The Roles of Interleukin-17 and T Helper 17 Cells in Intestinal Barrier Function

This book is dedicated to inflammatory bowel disease, and the authors discuss the advances in the pathogenesis of inflammatory bowel disease, as well as several new parameters involved in the etiopathogeny of Crohn's disease and ulcerative colitis, such as intestinal barrier dysfunction and the roles of TH 17 cells and IL 17 in the immune response in inflammatory bowel disease. The book also focuses on several relevant clinical points, such as pregnancy during inflammatory bowel disease and the health-related quality of life as an end point of the different treatments of the diseases. Finally, advances in management of patients with inflammatory bowel disease are discussed, especially in a complete review of the recent literature.


Introduction
Inflammatory bowel diseases (IBD) are caused by chronic inflammation of the gastrointestinal tract, affecting as many as 1.4 million persons in the United States, and 2.2 million persons in Europe (Loftus, 2004). Crohn's disease (CD) and ulcerative colitis (UC), the two major forms of IBD, affect different regions of the intestinal tract and have distinct cytokine profiles. In CD, transmural inflammation can occur over the entire length of the gastrointestinal tract, whereas UC inflammation is restricted to the mucosa of the colon. The T helper (T h ) paradigm was established by Mosmann et al (1986) who observed distinct cytokine patterns were produced by two types of fully differentiated effector T cells which they termed termed T h 1 and T h 2 cells. The initial cytokine profiles observed in IBD helped to classify CD as a T h 1 disease, due to the increased production of the main T h 1 effector cytokine, interferon-gamma (IFN-). UC was slightly more difficult to classify because levels of a central T h 2 effector cytokine, IL-4, are not increased; however, other T h 2 effector cytokines, such as IL-5 and IL-13 are produced at higher levels (Fuss et al, 1996). Therefore, UC is not considered fully T h 2, but rather a T h 2-like disease . Conventional IBD therapies, including corticosteroids and anti-tumor necrosis factor-alpha (TNF-α) therapy, are aimed at reducing nonspecific inflammation. TNF-α is a central proinflammatory cytokine that contributes to the pathology of many autoimmune disorders. Anti-TNF-α was the first biological therapy introduced for patients with IBD in the late 1990s, and corticosteroid-refractory or fistulizing CD and refractory UC generally respond very well to anti-TNF-α treatment (Hoentjen & van Bodegraven, 2009;Rutgeerts et al., 2006). The initial identification of disease-specific inflammatory mediators in CD and UC, T h 1 and T h 2-associated cytokines respectively, lead to the development of more specific antiinflammatory treatment options, and the efficacies of these new biological agents have in turn helped evolve our understanding of IBD pathogenesis. Using mouse models of intestinal inflammation that resemble CD, and targeting the main cytokine that drives T h 1 cellular development, IL-12, with an antibody to the IL-12p40 subunit either prevented the development of colitis, or completely cured established colitis (Liu et al., 2001;Neurath et al, 1995). These observations further supported the link between CD and T h 1 responses, in addition to warranting the development of an anti-IL-12p40 antibody for human patients with CD. In clinical trials, anti-IL-12p40 therapy induced clinical responses and remissions in patients with active CD (Mannon et al., 2005;Sandborn et al., 2008), which lead to its acceptance as a new therapy for CD.
www.intechopen.com (Fujino et al., 2003;Rovedatti et al., 2009). Furthermore, polymorphisms in the IL-17A and IL-17F g e n e s h a v e b e e n l i n k e d t o U C a n d a n i m a l m o d e l s i n d i c a t e t h a t t h e y a r e fundamentally involved in the etiology of IBD (Arisawa et al., 2008). However, their precise roles in pathogenesis are not entirely clear. This chapter will focus on the cytokines IL-17A and IL-17F, and review what is known about their contributions to mucosal barrier function in the gastrointestinal tract with special emphasis on IBD.

The intestinal mucosal barrier
The gastrointestinal tract forms the largest surface in contact with the external environment. The intestinal mucosal barrier separates the internal intestinal tissues from an estimated 10 14 organisms (Savage, 1977), and is composed of a physical barrier as well as specialized immune cells, primed to react if the physical barrier is breached.

Anatomy and function of the physical barrier
The physical barrier is comprised of an outer mucus layer less than a millimeter thick, and a single layer of epithelial cells joined together by tight junctions (Figure 1). The main structural component of the outer mucus layer is the heavily O-glycosylated glycoprotein MUC2, which is produced by goblet cells and gives mucus its viscous properties. The outer mucus layer was recently discovered to contain within it two distinct layers: an outer loose mucus layer with high numbers of commensal bacteria, and a dense inner layer that is sterile, containing high concentrations of antimicrobial molecules including nonspecific antimicrobial peptides and specific antimicrobial immunoglobulins (IgA) (Johansson et al., 2008). Commensal bacteria contribute to the function of the mucosal barrier by inducing the production of IgA, recruiting intraepithelial lymphocytes, and providing a physical blockade to prevent the colonization of pathogens (Umesaki et al., 1999). The second component of the physical mucosal barrier is the single layer of epithelial cells supporting the outer mucus layer. The majority of epithelial cells are transporting enterocytes, but specialized epithelial cell types contribute to mucosal barrier integrity by producing the main constituents of the mucus layer, which minimizes microbial contact with the epithelium. Additionally, epithelial cells have a dense glycocalyx overlaying microvillar projections that prevent microbial attachment (Linden et al., 2008;L. Shen & Turner, 2006). The epithelial barrier needs to be selective to allow the absorption of essential nutrients while preventing the entry of potentially noxious compounds. As depicted in Figure 1, tight junctions that connect the epithelial cells allow the cellular barrier to respond to changes in the environment by regulating the tight junction protein composition, which leads to general or ion-selective changes in paracellular permeability (Arrieta et al., 2006). In both animal models of IBD and the clinical disease in humans, changes in the physical mucosal barrier have been observed. In patients suffering from UC, MUC2 protein levels are significantly decreased during active phases of the disease, resulting in a thinner protective mucus layer (Hinoda et al., 1998;Tytgat et al., 1996). In animal models of chronic intestinal inflammatory conditions that cycle between active and quiescent phases, paracellular permeability remains increased regardless of the inflammatory state, whereas transcellular permeability is only increased during active inflammation (Porras et al., 2006). Similar observations have been made in humans, where patients with quiescent CD have significantly increased intestinal permeability when compared to controls (Wyatt et al., 1993). It is believed that the sustained increase in paracellular permeability, indicative of epithelial cell layer dysfunction, contributes to the chronic nature of the disease. The protein composition of the tight junction is dynamic, and different claudin-family proteins as well as varying levels of occludin and the junctional adhesion molecule (JAM) allow for specific alterations of paracellular permeability. The foundation of the adherens junction is formed by contacts between epithelial cadherin (E-cadherin)-catenin complexes, which functions to connect neighboring epithelial cells and maintain cell polarity.

Immune cells of the mucosal barrier: Surveillance and tolerance
In addition to the physical boundary, there are immune cells and gut associated lymphoid tissues (GALT) situated within and below the epithelium. In a healthy intestine these cells and tissues strike a balance between immunity and tolerance, and maintain barrier function. The intestinal tract harbors vast populations of leukocytes. Innate immune cells typically mediate the first line of host defense, and in the intestine these include dendritic cells, macrophages, natural killer (NK) cells, T cells, NKT cells and polymorphonuclear cells (Meresse & Cerf-Bensussan, 2009). Innate immunity evolved to recognize molecular signatures within the products of microbes that are essential to microbial survival. The innate immune system is comprised of pathogen recognition receptors (PRRs), such as toll-like receptors (TLR) and nucleotide-binding and oligomerization domain-like receptors (NLR), which recognize pathogen-associated molecular patterns (PAMPs). Binding of PAMPs to their cognate PRR activates signaling pathways that in turn activate host defense mechanisms. Different cell types have distinct immune functions; they express different combinations and levels of PRRs, and the downstream targets of PRR signaling are cell specific (Wells et al., 2010). Consequently, PAMP-PRR signaling mediates cell specific responses that enable the surrounding tissue to adapt to the dynamic intestinal environment. Additionally, intestinal tissues are unique in that they harbor large numbers of adaptive immune cells expressing effector or memory phenotypes (Mowat, 2003). These include IgA and IgG secreting plasma B cells and canonical T cells located in the lamina propria.
Adaptive immune responses are antigen specific and typically facilitate expeditious removal of pathogens. In the gut however, adaptive immune tolerance is crucial for maintaining quiescent relationships with the microbial flora and food antigens. In this regard, the intestine is a prime inductive site for large numbers of adaptive T reg cells that home back to the intestinal mucosa, where they help to maintain intestinal tolerance (Belkaid & Oldenhove, 2008;Coombes et al., 2007). Thus, innate and adaptive immune cells in the gut are primed for action so that they can maintain a tolerant immune environment, while still being able to rapidly respond to invading pathogens.

Interleukin-17
IL-17 is a central pro-inflammatory cytokine at mucosal surfaces, with important functions in innate and adaptive immunity, as well as host defense against extracellular pathogens.
Originally named cytotoxic T-lymphocyte antigen (CTLA)-8, IL-17 was first described in the mid 1990s (prior to the identification of T h 17 cells) as a cytokine produced by activated CD4 + T cells that acts on stromal cells to up regulate inflammatory and hematopoietic processes (Fossiez et al., 1996;Rouvier et al., 1993;Yao et al., 1995a). IL-17 is now best known as the signature cytokine secreted from the recently characterized T h 17 cells, however numerous innate cells can also produce IL-17, including innate-like γδ intraepithelial lymphocytes (IEL), natural killer (NK) T cells, lymphoid tissue inducer (LTi)-like cells, Paneth cells, and neutrophils, as well as other unidentified cell types (Buonocore et al., 2010;Cua & Tato, 2010;Doisne et al., 2011;L. Li et al., 2010;Maele et al., 2010;Michel et al., 2007;Shibata et al., 2007;Takahashi et al., 2006;Takatori et al., 2008). In the context of the intestinal mucosa, γδ IELs are currently the best-characterized innate sources of IL-17. γδ IELs reside at the intestinal mucosal surface between epithelial cells on the basolateral side of tight junctions. They play an essential role in the restitution of epithelial cells following mucosal injury through the production of growth factors, a distinct ability that does not occur in other mucosal T cell populations (Y. Chen et al, 2002). Additionally, γδ IELs play an essential role in controlling bacterial penetration across injured mucosal surfaces, and recruiting neutrophils following Escherichia coli infection by acting as the major source of early IL-17 (Ismail et al., 2009;Shibata et al., 2007). Importantly, since the discovery of IL-17 additional IL-17 family members have been identified. The IL-17 cytokine family consists of six members in mammals: IL-17A (also called IL-17), IL-17B, IL-17C, IL-17D, IL-17E (also called IL-25), and IL-17F (X. Zhang et al., 2011). IL-17F shares 50% sequence homology with IL-17A, is also produced by T h 17 cells, binds the same receptor as IL-17A and in turn shares certain biological activities (Hymowitz et al., 2001). IL-17A and IL-17F are either produced as homodimeric cytokines or as heterodimers composed of IL-17A/F (Wright et al., 2007). When acting on fibroblasts, endothelial cells, or epithelial cells, both IL-17A and IL-17F induce the production of proinflammatory cytokines (notably IL-6 and IL-8), chemokines, antimicrobial peptides, and matrix metalloproteinases (Iwakura et al., 2011;Starners et al., 2001). Despite their similar pro-inflammatory actions, IL-17A and IL-17F appear to have distinct roles in mediating inflammatory processes and autoimmune diseases (discussed later). IL-17B, IL-17C, and IL-17D are the least well-characterized members of the IL-17 family. IL-17B and IL-17C have 27% homology with IL-17A, but are not produced by activated T cells and do not induce the same pro-inflammatory cytokines as IL-17A and IL-17F (H. Li et al., 2000). IL-17D, which is most similar to IL-17B with 27% sequence identity, is highly expressed in skeletal muscle, brain, adipose, heart, and lung tissue, but poorly expressed in activated T cells. However, similar to IL-17A and IL-17F, IL-17D can induce the expression of IL-6 and IL-8 from endothelial cells (Starnes et al. 2002). Lastly, IL-17E has the most divergent primary sequence compared to IL-17A with 16% homology, and plays a role in pro-allergic type 2 immune responses (Angkasekwinai et al., 2007;Lee et al., 2001;Pan et al., 2001). Despite the varying degrees of sequence homology and varying functions, the C-terminal region of each IL-17 family member is quite conserved, containing 4 cysteine and 2 serine residues. Three IL-17 crystal structures have been resolved thus far: IL-17A with its neutralizing antibody, IL-17F, and IL-17F with its receptor IL-17RA. These structures have demonstrated the 6 conserved residues adopt a cysteine knot fold, which differs from the canonical cysteine knot found in TGF-β and neurotrophin proteins due to the absence of two cysteine residues (Ely et al., 2009;Gerhardt et al., 2009;Hymowitz et al, 2001).

IL-17 receptor and signaling
Cytokine receptors are generally classified into six main categories: IL-1 receptors, class I cytokine receptors, class II cytokine receptors, TNF receptors, tyrosine kinase receptors and chemokine receptors (Wang et al., 2009). The IL-17 receptors do not belong to any of these categories based on their unique structure and cytokine interaction (X. Zhang et al., 2011). The IL-17 receptor family contains 5 members: IL-17RA (or IL-17R), IL-17RB, IL-17RC, IL-17RD, and IL-17RE. IL-17B is known to signal through IL-17RB, IL-17C through IL-17RE, and IL-17E through IL-17RA/IL-17RB (Iwakura et al., 2011;Wright et al., 2008). The receptor for IL-17D remains unknown. IL-17RA and IL-17RC are normally required for IL-17A, IL-17F, and IL-17A/F signaling (Iwakura et al., 2011). However, the IL-17RA is highly expressed on mouse T cells, while IL-17RC is undetectable, and only IL-17A but not IL-17F can induce signaling (Ishigame et al., 2009). Thus, it appears in some cell types IL-17RC is dispensable for IL-17RA signaling. This has lead to the hypotheses that IL-17RA forms either a homodimeric signaling complex or that other subunits can pair with IL-17RA in some cell types that do not express IL-17RC (Gaffen, 2009). Clarification of the receptor complexes for IL-17A and IL-17F is important for understanding how a cell or tissue responds to IL-17A versus IL-17F and will undoubtedly reveal crucial aspects of tissue specific T h 17 responses. Signaling through IL-17 receptors triggers pathways that are usually associated with innate immune signaling (F. Shen et al., 2005;Park et al., 2005). Classical T h 1 and T h 2 cytokines activate JAK/STAT signaling, however IL-17A and IL-17F mediate signaling through nuclear factor (NF)-κB, NF-κB activator 1 (Act1) and tumor necrosis factor (TNF) receptor associated factor 6 (TRAF6) (Chang et al., 2006;Schwandner et al., 2000; Yao e t a l . , 1 9 9 5 b ) . T h i s m o d e o f s i g n a l i n g i s s i m i l a r t o t h o s e u s e d b y T L R s a n d the IL-1 receptor family, which function in innate immunity. Furthermore, IL-17A and IL-17F generally induce events that are typical of early inflammation (Gaffen, 2009). Upon receptor binding, IL-17A and IL-17F induce expression of many pro-inflammatory genes including: the cytokines TNF, IL-1, IL-6, granulocye-colony stimulating factor (G-CSF) and granulocyte-macrophage (GM)-CSF; the chemokines CXCL1, CXCL5, IL-8, CCL2, and CCL7; antimicrobial defensins, and S100 proteins; as well as matrix metalloproteinases (MMP)-1, -3, and -13 (Iwakura et al., 2011). In this regard, signaling by IL-17A and IL-17F through an IL-17 receptor complex is considered to mediate innate-like inflammatory events.

Interleukin-17 and the mucosal barrier
Although the majority of the defined roles played by IL-17 in mucosal barrier function are related to innate and adaptive immune functions, IL-17 has also been found to directly regulate components of the physical mucosal barrier. In colonic epithelial monolayers IL-17A enhances tight junction formation by increasing claudins 1 and 2 association with the membrane (Kinugasa et al., 2000). Direct application of IL-17A to T84 monolayers increased transepithelial resistance and decreased manitol flux through monolayers. Thus, IL-17A may have an important role in maintaining tight junctions and epithelial restitution during repair processes. In airway epithelial cells IL-17A induces mucin gene expression, and it may have similar inductive effects in the intestine on goblet cells . IL-17A also induces expression of -defensins in the colon (Ishigame et al., 2009). Furthermore, in subepithelial myofibroblasts, which sit just below the epithelium, IL-17A reduced TNF-induced secretion of pro-inflammatory cytokines, demonstrating that IL-17 is not implicitly a pro-inflammatory cytokine. Additionally, IL-17 receptor-deficient mice show increased dissemination of S. typhimurium from the gut (Raffatellu et al., 2008). Taken together, it appears IL-17 can dynamically regulate components of the physical intestinal epithelial barrier, and the barrier is dysfunctional when IL-17 signaling is impaired.

Induction of adaptive immunity
There are numerous locations in the gut where adaptive immune responses are initiated. These include organized lymphoid tissue such as Peyer's patches and isolated lymphoid follicles (ILF) that are embedded directly in the epithelial wall, and mesenteric lymph nodes (MLN), which are connected to the intestinal mucosa by draining lymphatic vessels ( Figure  2, Mowat, 2003). Furthermore, there is evidence that adaptive responses occur directly in the lamina propria via dendritic cell and epithelial cell signaling (He et al., 2007). Under homeostatic conditions, intestinal luminal contents are constitutively sampled and processed by professional antigen presenting cells (pAPC). pAPC present processed antigen to the naive T cell population, which has an infinite repertoire of antigen-specific receptors. Upon presentation of antigen to a T cell bearing a cognate receptor, the pAPC drives an antigen specific T cell response. Depending on the accompanying signals from the pAPC and surrounding environment, the T cell may become activated into an effector cell, anergic (unresponsive to antigen) or apoptotic. Classically there are three types of cells that act as pAPC: B cells, macrophages and dendritic cells. Arguably, antigen acquisition by dendritic cells is most critical for priming adaptive immune responses, as dendritic cells are the most efficient class of pAPC. In Peyer's patches and ILF, antigen is transported from the lumen by microfold (M) cells to dendritic cells located in the follicle associated epithelium or the underlying subepithelial dome. From there, dendritic cells move into local T cell/follicular areas or drain to the MLN to initiate adaptive responses (Artis, 2008;Kelsall, 2008;Mowat, 2003). The other site for antigen entry is the non-follicular associated epithelium overlying the lamina propria. Under normal conditions antigen is moved across the non-follicular associated epithelium by receptor-mediated transport (Kelsall, 2008) and by dendritic cells located in the lamina propria, which project dendrites through the tight junctions into the lumen (Figure 2, Chieppa et al., 2006;Rescigno et al., 2001). When the epithelium is damaged, as occurs in IBD and pathogenic infections, antigens also enter directly. Dendritic cells are equipped to recognize microbial products with an array of PRR and in doing so, undergo a process of maturation in order to become proficient antigen presenters for naive T cells. In addition to down-regulating their phagocytic machinery and upregulating antigen processing pathways, dendritic cells secrete an array of immunomodulatory cytokines. At first, they express a mixed cytokine profile. However, a dominant cytokine profile emerges and this dictates the type of adaptive immunity that develops (Wilson et al., 2009). The specific constellation of PRRs that are engaged on a dendritic cell is what determines their cytokine profile.

Adaptive immune cells
The defining feature of an adaptive immune system is antigen-specific immunity. The first encounter with antigen leads to clonal expansion of a few antigen-specific lymphocytes, which target immune responses towards their cognate antigens. Some of these cells become longlived memory cells and they enable the immune system to remember antigen that has already been encountered, so that upon re-exposure a tailored immune response is quickly recalled. The cells of the adaptive immune system are T and B-lymphocytes. Each lymphocyte bears a surface receptor of a single specificity that binds antigen in a highly specific manner. T and B cell development generates an infinitely diverse repertoire of T and B cell receptors, so that in www.intechopen.com theory any possible antigen can be recognized. Classical naive T cells express an αβ T cell receptor and a co-receptor, which comes in two flavors: CD4 or CD8. Accordingly, CD4 expressing T cells are called CD4 + T cells and CD8 expressing T cells are called CD8 + T cells.
CD4 + T cells are also called T helper cells because following establishment of the adaptive phase by innate defenses, CD4 + T cells become the central coordinators of the adaptive immune response. The primary effector function of CD4 + T cells is to help and regulate other immune cells. Upon encountering their cognate antigen on mature pAPCs, CD4 + T cells proliferate and differentiate into antigen-specific effector cells.

T Helper cell differentiation
There are currently four well characterized lineages of T h cells: T h 1, T h 2, T reg , and T h 17 ( Figure 3). Naïve CD4 + T cells differentiate into T h 1 cells in the presence of IFNγ and IL-12, which enhances the expression of the principal T h 1 transcription factors, T-box family of transcription factors (T-bet) and the signal transducers and activators of transcription protein 4 (STAT4). Effector cytokines produced by T h 1 cells include IFNγ, TNFα and IL-2, which help to clear intracellular pathogens. T h 2 cells differentiate in the presence of IL-4, which activates STAT6 and leads to the expression of the transcription factor GATA binding protein 3 (GATA3). T h 2-derived cytokines, including IL-4, IL-5 and IL-13, are important in mediating asthma and allergic responses (Zhu & Paul, 2010). Fig. 3. T helper cell differentiation. After encountering an antigen-presenting cell (APC) within the periphery, naïve T helper (T h 0) cells are able to differentiate into one of four T h subsets based on the cytokine milieu present. In the presence of interleukin (IL)-12, the activation of transcription factors STAT4 and T-bet lead to T h 1 development, whereas IL-4 results in the activation of STAT6 and GATA3, leading to T h 2 development. In the presence of TGF-β, T h 0 cells will differentiate into inducible T regulatory (iT reg ) cells following transcription of STAT5 and Foxp3, unless IL-6 (in mouse) or IL-21 (in human) is present in addition to TGF-β, in which case T h 17 cells will develop following ROR-γt transcription.
Most T reg cells, termed natural T reg (nT reg ) cells, are fully differentiated before leaving the thymus, upon TCR stimulation and encountering IL-2 or IL-15. This results in the activation of STAT5 and leads to forkhead box (Fox)p3 expression, the characteristic transcription factor of T reg cells (Burchill et al., 2008). Once these cells leave the thymus, they can home to mucosal surfaces, including the GI tract where the presence of TGF-β helps them to maintain their regulatory phenotype (Barnes & Powrie, 2009). TGF-is also able to induce the expression of Foxp3 in naïve T cells within the periphery, resulting in inducible T reg (iT reg ) cells. Primarily through the production of IL-10, nT reg and iT reg cells share the same suppressive phenotype and function to maintain peripheral tolerance and prevent autoimmunity (Maloy et al., 2003;Read et al., 2000;Zheng & Rudensky, 2007). T h 17 cellular differentiation also depends on TGF-β, however with the additional presence of IL-6 in mice (Veldhoen et al., 2006), or IL-21 in humans (L. , Foxp3 expression is inhibited and STAT3 activation leads to expression of the transcriptional regulator retinoic acid receptor-related orphan receptor-γt (RORγt), which drives T h 17 differentiation (Ivanov et al., 2006). Once differentiated, T h 17 cells are highly responsive to IL-21 and IL-23, cytokines that function to maintain the T h 17 phenotype. The principle effector cytokines produced by T h 17 cells include IL-17A, IL-17F, IL-21, and IL-22.

Role of IL-17 in enteric infections
Several murine models of infectious disease highlight the presence and importance of IL-17 in intestinal inflammation: Helicobacter hepaticus, Salmonella enterica serotype typhimurium, and Citrobacter rodentium. In H. hepaticus-induced typhlocolitis, a model of T-cell independent innate inflammation, local increases in IL-23 induced the secretion of IL-17 from non-T cell sources . A similar study using the same H. hepaticus model of bacteria-driven innate colitis confirmed the IL-23-dependent increases in IL-17, and went on to characterize the IL-17-producing cells. This led to the identification of a novel innate lymphoid cell population that accumulates in the inflamed colon, and is able to mediate acute and chronic innate colitis in response to IL-23 stimulation (Buonocore et al., 2010).
In the second infectious model with S. typhimurium, initial inflammatory responses are important to contain the infection as localized gastroenteritis, and prevent the systemic spread of bacteria. Macrophages and dendritic cells infected with S. typhimurium are a major source of IL-23, and five hours post-S. typhimurium infection, IL-17 expression is markedly up regulated (Raffetulla et al., 2008(Raffetulla et al., , 2009). The increased IL-17 production resulted in IL-17-dependent intestinal epithelial induction of antimicrobial peptides ). In IL-23p19-deficient mice, the increased expression of IL-17 during S. typhimurim infection was abrogated. Although αβ T cells were found to be the predominant cell type expressing the IL-23R, there was a marked increase in γδ T cells expressing the IL-23R during S. typimurium infection. γδ T cell-deficient mice demonstrated a blunted expression of IL-17, suggesting that γδ T cells are a significant source, but not the only source of IL-17 during an acute bacterial infection (Godinez et al., 2009). Lastly, C. rodentium is a non-invasive bacterium that transiently colonizes the large intestine of mice. In addition to serving as a model for attaching/effacing bacteria, C. rodentium infection can be used a model of IBD, as the infection-associated pathology shares many features with IBD (Mundy et al., 2005). The first evidence of IL-17 involvement in C. rodentium infection implicated its importance during the peak and late stages of infection, demonstrating a role for adaptive T h 17 cells in clearing the infection (Symonds et al., 2009; www.intechopen.com Zheng et al., 2008). More recent evidence also suggests there is an early T h 17-like response during C. rodentium infection that is dependent on the activation of the innate immune receptors Nod1 and Nod2 (Geddes et al., 2011). Whether or not this will directly relate to the NOD2 coding variants identified as risk factors for IBD (Hugot & Cho, 2002) remains to be explored.

Role of IL-17 in IBD pathogenesis
Since the discovery of IL-23 as a critical regulator of T h 17 responses and that there are increased numbers of T h 17 cells in IBD patients (Kleinschek et al., 2009), the importance of T h 17 cells and their effector cytokines has been an active area of IBD research. To help elucidate the precise role of the T h 17 subset, three principle animal models of intestinal inflammation resembling CD have been employed: T cell transfer models of colitis, trinitobenzene sulfonic acid (TNBS)-induced colitis, and dextran sulfate sodium (DSS)induced colitis. With the T cell-transfer model, the initiation of colitis via an adaptive immune response is modeled through the transfer of naïve CD4 + T cells (CD45RB high ) to immune-deficient mice that lack T cells and B cells, such as recombination activating gene (RAG)-deficient mice, or severe combined immune-deficient (SCID) mice. The naïve cells introduced develop into pro-inflammatory effector T cells in the absence of a mature immune cell population (CD45RB low ) containing T reg cells, and spontaneous intestinal inflammation develops (Powrie et al., 1994a). TNBS-induced colitis is also dependent on the adaptive immune system, where mucosal inflammation following the administration of the haptenizing agent TNBS is mediated by T h 1 and T h 17 responses (Alex et al, 2009). In contrast to the latter two models, DSS-induced colitis does not require T cells to initiate inflammation. DSS is thought to disrupt the epithelial barrier, resulting in the activation of lamina propria cells by the normal microflora. In the acute DSS model both T h 1 and T h 17 cells accumulate; however, if the DSS is given in several cycles to establish chronic inflammation, the cytokine profile shifts towards T h 2 (Alex et al., 2009). Therefore, acute DSS can be used as a model for CD whereas chronic DSS is more representative of UC.

The IL-23/T h 17 axis and IBD
IL-23 has been found to critically mediate intestinal inflammation through both adaptive and innate immune pathways. Interestingly, an uncommon coding variant of the IL23R gene, which encodes a subunit of the IL-23 receptor, was found to confer strong protection against both CD and UC (Duerr et al., 2006). T cell transfer models show that IL-23 is required for spontaneous development of colitis by activated CD4 + T cells (Elson et al., 2007;Hue et al., 2006). Similarly, RAG deficient mice that are also IL-23p19 or IL-12p40 deficient (do not produce IL-23) do not develop spontaneous intestinal inflammation, whereas RAG deficient mice that lack IL-12p35 (do not produce IL-12) still develop colitis . In these experimental systems IL-23 and not IL-12 drives intestinal inflammation. Interestingly, though IL-23p19 deficient mice fail to develop intestinal inflammation, they still develop a systemic inflammatory response . This demonstrates that IL-23 driven inflammation by CD4 + T cells is localized to the gut. A transfer model with bacteria-reactive CD4 + T cells showed that neutralization of IL-23p19 with a monoclonal antibody attenuates intestinal inflammation and that individually, bacteria-reactive T h 17 cells induce more inflammation than bacteria-reactive T h 1 cells (Elson et al., 2007). The latter study also highlights that T h 1 and T h 17 cells have an overlapping ability to promote www.intechopen.com pathologic responses. Although IL-12 as a T h 1 inducing cytokine is dispensable for initiating colitis, T h 1 responses should not be considered insignificant in inflammatory bowel disease. Previous studies have shown that neutralization of IFN-γ (signature T h 1 cytokine) prevents intestinal inflammation and severe wasting, and transfer of IFN-γ deficient T cells into RAG deficient mice fails to induce colitis (Ito & Fathman, 1997;O'Connor et al., 2009;Powrie et al., 1994b). Taken together, these results suggest that although IFN-γ still appears to be the main effector cytokine driving the cell-transfer colitis model, IL-23 and T h 17 responses are essential to support the development of chronic inflammation.

Contributions of IL-17A and IL-17F to IBD
There are multiple lines of evidence to suggest that blocking IL-17A and IL-17F would prevent intestinal inflammation as both cytokines robustly induce neutrophil recruitment and pro-inflammatory cytokines, blocking IL-23 prevents development of pathogenic T h 17 cells and colitis in animal models, and blocking IL-23 signaling is beneficial for treating CD. Along these same lines, IL-17R-deficient mice are significantly protected from TNBSinduced colitis, despite no change in the levels of IL-23 or IL-12 and IFN-γ (Z. Zhang et al., 2006). Thus, it was unexpected that neutralization of IL-17A exacerbated intestinal inflammation in the dextran sodium sulfate (DSS) colitis model (Ogawa et al., 2004). Animals treated with an IL-17A monoclonal antibody had enhanced inflammatory cell infiltrates into the mucosa and submucosa, more severe mucosal injury and drastically increased weight loss. Moreover, addition of IL-17A attenuated the response (Ogawa et al, 2004). These results were confirmed in IL-17A knockout mice, which also developed more severe DSS-induced colitis (X. . Interestingly, this same study showed that IL-17F knockout mice, unlike IL-17A knockouts, were protected from DSS-induced colitis. Colons of IL-17F deficient mice showed little pathology and extremely low levels of proinflammatory cytokines . Using a T cell transfer model, IL-17A secretion by T h 17 cells was also protective against the development of intestinal inflammation, as IL-17A deficient T cells transferred into RAG deficient mice caused more severe disease than transferred wildtype T cells (O'Connor et al, 2009). Additionally, IL-17A has been shown to directly inhibit T h 1 cells and suppress T h 1 mediated intestinal inflammation (Awasthi & Kuchroo, 2009). Taken together, these data suggest that IL-17A has protective roles in acute tissue inflammation and that IL-17F has pathogenic functions. However, there has also been some evidence that IL-17A is not protective. T cells deficient in ROR-γt, and therefore unable to differentiate into T h 17 effector cells, were unable to induce colitis when transferred to RAG-deficient mice, but treatment with IL-17A caused colitis after the transfer of ROR-tdeficient cells (Leppkes et al., 2009). Therefore, additional work on the mechanisms, function, and regulation of IL-17A/F in the context of intestinal inflammation is required before confident and definitive conclusions can be drawn.

Conclusion
Knowledge of T h 17 cells and their characteristic cytokines IL-17A and IL-17F has rapidly progressed. Likewise, significant progress has been made towards understanding their role in regulating the gut environment. However, there are numerous outstanding questions. The T h 17 subset is unequivocally associated with chronic inflammatory bowel diseases, and the current belief is that they are instigated by a loss of tolerance to the intestinal microflora.
In addition to T h 17 cells, dysregulated T h 1 and Foxp3 + iTreg responses are also involved.
Yet, the precise nature of the relationship between T h 17 cells and T h 1 as well as T h 17 cells and Foxp3 + iTregs is unclear. Furthermore, in the gut there appears to be multiple cellular sources of IL-17A and IL-17F, in addition to heterogeneous expression of their receptors, IL-17RA and IL-17RC. Our understanding of how IL-17A and IL-17F mediate their cell specific effects and how this plays out during steady states, infectious disease and chronic inflammation in the intestinal tract is currently in progress. Beneficial results have been obtained using antibodies to neutralize IL-12p40 in Crohn's disease and genome wide association studies implicate the IL-23-T h 17 axis in both Crohn's disease and ulcerative colitis. Together these data suggest therapies specifically targeting T h 17 responses might provide better treatments. However, animal models have also shown IL-17A and IL-17F to critically mediate host protection and components of normal barrier function. Thus given these roles, targeted interventions of IL-17A and IL-17F will need careful consideration. Inflammatory bowel diseases are a complex set of diseases involving pre-disposing genetic factors and environmental triggers. The emerging IL-23-T h 17 axis represents one significant component of these diseases among several. Though progress has been made, a substantial amount of work remains to identify pathways and mechanisms that connect T h 17 cells, IL-17A and IL-17F to the etiology of inflammatory bowel diseases. In particular, genome wide association studies have established a key role for innate immunity in these diseases. Most well known are NOD2 and autophagy genes ATG16L and IRGM involved in bacterial detection and processing. In this regard, much less is known about IL-23, IL-17A and IL-17F in aberrant innate immune responses. For now we can ascertain that both innate and adaptive immunity coordinate an imbalanced relationship between host and microflora that leads to chronic intestinal inflammation, and that T h 17 cells and the IL-17A/F cytokine network participate in both arms of the immune system that has gone awry.