Differentiation of B cell progenitors in vitro: generation of surface IgM+ B cells, including Ly-1 B cells, from Thy-1- asialoGM1+ cells in newborn liver.

We have established a stromal adherent cell line (ST2) from fetal liver that promotes growth and differentiation of early B lineage cells. Many cells in the "null population" (3-5%) from newborn liver that lack antigens found on mature erythroid, myeloid or lymphoid cells proliferate extensively on this ST2 layer. Further division of this cell fraction on the basis of Thy-1 and asialoGM1 (aGM1) expression discriminates cells that predominantly proliferate from those that differentiate on the ST2 layer. Among four populations, Thy-1+ aGM1- cells proliferate most but yield few B220+ cells. In contrast, Thy-1- aGM1+ cells proliferate to a very limited extent, but most (greater than 90%) start to express B220 and a large fraction (up to 50%) become surface IgM+ after 2 weeks of culture. These B cells include cells expressing the pan-T cell molecule Ly-1, that is, Ly-1 B cells. Curiously, this Thy-1- aGM1+ cell population is largely absent from bone marrow in adult mice.