Identification of leukemia cell-derived inhibitory activity (LIA) in conditioned media from human myeloid leukemic cell line ML-2

Conditioned media from the human myeloid leukemic cell line ML-2 contain a factor that inhibits the entry of normal CFU-GM into S phase of mitotic cycle as measured by the 3H-TdR suicide technique. This factor was detected in conditioned media prepared by incubating 5×106 ML-2 cells/ml or 1×106 ML-2 cells/ml in serum-free RPMI for 5 or 24 hours respectively, and was isolated by ultrafiltration through an XM300 Diaflo membrane followed by chromatography on Sepharose 6B. Ferritin, prepared from human placenta, had the same inhibitory effect on CFU-GM. Antibodies against human placental ferritin completely inactivated the inhibitory effect of both human placental ferritin and the factor released from ML-2 cells. The inhibitory activity produced by the cell-line ML-2 was considered as LIA (leukemia cell-derived inhibitory activity) earlier found in HL-60 cell line and AML and CML cells.