The mucosal immune system: from specialized immune defense to inflammation and allergy.

The mucosal immune system is a first line of defense against foreign antigens, including microbial and dietary antigens. Under normal circumstances, the mucosal immune system employs tightly regulated dynamic mucosal intra- and internets consisting of inductive and effector sites for the induction of an appropriate immunological homeostasis between the host and mucosal environments. The common mucosal immune system (CMIS), which interconnects between inductive (e.g. Peyer patch) and effector (e.g. intestinal lamina propria) tissues for the induction of the IgA response, is well characterized. Recent results provide strong evidence for the presence of a CMIS-independent IgA induction pathway. Two distinct subsets of mucosal IgA-committed B cells termed B-1 and B-2, are associated with CMIS-independence and CMIS-dependent cascades respectively. In some cases, the breakdown of this tightly regulated mucosal immune system leads to pathological responses to different gut environmental antigens. As a result, disorders such as inflammatory bowel disease (e.g. IBD) and allergic gastroenteropathy can be evoked in the gastrointestinal tissues. Recently, many studies have described possible molecular and cellular mechanisms for this dysfunction in the gastrointestinal tissues by using murine models with specific gene manipulation. In this review we summarize recent findings from our group concerning the CMIS-dependent and CMIS-independent IgA induction pathways and gastrointestinal diseases (IBD and intestinal allergic diseases). These observations may provide useful information for the development of new mucosal immune therapy.