Inflammatory bowel disease: the classic gastrointestinal autoimmune disease

Inflammatory bowel disease (IBD) is an idiopathic disease thought to be caused by a dysregulated immune response to host intestinal microflora. The role of genetic factors is indicated by familial clustering of cases and higher incidence in monozygotic twins. An interaction between genetic and environmental factors is thought to play a role in the pathogenesis of these disorders. Changes in diet, antibiotic use and intestinal colonization have likely contributed to increased prevalence of inflammatory bowel disease in the past century. Environmental factors or infections are thought to alter the barrier function of the epithelium, leading to loss of immune tolerance to intestinal antigens. This loss of tolerance activates dendritic cells, triggering their transport to mesenteric lymph nodes, where they promote differentiation of naïve T cells to TH-1, TH-2, TH-17 cells or T regulatory cells. Production of proinflammatory cytokines and chemokines then follows. Circulating effector and regulatory cells enter the intestine through a highly selective mechanism that involves interaction with the vascular endothelium, diapedesis through the vessel wall and migration to the lamina propria. There are several genes implicated in IBD. Mutations in certain genes can cause defective down regulation of the innate immune response, ineffective clearance of intracellular bacteria and proliferation of both luminal and mucosal-adherent commensal bacteria. IBD is a chronic relapsing inflammatory condition that is immune mediated. Results from research in animal models, human genetics, basic science and clinical trials provide evidence that it is heterogeneous, characterized by various genetic abnormalities, leading to a dysregulated and overly aggressive T cell response to commensal enteric bacteria. Different genetic abnormalities can be characterized as causing defects in mucosal barrier function, immunoregulation or bacterial clearance. Advances in our understanding of the interplay between components of innate and adaptive immune response will be central to future progress.