IEC represent a first line of defense to invasion by pathogens [13]. Intestinal epithelial cells are equipped with a diverse array of PRR that, upon ligand binding results in the induction of a diverse set of chemokines that recruit circulating leukocytes to initiate innate immune responses [9]. Colonic epithelial cells, for example, express surface TLRs, including TLR2, 3, 4, and 5, that are selectively polarized to the basolateral surface [14], ensuring that only invasive bacteria that have breached the barrier trigger TLR’s in the epithelium (see Fig. 11.1).

While specialized gut-associated lymphoid tissues (GALT) , including Peyer’s patches, exist throughout the GI tract and are dedicated to sampling luminal contents at steady state, in fact any IEC can adopt such functions under inflammatory conditions. In addition to relaying luminal signals to the lamina propria, Mayer et al. have provided significant evidence that IEC can recognize, process and even present antigens to antigen-specific lymphocytes [1]. IEC constitutively express MHC class I along the length of the small and large intestine as well as MHC class II in the small intestine, the latter of which can be significantly induced by cytokines such as IFNγ [15]. Polarized antigen sorting and processing in late endosomes is similarly regulated by IFNγ, where antigens are subsequently processed into appropriate immunogenic peptides for presentation to CD4+ T-cells [1]. While IEC do not express classical co-stimulatory molecules such as CD80 or CD86, IEC from IBD patients express novel members of the B7 family (B7h and B7H1) that when paired CD28 or CD152 can produce a co-stimulatory signals [16].

In addition to expression of MHC class I and II, IEC have been shown to express functional non-classical MHC molecules, particularly CD1d [17]. The CD1 family of molecules present self and microbial glycolipids to CD1-restricted T cells (NKT cells in the case of CD1d). Ligation of IEC CD1d has been shown to result in the production of the immunoregulatory cytokine IL-10 [18], for which IEC have been demonstrated to express IL-10 receptors on the luminal surface [19]. Olszak et al., recently demonstrated that IEC -specific deletion of CD1d resulted in a severe NKT cell-mediated intestinal inflammation in a mouse model [20]. Moreover, the decreased expression of CD1d on IEC of patients with IBD may contribute to perpetuation of intestinal inflammation [21].

The successful inflammatory response is initiated by recruitment of leukocytes to sites of infection/injury. Leukocyte migration into and across the epithelium is orchestrated through a highly coordinated series of steps, mediated by cell adhesion molecules and integrins. The molecular details of this cascade have been extensively summarized elsewhere [22, 23]. These seminal studies have revealed that IEC surface molecules and IEC secreted factors play a central role in recruiting and coordinating leukocyte migration to the mucosa (Fig. 11.1).

Crypt abscesses , the accumulation of large numbers of leukocytes to the luminal surface, represent one of the pathological hallmarks of active IBD [22]. There is significant recent interest in understanding how such crypt abscesses might impact mucosal tissue function, particularly related to inflammatory resolution or progression toward chronicity. A recent study examined the influence of neutrophil (polymorphonuclear leukocyte, PMN ) transepithelial migration on epithelial gene programming and the resolution response [24]. In this study, gene expression changes within the epithelium were attributable to the consumption of large amounts of O₂ by PMN through the activation of the NADPH oxidase. These studies revealed that O₂ consumption by activated PMN resulted in the stabilization of the transcription factor hypoxia-inducible factor (HIF) within the epithelium. Utilizing murine models of colitis, the authors demonstrated that both the presence of PMNs as well as PMN -elicited hypoxia were necessary for mucosal progressive resolution of inflammation. Depletion of PMNs led to exacerbated tissue destruction during colitis. These observations have also been validated in human patients. For example, human IBD specimens containing crypt abscesses were examined for the localized expression of the HIF gene target gene Glut1. Areas adjacent to the human crypt abscess revealed marked upregulation of Glut-1 relative to healthy controls. Also notable is the observation that patients lacking a functional NADPH oxidase (i.e., chronic granulomatous disease, CGD) often present with an IBD-like syndrome [25]. This NADPH oxidase complex is responsible for the generation of reactive oxygen species (ROS) and used by innate immune cells (esp. PMN ) to kill invading pathogens. CGD patients exhibit congenital defects in genes coding the subunits of the neutrophilNADPH oxidase complex (e.g., mutations in CYBA, CYBB, NCF, RAC1, and RAC2). Approximately 40 % of CGD patients develop IBD-like symptoms [26]. Such clinical observations suggest that CGD-associated IBD could represent a failure to resolve acute mucosal insults. These findings have given rise to significant interest in developing therapies around the concept of hypoxia-associated metabolism and HIF expression in the mucosa [27].

The mammalian gastrointestinal tract is home to trillions of bacteria. A finely regulated commensal relationship exists within the intestinal mucosa , where microbes, essential for host health, can also initiate and perpetuate mucosal disease [28]. As part of their contribution to overall innate immunity, IEC actively defend the mucosa through the production of antimicrobial peptides. As an example, actively produce defensins, a prominent class of antimicrobial peptides which are cationic, cysteine-rich, and possess broad antimicrobial activity [29, 30]. Defensins are classified as α- or β-defensins based on structural differences in cysteine bond pairing [31]. Crohn’s disease patients have been shown to express defects in α-defensin 5 and α-defensin 6 from Paneth cells of the small intestine [31]. The nature of this defect is not completely understood but is thought to relate to defects in the Nod2 and Atg16l1 genes. Up to 35 % of Crohn’s disease patients carry a mutation in NOD2 and correlates with defective secretion of defensins [31], which may contribute to the dysbiosis observed in some IBD patients.

Human β defensin-1 (hBD1) is notable within the IEC that it is constitutively secreted, whereas others are induced by inflammatory mediators [32, 33]. Constitutive expression of hBD1 was recently shown to depend on the tissue microenvironment (e.g., low oxygen levels found in the colon) [34]. Another distinguishing feature of hBD1 is that the full spectrum of its antimicrobial activity is only revealed when its disulfide bonds are reduced [35]. Reduction of the hBD1 disulfide bonds is accomplished by thioredoxin that localizes with hBD1 in the colonic mucus; oxidation of hBD1 is prevented by the low pO₂ environment of the lumen [36].

Among the other innate antimicrobial defense molecules expressed by IEC is bactericidal permeability-increasing protein (BPI) , originally found in neutrophil and eosinophil granule s [37]. Subsequently, BPI was found to be expressed in IEC [38]. Based on an original transcriptional profiling approach to identify lipid-mediator regulation of mucosal inflammation, BPI was found to be expressed in both human and murine epithelial cells of wide origin (oral, pulmonary, and gastrointestinal mucosa ). Additional studies in human and murine tissue ex vivo revealed that BPI is diffusely expressed along the crypt-villous axis [38, 39], and that epithelial BPI protein levels decrease along the length of the intestine [40]. As its name infers, BPI selectively exerts multiple antimicrobial actions against gram-negative bacteria, including cytotoxicity through damage to bacterial inner/outer membranes, neutralization of bacterial lipopolysaccharide (endotoxin), as well as functioning as an opsonin for phagocytosis of gram-negative bacteria by neutrophils [41, 42]. The high affinity of BPI for the lipid A region of LPS [43] targets its cytotoxic activity to gram-negative bacteria. Binding of BPI to the gram-negative bacterial outer membrane is followed by a time-dependent penetration of the molecule to the bacterial inner membrane where damage results in loss of membrane integrity, dissipation of electrochemical gradients, and bacterial death [44]. BPI binds the lipid A region of LPS with high affinity [45, 46] and thereby prevents its interaction with other (pro-inflammatory) LPS-binding molecules, including LBP and CD14 [47]. Since BPI binds the lipid A region common to all LPS, it is able to neutralize endotoxin from a broad array of gram-negative pathogens [42].

Intestinal alkaline phosphatase (ALPI ) represents another recently appreciated antimicrobial molecule expressed on apical (luminal) aspect of IEC [48]. In the past, this molecule was viewed as one of the better epithelial differentiation markers, with little understanding of the true function of this molecule in the mucosa . More recent studies have identified this molecule as a central player in microbial homeostasis [49–51]. Surface expressed ALPI was shown to retard gram-negative bacterial growth and to potently neutralize LPS through a mechanism involving dephosphorylation of 1,4′-bisphosphorylated glucosamine disaccharide of LPS lipid A [50, 51]. This observation was translated to a murine colitis model and revealed that the expression of ALPI strongly correlated with the resolution phase of inflammation. Moreover, inhibition of ALPI activity was shown to increase the severity of colitic disease [52]. Like those defining epithelial expression of BPI [38], these studies provide an example of the critical interface between inflammatory resolution and the importance of epithelial antimicrobial defense mechanisms (see Fig. 11.1).

The intestinal microbiota, in addition to aiding in digestion, produce a number of vitamins and benefit the host through the local synthesis of short-chain fatty acids (SCFAs) , including butyrate, propionate, and acetate. Butyrate can reach luminal concentrations of 30 mM in the colon and serves as a preferred metabolic substrate for colonic epithelial cells [53]. Butyrate is efficiently absorbed and metabolized by epithelia, and in contrast to other SCFAs, very little butyrate is released into portal circulation [53]. One factor contributing to the preference of the colonic epithelium for butyrate is that butyrate stimulates expression of pyruvate dehydrogenase kinases, which inhibit the pyruvate dehydrogenase complex [54]. This inhibition prevents conversion of glucose-derived pyruvate to acetyl-CoA. Yet, because formation of acetyl-CoA f rom butyrate is not dependent on pyruvate dehydrogenase, butyrate-derived acetyl-CoA is available for oxidative phosphorylation. Significant literature supports an immunological homeostatic role for SCFA in the distal gut [53, 55]. For example, the protection elicited by fiber and resistant starch in experimental colitis are thought to depend on SCFA production [56–58], and administration of exogenous butyrate promotes resistance to experimental colitis [59, 60]. Recent studies investigating dysbiosis in inflammatory bowel disease identified lower concentrations of luminal butyrate and reduced abundance of butyrate-producing organisms (e.g., certain Roseburia and Faecalibacterium species) with disease [61–63]. The importance of butyrate as the preferred epithelial substrate has been highlighted by demonstration that pharmacologic inhibition of β-oxidation induces colitis [64] and that mice with mitochondrial polymorphisms that maintain increased oxidative phosphorylation activity are resistant to colitis [65]. Several trials have evaluated the efficacy of butyrate in the treatment of human disease, primarily ulcerative colitis, with mixed results [53].

The intestinal microbiota shifts in fundamental ways during inflammation. It remains unclear exactly what these shifts in the microbiota might mean to tissue and immune function [66]. Microbial signals, such as those delivered by a mix of Clostridia species , induce mucosal tolerance by promoting the formation of regulatory T cells [67]. Moreover, studies have implicated SCFAs as critical products of tolerogenic Clostridia species [68]. In addition to functioning as a direct energy source, SCFAs can signal through a series of G-protein coupled receptors (GPR) to mediate their biological functions [69, 70]. In mice, deletion of Gpr41 and Gpr43 mediate protective immunity in inflammatory models [69, 70]. Also notable is the observation that treatment of mice with the SCFA propionate promotes colonic protection during inflammation [70] and that the major butyrate receptor (GPR109a) functions to suppress colonic carcinogenesis and inflammation [71]. Such studies clearly implicate that targeting SCFA and SCFA receptors/transporters as promising strategies for the development of new lines of treatment for IBD.

Multiple lines of evidence now support the concept that epithelial cells function as an integral part of the innate immune system (see Fig. 11.1). The intimate interactions between epithelial cells and the various components of the immune system contribute fundamentally to maintenance of health in the gut but also to the development of both acute and chronic inflammatory diseases , most particularly IBD. A further understanding of the genetic links to the luminal triggers associated with IBD will go far in developing effective therapies that enhance epithelial innate immune responses.