Acidic isoferritin inhibitory activity: A normal granulopoietic regulator within long term mouse bone marrow cultures

Long term murine bone marrow cultures maintain orderly granulopoiesis for several months. Assay of cell free supernatant obtained at weekly feedings from control cultures demonstrated endogeneously produced acidic isoferritin inhibitory activity. Addition of 10 9 M acidic isoferritin inhibitory activity to long term cultures gave a trend toward fewer nonadherent cells and fewer colony forming units-granulocyte/macrophage (CFU-GM) per flask, but this was not statistically significant. Media conditioned by each fraction of velocity sedimentation separated non-adherent cells gave two discrete peaks of inhibition at 5.6 and 6.8–9 mm/h, respectively. Acidic isoferritin inhibitory activity was restricted to the sharp peak at 5.6 mm/h. Media conditioned by each fraction of velocity sedimentation separated adherent layer cells did not contain assayable acidic isoferritin inhibitory activity. Tests for an endogeneously produced soluble inactivator of acidic isoferritin inhibitory activity were negative. The constitutive production and release of acidic isoferritin inhibitory activity by a restricted population of cells within long term marrow cultures supports the concept of this molecule as a normal hematopoietic regulator.