Mc3T3‐G2/PA6 preadipoctyes support in vitro proliferation of hemopoietic stem cells through a mechanism different from that of interleukin 3

Both MC3T3‐G2/PA6 preadipocytes and interleukin 3 (IL 3) can support in vitro proliferation of mouse hemopoietic stem cells (CFU‐S). We examined whether MC3T3‐G2/PA6 cells produce IL 3 and whether a common mechanism might underlie the action of both of these agents. We used cultured mast cells, DA‐1 cells, and FDC‐P2 cells as the targets of IL 3 and conditioned medium (CM) of WEHI‐3 cells as a source of IL 3. MC3T3‐C2/PA6 CM did not support the growth of the above cells. IL 3 mRNA was not detected in the preadipocytes. Since CM obtained from the cocultures of bone marrow cells and MC3T3‐G2/ PA6 cells did not have a significant effect on the growth of the IL 3‐dependent cells, none of the bone marrow cells seem to produce IL 3 under the influence of the preadipocytes. When the factor‐dependent cells were cocultured with MC3T3‐G2/PA6 cells, the former did not survive, whereas mast cells and DA‐1 cells intimately associated with the preadipocytes. Even when bone marrow cells, mast cells, and MC3T3‐G2/PA6 cells were cocultured, the number of CFU‐S increased, but not that of mast cells. These results seem to exclude the possibility of the action of IL 3 in the microenvironment provided by MC3T3‐G2/PA6 preadipocytes.