematic chemokine nomenclature. We report that CCL27 binds the previously orphan chemokine receptor GPR-2, as detected by calcium flux and chemotactic responses of GPR-2 transfectants. We renamed this receptor CCR10. Because of the skin-associated expression pattern of CCL27, we focused on the expression of CCL27 and CCR10 in normal skin compared with inflammatory and autoimmune skin diseases. CCL27 is constitutively produced by keratinocytes but can also be induced upon stimulation with TNF-alpha and IL-1beta. CCR10 is not expressed by keratinocytes and is instead expressed by melanocytes, dermal fibroblasts, and dermal microvascular endothelial cells. CCR10 was also detected in T cells as well as in skin-derived Langerhans cells. Taken together, these observations suggest a role for this novel ligand/receptor pair in both skin homeostasis as well as a potential role in inflammatory responses.     

A common mucosal chemokine (mucosae-associated epithelial chemokine/CCL28) selectively attracts IgA plasmablasts

IgA immunoblasts can seed both intestinal and nonintestinal mucosal sites following localized mucosal immunization, an observation that has led to the concept of a common mucosal immune system. In this study, we demonstrate that the mucosae-associated epithelial chemokine, MEC (CCL28), which is expressed by epithelia in diverse mucosal tissues, is selectively chemotactic for IgA Ab-secreting cells (ASC): MEC attracts IgA- but not IgG- or IgM-producing ASC from both intestinal and nonintestinal lymphoid and effector tissues, including the intestines, lungs, and lymph nodes draining the bronchopulmonary tree and oral cavity. In contrast, the small intestinal chemokine, TECK (CCL25), attracts an overlapping subpopulation of IgA ASC concentrated in the small intestines and its draining lymphoid tissues. Surprisingly, T cells from mucosal sites fail to respond to MEC. These findings suggest a broad and unifying role for MEC in the physiology of the mucosal IgA immune system.     

Epithelial cells as phagocytes: apoptotic epithelial cells are engulfed by mammary alveolar epithelial cells and repress inflammatory mediator release

Clearance of apoptotic cells is critical to tissue homeostasis and resolution of inflammatory lesions. Macrophages are known to remove dying cells and release anti-inflammatory mediators in response; however, many cells traditionally thought of as poor phagocytes can mediate this function as well. In the lactating mammary gland following weaning, alveolar epithelial cell death is massive, yet the gland involutes rapidly, attaining its prepregnancy state in a matter of days. We found histologic evidence of apoptotic cell phagocytosis by viable mammary epithelial cells (MEC) in the involuting mouse mammary gland. Cultured MEC were able to engulf apoptotic cells in vitro, utilizing many of the same receptors used by macrophages, including the phosphatidylserine receptor (PSR), CD36, the vitronectin receptor alpha(v)beta3, and CD91. In addition, MEC, like macrophages, produced TGFbeta in response to stimulation of the PSR by apoptotic cells or the anti-PSR ab 217G8E9, and downregulated endotoxin-stimulated proinflammatory cytokine production. These data support the hypothesis that amateur phagocytes play a significant role in apoptotic cell clearance and its regulation of inflammation.     

High level expression, activation, and antagonism of CC chemokine receptors CCR2 and CCR3 in Chinese hamster ovary cells

The specificity of leukocyte trafficking in inflammation is controlled by the interactions of chemokines with chemokine receptors. Reliable structure-function studies of chemokine-receptor interactions would benefit from cell lines that express consistent high levels of chemokine receptors. We describe herein two new Chinese hamster ovary (CHO) cell lines in which the genes for chemokine receptors CCR2 and CCR3 have been incorporated into identical positions in the host genome. CCR2 is the primary receptor for the chemokine monocyte chemoattractant protein-1 (MCP-1) whereas CCR3 is the primary receptor for the chemokines eotaxin-1, eotaxin-2 and eotaxin-3. Both receptors are expressed at >5,000,000 copies per cell, substantially higher levels than in previous cell lines, and both are competent for binding and activation by the cognate chemokines for these receptors. Using these cell lines we confirm that eotaxin-1 and eotaxin-3 can act as an agonist and an antagonist, respectively, of CCR2. In addition, we show that eotaxin-2 is an antagonist of CCR2 and MCP-1 is an agonist of CCR3. Comparison of the chemokine sequences reveals several positions that are identical in MCP-1 and eotaxin-1 but different in eotaxin-2 and eotaxin-3, suggesting that these amino acids play a role in CCR2 activation.     

Epithelial inflammation is associated with CCL28 production and the recruitment of regulatory T cells expressing CCR10

Mucosal tissues require constant immune surveillance to clear harmful pathogens while maintaining tolerance to self Ags. Regulatory T cells (Tregs) play a central role in this process and expression of alpha(E)beta(7) has been reported to define a subset of Tregs with tropism for inflamed tissues. However, the signals responsible for recruiting Tregs to epithelial surfaces are poorly understood. We have isolated a subset of CCR10-expressing CD25+CD4+Foxp3+ Tregs with potent anti-inflammatory properties from chronically inflamed human liver. The CCR10+ Tregs were detected around bile ducts that expressed increased levels of the CCR10 ligand CCL28. CCL28 was secreted by primary human cholangiocytes in vitro in response to LPS, IL-1beta, or bile acids. Exposure of CCR10+ Tregs to CCL28 in vitro stimulated migration and adhesion to mucosal addressin cell adhesion molecule-1 and VCAM-1. Liver-derived CCR10+ Tregs expressed low levels of CCR7 but high levels of CXCR3, a chemokine receptor associated with infiltration into inflamed tissue and contained a subset of alpha(E)beta7(+) cells. We propose that CXCR3 promotes the recruitment of Tregs to inflamed tissues and CCR10 allows them to respond to CCL28 secreted by epithelial cells resulting in the accumulation of CCR10+ Tregs at mucosal surfaces.     

CC chemokine ligands 25 and 28 play essential roles in intestinal extravasation of IgA antibody-secreting cells

CCL25 (also known as thymus-expressed chemokine) and CCL28 (also known as mucosae-associated epithelial chemokine) play important roles in mucosal immunity by recruiting IgA Ab-secreting cells (ASCs) into mucosal lamina propria. However, their exact roles in vivo still remain to be defined. In this study, we first demonstrated in mice that IgA ASCs in small intestine expressed CCR9, CCR10, and CXCR4 on the cell surface and migrated to their respective ligands CCL25, CCL28, and CXCL12 (also known as stromal cell-derived factor 1), whereas IgA ASCs in colon mainly expressed CCR10 and CXCR4 and migrated to CCL28 and CXCL12. Reciprocally, the epithelial cells of small intestine were immunologically positive for CCL25 and CCL28, whereas those of colon were positive for CCL28 and CXCL12. Furthermore, the venular endothelial cells in small intestine were positive for CCL25 and CCL28, whereas those in colon were positive for CCL28, suggesting their direct roles in extravasation of IgA ASCs. Consistently, in mice orally immunized with cholera toxin (CT), anti-CCL25 suppressed homing of CT-specific IgA ASCs into small intestine, whereas anti-CCL28 suppressed homing of CT-specific IgA ASCs into both small intestine and colon. Reciprocally, CT-specific ASCs and IgA titers in the blood were increased in mice treated with anti-CCL25 or anti-CCL28. Anti-CXCL12 had no such effects. Finally, both CCL25 and CCL28 were capable of enhancing alpha4 integrin-dependent adhesion of IgA ASCs to mucosal addressin cell adhesion molecule-1 and VCAM-1. Collectively, CCL25 and CCL28 play essential roles in intestinal homing of IgA ASCs primarily by mediating their extravasation into intestinal lamina propria.     

Production of MDC/CCL22 by human intestinal epithelial cells

The intestinal mucosa contains a subset of lymphocytes that produce Th2 cytokines, yet the signals responsible for the recruitment of these cells are poorly understood. Macrophage-derived chemokine (MDC/CCL22) is a recently described CC chemokine known to chemoattract the Th2 cytokine producing cells that express the receptor CCR4. The studies herein demonstrate the constitutive production of MDC/CCL22 in vivo by human colon epithelium and by epithelium of human intestinal xenografts. MDC/CCL22 mRNA expression and protein secretion was upregulated in colon epithelial cell lines in response to proinflammatory cytokines or infection with enteroinvasive bacteria. Inhibition of nuclear factor (NF)-kappaB activation abolished MDC/CCL22 expression in response to proinflammatory stimuli, demonstrating that MDC/CCL22 is a NF-kappaB target gene. In addition, tumor necrosis factor-alpha-induced MDC/CCL22 secretion was differentially modulated by Th1 and Th2 cytokines. Supernatants from the basal, but not apical, side of polarized epithelial cells induced a MDC/CCL22-dependent chemotaxis of CCR4-positive T cells. These studies demonstrate the constitutive and regulated production by intestinal epithelial cells of a chemokine known to function in the trafficking of T cells that produce anti-inflammatory cytokines.     

Mucosa-associated epithelial chemokine/CCL28 expression in the uterus attracts CCR10+ IgA plasma cells following mucosal vaccination via estrogen control

Previous studies demonstrated cross talk between mucosal and reproductive organs during secretory IgA Ab induction. In this study, we aimed to clarify the underlying mechanisms of this cross talk. We found significantly higher titers of Ag-specific secretory IgA Ab in the vaginal wash after mucosal vaccination by both the intranasal (i.n.) and the intravaginal routes but not by the s.c. route. Interestingly, Ag-specific IgA Ab-secreting cells (ASCs) were found mainly in the uterus but not in the cervix and vaginal canal after i.n. vaccination. The fact that most Ag-specific IgA ASCs isolated from the uteri of vaccinated mice migrated toward mucosa-associated epithelial chemokine (MEC)/CCL28 suggests dominant expression of CCR10 on the IgA ASCs. Further, IgA ASCs in the uteri of vaccinated mice were reduced drastically in mice treated with neutralizing anti-MEC/CCL28 Ab. Most intriguingly, the female sex hormone estrogen directly regulated MEC/CCL28 expression and was augmented by i.n. vaccination with cholera toxin or stimulators for innate immunity. Further, blockage of estrogen function in the uterus by oral administration of the estrogen antagonist raloxifene significantly inhibited migration of Ag-specific IgA ASCs after i.n. vaccination with OVA plus cholera toxin. Taken together, these data strongly suggest that CCR10(+) IgA ASCs induced by mucosal vaccination via the i.n. route migrate into the uterus in a MEC/CCL28-dependent manner and that estrogen might have a crucial role in the protection against genital infection by regulating MEC/CCL28 expression in the uterus.     

The role of thymus-expressed chemokine and its receptor CCR9 on lymphocytes in the regional specialization of the mucosal immune system

Chemokines play an important role in the migration of leukocytes at sites of inflammation, and some constitutively expressed chemokines may direct lymphocyte trafficking within lymphoid organs and peripheral tissues. Thymus-expressed chemokine (TECK or Ckbeta-15/CCL25), which signals through the chemokine receptor CCR9, is constitutively expressed in the thymus and small intestine but not colon, and chemoattracts a small fraction of PBLs that coexpress the integrin alpha(4)beta(7). Here we show that TECK is expressed in the human small bowel but not colon by endothelial cells and a subset of cells in intestinal crypts and lamina propria. CCR9 is expressed in the majority of freshly isolated small bowel lamina propria mononuclear cells (LPMC) and at significantly higher levels compared with colonic LPMC or PBL. TECK was selectively chemotactic for small bowel but not colonic LPMC in vitro. The TECK-induced chemotaxis was sensitive to pertussis toxin and partially inhibited by Abs to CCR9. TECK attracts predominantly the T cell fraction of small bowel LPMC, whereas sorted CD3(+)CCR9(+) and CD3(+)CCR9(-) lymphocytes produce similar Th1 or Th2 cytokines at the single cell level. Collectively, our data suggest that the selective expression of TECK in the small bowel underlie the homing of CCR9(+) intestinal memory T cells to the small bowel rather than to the colon. This regional specialization implies a segregation of small intestinal from colonic immune responses.