Evidence that intestinal IgA plasma cells in fi,x transgenic mice are derived from B-1

mice B6-Sp6 transgenic mice carry fully rearranged (BALB/c-derlved, Igh-C * allotype) n heavy chain and x light chain transgenes, specific for trinltrophenyl, on a C57BL background (Igh-C " allotype). FACS analyses show that the majority of B cells in peripheral lymphold organs and bone marrow (BM) express transgenic IgM exclusively. A small proportion of the B cells, however, express endogenous IgM, usually concomitant with transgenic IgM. Three criteria establish that the endogenous IgM expressing B cells belong to the B-1 cell lineage. (I) Endogenous IgM expressing B cells in B6-Sp6 mice have the same localization pattern as B-1 cells from normal animals: they are enriched In the peritoneal cavity, (ii) The endogenous lgM+ B cells have the phenotype of B-1 cells: the endogenous lgM+ peritoneal B cells express Mac-1 (CD 11b) and low levels of IgD, and most also express CD5 (Ly-1). (ill) B6-Sp6 BM poorly reconstitutes endogenous lgM+ B cells, Just as adult BM from normal mice poorly reconstitutes B-1 cells. In contrast, B cells which only express the transgene are readily reconstituted by B6-Sp6 BM. The few endogenous lgM+ cells In the B6-Sp6 BM recipients are located In the peritoneal cavity and have the phenotype of B-1