Mucosal Immune Responses to the Introduction of Gut Flora in Mice and the Establishment of a Murine Model of Crohn's Disease

Gut flora plays a key role in the maturation of intestinal mucosal immune systems such as the expression of class II MHC antigens on intestinal epithelial cells, and the cell expansion and functional maturation of both IgA-producing B lymphocytes in the lamina propria and TCR expressing intraepithelial lymphocytes in the intestinal epithelium in mice. In normal mice, the mucosal immune responses evoked through colonization of gut flora attained levels found in mice reared under normal gut flora-bearing conditions. However, in SAMP1/Yit mice, recently established as a murine model of Crohn's disease, transmural ileitis and cecetis developed following the introduction of commensal gut flora from normal mice, although no intestinal inflammation was observed under germfree conditions. These results suggested that commensal gut flora play critical roles in the development of Crohn's disease-like intestinal inflammation in SAMP1/ Yit mice. In this review, we focus on the specific interactions between the gut flora and mucosal immune systems that induce physiological or unphysiological mucosal immune responses.