B-1 cells and their reactivity with the murine intestinal microflora.

IgA secreting cells located in the lamina propria of the gut are a prominent feature of the mucosal immune system, which serves to protect the body from the continuous threat to infection by intestinal bacteria. In this review we summarize briefly the evidence that these IgA secreting cells have a dual origin and are derived either from conventional B cells or from B-1 cells. Furthermore, we show both at polyclonal and monoclonal levels that the major antigenic target of B-1 cell derived IgA are normal intestinal bacteria. Coating of intestinal bacteria with IgA is thought to result in immune exclusion, as shown for pathogenic bacteria. However, the bacterial microflora of the gut is an extremely stable ecosystem, despite the fact that the majority of intestinal bacteria are coated with IgA. We speculate here that these apparent contradictory functions of the humoral immune system, i.e. removal of bacteria and maintaining the normal gut flora might be exerted by IgA antibodies produced by the two B-cell lineages. The fixed and biased repertoire of B-1 cells might play a role in maintaining the normal intestinal flora. When pathogenic bacteria penetrate into the gut, conventional B cells may be induced in the Peyer's patches to produce high affinity, narrowly tuned IgA antibodies, leading to immune exclusion.