Interleukin 3 alone does not support the proliferation of bone marrow cells from A/J mice: a novel system for studying the synergistic activities of IL‐3

The number of colonies produced by bone marrow cells in response to interleukin 3 (IL-3) in soft agar cultures varies according to the strain of the donor mice. A/J, AKR, A.TH and A.TL bone marrow cells are particularly hyporesponsive, producing only occasional colonies in the presence of IL-3. Bone marrow cells from all strains of mice, including A/J, produce distinctively large colonies in response to the combination of IL-3 and macrophage colony stimulating factor (M-CSF). In cultures of A/J bone marrow cells, the synergy between IL-3 and M-CSF is further reflected in an increase in both the number and the variety of colonies produced. The increase in colony numbers may be due to the priming of a population of A/J colony-forming-cells (CFCs) by IL-3, enabling them to respond to M-CSF. In support of this notion, IL-3 enhanced the level of c-fms (M-CSF receptor) messenger RNA in cultures of A/J bone marrow cells. It is also possible that a subpopulation of CFCs requires both IL-3 and M-CSF as co-mitogens. The A/J strain provides a novel system for studying the mechanisms involved in the interaction between IL-3 and M-CSF in haemopoiesis.