The influence in vivo of natural murine interleukin-3 on the proliferation of myeloid progenitor cells in mice recovering from sublethal dosages of cyclophosphamide.

Purified natural murine interleukin-3 (IL-3) was assessed for its effects in vivo in mice pretreated 7 days earlier with a sublethal dosage of cyclophosphamide. The multipotential (CFU-GEMM), erythroid (BFU-E) and granulocyte-macrophage (CFU-GM) progenitor cells in these mice were in a slowly- or non-cycling state. Three hours after the i.v. administration of 200 units IL-3 into these mice, the hematopoietic progenitor cells in the marrow and spleen were placed into rapid cell-cycle. At this time, no effects were noted on marrow or spleen nucleated cellularity, numbers of progenitor cells per organ, or peripheral blood counts. No endotoxin was detected in the IL-3 preparation, by Limulus lysate assay. Treatment of IL-3 in vitro at 100 degrees C for 20 min partially decreased its colony stimulating activity in vitro and completely inactivated its proliferation stimulating effects in vivo. These findings suggest that the effects of IL-3 in vivo were not due to contaminating endotoxin or to a non-specific protein effect. These studies do not allow us to conclude whether the effects of IL-3 in vivo are directly on the progenitors and/or are indirect effects mediated by accessory cells.