The aetiological factors that are involved in the pathogenesis of IBD have not yet been fully determined. However, accumulating evidence suggests that both genetic and environmental factors contribute to the pathogenesis of IBD (79, 80). The current hypothesis for the pathogenesis of IBD suggests that it results from the interaction of environmental factors, mainly the intestinal microbiota, and inappropriate immune responses in genetically predisposed individuals (81). Recent genome-wide association studies have identified approximately 100 genomic loci that confer susceptibility to IBD. These susceptibility loci contain candidate genes that encode proteins that are required for various crucial cellular functions such as microbial recognition, cytokine signalling and epithelial barrier function. Among these genes is Rel that encodes the NFκB family member c-Rel (82, 83). Several of the identified candidate genes have been shown to be specifically associated with Crohn’s Disease such as NOD2 (84), ATG16L1 (85) and IRGM (86). These genes
45 encode key proteins involved in microbial recognition (eg NOD2) and autophagy (eg NOD2, ATG16 and IRGM). NOD2 is expressed in immune and epithelial cells of the intestine and it is important for microbial sensing and subsequent NFκB activation to initiate an adequate innate immune response which required for bacterial clearance (87). Moreover, autophagy (intracellular bacterial processing) is also required for the clearance of invading bacteria (88) and polymorphisms of NOD2, ATG16 and IRGM which are key regulators of this process (86, 89, 90) have been strongly associated with Crohn’s Disease.
In addition to defective bacterial sensing and autophagy, impairment of the intestinal mucosal barrier is also a key pathophysiological process that leads to intestinal inflammation and has been linked to the pathogenesis of IBD (91, 92). The mucosal barrier consists of several layers of protection, including outer mucosal barrier elements (such as commensal microbiota, the mucus layer and immunoglobulin A), intestinal epithelial cells and subepithelial immune cells. These elements communicate with and affect each other. The normal commensal microbiota contribute to maintaining a functional intestinal mucosal barrier. This results from inhibition of the colonisation of pathogenic bacteria and by influencing the development of both the intestinal epithelium and the gut associated lymphoid tissues (81, 93, 94). However, this beneficial relationship between the microbiota and host can break down and this may result in chronic intestinal inflammation. This abnormal reaction may occur as a consequence of dysbiosis and an altered gut microbial ecosystem which are common findings in IBD. Several studies have shown that the gut microbiota represent a crucial factor in inducing inflammation in IBD
46 (95, 96). Several environmental factors have also been shown to be associated with IBD such as diet and life style (eg smoking). These factors also influence the composition of the gut microbiota (96, 97). Defects in the intestinal epithelial cell barrier have been shown to be essential for the abnormal response of the mucosal immune system to microbiota that leads to chronic intestinal inflammation (98). Disruption of the intestinal epithelium is a hallmark of IBD and is associated with increased intestinal epithelial cell apoptosis (81). As a consequence, subepithelial immune cells are exposed to luminal bacteria resulting in an uncontrolled inflammatory response (98). Indeed, a large body of evidence from human studies has indicated the importance of these factors in the pathogenesis of IBD. However, animal models have also provided insights into the pathogenesis of IBD.
In order for an animal to act as a model of IBD, several characteristic features of human IBD should be demonstrated such as characteristic histopathology and accompanying clinical features. Although none of the available animal models is exactly identical to human IBD, they do represent several aspects of the human disease. They can therefore provide valuable information about the pathogenesis of the disease and are useful tools for evaluating the therapeutic potential of new therapies for IBD before they are tested in human clinical trials. In the last two decades, a large number of animal models of IBD have been developed. These can be classified into four categories: spontaneous models, genetically engineered models, inducible models and adoptive transfer models in immunocompromised hosts (99-101). Colitis develops spontaneously in certain strains of mice. For instance, C3H/HejBir mice spontaneously develop colitis by the age of 3 - 4 weeks
47 and this disappears by the time they reach 10 - 12 weeks old (102). A spontaneous IBD animal model has been also demonstrated in SAMP1/Yit mice, which develop chronic terminal ileitis, resembling Crohn’s disease (103). The genetically engineered models include two subgroups, knockout and transgenic. IL-7 transgenic mice develop chronic colitis when they are 4-11 weeks old and this resembles ulcerative colitis in humans (104). Genetic deletion of IL-10 has also been shown to induce spontaneous chronic colitis in mice (105). In adaptive transfer models, intestinal inflammation occurs as a result of selective transfer of certain cell types to immunodeficient host animals such as the adoptive transfer of CD45 RBhigh T-helper cells into SCID mice (severe combined immunodeficient mice) (106). Inducible colitis models are however probably the most widely used by researchers for studying IBD pathogenesis and the effects of new therapeutic agents. Therefore, we have focused on inducible models in this thesis as described below.