IMMUNOFLUORESCENCE ANALYSIS OF B-1 CELL ONTOGENY IN THE MOUSE

In order to further understand the developmental aspects of B-1 cells, we characterized the ontogeny of this B cell population in the spleen and peritoneal cavity of BALB/c mice. Although there are B-1 cells in the spleen within the first 1 - 3 weeks after birth, they do not at a ny stage represent the majority of splenic B cells. Splenic B-1 cells reach peak levels at approximately 9 days after birth. The mesenteric lining that covers the small intestine of 7-day-old mice contains a po pulation of IgM+ B cells, while at the same age, there are few lymphoi d cells in the peritoneal cavity. Between 7 and 8 days after birth the re is an influx of B cells into the peritoneal cavity. At 8 days, the first detectable peritoneal B cells appear to be of the B-1 type based on expression of IL-5 receptor and CD5. However, these peritoneal B-1 cells do not express Mac-1. This antigen is not expressed by the majo rity of peritoneal B-1 cells until 3 weeks. This study indicates that the majority of early splenic B cells are not B-1 cells and it suggest s that the mesenteric tissues surrounding the gut contain B lymphocyte s which traffic into the peritoneal cavity where they then reside.