Inhibitors of hematopoietic colonies are produced by certain rat fibroblastoid cell lines and are modulated by corticosteroids.

Both stimulatory (CSA) and inhibitory (INH) factors may contribute to hematopoietic regulation, but little is known about how their physiologic balance is maintained. Previously we have shown that antigen-defined fibroblastoid cells cultured from rat lung (ST3-/ST4+) constitutively produce INH, and those derived from bone marrow (ST3+/ST4-) respond to macrophage cytokines to release both CSA and INH into their conditioned media (CM). Here we show that this pattern was maintained in cell strains ("ST3" and "ST4") propagated from the primary cultures, and that the presence of CSA was measured in "ST4" CM if the inhibitory > 100 kd fraction was removed. Two subclones of the "ST3" line, called 2A and 9D, were selected for high or low expression of the ST3 antigen, respectively. Both produced CSA, but only 9D produced the > 100 kd inhibitor. In the CM of cells cultured in the presence of hydrocortisone, there was less INH detected but CSA was not changed. From these data, however, we cannot assess how many individual cell products might be contributing to the INH activity. These results demonstrate that the appearance of inhibitory activity in the growth media differs among fibroblast subpopulations, and that it can be modified by natural regulators such as corticosteroids.