Immuno-bacterial homeostasis in the gut: New insights into an old enigma

The intestinal mucosa is the interface between the immune system and the massive antigenic load represented by the commensal enteric bacteria. These commensal bacteria drive the development of the mucosal immune system, and in turn most of the lymphocytes in the intestinal mucosa appear to be specific for enteric bacteria antigens. Proper regulation of the responses of these anti-bacterial lymphocytes are extremely important because T cell effectors reactive to enteric bacterial antigens have been shown to cause chronic intestinal inflammation in an adoptive transfer system. The cells and molecules important in regulating mucosal immune response are now being identified. Insights into the mechanisms of mucosal regulation have come from a number of genetically manipulated mouse strains which develop inflammatory bowel disease in response to the enteric bacterial flora. CD4⁺ T cells with regulatory function in the mucosa are being identified; other cell types such as CD8⁺ T cells. NK cells, and B cells may also have a role in mucosal immune regulation. A model for T cell-immune homeostasis in the intestinal mucosa is presented.