The key role of macrophages in the immunopathogenesis of inflammatory bowel disease

Macrophages are important in the host's immunological and inflammatory resp onses. There is a large population of these cells in the normal intestinal mucosa where they represent the major antigen presenting cell population ca pable of determining the type of T cell responses that develop to luminal a ntigens. Studies suggest that the normal intestinal macrophages cannot be e asily induced to mediate acute inflammatory responses. In active inflammato ry bowel disease there is an increase in the mucosal macrophage population, derived from circulating monocytes. These recruited macrophages are phenot ypically different from the resident population of cells and play a major r ole in mediating the chronic mucosal inflammation seen in patients with ulc erative colitis and Crohn's disease. They secrete many cytokines that are i mportant in the proinflammatory responses, such as interleukin (IL)-1, IL-6 , IL-8, IL-12, IL-18, and tumor necrosis factor-alpha. They also release re active metabolites of oxygen and nitrogen and proteases that degrade the ex tracellular matrix. Macrophages also appear to be important during resoluti on of inflammation and repair of the intestinal mucosa that occurs during d isease remission.