Suppression of Normal Granulopoiesis In Vitro by a Leukemia - Associated Inhibitor ( LA ! ) of Acute and Chronic Leukemia

To determine if normal granulopoiesis can be suppressed by substances produced by cells from patients with leukemia. conditioned media from such cells were assayed for their ability to reduce the fraction of normal CFU-c in S-phase or inhibit cluster/colony formation in agar cultures. Low-density (<1 .077 g/ml) marrow or blood cells from patients with leukemia. but not from normals. were shown to release a high molecular weight substance (>300.000 daltons) called “leukemia-associated inhibitor” (LAl) that reduces the fraction of normal marrow CFU-c in S-phase. This effect appeared to be reversible. In 10/12 patients with CML, 12/15 with AMI, 1 /7 with CLI. and 2/6 with ALL. significant amounts of LAI were detected. LAI also reduces the labeling index of normal myeloblasts. LAI is resistant to heating to 6O C for 10 mm. Conditioned media also contain a substance that inhibits cluster/colony formation. which is distinguished from LAI by its heat lability. The production of LAI is dependent on an active cell metabolism and is abolished at +4CC or by cycloheximide. LAI is also released in the absence of fetal calf serum and after trypsinization of the cells. Its production reaches a plateau after 3-5 hr of incubation at an optimal cell concentration of 5 x 10 cells/mI. When CML or AML cells were used as a target. 9/1 1 patients were found to be resistant to the action of LAI. LAI may be responsible for suppression of normal granulopoiesis in leukemia and provide a growth advantage for the leukemic cells.