Stromal cell independent growth of bipotent B cell--macrophage precursors from murine fetal liver.

We have established stromal cell independent culture conditions under which single murine hematopoietic progenitor cells give rise to B lymphocytes and macrophages. IL-11, mast cell growth factor, and IL-7 are sufficient for the growth of B lineage cells from AA4.1+B220-Mac-1-Ly6A+ fetal liver cells isolated at day 12 of gestation. These conditions are also sufficient for commitment to myeloid differentiation, although CSF-1 is required for substantial growth of macrophages. The progenitors responding under these culture conditions: (i) require 8 days in culture before they become competent to respond to B cell mitogens in the presence of stromal cells and (ii) undergo a minimum of three cell divisions prior to Ig heavy chain allotype restriction. The findings suggest that the stromal cell dependency of immature hematopoietic cells may reflect a requirement for stromal cell derived growth factors. Further, this provides an in vitro system in which commitment and differentiation can be studied in single cells.