ABSTRACT
Our experiments have addressed the regulation of early hematopoietic progenitor cell expansion by interleukin 3 (IL-3) and interleukin 1 beta (IL-1 beta) and its modulation by bone marrow fibroblasts in vitro. In a two-stage assay utilizing serum-deprived (SD) presuspension cultures of CD34-enriched bone marrow (BM) cells followed by clonal cultures, absolute numbers of granulocyte-macrophage progenitor cells (day-14 granulocyte-macrophage colony-forming units [CFU-GM]) increased progressively to 164% and 204% of input levels after 12 days of culture in the presence of IL-3 alone or in combination with IL-1 beta, respectively. Multilineage (granulocyte erythrocyte macrophage megakaryocyte colony-forming units, CFU-GEMM) and erythroid (erythroid burst-forming units, BFU-E) progenitor cell numbers increased above or were maintained at input levels after 4 and 7 days of liquid culture in the presence of IL-3 and IL-3 plus IL-1 beta, respectively, but in contrast to granulocyte-macrophage colony-forming units (CFU-GM) they were essentially undetectable after 12 days of culture. Progenitors more primitive than colony-forming cells (pre-CFU) were assessed in SD-presuspension cultures of CD34-enriched BM cells purged with mafosfamide to eliminate base-line CFU-GM, CFU-GEMM, and BFU-E. Under these conditions and in the absence of stromal elements, CFU-GM but neither CFU-GEMM nor BFU-E developed in response to cytokines alone. In the additional presence of passaged bone marrow fibroblasts, however, IL-3 plus IL-1 beta and to a lesser degree IL-3 alone induced a pronounced amplification of BFU-E and CFU-GEMM, indicating that their development from a more primitive progenitor compartment requires growth activities in addition to IL-3 and IL-1 beta that are provided by marrow-derived stromal cells such as fibroblasts.