Functional maturation of murine B lymphocyte precursors. II. Analysis of cells required from the bone marrow microenvironment.

The development of mature B cells in cultures of early B cell precursors depends on the presence of a confluent adherent bone marrow (aBM) cell layer. Adherent and sIgM+ cell-depleted bone marrow (BM) from untreated or 5-fluorouracil-pretreated donors or day 12 fetal liver cells were used as precursor cell populations. When adherent cells from thymus or highly enriched BM-derived macrophages were co-cultured with precursor cells, mature B cells were not developed. Similarly, aBM cell layers generated in the presence of hydrocortisone and horse serum were unable to support aBM cell-dependent precursor differentiation, even though cortisone was removed before the addition of precursor cells. In contrast, this type of microenvironment promoted the differentiation of precursor of myeloid cell lineages. Repeated treatment of established aBM cell populations with a monoclonal anti-macrophage antibody (31.3, known to recognize a surface marker on a subset of BM macrophages) and complement abolished the capacity of otherwise functional aBM cells to sustain the development of B cell precursors. Macrophage-depleted aBM cells regained their function after supplementation with highly enriched BM-derived macrophages grown in vitro. Limiting dilution analysis of aBM cells in microcultures containing saturating numbers of early B cell progenitors also suggests the participation of more than one cell type in the BM cell population. In conclusion, differentiation of early B cell progenitors requires macrophages in addition to at least one additional cell type contained in the aBM cell population.