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## **Meet the editor**

Sami Karoui was born in Tunis in 1968. He studied at the Faculty of Medicine of Tunis, and he continued his study of Gastroenterology and Hepatology at the University Hospital in Tunis. He received his diploma in Gastroenterology in 1998, and he has been working in the Department of Gastroenterology A (La Rabta Hospital – Tunis) and in the Faculty of Medicine of Tunis since 2008 as

professor of Gastroenterology and Hepatology.

Contents

**Preface IX**

Chapter 2 **Manning the Barricades:**

**Part 1 Advances in Etiopathogeny of Crohn's Disease 1**

**by Proinflammatory Cytokines in Crohn's Disease 3**  Amira Seltana, Manon Lepage and Jean-François Beaulieu

**Role of the Gut Epithelium in Crohn's Disease 17**

*IL23R* **Genes in Polish Crohn's Disease (CD) Patients**

Szymon Hryhorowicz, Marcin A. Kucharski, Ryszard Slomski, Krzysztof Linke and Agnieszka Dobrowolska-Zachwieja

**the Disease and Extra-Intestinal Symptoms? 39**  Ludwika Jakubowska-Burek, Elzbieta Kaczmarek, Justyna Hoppe-Golebiewska, Marta Kaczmarek-Rys,

Chapter 1 **Alteration of the Crypt Epithelial-Stromal Interface**

Erik P. Lillehoj and Erik P.H. De Leeuw

Chapter 3 **Genotyping of** *CARD15/NOD2***,** *ATG16L1* **and**

Chapter 4 **Inflammatory Bowel Disease G-Prote**

**Part 2 Diagnosis of Crohn's Disease 87**

Amandine Gagneux-Brunon, Bernard Faulques and Xavier Roblin

Chapter 5 **Be or Not to Be a Crohn's Disease:**

Beyazit Zencirci

**– Are They Related to the Localization of**

**in Coupled Receptors (GPCRs) Expression Profiling with Microfluidic Cards 59**  Nathalie Taquet, Claude Philippe,

Jean-Marie Reimund and Christian D. Muller

**CD and Its Numerous Differential Diagnosis 89**

Chapter 6 **Crohn's Disease: From an Anesthetist's Perspective 103**

### Contents

#### **Preface XI**

	- **Part 2 Diagnosis of Crohn's Disease 87**

X Contents

Chapter 8 *Mycobacterium avium* **ssp.**  *paratuberculosis* **vs Crohn's Disease 129**  Isabel Azevedo Carvalho, Maria de Lourdes de Abreu Ferrari and Maria Aparecida Scatamburlo Moreira

### Preface

Crohn's disease (CD) is a lifelong disease arising from interaction between genetic and environmental factors. The precise aetiology of CD is unknown, and therefore, a causal treatment is not yet available. During the last few years, there have been many advances concerning etiopathogeny, diagnosis tools, and management of CD.

Inflammatory bowel disease, and particularly CD, results from an inappropriate inflammatory response to intestinal microbes in a genetically susceptible host. The intestinal lamina propria contains a complex population of immune cells that balance the requirement for immune tolerance of luminal microbiota with the need to defend against pathogens and the excessive entry of luminal microbiota. The hallmark of active CD is a pronounced infiltration into the lamina propria of innate immune cells (neutrophils, macrophages, dendritic cells, and natural killer T cells), and adaptive immune cells (B cells and T cells). Increased numbers and activation of these cells in the intestinal mucosa elevate local levels of TNF alpha, interleukin 1 ß, interferon γ, and cytokines of the interleukin-23-Th17 pathway. CD is associated with a Th1 T cell mediated response, characterized by enhanced production of interferon γ and TNF α. IL-12 and possibly IL-23 govern the Th1 cell differentiation, but optimal induction and stabilization of polarized Th1 cells would require additional cytokines.

Genetic factors are intimately involved in the pathogenesis of CD. To data, CARD15/NOD2 in the only confirmed CD susceptibility gene identified. There are a number of other likely candidates of genes and loci that have been described, although details remain less clear, and their roles less well characterized. Potential application of genetic testing in CD is a prediction of disease susceptibility, choice and response to therapy, and disease prevention with specific interventions. More recently, associations with CD have been established for ATG16L1 and immunity-related GTPase M Protein, two genes involved in autophagy.

Clinical features of CD vary depending on the location, behaviour, and severity of disease, as well as extra-intestinal manifestations and medication. The diagnosis is confirmed by clinical evaluation, and a combination of endoscopic, histological, radiological, and/or biochemical investigations. Ileocolonoscopy with multiple biopsy specimens is well established as the first line procedure for the diagnosis of CD. In some cases, the diagnosis can be difficult, confounding by ileocoecal tuberculosis,

#### X Preface

yersiniosis, and ulcerative colitis is a case of involvement of the colon. Histological, bacteriological, and some biochemical markers can help the clinicians to establish the right diagnosis and to prescribe the optimal treatment.

The main goals of therapy in CD are to induce a clinical remission and then maintain that remission over time. Secondary management goals include mucosal healing, maintaining the quality of life of patients, restoring nutritional balance, preventing the complications of the disease, and timing surgical therapy to minimize the morbidity of the disease. Specific therapy of CD depends upon disease severity, location, and complications. In fact, current practice guidelines recommend using a sequential approach to treatment according to the severity of the clinical presentation and associated complications. Categories of severity are based on the type and acuity of symptoms, as well as response to medications, and include mild to moderate, moderate to severe, and severe to fulminant disease. Sulfasalazine and Mesalamine can still be considered first-line therapies for selected patients with mildly to moderately active CD. Studies have conclusively shown the benefit of corticosteroids in induction of remission in patients with CD. Patients unable to discontinue corticosteroids should be considered for alternative therapies such as azathioprine, methotrexate, or infliximab. Anti-TNF α antibodies and Natalizumab were both superior to a placebo in inducing remission and in preventing relapses in luminal CD patients.

> **Dr Sami Karoui** Department of Gastroenterology A La Rabta Hospital, Tunis Tunisia

X Preface

patients.

yersiniosis, and ulcerative colitis is a case of involvement of the colon. Histological, bacteriological, and some biochemical markers can help the clinicians to establish the

The main goals of therapy in CD are to induce a clinical remission and then maintain that remission over time. Secondary management goals include mucosal healing, maintaining the quality of life of patients, restoring nutritional balance, preventing the complications of the disease, and timing surgical therapy to minimize the morbidity of the disease. Specific therapy of CD depends upon disease severity, location, and complications. In fact, current practice guidelines recommend using a sequential approach to treatment according to the severity of the clinical presentation and associated complications. Categories of severity are based on the type and acuity of symptoms, as well as response to medications, and include mild to moderate, moderate to severe, and severe to fulminant disease. Sulfasalazine and Mesalamine can still be considered first-line therapies for selected patients with mildly to moderately active CD. Studies have conclusively shown the benefit of corticosteroids in induction of remission in patients with CD. Patients unable to discontinue corticosteroids should be considered for alternative therapies such as azathioprine, methotrexate, or infliximab. Anti-TNF α antibodies and Natalizumab were both superior to a placebo in inducing remission and in preventing relapses in luminal CD

**Dr Sami Karoui**

Tunisia

La Rabta Hospital, Tunis

Department of Gastroenterology A

right diagnosis and to prescribe the optimal treatment.

**Part 1**

**Advances in Etiopathogeny of Crohn's Disease**

## **Part 1**

## **Advances in Etiopathogeny of Crohn's Disease**

**1**

**Crohn's Disease**

*Université de Sherbrooke*

*Canada*

**Alteration of the Crypt Epithelial-Stromal**

Amira Seltana, Manon Lepage and Jean-François Beaulieu

**Interface by Proinflammatory Cytokines in**

The intestinal epithelium is a very dynamic tissue, being completely renewed over a 3-5 day period in the human. Epithelial cells in the intestine lie on a specialized layer of extracellular matrix material referred to as the basement membrane (BM). Intestinal epithelial cell interactions with BM molecules such as collagens, laminins, fibronectin and tenascin regulate crucial functions of the normal intestinal epithelial renewal process such as cell adhesion, migration, signalization and gene expression as well as anoikis. BM molecules can be secreted by epithelial cells but a number of them are exclusively synthesized and deposited by the subepithelial myofibroblasts. In this chapter, we will review alterations at the epithelial-stromal interface in Crohn's disease (CD) with a specific emphasis on epithelial cell and myofibroblast susceptibility to proinflammatory cytokines in the crypt

The epithelial BM of the human intestine has been found to contain all the major components specific to most basement membranes as well as a number of non-exclusive BM components. There is good evidence that both types of BM components play an active role in intestinal epithelial cell biology through their interaction with specific cell membrane integrin and non-integrin receptors, which mediate cell adhesion, migration, cell cycle and gene expression. Current knowledge about epithelial BM composition in the normal human intestine is summarized below. More detailed information on BM molecules and their receptors in the intestine can be found elsewhere (Beaulieu 1997a, Beaulieu 2001, Lussier et

As illustrated in Figure 1, exclusive BM components include the type IV collagens and laminins. These macromolecules are complex protein families composed of various subunits. Detailed analysis of various genetic forms of type IV collagens and laminins revealed the presence of at least two distinct types of type IV collagen heterotrimers based on the expression of the 1(IV)/2(IV) and 5(IV)/6(IV) chains (Beaulieu 1992, Beaulieu et al.

**2. Epithelial BM molecules in the normal human intestine**

al. 2000, Ménard et al. 2006, Teller&Beaulieu 2001).

**2.1 Exclusive and non-exclusive BM components**

**1. Introduction**

region.

### **Alteration of the Crypt Epithelial-Stromal Interface by Proinflammatory Cytokines in Crohn's Disease**

Amira Seltana, Manon Lepage and Jean-François Beaulieu *Université de Sherbrooke Canada*

#### **1. Introduction**

The intestinal epithelium is a very dynamic tissue, being completely renewed over a 3-5 day period in the human. Epithelial cells in the intestine lie on a specialized layer of extracellular matrix material referred to as the basement membrane (BM). Intestinal epithelial cell interactions with BM molecules such as collagens, laminins, fibronectin and tenascin regulate crucial functions of the normal intestinal epithelial renewal process such as cell adhesion, migration, signalization and gene expression as well as anoikis. BM molecules can be secreted by epithelial cells but a number of them are exclusively synthesized and deposited by the subepithelial myofibroblasts. In this chapter, we will review alterations at the epithelial-stromal interface in Crohn's disease (CD) with a specific emphasis on epithelial cell and myofibroblast susceptibility to proinflammatory cytokines in the crypt region.

#### **2. Epithelial BM molecules in the normal human intestine**

The epithelial BM of the human intestine has been found to contain all the major components specific to most basement membranes as well as a number of non-exclusive BM components. There is good evidence that both types of BM components play an active role in intestinal epithelial cell biology through their interaction with specific cell membrane integrin and non-integrin receptors, which mediate cell adhesion, migration, cell cycle and gene expression. Current knowledge about epithelial BM composition in the normal human intestine is summarized below. More detailed information on BM molecules and their receptors in the intestine can be found elsewhere (Beaulieu 1997a, Beaulieu 2001, Lussier et al. 2000, Ménard et al. 2006, Teller&Beaulieu 2001).

#### **2.1 Exclusive and non-exclusive BM components**

As illustrated in Figure 1, exclusive BM components include the type IV collagens and laminins. These macromolecules are complex protein families composed of various subunits. Detailed analysis of various genetic forms of type IV collagens and laminins revealed the presence of at least two distinct types of type IV collagen heterotrimers based on the expression of the 1(IV)/2(IV) and 5(IV)/6(IV) chains (Beaulieu 1992, Beaulieu et al.

Alteration of the Crypt Epithelial-Stromal Interface

by Proinflammatory Cytokines in Crohn's Disease 5

epithelial renewing units consist of spatially well-separated proliferative and differentiated cell populations. Furthermore, the renewing units differ along the proximal-distal gradient, the crypt-to-villus axis of the small intestine being replaced by a gland-to-surface epithelial axis in the colon (Beaulieu 1997b, Ménard&Beaulieu 1994, Ménard et al. 2006) (Fig. 2).

Fig. 2. Development and characteristics of the epithelium in the human small and large intestines. The human intestine develops relatively early during ontogeny. Villi develop between 9 and 11 weeks of gestation while crypts form around 16 weeks so that typical adult-like crypt-villus architecture is already established in the small intestine at midgestation (18 to 20 weeks). Similar crypt-villus architecture is transitorily present in the developing colon up to mid-gestation. However, at birth, the villi have disappeared and the typical adult gland-to-surface epithelial architecture has been established (proximal-distal gradient). At maturity, the proliferative cell populations responsible for the renewing of the epithelium are located in the lower ⅔ of the small intestinal crypts and the lower ½ of the colonic glands. The functional cells of these renewing units are located on the villus and the

surface epithelium of the small and large intestines, respectively (Adapted from

Alterations of epithelial BM composition have been reported in various intestinal pathologies (Belanger&Beaulieu 2000, Teller&Beaulieu 2001). Although none of these alterations has yet been demonstrated to be the primary defect in any intestinal disease, they are not exclusively secondary to the disruption of the epithelial-stromal interface. In colorectal cancers for instance, laminin alterations are thought to play an active role in

**3. Alterations of epithelial BM composition in CD**

(Teller&Beaulieu 2001).

1994, Simoneau et al. 1998) and the 4 main laminins, LM-111, LM-211, LM-332 and LM-511 (Beaulieu 1992, Beaulieu&Vachon 1994, Teller et al. 2007).

A second interesting feature of the intestinal epithelial BM is the presence of a relatively large number of non-exclusive BM components, such as fibronectin, tenascin-C, osteopontin and type VI collagen that have been found to be integral epithelial BM components (Aufderheide&Ekblom 1988, Beaulieu et al. 1991, Beaulieu 1992, Groulx et al. 2011, Simon-Assmann et al. 1990b) although they can be found also in the underlying interstitial matrix (Fig.1).

Fig. 1. The intestinal epithelial BM. The BM, which is located at the interface between the epithelial cells (e) and the subepithelial myofibroblasts (m), contains BM-specific macromolecules (e.g. type IV collagens and laminins) as well as non-exclusive BM components (e.g. tenascin-C and type VI collagen). Both types of components can originate from the epithelial cells and/or the subepithelial myofibroblasts (white arrows).

The third interesting phenomenon relative to the epithelial BM in the intestine is the dual tissular origin of the BM components. Indeed, while the type IV collagen 5(IV) and 6(IV) chains as well as type VI collagen are expressed at least in part by epithelial cells, the major type IV collagen chains 1(IV) and 2(IV) are exclusively of stromal origin (Beaulieu et al. 1994, Groulx et al. 2011, Simon-Assmann et al. 1990a, Simoneau et al. 1998, Vachon et al. 1993), presumably synthesized by the subepithelial myofibroblasts (Fig. 1). Analysis of the tissular origin of the laminins also revealed dual epithelial/stromal origin for laminins LM-111 and LM-332 (epithelial), LM211 (stromal) and LM511 (both) (Perreault et al. 1998, Teller et al. 2007).

#### **2.2 Spatial and temporal BM microenvironments**

Spatial and temporal patterns of expression for BM components in the intact intestinal epithelium have been very informative in evaluating the potential role of individual macromolecules in the regulation of cell functions, most notably cell growth and differentiation, under a normal environment. Indeed, during development, the process of endodermal differentiation into a functional epithelium coincides with the morphogenesis of the villi and the crypts in both the small and large intestines. In the mature intestine, the

1994, Simoneau et al. 1998) and the 4 main laminins, LM-111, LM-211, LM-332 and LM-511

A second interesting feature of the intestinal epithelial BM is the presence of a relatively large number of non-exclusive BM components, such as fibronectin, tenascin-C, osteopontin and type VI collagen that have been found to be integral epithelial BM components (Aufderheide&Ekblom 1988, Beaulieu et al. 1991, Beaulieu 1992, Groulx et al. 2011, Simon-Assmann et al. 1990b) although they can be found also in the underlying interstitial matrix

Fig. 1. The intestinal epithelial BM. The BM, which is located at the interface between the

components (e.g. tenascin-C and type VI collagen). Both types of components can originate

The third interesting phenomenon relative to the epithelial BM in the intestine is the dual tissular origin of the BM components. Indeed, while the type IV collagen 5(IV) and 6(IV) chains as well as type VI collagen are expressed at least in part by epithelial cells, the major type IV collagen chains 1(IV) and 2(IV) are exclusively of stromal origin (Beaulieu et al. 1994, Groulx et al. 2011, Simon-Assmann et al. 1990a, Simoneau et al. 1998, Vachon et al. 1993), presumably synthesized by the subepithelial myofibroblasts (Fig. 1). Analysis of the tissular origin of the laminins also revealed dual epithelial/stromal origin for laminins LM-111 and LM-332 (epithelial), LM211 (stromal) and LM511 (both) (Perreault et al. 1998, Teller

Spatial and temporal patterns of expression for BM components in the intact intestinal epithelium have been very informative in evaluating the potential role of individual macromolecules in the regulation of cell functions, most notably cell growth and differentiation, under a normal environment. Indeed, during development, the process of endodermal differentiation into a functional epithelium coincides with the morphogenesis of the villi and the crypts in both the small and large intestines. In the mature intestine, the

epithelial cells (e) and the subepithelial myofibroblasts (m), contains BM-specific macromolecules (e.g. type IV collagens and laminins) as well as non-exclusive BM

from the epithelial cells and/or the subepithelial myofibroblasts (white arrows).

**2.2 Spatial and temporal BM microenvironments**

(Beaulieu 1992, Beaulieu&Vachon 1994, Teller et al. 2007).

(Fig.1).

et al. 2007).

epithelial renewing units consist of spatially well-separated proliferative and differentiated cell populations. Furthermore, the renewing units differ along the proximal-distal gradient, the crypt-to-villus axis of the small intestine being replaced by a gland-to-surface epithelial axis in the colon (Beaulieu 1997b, Ménard&Beaulieu 1994, Ménard et al. 2006) (Fig. 2).

Fig. 2. Development and characteristics of the epithelium in the human small and large intestines. The human intestine develops relatively early during ontogeny. Villi develop between 9 and 11 weeks of gestation while crypts form around 16 weeks so that typical adult-like crypt-villus architecture is already established in the small intestine at midgestation (18 to 20 weeks). Similar crypt-villus architecture is transitorily present in the developing colon up to mid-gestation. However, at birth, the villi have disappeared and the typical adult gland-to-surface epithelial architecture has been established (proximal-distal gradient). At maturity, the proliferative cell populations responsible for the renewing of the epithelium are located in the lower ⅔ of the small intestinal crypts and the lower ½ of the colonic glands. The functional cells of these renewing units are located on the villus and the surface epithelium of the small and large intestines, respectively (Adapted from (Teller&Beaulieu 2001).

#### **3. Alterations of epithelial BM composition in CD**

Alterations of epithelial BM composition have been reported in various intestinal pathologies (Belanger&Beaulieu 2000, Teller&Beaulieu 2001). Although none of these alterations has yet been demonstrated to be the primary defect in any intestinal disease, they are not exclusively secondary to the disruption of the epithelial-stromal interface. In colorectal cancers for instance, laminin alterations are thought to play an active role in

Alteration of the Crypt Epithelial-Stromal Interface

by Proinflammatory Cytokines in Crohn's Disease 7

Fig. 3. Alterations of epithelial BM composition in the human intestinal mucosa under normal vs inflammatory conditions. In the normal small intestine (left), laminins LM-322 and LM-511 are found in the BM of the villus epithelial cells while laminin LM-211, is found in the BM of crypt epithelial cells. Tenascin-C (Tn-C) is found in the epithelial BM of both the crypts and villi. LM-332 and LM-511 of the epithelial BM are mainly produced by epithelial cells (E) while LM-211 and Tn-C are synthesized by the sub-epithelial

myofibroblasts (SEMF). In inflamed CD specimens (inflammatory conditions), laminins LM-332 and LM-511 remain expressed by epithelial cells of the atrophied villi while in the crypts, the other laminin, LM-211, as well as most of the Tn-C at the epithelial BM are lost and replaced by the neo-expression of laminins LM-332 and LM-511. Functionally, these events are associated with pro-inflammatory cytokines which a) stimulate LM-332 and LM-511 production by crypt epithelial cells and b) induce apoptosis and de-differentiation of the

sub-epithelial myofibroblasts (see sections 3.2 and 3.3 and Fig. 4 for more details).

invasion (Lohi 2001). Alteration in the distribution and/or expression of BM molecules were also observed in other intestinal pathologies such as tufting enteropathy (Goulet et al. 1995) and chronic inflammatory bowel conditions (Bouatrouss 1998). The alterations in the epithelial BM composition in the context of inflammatory bowel disease pathogenesis and as potential disease indicators will now be discussed.

#### **3.1 BM components in the crypts of inflamed specimens from CD patients**

Although clinically distinct, chronic inflammatory bowel diseases such as CD and ulcerative colitis share common histopathological features including mucosal inflammation, villous atrophy, crypt hypertrophy and epithelial cell injury (Chadwick 1991).

In CD, an important redistribution of laminins was observed at the epithelial-stromal interface of inflamed specimens as compared to non-inflamed specimens from the same patients (Bouatrouss et al. 2000). First, the crypt specific laminin, LM-211, was found to be essentially absent from the mucosa. Second, two other laminins, LM-332 and LM-511, remained strongly expressed in the epithelial BM of the atrophied and inflamed villi. However, a significant upregulation of both LM-332 and LM-511 expression was observed in the lower crypts of inflamed CD specimens. Furthermore, a significant reduction in the levels of tenascin-C was also observed in the crypt region of inflamed CD specimens (Francoeur et al. 2009). Incidentally, the intestinal mucosa is among the few sites where tenascin-C remains expressed in adulthood (Belanger&Beaulieu 2000). Interestingly, in ulcerative colitis, most of the epithelial BM surrounding the glands has been found to be devoid of immunoreactive laminin (Schmehl et al. 2000) and tenascin-C (Francoeur et al. 2009) in actively affected colonic tissues. Also, alterations in laminin and tenascin-C expression were not observed in celiac disease (Korhonen et al. 2000), an immune-mediated intestinal pathology that is also characterized by villus atrophy and crypt hyperplasia (Maki&Collin 1997), suggesting that the redistribution of laminins and tenascin-C in CD could be related to the chronic inflammatory condition.

From these studies, it can be concluded that important alterations in epithelial BM molecule expression occur in the intestinal mucosa of patients with inflammatory bowel diseases. The fact that these alterations appear to be mainly confined to the crypts in these diseases, as summarized for CD in Fig. 3, suggests that compositional changes in the crypt epithelial BM may be of functional importance in the pathogenesis of these afflictions.

#### **3.2 Effects of proinflammatory cytokines on human intestinal epithelial crypt cells**

One of the landmarks of inflammatory bowel conditions such as CD is the chronic imbalance between immunoregulatory and proinflammatory cytokines (Bouma&Strober 2003, Fiocchi 1997a, MacDonald&Monteleone 2006, Sartor 2006). Among the cytokines most frequently found to be elevated in the inflamed CD mucosa are interleukin-1 and 6 (IL-1 and IL-6), tumor necrosis factor (TNF) and interferon (IFN). Because they represent the main component of the intestinal barrier, epithelial cells have been one of the nonimmune cell types most extensively studied in inflammatory bowel diseases. While morphological and functional alterations in epithelial integrity were described several years ago (Dvorak&Dickersin 1980, Hollander 1993), more recent work has provided evidence that these changes are primarily mediated by cytokines released from adjacent inflammatory cells as well as from the epithelial cells themselves (Abraham&Medzhitov 2011, Dionne et al. 1999, Fiocchi 1997b, McKay&Baird 1999, Podolsky 2000).

invasion (Lohi 2001). Alteration in the distribution and/or expression of BM molecules were also observed in other intestinal pathologies such as tufting enteropathy (Goulet et al. 1995) and chronic inflammatory bowel conditions (Bouatrouss 1998). The alterations in the epithelial BM composition in the context of inflammatory bowel disease pathogenesis and as

Although clinically distinct, chronic inflammatory bowel diseases such as CD and ulcerative colitis share common histopathological features including mucosal inflammation, villous

In CD, an important redistribution of laminins was observed at the epithelial-stromal interface of inflamed specimens as compared to non-inflamed specimens from the same patients (Bouatrouss et al. 2000). First, the crypt specific laminin, LM-211, was found to be essentially absent from the mucosa. Second, two other laminins, LM-332 and LM-511, remained strongly expressed in the epithelial BM of the atrophied and inflamed villi. However, a significant upregulation of both LM-332 and LM-511 expression was observed in the lower crypts of inflamed CD specimens. Furthermore, a significant reduction in the levels of tenascin-C was also observed in the crypt region of inflamed CD specimens (Francoeur et al. 2009). Incidentally, the intestinal mucosa is among the few sites where tenascin-C remains expressed in adulthood (Belanger&Beaulieu 2000). Interestingly, in ulcerative colitis, most of the epithelial BM surrounding the glands has been found to be devoid of immunoreactive laminin (Schmehl et al. 2000) and tenascin-C (Francoeur et al. 2009) in actively affected colonic tissues. Also, alterations in laminin and tenascin-C expression were not observed in celiac disease (Korhonen et al. 2000), an immune-mediated intestinal pathology that is also characterized by villus atrophy and crypt hyperplasia (Maki&Collin 1997), suggesting that the redistribution of laminins and tenascin-C in CD

From these studies, it can be concluded that important alterations in epithelial BM molecule expression occur in the intestinal mucosa of patients with inflammatory bowel diseases. The fact that these alterations appear to be mainly confined to the crypts in these diseases, as summarized for CD in Fig. 3, suggests that compositional changes in the crypt epithelial BM

**3.2 Effects of proinflammatory cytokines on human intestinal epithelial crypt cells**  One of the landmarks of inflammatory bowel conditions such as CD is the chronic imbalance between immunoregulatory and proinflammatory cytokines (Bouma&Strober 2003, Fiocchi 1997a, MacDonald&Monteleone 2006, Sartor 2006). Among the cytokines most frequently found to be elevated in the inflamed CD mucosa are interleukin-1 and 6 (IL-1 and IL-6), tumor necrosis factor (TNF) and interferon (IFN). Because they represent the main component of the intestinal barrier, epithelial cells have been one of the nonimmune cell types most extensively studied in inflammatory bowel diseases. While morphological and functional alterations in epithelial integrity were described several years ago (Dvorak&Dickersin 1980, Hollander 1993), more recent work has provided evidence that these changes are primarily mediated by cytokines released from adjacent inflammatory cells as well as from the epithelial cells themselves (Abraham&Medzhitov

**3.1 BM components in the crypts of inflamed specimens from CD patients**

atrophy, crypt hypertrophy and epithelial cell injury (Chadwick 1991).

potential disease indicators will now be discussed.

could be related to the chronic inflammatory condition.

may be of functional importance in the pathogenesis of these afflictions.

2011, Dionne et al. 1999, Fiocchi 1997b, McKay&Baird 1999, Podolsky 2000).

Fig. 3. Alterations of epithelial BM composition in the human intestinal mucosa under normal vs inflammatory conditions. In the normal small intestine (left), laminins LM-322 and LM-511 are found in the BM of the villus epithelial cells while laminin LM-211, is found in the BM of crypt epithelial cells. Tenascin-C (Tn-C) is found in the epithelial BM of both the crypts and villi. LM-332 and LM-511 of the epithelial BM are mainly produced by epithelial cells (E) while LM-211 and Tn-C are synthesized by the sub-epithelial myofibroblasts (SEMF). In inflamed CD specimens (inflammatory conditions), laminins LM-332 and LM-511 remain expressed by epithelial cells of the atrophied villi while in the crypts, the other laminin, LM-211, as well as most of the Tn-C at the epithelial BM are lost and replaced by the neo-expression of laminins LM-332 and LM-511. Functionally, these events are associated with pro-inflammatory cytokines which a) stimulate LM-332 and LM-511 production by crypt epithelial cells and b) induce apoptosis and de-differentiation of the sub-epithelial myofibroblasts (see sections 3.2 and 3.3 and Fig. 4 for more details).

Alteration of the Crypt Epithelial-Stromal Interface

are lost in the inflamed mucosa of CD patients.

1999, Wang et al. 2006, Warhurst et al. 1998).

by Proinflammatory Cytokines in Crohn's Disease 9

bacteria and their products deposited in the subepithelial compartment when the epithelial barrier is disrupted. In turn, when stimulated, myofibroblasts can release various cytokines and chemokines (Fiocchi 1997b, Powell et al. 1999, Powell et al. 2005) and extracellular matrix molecules (Riedl et al. 1992) suggesting that intestinal subepithelial myofibroblasts can participate in the innate immune response (Otte et al. 2003, Saada et al. 2006). Furthermore, the subepithelial myofibroblast represents a key component of epithelial-stromal interactions in the intestine (Ménard et al. 2006, Powell et al. 2005), which can regulate both basic and healing epithelial cell functions through the secretion of paracrine factors such as the Wnts, BMPs, and TGF, which target epithelial cells (Ménard et al. 2006, Powell et al. 2005), the release of proteinases (Kruidenier et al. 2006, McKaig et al. 2003) and the production of extracellular matrix molecules that contribute to the epithelial BM such as laminin LM-211 and tenascin-C (Beaulieu 1997a, Perreault et al. 1998, Riedl et al. 1992, Teller et al. 2007, Vachon et al. 1993). Analysis of subepithelial myofibroblast characteristics in inflamed small intestinal mucosa of CD patients revealed a number of alterations in the crypt region (Francoeur et al. 2009). First, a disappearance of SMA positive cells was observed in a large proportion of the crypts while in others, the SMA cellular staining was abnormally thick and co-stained by desmin suggesting a reorganization/redifferentiation into smooth muscle cells. Characterization of the pericryptal myofibroblastic sheath in the colonic mucosa from patients with various pathologic conditions including CD, ulcerative colitis, acute infectious colitis and noninfectious colitis confirmed the disappearance of SMA positive cells (also desmin negative) in the inflamed mucosa from inflammatory bowel diseases but not in other pathological conditions. Analysis of the expression of tenascin-C, which is exclusively produced by the myofibroblasts and muscle cells of the human intestine (Belanger&Beaulieu 2000) in the crypt epithelial BM revealed a close correlation between myofibroblast disappearance (loss of normal SMA staining) and loss of tenascin-C staining (Francoeur et al. 2009). The significant reduction of SMA positive cells in the pericryptal region concomitant with a disappearance of tenascin-C (Francoeur et al. 2009) and laminin LM-211 (Bouatrouss et al. 2000) suggested that pericryptal myofibroblasts

To test this hypothesis, we used various preparations of myofibroblastic cells isolated from the human intestinal mucosa (Pinchuk et al. 2007, Teller et al. 2007, Vachon et al. 1993) as experimental cell models. Myofibroblasts are known to respond to various inflammatory signals (Otte et al. 2003, Saada et al. 2006) and have been shown to be altered in inflammatory bowel diseases (Fiocchi 1997b, Powell et al. 1999, Powell et al. 2005). For instance, mesenchymal cells isolated from the inflamed mucosa show higher levels of collagen production than their normal counterparts. We thus investigated the possibility that a loss of pericryptal myofibroblasts occurs in the inflamed regions of CD mucosa by testing myofibroblast susceptibility to the same panel of proinflammatory cytokines used above for epithelial cells (section 3.2). Individually, IL-1, IL-6, TNF and IFN as well as TGF exerted little or no effect on the growth and survival of intestinal myofibroblasts. However, cytokine combinations containing TNF and IFN induced significant caspasedependent apoptosis, suggesting that this mechanism may account for the loss of myofibroblasts in the inflamed CD mucosa. While not yet reported for myofibroblasts, the synergistic effect of TNF/IFN on various functions has been described in other cell types, namely intestinal epithelial cells (Begue et al. 2006, Francoeur et al. 2004, Ruemmele et al.

Interestingly, the pro-apoptotic effect of pro-inflammatory cytokines was only observed with myofibroblasts isolated from normal mucosa, myofibroblasts isolated from inflamed

Considering that changes in BM composition under inflammatory conditions are mainly observed in the lower part of the crypt (Fig. 3), our laboratory investigated the effect of proinflammatory cytokines on laminin expression in human intestinal epithelial cells using the well-characterized human intestinal epithelial crypt (HIEC) cell line as an experimental cell model representative of the human intestinal lower crypt (Benoit et al. 2010, Pageot et al. 2000, Quaroni&Beaulieu 1997). Individually, all tested cytokines including IL-1, IL-6, TNF and IFN as well as transforming growth factor (TGF) exerted relatively modest effects on laminin LM-332 and LM-511 production in HIEC cells. However in combination, a synergistic effect of TNF and IFN was observed on both laminin LM-332 and LM-511 production at protein and transcript levels (Francoeur et al. 2004). TNF and IFN synergy has also been reported in various intestinal epithelial cell models for chemokine production (Warhurst et al. 1998), alteration in epithelial barrier properties (Wang et al. 2006), and acquisition of susceptibility to Fas-induced apoptosis (Begue et al. 2006, Ruemmele et al. 1999). The TNF/IFN combination was also found to synergistically induce caspasedependent apoptosis in HIEC cells (Francoeur et al. 2004). Interestingly, caspase inhibitors completely prevented TNF/IFN-induced apoptosis but did not influence the induction of laminin expression indicating that the two events occurred independently.

The contribution of TGF in the synergistic effects of TNF/IFN on laminin production was investigated in light of the fact that levels of TGF have been reported to be elevated in the mucosa of CD patients (Babyatsky et al. 1996) and that this multifunctional cytokine can exert a crucial function on intestinal epithelial wound healing (Podolsky 2000), in part through its ability to stimulate extracellular matrix molecule expression (Verrecchia&Mauviel 2002). Indeed, in rodent intestinal crypt cells, the promotion of epithelial healing by specific cytokines such as IL-1, IFN and TGF was found to act under a bioactive TGF dependent mechanism (Dignass&Podolsky 1993). However, TGF was found to be significantly less potent than the TNF/IFN combination in human intestinal crypt cells on laminin production suggesting a TGF-independent mechanism (Francoeur et al. 2004).

In summary, the two proinflammatory cytokines TNF and IFN synergistically induce the expression of the specific BM molecules, laminin LM-332 and LM-511, in human intestinal crypt cells. The synergistic effect of the TNF/IFN combination on laminin production was found to be independent of the effect of these cytokines on cell apoptosis and appears to be controlled by an apparent TGF-independent mechanism.

#### **3.3 Effects of proinflammatory cytokines on human pericryptal subepithelial myofibroblasts**

Along with the epithelial cells, myofibroblasts are another important nonprofessional immune mucosal cell type for which evidence supports a participation in the pathogenesis of inflammatory bowel diseases (Fiocchi 1997b, Macdonald&Monteleone 2005). The myofibroblast is an intermediate cell type between the smooth muscle cell and the fibroblast that is characterized by alpha smooth muscle actin (SMA) and vimentin expression (Gabbiani 2003). While there is evidence that intramucosal myofibroblasts can be involved in CD (Vallance et al. 2005), it is the intestinal subepithelial myofibroblasts present immediately subjacent to the epithelial BM (Powell et al. 2005) in the form of a pericryptal sheath that have been the primary focus of attention in the pathogenesis of inflammatory bowel diseases. Because of their vicinity to the basal surface of epithelial cells, they are potential targets for

Considering that changes in BM composition under inflammatory conditions are mainly observed in the lower part of the crypt (Fig. 3), our laboratory investigated the effect of proinflammatory cytokines on laminin expression in human intestinal epithelial cells using the well-characterized human intestinal epithelial crypt (HIEC) cell line as an experimental cell model representative of the human intestinal lower crypt (Benoit et al. 2010, Pageot et al. 2000, Quaroni&Beaulieu 1997). Individually, all tested cytokines including IL-1, IL-6, TNF and IFN as well as transforming growth factor (TGF) exerted relatively modest effects on laminin LM-332 and LM-511 production in HIEC cells. However in combination, a synergistic effect of TNF and IFN was observed on both laminin LM-332 and LM-511 production at protein and transcript levels (Francoeur et al. 2004). TNF and IFN synergy has also been reported in various intestinal epithelial cell models for chemokine production (Warhurst et al. 1998), alteration in epithelial barrier properties (Wang et al. 2006), and acquisition of susceptibility to Fas-induced apoptosis (Begue et al. 2006, Ruemmele et al. 1999). The TNF/IFN combination was also found to synergistically induce caspasedependent apoptosis in HIEC cells (Francoeur et al. 2004). Interestingly, caspase inhibitors completely prevented TNF/IFN-induced apoptosis but did not influence the induction of

laminin expression indicating that the two events occurred independently.

controlled by an apparent TGF-independent mechanism.

(Francoeur et al. 2004).

**myofibroblasts**

The contribution of TGF in the synergistic effects of TNF/IFN on laminin production was investigated in light of the fact that levels of TGF have been reported to be elevated in the mucosa of CD patients (Babyatsky et al. 1996) and that this multifunctional cytokine can exert a crucial function on intestinal epithelial wound healing (Podolsky 2000), in part through its ability to stimulate extracellular matrix molecule expression (Verrecchia&Mauviel 2002). Indeed, in rodent intestinal crypt cells, the promotion of epithelial healing by specific cytokines such as IL-1, IFN and TGF was found to act under a bioactive TGF dependent mechanism (Dignass&Podolsky 1993). However, TGF was found to be significantly less potent than the TNF/IFN combination in human intestinal crypt cells on laminin production suggesting a TGF-independent mechanism

In summary, the two proinflammatory cytokines TNF and IFN synergistically induce the expression of the specific BM molecules, laminin LM-332 and LM-511, in human intestinal crypt cells. The synergistic effect of the TNF/IFN combination on laminin production was found to be independent of the effect of these cytokines on cell apoptosis and appears to be

Along with the epithelial cells, myofibroblasts are another important nonprofessional immune mucosal cell type for which evidence supports a participation in the pathogenesis of inflammatory bowel diseases (Fiocchi 1997b, Macdonald&Monteleone 2005). The myofibroblast is an intermediate cell type between the smooth muscle cell and the fibroblast that is characterized by alpha smooth muscle actin (SMA) and vimentin expression (Gabbiani 2003). While there is evidence that intramucosal myofibroblasts can be involved in CD (Vallance et al. 2005), it is the intestinal subepithelial myofibroblasts present immediately subjacent to the epithelial BM (Powell et al. 2005) in the form of a pericryptal sheath that have been the primary focus of attention in the pathogenesis of inflammatory bowel diseases. Because of their vicinity to the basal surface of epithelial cells, they are potential targets for

**3.3 Effects of proinflammatory cytokines on human pericryptal subepithelial**

bacteria and their products deposited in the subepithelial compartment when the epithelial barrier is disrupted. In turn, when stimulated, myofibroblasts can release various cytokines and chemokines (Fiocchi 1997b, Powell et al. 1999, Powell et al. 2005) and extracellular matrix molecules (Riedl et al. 1992) suggesting that intestinal subepithelial myofibroblasts can participate in the innate immune response (Otte et al. 2003, Saada et al. 2006). Furthermore, the subepithelial myofibroblast represents a key component of epithelial-stromal interactions in the intestine (Ménard et al. 2006, Powell et al. 2005), which can regulate both basic and healing epithelial cell functions through the secretion of paracrine factors such as the Wnts, BMPs, and TGF, which target epithelial cells (Ménard et al. 2006, Powell et al. 2005), the release of proteinases (Kruidenier et al. 2006, McKaig et al. 2003) and the production of extracellular matrix molecules that contribute to the epithelial BM such as laminin LM-211 and tenascin-C (Beaulieu 1997a, Perreault et al. 1998, Riedl et al. 1992, Teller et al. 2007, Vachon et al. 1993).

Analysis of subepithelial myofibroblast characteristics in inflamed small intestinal mucosa of CD patients revealed a number of alterations in the crypt region (Francoeur et al. 2009). First, a disappearance of SMA positive cells was observed in a large proportion of the crypts while in others, the SMA cellular staining was abnormally thick and co-stained by desmin suggesting a reorganization/redifferentiation into smooth muscle cells. Characterization of the pericryptal myofibroblastic sheath in the colonic mucosa from patients with various pathologic conditions including CD, ulcerative colitis, acute infectious colitis and noninfectious colitis confirmed the disappearance of SMA positive cells (also desmin negative) in the inflamed mucosa from inflammatory bowel diseases but not in other pathological conditions. Analysis of the expression of tenascin-C, which is exclusively produced by the myofibroblasts and muscle cells of the human intestine (Belanger&Beaulieu 2000) in the crypt epithelial BM revealed a close correlation between myofibroblast disappearance (loss of normal SMA staining) and loss of tenascin-C staining (Francoeur et al. 2009). The significant reduction of SMA positive cells in the pericryptal region concomitant with a disappearance of tenascin-C (Francoeur et al. 2009) and laminin LM-211 (Bouatrouss et al. 2000) suggested that pericryptal myofibroblasts are lost in the inflamed mucosa of CD patients.

To test this hypothesis, we used various preparations of myofibroblastic cells isolated from the human intestinal mucosa (Pinchuk et al. 2007, Teller et al. 2007, Vachon et al. 1993) as experimental cell models. Myofibroblasts are known to respond to various inflammatory signals (Otte et al. 2003, Saada et al. 2006) and have been shown to be altered in inflammatory bowel diseases (Fiocchi 1997b, Powell et al. 1999, Powell et al. 2005). For instance, mesenchymal cells isolated from the inflamed mucosa show higher levels of collagen production than their normal counterparts. We thus investigated the possibility that a loss of pericryptal myofibroblasts occurs in the inflamed regions of CD mucosa by testing myofibroblast susceptibility to the same panel of proinflammatory cytokines used above for epithelial cells (section 3.2). Individually, IL-1, IL-6, TNF and IFN as well as TGF exerted little or no effect on the growth and survival of intestinal myofibroblasts. However, cytokine combinations containing TNF and IFN induced significant caspasedependent apoptosis, suggesting that this mechanism may account for the loss of myofibroblasts in the inflamed CD mucosa. While not yet reported for myofibroblasts, the synergistic effect of TNF/IFN on various functions has been described in other cell types, namely intestinal epithelial cells (Begue et al. 2006, Francoeur et al. 2004, Ruemmele et al. 1999, Wang et al. 2006, Warhurst et al. 1998).

Interestingly, the pro-apoptotic effect of pro-inflammatory cytokines was only observed with myofibroblasts isolated from normal mucosa, myofibroblasts isolated from inflamed

Alteration of the Crypt Epithelial-Stromal Interface

**4. Conclusions**

**5. Acknowledgements**

**6. References**

by Proinflammatory Cytokines in Crohn's Disease 11

It is becoming more and more evident that under inflammatory conditions, the intestinal immune response relies on a complex interplay between immune and nonprofessional immune cells (Fiocchi 1997b, Macdonald&Monteleone 2005). Proinflammatory cytokines, namely TNF and IFN in combination, were shown to induce human intestinal crypt epithelial cell apoptosis and altered expression and distribution of laminins LM-332 and LM-511 in the crypts (Francoeur et al. 2004), two events that contribute to the disruption of epithelial cell homeostasis (Bouatrouss et al. 2000, Teller&Beaulieu 2001). As summarized in Fig. 4, the proinflammatory conditions that prevail in the intestinal mucosa of patients affected with inflammatory bowel diseases also disrupt the pericryptal sheath, as shown by the loss of myofibroblasts and altered expression of laminin LM-211 and tenascin-C (Francoeur et al. 2009). Taken together, these data suggest that the entire epithelial-stromal

The authors wish to thank the other members of the laboratory for discussion, namely Drs Caroline Francoeur and Yamina Bouatrouss who were particularly involved in the original

The work was supported by grants from the Canadian Institutes for Health Research. JFB holds the Canadian Research Chair in Intestinal Physiopathology and is a member of the

Abraham C, Medzhitov R (2011). Interactions between the host innate immune system and

Andoh A, Bamba S, Fujiyama Y, Brittan M, Wright NA (2005). Colonic subepithelial

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Beaulieu JF (1992). Differential expression of the VLA family of integrins along the crypt-

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microbes in inflammatory bowel disease. *Gastroenterology*, Vol.140, No.6, (May),

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components during organogenesis in the human small intestine. *Anat Embryol*

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crypt-villus axis in the human small intestine. *Gastroenterology*, Vol.106, No.4,

interface is affected in the intestine under proinflammatory conditions.

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mucosa being apoptosis-resistant (Francoeur et al. 2009). Distinct intrinsic properties of myofibroblasts isolated from inflammatory bowel disease vs non-inflammatory bowel disease patients remains to be elucidated but is not without precedent (Lawrance et al. 2001). The contribution of bone marrow-derived myofibroblasts in the regenerative process in inflammatory bowel disease (Andoh et al. 2005, Brittan et al. 2007) may explain the resistance to cytokine-mediated apoptosis.

The high levels of TGF in the inflamed mucosa may need to be further investigated considering its anti-inflammatory effect in inflammatory bowel diseases (Fiocchi 2001) and its promoting effect on myofibroblast differentiation (Simmons et al. 2002). Indeed, although not directly tested on myofibroblasts for apoptosis, TGF was found to significantly enhance SMA expression in intestinal myofibroblasts while all other cytokines reduced SMA expression. Interestingly, TGF completely reversed the down-regulation of SMA expression triggered by individual proinflammatory cytokines but failed to prevent the SMA down-regulation induced by the TNF/IFN combination, suggesting a dedifferentiation mechanism (Francoeur et al. 2009).

In summary, these studies showed that the myofibroblasts of the intestinal pericryptal sheath are a target for proinflammatory cytokines in active inflammatory bowel diseases. The disappearance of laminin LM-211 and tenascin-C in the pericryptal region of the inflamed CD mucosa appears to be a direct consequence of the alteration of the myofibroblastic sheath. While proinflammatory cytokines appear to be responsible for the net reduction of the number of pericryptal myofibroblasts, the mechanisms involved include caspase-dependent apoptosis and dedifferentiation toward a fibroblastic phenotype.

Fig. 4. Functional alterations of human intestinal epithelial-stromal components resulting from the synergistic effects of TNF and IFN. The TNF/IFN combination was found to trigger myofibroblast caspase-dependent apoptosis and myofibroblast dedifferentiation, which results in the disappearance of a significant portion of the pericryptal sheath (1) and, concomitantly, the loss of two extracellular matrix molecules specifically synthesized by these cells: laminin LM-211 and tenascin-C (Tn-C) (2). The TNF/IFN combination was found to also elicit caspase-dependent apoptosis in epithelial crypt cells (3) as well as induction of the expression of the laminins LM-332 and LM-511 (4), two laminins normally not expressed in the lower crypt. Expression of LM-511 may compensate for the disappearance of LM-211.

### **4. Conclusions**

10 Crohn's Disease

mucosa being apoptosis-resistant (Francoeur et al. 2009). Distinct intrinsic properties of myofibroblasts isolated from inflammatory bowel disease vs non-inflammatory bowel disease patients remains to be elucidated but is not without precedent (Lawrance et al. 2001). The contribution of bone marrow-derived myofibroblasts in the regenerative process in inflammatory bowel disease (Andoh et al. 2005, Brittan et al. 2007) may explain the

The high levels of TGF in the inflamed mucosa may need to be further investigated considering its anti-inflammatory effect in inflammatory bowel diseases (Fiocchi 2001) and its promoting effect on myofibroblast differentiation (Simmons et al. 2002). Indeed, although not directly tested on myofibroblasts for apoptosis, TGF was found to significantly enhance SMA expression in intestinal myofibroblasts while all other cytokines reduced SMA expression. Interestingly, TGF completely reversed the down-regulation of SMA expression triggered by individual proinflammatory cytokines but failed to prevent the SMA down-regulation induced by the TNF/IFN combination, suggesting a

In summary, these studies showed that the myofibroblasts of the intestinal pericryptal sheath are a target for proinflammatory cytokines in active inflammatory bowel diseases. The disappearance of laminin LM-211 and tenascin-C in the pericryptal region of the inflamed CD mucosa appears to be a direct consequence of the alteration of the myofibroblastic sheath. While proinflammatory cytokines appear to be responsible for the net reduction of the number of pericryptal myofibroblasts, the mechanisms involved include

caspase-dependent apoptosis and dedifferentiation toward a fibroblastic phenotype.

Fig. 4. Functional alterations of human intestinal epithelial-stromal components resulting from the synergistic effects of TNF and IFN. The TNF/IFN combination was found to trigger myofibroblast caspase-dependent apoptosis and myofibroblast dedifferentiation, which results in the disappearance of a significant portion of the pericryptal sheath (1) and, concomitantly, the loss of two extracellular matrix molecules specifically synthesized by these cells: laminin LM-211 and tenascin-C (Tn-C) (2). The TNF/IFN combination was found to also elicit caspase-dependent apoptosis in epithelial crypt cells (3) as well as induction of the expression of the laminins LM-332 and LM-511 (4), two laminins normally

not expressed in the lower crypt. Expression of LM-511 may compensate for the

disappearance of LM-211.

resistance to cytokine-mediated apoptosis.

dedifferentiation mechanism (Francoeur et al. 2009).

It is becoming more and more evident that under inflammatory conditions, the intestinal immune response relies on a complex interplay between immune and nonprofessional immune cells (Fiocchi 1997b, Macdonald&Monteleone 2005). Proinflammatory cytokines, namely TNF and IFN in combination, were shown to induce human intestinal crypt epithelial cell apoptosis and altered expression and distribution of laminins LM-332 and LM-511 in the crypts (Francoeur et al. 2004), two events that contribute to the disruption of epithelial cell homeostasis (Bouatrouss et al. 2000, Teller&Beaulieu 2001). As summarized in Fig. 4, the proinflammatory conditions that prevail in the intestinal mucosa of patients affected with inflammatory bowel diseases also disrupt the pericryptal sheath, as shown by the loss of myofibroblasts and altered expression of laminin LM-211 and tenascin-C (Francoeur et al. 2009). Taken together, these data suggest that the entire epithelial-stromal interface is affected in the intestine under proinflammatory conditions.

#### **5. Acknowledgements**

The authors wish to thank the other members of the laboratory for discussion, namely Drs Caroline Francoeur and Yamina Bouatrouss who were particularly involved in the original work and Elizabeth Herring for proofreading the manuscript.

The work was supported by grants from the Canadian Institutes for Health Research. JFB holds the Canadian Research Chair in Intestinal Physiopathology and is a member of the FRSQ-funded Centre de Recherche Clinique Etienne-LeBel of the CHUS.

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**2**

*USA*

**Manning the Barricades:**

Erik P. Lillehoj1 and Erik P.H. De Leeuw2

*2Institute of Human Virology and Department of Biochemistry & Molecular Biology of the University*

*of Maryland Baltimore School of Medicine,*

*1Department of Pediatrics and*

**Role of the Gut Epithelium in Crohn's Disease**

Crohn's disease is a chronic inflammation of the gut that affects an estimated 800,000 people in North-America alone. Crohn's disease most commonly affects the ileum, and to a lesser extent, the colon, however can be found throughout the entire gastro-intestinal tract (Shanahan, 2002). The cause of this disease is as yet largely unknown, despite tremendous progress in research efforts over the last decade. It is increasingly clear that inflammation and disease progression involves a complex interplay between the environment, host genes and microbes (Baumgart and Carding, 2007). Increasingly, and predominantly based on genomic analyses, involvement of components of the innate immune system have been recognized in inflammatory bowel disease (Baumgart and Sandborn, 2007). The first major susceptibilty locus that was identified for Crohn's disease was the IBD1 locus, encoding nucleotide oligomerization domain 2 or NOD2 (Hugot et al., 2001; Ogura et al., 2001). Various variations in genotypes and single nucleotide polymorphisms have been identifed in NOD2 that are strongly associated with Crohn's disease development (Economou et al., 2004; Lesage et al., 2002). In a recent genomewide study, a total of 71 loci were identified to be associated with Crohn's disease, with the potential involvement of more genes (Franke et al., 2010). Among the genes identified were the autophagy-related 16-1 or *ATG16L1* gene and the interleukin-23 (IL-23) receptor gene. Autophagy is a mechanism that regulates protein degradation and is essential for immune balance. Disturbance of this mechanism may lead to inflammation or disease and therapeutic applications of manipulating this mechanism are under investigation (Fleming et al., 2011). The IL-23 receptor is a key feature of the Th17 subset of T helper cells which are a critical component of the antibacterial defense (Abraham and Cho, 2009). Both IL-17 and the IL-23

receptor ligand are currently targeted for therapy (De Nitto et al., 2010).

The epithelium of the gut is an important component of innate immunity. Epithelial cells perform an essential, yet selective barrier function, physically separating the gut lumen from underlying cells and tissues (Peyrin-Biroulet et al., 2008). This physical barrier limits the exposure of microbes and infectious agents to the underlying mucosal immune system, while at the same time allowing exchange and uptake of fluids and nutrients. More than a physical barrier, the gut epithelium actively participates in host defense. Epithelial cells

**1. Introduction**

### **Manning the Barricades: Role of the Gut Epithelium in Crohn's Disease**

Erik P. Lillehoj1 and Erik P.H. De Leeuw2

*1Department of Pediatrics and 2Institute of Human Virology and Department of Biochemistry & Molecular Biology of the University of Maryland Baltimore School of Medicine, USA*

#### **1. Introduction**

16 Crohn's Disease

Wang F, Schwarz BT, Graham WV, Wang Y, Su L, Clayburgh DR *et al* (2006). IFN-gamma-

barrier dysfunction. *Gastroenterology*, Vol.131, No.4, (pp. 1153-1163. , Warhurst AC, Hopkins SJ, Warhurst G (1998). Interferon gamma induces differential

*Gut*, Vol.42, No.2, (pp. 208-213,

induced TNFR2 expression is required for TNF-dependent intestinal epithelial

upregulation of alpha and beta chemokine secretion in colonic epithelial cell lines.

Crohn's disease is a chronic inflammation of the gut that affects an estimated 800,000 people in North-America alone. Crohn's disease most commonly affects the ileum, and to a lesser extent, the colon, however can be found throughout the entire gastro-intestinal tract (Shanahan, 2002). The cause of this disease is as yet largely unknown, despite tremendous progress in research efforts over the last decade. It is increasingly clear that inflammation and disease progression involves a complex interplay between the environment, host genes and microbes (Baumgart and Carding, 2007). Increasingly, and predominantly based on genomic analyses, involvement of components of the innate immune system have been recognized in inflammatory bowel disease (Baumgart and Sandborn, 2007). The first major susceptibilty locus that was identified for Crohn's disease was the IBD1 locus, encoding nucleotide oligomerization domain 2 or NOD2 (Hugot et al., 2001; Ogura et al., 2001). Various variations in genotypes and single nucleotide polymorphisms have been identifed in NOD2 that are strongly associated with Crohn's disease development (Economou et al., 2004; Lesage et al., 2002). In a recent genomewide study, a total of 71 loci were identified to be associated with Crohn's disease, with the potential involvement of more genes (Franke et al., 2010). Among the genes identified were the autophagy-related 16-1 or *ATG16L1* gene and the interleukin-23 (IL-23) receptor gene. Autophagy is a mechanism that regulates protein degradation and is essential for immune balance. Disturbance of this mechanism may lead to inflammation or disease and therapeutic applications of manipulating this mechanism are under investigation (Fleming et al., 2011). The IL-23 receptor is a key feature of the Th17 subset of T helper cells which are a critical component of the antibacterial defense (Abraham and Cho, 2009). Both IL-17 and the IL-23 receptor ligand are currently targeted for therapy (De Nitto et al., 2010).

The epithelium of the gut is an important component of innate immunity. Epithelial cells perform an essential, yet selective barrier function, physically separating the gut lumen from underlying cells and tissues (Peyrin-Biroulet et al., 2008). This physical barrier limits the exposure of microbes and infectious agents to the underlying mucosal immune system, while at the same time allowing exchange and uptake of fluids and nutrients. More than a physical barrier, the gut epithelium actively participates in host defense. Epithelial cells

Manning the Barricades: Role of the Gut Epithelium in Crohn's Disease 19

optimized for antigen sampling and transport and are in close proximity with underlying

Goblet cells are glandular simple columnar epithelial cells that are found scattered among the epithelia of the intestinal and respiratory tracts, as well as the urogenital, visual, and auditory systems. The primary function of goblet cells is to secrete mucin into the lumen of the gut and airways. The majority of the cytoplasm of goblet cells is occupied by secretory granules containing a variety of proteins that form the mucus layer upon granule exocytosis. Rough endoplasmic reticulum, mitochondria, nucleus, and other organelles are located in the basal portion of the cell. The apical plasma membrane of goblet cells contains microvilli

Mucus comprises a visoelastic layer of fluid that plays an important defensive role against foreign environmental substances. Mucus covers all exposed epithelia of mammals, as well as the epidermis of amphibians and the gills of fish. In addition to trapping and removing foreign substances, mucus serves to lubricate the some epithelial surfaces, principally those of the gastro-intestinal tract. A layer of mucus along the inner walls of the stomach is vital to protect gastric epithelial cells from the highly acidic environment. The average human body produces about one liter of mucus per day (Thorton, 2008). Mucus consists of water, salts and various macromolecules, including mucins, proteinases, proteinase inhibitors, proteoglycans, and defensive proteins. In the latter category are proteins such as lysozyme, lactoferrin, and immunoglobulins. Proper concentrations of these components are required for the optimum function of mucus, and an alteration in the quality or quantity of the

Mucins are the primary protein constituents of mucus (Lillehoj and Kim, 2002). These high molecular weight glycoproteins contain variable numbers of tandem repeats (VNTRs) in which serine, threonine, and/or proline residues are highly enriched. Serines and threonines are responsible for extensive mucin glycosylation that contributes to size and charge heterogeneity of the molecules. Glycosylation within the VNTR takes place between the serine/threonine moieties of the peptide backbone and N-acetylgalactosamine of the oligosaccharides, characteristic of O-linked glycoproteins. In addition, a limited amount of N-linked glycosylation between asparagines residues of the protein backbone and Nacetylglucosamine of the oligosaccharides also are present. Mucins can be broadly classified as either gel-forming/secreted mucins or membrane mucins. Gel-forming mucins are produced by goblet cells and account for the visoelastic property of the mucus layer as a result of protein cross-linking between mucin monomers. Cross-linking occurs following disulfide bonding between cysteine-rich D domains in the NH2- and COOH-termini of the proteins. Membrane mucins are expressed in a polarized fashion on the apical surface of all epithelial cells. Eighteen mucin (MUC) genes have been cloned and the particular distribution of mucin gene expression varies by epithelial type. In the gastro-intestinal tract, 15 mucin glycoproteins are present (McGuckin et al., 2011). These include both gel-forming

individual constituents of mucus may lead to pathological conditions.

components of adaptive immunity.

to increase the surface area for secretion.

**2. Goblet cells**

**2.1 Mucus**

**2.2 Mucins**

form a critical link between mucosal immunity and the microbial intestinal flora via germline encoded receptors and specific signaling pathways (Abreu, 2010; Koch and Nusrat, 2009; Wells et al., 2010). For example, epithelial cells from distinct lineages express NOD2 or ATG16L1, critical for recognition and clearance of intracellular microbes and linked to Crohn's disease as mentioned (Bevins, 2004, 2005; Kaser and Blumberg, 2011). Barrier functions of the gastro-intestinal tract is regulated by chemokines and cytokines released in underlying compartments as well (Zimmerman et al., 2008). The exact sites and mechanisms of how cytokines affect epithelial permeability is not known, however it involves mainly the Th1 cytokines tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ). Therapy directed against these cytokines is currently widely applied in the clinic (Ford et al., 2011). Additionally, specialized epithelial cells have evolved in the gut that are critical in three areas: **1**) Goblet cells secrete mucins and are a source of trefoil peptides, important for mucosal repair (McGuckin et al., 2011); **2**) Paneth cells secrete antimicrobial peptides (Bevins, 2006; Ouellette, 2011); and **3**) M cells transport antigen and micro-organisms, thus sampling the gut lumen (Miller et al., 2007). In this chapter, we will discuss in detail the active barrier function of the indivdual cellular components of the intestinal epithelium in context of immune homeostasis and Crohn's disease.

Fig. 1. Epithelial cells of the ileum.

The ileum is predominantly populated by columnar enterocytes or columnar cells which provide essential barrier function to the gut, separating the lumen from underlying tissue. Specialized goblet cells produce mucus, the first line of defence against microorganisms, but also microhabitat for bacteria. Paneth cells produce a host of antimicrobial factors resulting in a relatively sterile environment in the crypt base. Poliplurent stem cells continually selfreplicate and differentiate to ensure high turnover rate of epithelial cells. M cells are optimized for antigen sampling and transport and are in close proximity with underlying components of adaptive immunity.

#### **2. Goblet cells**

18 Crohn's Disease

form a critical link between mucosal immunity and the microbial intestinal flora via germline encoded receptors and specific signaling pathways (Abreu, 2010; Koch and Nusrat, 2009; Wells et al., 2010). For example, epithelial cells from distinct lineages express NOD2 or ATG16L1, critical for recognition and clearance of intracellular microbes and linked to Crohn's disease as mentioned (Bevins, 2004, 2005; Kaser and Blumberg, 2011). Barrier functions of the gastro-intestinal tract is regulated by chemokines and cytokines released in underlying compartments as well (Zimmerman et al., 2008). The exact sites and mechanisms of how cytokines affect epithelial permeability is not known, however it involves mainly the Th1 cytokines tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ). Therapy directed against these cytokines is currently widely applied in the clinic (Ford et al., 2011). Additionally, specialized epithelial cells have evolved in the gut that are critical in three areas: **1**) Goblet cells secrete mucins and are a source of trefoil peptides, important for mucosal repair (McGuckin et al., 2011); **2**) Paneth cells secrete antimicrobial peptides (Bevins, 2006; Ouellette, 2011); and **3**) M cells transport antigen and micro-organisms, thus sampling the gut lumen (Miller et al., 2007). In this chapter, we will discuss in detail the active barrier function of the indivdual cellular components of the intestinal epithelium in

The ileum is predominantly populated by columnar enterocytes or columnar cells which provide essential barrier function to the gut, separating the lumen from underlying tissue. Specialized goblet cells produce mucus, the first line of defence against microorganisms, but also microhabitat for bacteria. Paneth cells produce a host of antimicrobial factors resulting in a relatively sterile environment in the crypt base. Poliplurent stem cells continually selfreplicate and differentiate to ensure high turnover rate of epithelial cells. M cells are

context of immune homeostasis and Crohn's disease.

Fig. 1. Epithelial cells of the ileum.

Goblet cells are glandular simple columnar epithelial cells that are found scattered among the epithelia of the intestinal and respiratory tracts, as well as the urogenital, visual, and auditory systems. The primary function of goblet cells is to secrete mucin into the lumen of the gut and airways. The majority of the cytoplasm of goblet cells is occupied by secretory granules containing a variety of proteins that form the mucus layer upon granule exocytosis. Rough endoplasmic reticulum, mitochondria, nucleus, and other organelles are located in the basal portion of the cell. The apical plasma membrane of goblet cells contains microvilli to increase the surface area for secretion.

#### **2.1 Mucus**

Mucus comprises a visoelastic layer of fluid that plays an important defensive role against foreign environmental substances. Mucus covers all exposed epithelia of mammals, as well as the epidermis of amphibians and the gills of fish. In addition to trapping and removing foreign substances, mucus serves to lubricate the some epithelial surfaces, principally those of the gastro-intestinal tract. A layer of mucus along the inner walls of the stomach is vital to protect gastric epithelial cells from the highly acidic environment. The average human body produces about one liter of mucus per day (Thorton, 2008). Mucus consists of water, salts and various macromolecules, including mucins, proteinases, proteinase inhibitors, proteoglycans, and defensive proteins. In the latter category are proteins such as lysozyme, lactoferrin, and immunoglobulins. Proper concentrations of these components are required for the optimum function of mucus, and an alteration in the quality or quantity of the individual constituents of mucus may lead to pathological conditions.

#### **2.2 Mucins**

Mucins are the primary protein constituents of mucus (Lillehoj and Kim, 2002). These high molecular weight glycoproteins contain variable numbers of tandem repeats (VNTRs) in which serine, threonine, and/or proline residues are highly enriched. Serines and threonines are responsible for extensive mucin glycosylation that contributes to size and charge heterogeneity of the molecules. Glycosylation within the VNTR takes place between the serine/threonine moieties of the peptide backbone and N-acetylgalactosamine of the oligosaccharides, characteristic of O-linked glycoproteins. In addition, a limited amount of N-linked glycosylation between asparagines residues of the protein backbone and Nacetylglucosamine of the oligosaccharides also are present. Mucins can be broadly classified as either gel-forming/secreted mucins or membrane mucins. Gel-forming mucins are produced by goblet cells and account for the visoelastic property of the mucus layer as a result of protein cross-linking between mucin monomers. Cross-linking occurs following disulfide bonding between cysteine-rich D domains in the NH2- and COOH-termini of the proteins. Membrane mucins are expressed in a polarized fashion on the apical surface of all epithelial cells. Eighteen mucin (MUC) genes have been cloned and the particular distribution of mucin gene expression varies by epithelial type. In the gastro-intestinal tract, 15 mucin glycoproteins are present (McGuckin et al., 2011). These include both gel-forming

Manning the Barricades: Role of the Gut Epithelium in Crohn's Disease 21

to the mucus gel overlying the intestinal epithelium. In mice, the cysteine-rich EGF-like domains inhibit apoptosis and stimulate cell migration, implying a regulatory role in

MUC1 was the first mucin gene to be cloned (Gendler et al., 1990; Lan et al., 1990). Several studies have provided evidence that MUC1 plays a critical role in the intestinal tract. First, mice deficient in MUC1 expression have reduced amounts of intestinal mucus (Parmley and Gendler, 1998). Second, lack of intestinal MUC1 mucin in knockout mice impairs cholesterol uptake and absorption (Wang et al., 2004). Similar to MUC3, MUC1 consists of a large extracellular domain which is heavily glycosylated through N-acetylgalactosamine Olinkages, a single-pass transmembrane region, and a cytoplasmic CT (Figure 3). The MUC1 ectodomain serves as a binding site for pathogenic microorganisms, including *Pseudomonas aeruginosa* (Kato et al.; Lillehoj et al., 2001), *Helicobacter pyori* (Linden et al., 2004; Linden et al., 2009), *Campylobacter jejuni* (McAuley et al., 2007), *Escherichia coli* (Parker et al., 2010; Sando et al., 2009), and *Salmonella enterica* (Parker et al., 2010). During intracellular biosynthesis, the MUC1 ectodomain is autoproteolytically cleaved in its SEA domain to yield two noncovalently associated protein chains. The 72-amino acid CT domain of MUC1 contains 7 evolutionally conserved tyrosine residues. Many of these tyrosines are phosphorylated, leading to MUC1 interaction with receptor and cytosolic kinases as well as various adapter

5' 3'

1 2 3 4 56 7

NH2 VNTR CT COOH <sup>T</sup> L SEA <sup>M</sup>

Upper, the MUC1 gene is located on chromosome 1q21 between the genes for thrombospondin 3 (THBS3) and tripartite motif containing 46 (TRIM46). Lower, the seven exon structure of the MUC1 gene and corresponding protein regions. L, leader peptide; VNTR, variable number of tandem repeats;

SEA, sperm protein, enterokinase, and agrin; TM, transmembrane; CT, cytoplasmic tail.

Fig. 3. Genomic organization of MUC1.

*TRIM46*

maintaining the structure and function of the intestinal epithelial layer.

**2.2.3 MUC1, a membrane mucin with signaling potential**

(MUC2, MUC5AC, MUC5B, MUC7, and MUC19) and membrane (MUC1, MUC3, MUC4, MUC12, MUC13, MUC15, MUC16, and MUC17) mucins.

#### **2.2.1 MUC2, the major gel-forming mucin of the intestinal tract**

MUC2 is the major component of the secreted mucus barrier in the small and large intestines (Figure 2). MUC2 knockout mice spontaneously develop colitis, indicating that MUC2 is critical for colonic protection (Van der Sluis et al., 2006). The MUC2 gene product is a very large, greater than 5,100 amino acids in length, and contains two VNTRs with different amino acid sequences (Gum et al., 1994). The VNTR domain contains 50 -100 threonine/proline-rich 23 amino acid continuous repeats, while the second is composed of a 347 residue irregular and discontinuous serine/threonine/proline-rich repeat. MUC2 contains four cysteine-rich D domains, three located at the NH2-terminus and the fourth at the COOH-terminus of the protein. This D domain organization is similar to that seen in von Willebrand factor, a glycoprotein involved in hemostasis. The MUC2 D domains contain a characteristic -cysteine-X-X-cysteine- sequence (where X is any amino acid) that mediates mucin oligomerization through disulfide bonding. The glycan moieties of MUC2 contain an equal fraction of neutral (40%) and sialylated (40%) residues with the remainder being sulphated (Karlsson et al., 1996). Mass spectrometry identified the sulfate group attached to C-6 of the N-acetylglucosamine moiety.

Upper, MUC2 cDNA. D1, D2, D3 = dimerization domains; yellow = cysteine-rich regions; blue = cysteine knot. Lower, MUC2 protein. Tan = non-repeat NH2–terminal region; red = VNTRs; green = non-repeat COOH-terminal region.

Fig. 2. Schematic structure of MUC2.

#### **2.2.2 MUC3, the major membrane mucin of the intestinal tract**

MUC3 is the most abundantly expressed membrane mucin in the small intestine (Kim and Ho). Here, MUC3 expression on epithelial cells shows a maturational gradient with increasing expression from the crypt to villus. The MUC3 protein consists of two subunits, an extracellular region containing heavily O-glycosylated VNTR domains and two epidermal growth factor (EGF)-like domains. The EGF-like regions are separated by a SEA (sperm protein, enterokinase, and agrin) module, containing a proteolytic cleavage site during biosynthesis. A membrane-spanning, hydrophilic region that is responsible for incorporation of MUC3 into the lipid bilayer and an intracellular cytoplasmic tail (CT) with potential phosphorylation sites involved in signalling, lie distal to the SEA domain. The MUC3 ectodomain may be shed from the cell surface by the activation of membraneassociated metalloproteinases, by the separation of two subunits in the SEA domain, or by alternative splicing of its mRNA. Despite the mechanism involved, shed MUC3 contributes

(MUC2, MUC5AC, MUC5B, MUC7, and MUC19) and membrane (MUC1, MUC3, MUC4,

MUC2 is the major component of the secreted mucus barrier in the small and large intestines (Figure 2). MUC2 knockout mice spontaneously develop colitis, indicating that MUC2 is critical for colonic protection (Van der Sluis et al., 2006). The MUC2 gene product is a very large, greater than 5,100 amino acids in length, and contains two VNTRs with different amino acid sequences (Gum et al., 1994). The VNTR domain contains 50 -100 threonine/proline-rich 23 amino acid continuous repeats, while the second is composed of a 347 residue irregular and discontinuous serine/threonine/proline-rich repeat. MUC2 contains four cysteine-rich D domains, three located at the NH2-terminus and the fourth at the COOH-terminus of the protein. This D domain organization is similar to that seen in von Willebrand factor, a glycoprotein involved in hemostasis. The MUC2 D domains contain a characteristic -cysteine-X-X-cysteine- sequence (where X is any amino acid) that mediates mucin oligomerization through disulfide bonding. The glycan moieties of MUC2 contain an equal fraction of neutral (40%) and sialylated (40%) residues with the remainder being sulphated (Karlsson et al., 1996). Mass spectrometry identified the sulfate group attached to

**D1 D2 D3 D4**

Upper, MUC2 cDNA. D1, D2, D3 = dimerization domains; yellow = cysteine-rich regions; blue = cysteine knot. Lower, MUC2 protein. Tan = non-repeat NH2–terminal region; red = VNTRs;

**2.2.2 MUC3, the major membrane mucin of the intestinal tract**

**Tandem repeats**

**NH2 COOH**

MUC3 is the most abundantly expressed membrane mucin in the small intestine (Kim and Ho). Here, MUC3 expression on epithelial cells shows a maturational gradient with increasing expression from the crypt to villus. The MUC3 protein consists of two subunits, an extracellular region containing heavily O-glycosylated VNTR domains and two epidermal growth factor (EGF)-like domains. The EGF-like regions are separated by a SEA (sperm protein, enterokinase, and agrin) module, containing a proteolytic cleavage site during biosynthesis. A membrane-spanning, hydrophilic region that is responsible for incorporation of MUC3 into the lipid bilayer and an intracellular cytoplasmic tail (CT) with potential phosphorylation sites involved in signalling, lie distal to the SEA domain. The MUC3 ectodomain may be shed from the cell surface by the activation of membraneassociated metalloproteinases, by the separation of two subunits in the SEA domain, or by alternative splicing of its mRNA. Despite the mechanism involved, shed MUC3 contributes

MUC12, MUC13, MUC15, MUC16, and MUC17) mucins.

C-6 of the N-acetylglucosamine moiety.

green = non-repeat COOH-terminal region. Fig. 2. Schematic structure of MUC2.

**2.2.1 MUC2, the major gel-forming mucin of the intestinal tract**

to the mucus gel overlying the intestinal epithelium. In mice, the cysteine-rich EGF-like domains inhibit apoptosis and stimulate cell migration, implying a regulatory role in maintaining the structure and function of the intestinal epithelial layer.

#### **2.2.3 MUC1, a membrane mucin with signaling potential**

MUC1 was the first mucin gene to be cloned (Gendler et al., 1990; Lan et al., 1990). Several studies have provided evidence that MUC1 plays a critical role in the intestinal tract. First, mice deficient in MUC1 expression have reduced amounts of intestinal mucus (Parmley and Gendler, 1998). Second, lack of intestinal MUC1 mucin in knockout mice impairs cholesterol uptake and absorption (Wang et al., 2004). Similar to MUC3, MUC1 consists of a large extracellular domain which is heavily glycosylated through N-acetylgalactosamine Olinkages, a single-pass transmembrane region, and a cytoplasmic CT (Figure 3). The MUC1 ectodomain serves as a binding site for pathogenic microorganisms, including *Pseudomonas aeruginosa* (Kato et al.; Lillehoj et al., 2001), *Helicobacter pyori* (Linden et al., 2004; Linden et al., 2009), *Campylobacter jejuni* (McAuley et al., 2007), *Escherichia coli* (Parker et al., 2010; Sando et al., 2009), and *Salmonella enterica* (Parker et al., 2010). During intracellular biosynthesis, the MUC1 ectodomain is autoproteolytically cleaved in its SEA domain to yield two noncovalently associated protein chains. The 72-amino acid CT domain of MUC1 contains 7 evolutionally conserved tyrosine residues. Many of these tyrosines are phosphorylated, leading to MUC1 interaction with receptor and cytosolic kinases as well as various adapter

Upper, the MUC1 gene is located on chromosome 1q21 between the genes for thrombospondin 3 (THBS3) and tripartite motif containing 46 (TRIM46). Lower, the seven exon structure of the MUC1 gene and corresponding protein regions. L, leader peptide; VNTR, variable number of tandem repeats; SEA, sperm protein, enterokinase, and agrin; TM, transmembrane; CT, cytoplasmic tail.

Fig. 3. Genomic organization of MUC1.

Manning the Barricades: Role of the Gut Epithelium in Crohn's Disease 23

Trefoil peptides, or trefoil factors (TFFs), are a group of molecules that are characterized by having at least one copy of the trefoil motif, a 40-amino acid domain that contains three conserved disulfide bonds (Wong et al., 1999). Trefiol peptides are stable secretory proteins expressed in the gastro-intestinal tract. Their functions are not well defined, but they may protect the mucosa from insults, stabilize the mucus layer, and regulate healing of the epithelium. The close physical association between trefoil peptides and mucins supports these possible roles. The trefoil domain is found in a variety of extracellular eukaryotic proteins, including TFF1 (or protein pS), a protein secreted by the stomach mucosa, TFF2 (or spasmolytic polypeptide), a protein of about 115 residues that inhibits gastro-intestinal motility, and TFF3 (or intestinal trefoil factor, ITF). Other proteins with trefoil domains are *Xenopus laevis* stomach proteins xP1 and xP4, *Xenopus* integumentary mucins A.1 and C.1, *Xenopus* skin protein xp2, zona pellucida sperm-binding protein B (ZP-B), and intestinal sucrase-isomaltase. TFF1 and TFF3 contain one trefoil domain, TFF2 contains two domains, and the *Xenopus* proteins contain multiple copies. All three human proteins are clustered on chromosome 21q22.3. Overexpression of human TFF1 in mice was reported to reduce their susceptibility to dextran sodium sulfate (DSS)-induced colitis and TFF-deficient mice exhibited increased disease susceptibility (Mashimo et al., 1996; Playford et al., 1996). Unfortunately, however, these animal studies have not been translated into an effective

Paneth cells are specialized intestinal epithelial cells located at the base of ileal crypts in healthy individuals (Bevins, 2004; Ouellette, 2011). These cells are pivotal in maintaining the balance between the host and the microbiome. These cells act as sentinels for the detection of microbial molecules which are recognized by Toll-like receptors (TLRs), germ-line encoded receptors specific for bacterial and viral antigens. Genetic polymorphisms in these receptors and their signaling pathways affect Paneth cell function and have been associated with Crohn's disease (Inohara et al., 2005; Kobayashi et al., 2005). Paneth cell function is regulated by two additional mechanisms, the so-called unfolded protein response or UPR and autophagy, a process involved in clearance of intracellular microbes. The process of autophagy is induced by stress in the endoplasmatic reticulum (ER), which in turn is activated by UPR. Genetic mutations in proteins involved in both of these mechanisms have been linked to Crohn's disease as well. Variations in the autophagy protein ATG16L1 were identified in genome-wide studies and found to be associated with increased risk of disease development (Hampe et al., 2007; Rioux et al., 2007). Alterations in the gene encoding the UPR transcription factor protein Xbox-binding protein 1 or XBP-1 are signifiantly associated with inflammatory bowel disease in humans (Kaser et al., 2008). Further, loss of XBP-1 decreases the number of Paneth cells and thus the antimicrobial capacity of the intestine and leads to spontaneous enteritis in mice (Kaser and Blumberg, 2009; Kaser et al., 2008). Paneth cells are equipped with a vast arsenal of antimicrobial agents which are deployed following the recognition of potential microbial threats. These include enzymes, such as lysozyme , trypsin, phospholipase A2 and matrix metalloproteases, cytokines such as TNF-α and IL-17, as well as the bactericidal defensin peptides (Figure 4). In the following sections, we will discuss in detail the role of defensins as effectors of the innate immune system and their

**2.5 Trefoil peptides**

clinical therapy (Mahmood et al., 2005).

involvement in epithelial mucosal barrier function.

**3. Paneth cell**

proteins, including phosphoinositide 3-kinase (PI3K), Shc, phospholipase C-γ (PLC-γ), c-Src, and Grb-2 (Hattrup and Gendler, 2008; Theodoropoulos and Carraway, 2007). Binding of PI3K, c-Src, and Grb-2 to the CT have been experimentally verified, while Shc and PLC-γ are only inferred based upon the presence of the predicted amino acid sequence motifs. Other proteins bind to non-tyrosine sites, including glycogen synthase kinase 3β (GSK3β), protein kinase C-δ (PKC-δ), and β-catenin. Consensus sequences resembling an ITAM (immunoreceptor tyrosine-based activation motif) and ITIM (immunoreceptor tyrosinebased inhibitory motif) are also present in the MUC1 CT region. Estrogen receptor α (ERα), p53, p120ctn, ErbB1-4, adenomatous polyposis coli (APC), heat shock protein 70 (Hsp70), and Hsp90 also have been reported as binding partners of the CT, but specific amino acid residues have not been identified. Analysis of downstream signaling events indicated that the MUC1 CT activated a Ras → MEK1/2 → ERK1/2 pathway, but the mechanism is unclear.

#### **2.3 Mucus proteoglycans**

Proteoglycans are large molecular weight glycoconjugates characterized by variable numbers of glycan repeats (Meisenberg, 2006). The basic proteoglycan unit consists of a core protein with one or more covalently attached glycosaminoglycan chain(s) to a serine residue. The serine residue is generally in the sequence -serine-glycine-X-glycine-, although not every protein with this sequence has an attached glycan moiety. The chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions, due to the occurrence of sulfate and uronic acid groups. As a result of the later modifications, proteoglycans are highly acidic in physiologic conditions allowing them to bind to cations, such as Na+, K+, and Ca2+. Three types of proteoglycans were shown to be secreted into mucus by epithelial cells cultured *in vitro*, hyaluronic acid containing proteoglycans, chondroitin sulfate containing proteoglycans and heparan sulfate containing proteoglycans (Kim, 1985; Paul et al., 1988; Wu et al., 1985). While the physiologic roles of proteoglycans in mucus remain largely unknown, suggested functions include epithelial development, remodeling, inflammation, and host defense (Forteza et al., 2001; Huang et al., 1999; Ohkawara et al., 2000; Zhao et al., 1999).

#### **2.4 Mucus proteinases and proteinase inhibitors**

A number of proteinases are present in mucus, all of which known to be associated with inflammation and derived from inflammatory cells. Among these are elastase and various cathepsins from neutrophils and chymase and tryptase from mast cells. Neutrophil elastase has been shown to cause destruction of elastin (Snider et al., 1984), stimulate mucin release from goblet cells (Kim et al., 1987), and induce chemotaxis via production of IL-8 by the underlying epithelial cells ((Nakamura et al., 1992). Excess elastase released from neutrophils during injury and inflammation is balanced by several proteinase inhibitors, including α1-anti-trypsin, soluble leukocyte protease inhibitor (sLPI), and elafin ((Perlmutter and Pierce, 1989; Sallenave et al., 1993; Thompson and Ohlsson, 1986). Attenuated induction of sLPI and elafin has been reported in Crohn's disease (Schmid et al., 2007). Chymase and trypase are proteinases produced by mast cells, the former being responsible for disruption of the epithelial cell barrier allowing antigens and inflammatory mediators to enter the intestinal mucosa, while the latter is responsible for stimulating mucus secretion as well as TGF-β release from the extracellular matrix (Sommerhoff et al., 1990; Taipale et al., 1995).

#### **2.5 Trefoil peptides**

22 Crohn's Disease

proteins, including phosphoinositide 3-kinase (PI3K), Shc, phospholipase C-γ (PLC-γ), c-Src, and Grb-2 (Hattrup and Gendler, 2008; Theodoropoulos and Carraway, 2007). Binding of PI3K, c-Src, and Grb-2 to the CT have been experimentally verified, while Shc and PLC-γ are only inferred based upon the presence of the predicted amino acid sequence motifs. Other proteins bind to non-tyrosine sites, including glycogen synthase kinase 3β (GSK3β), protein kinase C-δ (PKC-δ), and β-catenin. Consensus sequences resembling an ITAM (immunoreceptor tyrosine-based activation motif) and ITIM (immunoreceptor tyrosinebased inhibitory motif) are also present in the MUC1 CT region. Estrogen receptor α (ERα), p53, p120ctn, ErbB1-4, adenomatous polyposis coli (APC), heat shock protein 70 (Hsp70), and Hsp90 also have been reported as binding partners of the CT, but specific amino acid residues have not been identified. Analysis of downstream signaling events indicated that the MUC1 CT activated a Ras → MEK1/2 → ERK1/2 pathway, but the mechanism is

Proteoglycans are large molecular weight glycoconjugates characterized by variable numbers of glycan repeats (Meisenberg, 2006). The basic proteoglycan unit consists of a core protein with one or more covalently attached glycosaminoglycan chain(s) to a serine residue. The serine residue is generally in the sequence -serine-glycine-X-glycine-, although not every protein with this sequence has an attached glycan moiety. The chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions, due to the occurrence of sulfate and uronic acid groups. As a result of the later modifications, proteoglycans are highly acidic in physiologic conditions allowing them to bind to cations, such as Na+, K+, and Ca2+. Three types of proteoglycans were shown to be secreted into mucus by epithelial cells cultured *in vitro*, hyaluronic acid containing proteoglycans, chondroitin sulfate containing proteoglycans and heparan sulfate containing proteoglycans (Kim, 1985; Paul et al., 1988; Wu et al., 1985). While the physiologic roles of proteoglycans in mucus remain largely unknown, suggested functions include epithelial development, remodeling, inflammation, and host defense (Forteza et al., 2001; Huang et al.,

A number of proteinases are present in mucus, all of which known to be associated with inflammation and derived from inflammatory cells. Among these are elastase and various cathepsins from neutrophils and chymase and tryptase from mast cells. Neutrophil elastase has been shown to cause destruction of elastin (Snider et al., 1984), stimulate mucin release from goblet cells (Kim et al., 1987), and induce chemotaxis via production of IL-8 by the underlying epithelial cells ((Nakamura et al., 1992). Excess elastase released from neutrophils during injury and inflammation is balanced by several proteinase inhibitors, including α1-anti-trypsin, soluble leukocyte protease inhibitor (sLPI), and elafin ((Perlmutter and Pierce, 1989; Sallenave et al., 1993; Thompson and Ohlsson, 1986). Attenuated induction of sLPI and elafin has been reported in Crohn's disease (Schmid et al., 2007). Chymase and trypase are proteinases produced by mast cells, the former being responsible for disruption of the epithelial cell barrier allowing antigens and inflammatory mediators to enter the intestinal mucosa, while the latter is responsible for stimulating mucus secretion as well as TGF-β release from the extracellular matrix (Sommerhoff et al.,

unclear.

**2.3 Mucus proteoglycans**

1990; Taipale et al., 1995).

1999; Ohkawara et al., 2000; Zhao et al., 1999).

**2.4 Mucus proteinases and proteinase inhibitors**

Trefoil peptides, or trefoil factors (TFFs), are a group of molecules that are characterized by having at least one copy of the trefoil motif, a 40-amino acid domain that contains three conserved disulfide bonds (Wong et al., 1999). Trefiol peptides are stable secretory proteins expressed in the gastro-intestinal tract. Their functions are not well defined, but they may protect the mucosa from insults, stabilize the mucus layer, and regulate healing of the epithelium. The close physical association between trefoil peptides and mucins supports these possible roles. The trefoil domain is found in a variety of extracellular eukaryotic proteins, including TFF1 (or protein pS), a protein secreted by the stomach mucosa, TFF2 (or spasmolytic polypeptide), a protein of about 115 residues that inhibits gastro-intestinal motility, and TFF3 (or intestinal trefoil factor, ITF). Other proteins with trefoil domains are *Xenopus laevis* stomach proteins xP1 and xP4, *Xenopus* integumentary mucins A.1 and C.1, *Xenopus* skin protein xp2, zona pellucida sperm-binding protein B (ZP-B), and intestinal sucrase-isomaltase. TFF1 and TFF3 contain one trefoil domain, TFF2 contains two domains, and the *Xenopus* proteins contain multiple copies. All three human proteins are clustered on chromosome 21q22.3. Overexpression of human TFF1 in mice was reported to reduce their susceptibility to dextran sodium sulfate (DSS)-induced colitis and TFF-deficient mice exhibited increased disease susceptibility (Mashimo et al., 1996; Playford et al., 1996). Unfortunately, however, these animal studies have not been translated into an effective clinical therapy (Mahmood et al., 2005).

#### **3. Paneth cell**

Paneth cells are specialized intestinal epithelial cells located at the base of ileal crypts in healthy individuals (Bevins, 2004; Ouellette, 2011). These cells are pivotal in maintaining the balance between the host and the microbiome. These cells act as sentinels for the detection of microbial molecules which are recognized by Toll-like receptors (TLRs), germ-line encoded receptors specific for bacterial and viral antigens. Genetic polymorphisms in these receptors and their signaling pathways affect Paneth cell function and have been associated with Crohn's disease (Inohara et al., 2005; Kobayashi et al., 2005). Paneth cell function is regulated by two additional mechanisms, the so-called unfolded protein response or UPR and autophagy, a process involved in clearance of intracellular microbes. The process of autophagy is induced by stress in the endoplasmatic reticulum (ER), which in turn is activated by UPR. Genetic mutations in proteins involved in both of these mechanisms have been linked to Crohn's disease as well. Variations in the autophagy protein ATG16L1 were identified in genome-wide studies and found to be associated with increased risk of disease development (Hampe et al., 2007; Rioux et al., 2007). Alterations in the gene encoding the UPR transcription factor protein Xbox-binding protein 1 or XBP-1 are signifiantly associated with inflammatory bowel disease in humans (Kaser et al., 2008). Further, loss of XBP-1 decreases the number of Paneth cells and thus the antimicrobial capacity of the intestine and leads to spontaneous enteritis in mice (Kaser and Blumberg, 2009; Kaser et al., 2008). Paneth cells are equipped with a vast arsenal of antimicrobial agents which are deployed following the recognition of potential microbial threats. These include enzymes, such as lysozyme , trypsin, phospholipase A2 and matrix metalloproteases, cytokines such as TNF-α and IL-17, as well as the bactericidal defensin peptides (Figure 4). In the following sections, we will discuss in detail the role of defensins as effectors of the innate immune system and their involvement in epithelial mucosal barrier function.

Manning the Barricades: Role of the Gut Epithelium in Crohn's Disease 25

immature dendritic cells and memory T lymphocytes (Yang et al., 1999; Yang et al., 2001).  This interaction mediated signaling for dendritic cell maturation and triggered a polarized immune response *in vivo* (Biragyn et al., 2002). In the case of human defensin-2 (HBD-2), the observed chemotaxis of immature dendritic cells and memory T cells was shown to result from directly binding the chemokine receptor CCR6 (Yang et al., 1999). Subsequently, a murine -defensin was shown to recruit tumor-infiltrating dendritic cell precursors through CCR6 also (Conejo-Garcia et al., 2004). In contrast to these earlier studies, it was reported recently that -defensins chemoattract mast cells and macrophages but not dendritic cells and lymphocytes and that CCR6 was not involved (Soruri et al., 2007). Specific receptors for the chemotactic activity of -defensins have not been identified. Several studies however have shown that also for -defensins this activity is blocked by pertussis toxin, indicating the involvement of G*<sup>i</sup>*�-coupled receptors (Chertov et al., 1996;

There is increasing evidence that aberrant defensin expression is correlated to inflammation of the gastro-intestinal tract. A specific deficiency of the enteric -defensin HD-5 was observed in patients suffering from ileal Crohn's disease (Wehkamp et al., 2005b). Interestingly, the HD-5 deficiency was more pronounced in patients carrying loss-offunction mutations in the cellular receptor NOD2, an intracellular receptor for the bacterial peptidoglycan component muramyl dipeptide (Inohara et al., 2005). NOD2 is predominantly expressed in the distal part of the ileum in a number of cell types including Paneth cells, which are the sole source of HD-5 (Bevins, 2006; Porter et al., 2002). In addition to recognition of bacterial ligands, NOD2 monitors the expression of enteric -defensins. Genetic polymorphisms in the *NOD2/CARD15* gene have been identified to be tightly linked with susceptibility to Crohn's disease (Hugot et al., 2001; Ogura et al., 2001) and with

A number of recent animal model studies have underscored the importance of NOD2 and defensin expression in relation to infection. Compared with wild-type mice, NOD2 deficient mice showed reduced expression of certain -defensins, resulting in increased susceptibility to oral infection by *Listeria monocytogenes* (Kobayashi et al., 2005). Similarly, mice that lack mature cryptdins (the murine orthologue for -defensins) are more susceptible to ileal colonization by non-invasive *Escherichia coli* (Wilson et al., 1999). Paneth cell expression of HD-5 rendered mice markedly resistant to oral, but not peritoneal, challenge with a virulent strain of *Salmonella typhymurium* (Salzman et al., 2003). Interestingly, HD-5 transgenic mice showed a striking loss of segmented filamentous bacteria and had fewer IL-17-producing lamina propria T cells (Salzman et al., 2010). These findings are in support of the notion that defensin deficiency may alter the microbiome, which in turn affects the adaptive immune response of the host. IL-17-producing T cells, however, were also observed in wild-type mice with functional defensins, in the specific absence of this class of bacteria. Additionally, HD-5 was shown to slightly improve mortality in lethal DSS-induced colitis in mice by intraperitoneal injection; however no effect on disease was noted when the defensin was administered orally (Ishikawa et al., 2009). This may suggest that HD-5 directly affects

components of adaptive immunity in addition to affecting the microbiome.

Yang et al., 2000).

decreased defensin expression.

**3.1.1 Alpha-Defensins and gastro-intestinal inflammation**

(Left) Light microscope image of isolated human ileal crypt. Paneth cells are localized at the base of the crypt as indicated. The box on the right lists confirmed compounds localized in dense secretory granules.

Fig. 4. Localization and cellular contents of Paneth cells.

#### **3.1 Defensins**

Defensins constitute a major family of antimicrobial peptides that play a protective role against microbial invasion of various epithelial surfaces, including the skin, respiratory tract and gastro-intestinal tract. Primarily, these small cationic peptides act as effectors of the innate immune system with the ability to kill a variety of microbial pathogens, including bacteria, fungi and viruses (Ganz, 2003; Zasloff, 2002). Based on a difference in disulfide connectivity of six conserved cysteine residues, defensins have been divided in two families, termed and . Both families are believed to have evolved from a common ancestral defensin gene (Patil et al., 2004; Schutte et al., 2002), and share similar tertiary structures despite low amino acid sequence identity (Hill et al., 1991; Pazgiera et al., 2006; Szyk et al., 2006). In humans, -defensins are widely expressed in epithelial cells. Defensins of the family are expressed predominantly in neutrophils (termed human neutrophil peptides, or HNPs) or in ileal Paneth cells in the case of Human Defensin 5 and 6 (HD-5 and HD-6) (Porter et al., 2002; Selsted and Ouellette, 2005).

In addition to their antimicrobial activities, increasing evidence suggests that defensins play a significant role in innate and adaptive immunity. Such functions include chemoattraction and immune cell activation and promotion of cell proliferation, often involving interactions with cellular receptors (Aarbiou et al., 2002; Biragyn et al., 2002; Grigat et al., 2007; Yang et al., 1999). The capacity to chemoattractant monocytes was first described for HNPs (Territo et al., 1989). Subsequently, HNPs were shown to chemoattract different subsets of T lymphocytes and immature dendritic cells (Chertov et al., 1997; Yang et al., 2000). Similar functions were reported for -defensins, which were shown to selectively chemoattract

(Left) Light microscope image of isolated human ileal crypt. Paneth cells are localized at the base of the crypt as indicated. The box on the right lists confirmed compounds localized in dense secretory

Defensins constitute a major family of antimicrobial peptides that play a protective role against microbial invasion of various epithelial surfaces, including the skin, respiratory tract and gastro-intestinal tract. Primarily, these small cationic peptides act as effectors of the innate immune system with the ability to kill a variety of microbial pathogens, including bacteria, fungi and viruses (Ganz, 2003; Zasloff, 2002). Based on a difference in disulfide connectivity of six conserved cysteine residues, defensins have been divided in two families, termed and . Both families are believed to have evolved from a common ancestral defensin gene (Patil et al., 2004; Schutte et al., 2002), and share similar tertiary structures despite low amino acid sequence identity (Hill et al., 1991; Pazgiera et al., 2006; Szyk et al., 2006). In humans, -defensins are widely expressed in epithelial cells. Defensins of the family are expressed predominantly in neutrophils (termed human neutrophil peptides, or HNPs) or in ileal Paneth cells in the case of Human Defensin 5 and 6 (HD-5 and HD-6)

In addition to their antimicrobial activities, increasing evidence suggests that defensins play a significant role in innate and adaptive immunity. Such functions include chemoattraction and immune cell activation and promotion of cell proliferation, often involving interactions with cellular receptors (Aarbiou et al., 2002; Biragyn et al., 2002; Grigat et al., 2007; Yang et al., 1999). The capacity to chemoattractant monocytes was first described for HNPs (Territo et al., 1989). Subsequently, HNPs were shown to chemoattract different subsets of T lymphocytes and immature dendritic cells (Chertov et al., 1997; Yang et al., 2000). Similar functions were reported for -defensins, which were shown to selectively chemoattract

Fig. 4. Localization and cellular contents of Paneth cells.

(Porter et al., 2002; Selsted and Ouellette, 2005).

granules.

**3.1 Defensins**

immature dendritic cells and memory T lymphocytes (Yang et al., 1999; Yang et al., 2001).  This interaction mediated signaling for dendritic cell maturation and triggered a polarized immune response *in vivo* (Biragyn et al., 2002). In the case of human defensin-2 (HBD-2), the observed chemotaxis of immature dendritic cells and memory T cells was shown to result from directly binding the chemokine receptor CCR6 (Yang et al., 1999). Subsequently, a murine -defensin was shown to recruit tumor-infiltrating dendritic cell precursors through CCR6 also (Conejo-Garcia et al., 2004). In contrast to these earlier studies, it was reported recently that -defensins chemoattract mast cells and macrophages but not dendritic cells and lymphocytes and that CCR6 was not involved (Soruri et al., 2007). Specific receptors for the chemotactic activity of -defensins have not been identified. Several studies however have shown that also for -defensins this activity is blocked by pertussis toxin, indicating the involvement of G*<sup>i</sup>*�-coupled receptors (Chertov et al., 1996; Yang et al., 2000).

#### **3.1.1 Alpha-Defensins and gastro-intestinal inflammation**

There is increasing evidence that aberrant defensin expression is correlated to inflammation of the gastro-intestinal tract. A specific deficiency of the enteric -defensin HD-5 was observed in patients suffering from ileal Crohn's disease (Wehkamp et al., 2005b). Interestingly, the HD-5 deficiency was more pronounced in patients carrying loss-offunction mutations in the cellular receptor NOD2, an intracellular receptor for the bacterial peptidoglycan component muramyl dipeptide (Inohara et al., 2005). NOD2 is predominantly expressed in the distal part of the ileum in a number of cell types including Paneth cells, which are the sole source of HD-5 (Bevins, 2006; Porter et al., 2002). In addition to recognition of bacterial ligands, NOD2 monitors the expression of enteric -defensins. Genetic polymorphisms in the *NOD2/CARD15* gene have been identified to be tightly linked with susceptibility to Crohn's disease (Hugot et al., 2001; Ogura et al., 2001) and with decreased defensin expression.

A number of recent animal model studies have underscored the importance of NOD2 and defensin expression in relation to infection. Compared with wild-type mice, NOD2 deficient mice showed reduced expression of certain -defensins, resulting in increased susceptibility to oral infection by *Listeria monocytogenes* (Kobayashi et al., 2005). Similarly, mice that lack mature cryptdins (the murine orthologue for -defensins) are more susceptible to ileal colonization by non-invasive *Escherichia coli* (Wilson et al., 1999). Paneth cell expression of HD-5 rendered mice markedly resistant to oral, but not peritoneal, challenge with a virulent strain of *Salmonella typhymurium* (Salzman et al., 2003). Interestingly, HD-5 transgenic mice showed a striking loss of segmented filamentous bacteria and had fewer IL-17-producing lamina propria T cells (Salzman et al., 2010). These findings are in support of the notion that defensin deficiency may alter the microbiome, which in turn affects the adaptive immune response of the host. IL-17-producing T cells, however, were also observed in wild-type mice with functional defensins, in the specific absence of this class of bacteria. Additionally, HD-5 was shown to slightly improve mortality in lethal DSS-induced colitis in mice by intraperitoneal injection; however no effect on disease was noted when the defensin was administered orally (Ishikawa et al., 2009). This may suggest that HD-5 directly affects components of adaptive immunity in addition to affecting the microbiome.

Manning the Barricades: Role of the Gut Epithelium in Crohn's Disease 27

the expression of TLRs, integrins and microbial adhesion molecules such as galectin-9 (Kyd

Microfold (M) cells specialize in antigen sampling of the gut lumen and act as a selective conduit to underlying components of adaptive immunity without compromising epithelial barrier function.

M cells do not harbor many lysosomes and do not express major histocompatibility (MHC) class II molecules, suggesting that most antigens that are transported are not degraded (Owen et al., 1986; Pickard and Chervonsky, 2010). Because of their relatively weak defenses compared with other sites of the ileal mucosa, M cells are exploited by pathogens as a potential entry site for infection. Such pathogens include EHEC and EPEC strains of *Escherichia coli* (Fitzhenry et al., 2002; Phillips et al., 2000), as well as *Shigella flexneri* and *Salmonella typhymurium* (Jensen et al., 1998). Viruses may also use M cells as a point of entry and specific receptors for HIV (Fotopoulos et al., 2002) and reovirus (Helander et al., 2003)

It is technically challenging to study human M cells *in vitro*, mainly because of the absence of clear cellular markers. Differentiation of enterocytes into M cells likely requires epithelial cell-T lymphocyte cross-talk as indicated by a co-culture model of these two types of cells (Kerneis et al., 1997). Most of our current knowledge on M cells and their role in gastrointestinal disease comes from animal studies. Various models of chemically induced intestinal inflammation have been used to study M cells, the FAE and interplay with the underlying Peyer's patches. In an indomethacin-induced enteritis model in rats, M cell numbers increased initially and showed increased apoptosis in inflamed tissue only (Kucharzik et al., 2000; Lugering et al., 2004). In the DSS-induced model of colitis in mice, increased severity of disease was associated with lack of both Peyer's patches and lymph

Fig. 5. Structure of the follicle-associated epithelium.

**4.1 M cells and gastro-intestinal inflammation**