Immune-bacterial homeostasis in the gut: new insights into an old enigma

The intestinal mucose is the interface between the immune system and the ma ssive antigenic load represented by the commensal enteric bacteria. These c ommensal bacteria drive the development of the mucosal immune system, and i n turn most of the lymphocytes in the intestinal mucosa appear to be specif ic for enteric bacteria antigens. Proper regulation of the responses of the se anti-bacterial lymphocytes are extremely important because T cell effect ers reactive to enteric bacterial antigens have been shown to cause chronic intestinal inflammation in an adoptive transfer system. The cells and mole cules important in regulating mucosal immune response are now being identif ied. Insights into the mechanisms of mucosal regulation have come from a nu mber of genetically manipulated mouse strains which develop inflammatory bo wel disease in response to the enteric bacterial flora. CD4(+) T cells with regulatory function in the mucosa are being identified; other cell types s uch as CD8(+) T cells. NK cells, and B cells may also have a role in mucosa l immune regulation. A model for T cell-immune homeostasis in the intestina l mucosa is presented.