The Ly-1+ Mac-1+ B-220+ CC11+ progenitor clones LyD9 and LyB9 were previously shown to give rise to Ly-1+IgM+ B lymphocytes either in vivo or in vitro by co-culture with nonlymphoid accessory cells from spleen and lipopolysaccharide. The clones did not generate T lymphocytes either in vivo or in vitro. We now find that both LyD9 and LyB9 progenitors are induced to differentiate in vitro by co-culture with the RP. 0. 10 bone marrow stroma clone or with heterogeneous marrow-adherent stroma cell populations obtained from adult mice into Ly-1-IgM+ B lymphocytes as well as myeloid GM1. 2+ Mac-1+ cells. We could obtain evidence that a high proportion of LyD9 and LyB9 cells already switched off expression of Ly-1 and CC11 (interleukin 3 receptor) surface molecules 2 days after initiation of the cultures, and by days 8-10 of culture no detectable Ly-1+ cells and only about 20% CC11+ cells were observed. Ly-1 surface expression could not be re-induced on the progeny of LyD9 and LyB9 progenitors generated under the influence of marrow stroma cells. Remarkably, the LyD9 and LyB9 progenitors gave rise to both Ly-1+IgM+ and Ly-1-IgM+ B cells upon culture with heterogeneous stroma monolayers obtained from 18-day fetal liver. Finally, the differentiating property of the stroma cells for the LyD9 and LyB9 progenitors could not be replaced with soluble factors produced either spontaneously or after stimulation by the marrow stroma cells. Our results show the importance of epigenetic influences provided by a given microenvironment on the developmental potential of B cell progenitors. They provide direct evidence that the same pro-B lymphocyte can give rise to both Ly-1+ and Ly-1-IgM+ B cells depending on both the time of development and the tissue of origin of stroma cells with which the B cell progenitor interacts. Also, the results strongly suggest that cell contact between the stroma cell and the B cell progenitor is essential to induce rearrangement and expression of the Ig genes in pro-B lymphocytes VSports手机版. .