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 lymp hocytes 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-Ly6 A+ fetal liver cells isolated at day 12 of gestation. These conditions are also sufficient for commitment to myeloid differentiation, althou gh CSF-1 is required for substantial growth of macrophages. The progen itors 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 cel l divisions prior to Ig heavy chain allotype restriction. The findings suggest that the stromal cell dependency of immature hematopoietic ce lls may reflect a requirement for stromal cell derived growth factors. Further, this provides an in vitro system in which commitment and dif ferentiation can be studied in single cells.