Effects of bone marrow fibroblasts on the proliferation and differentiation of myeloid leukemic cell lines

The effects of normal bone marrow fibroblasts (BM FB) on proliferation and differentiation of 10 myeloid leukemic cell lines were investigated in a serum-free co-culture system. The proliferation of three of the cell lines was supported by BM FB. Three of the myeloid cell lines were inhibited 40-70%. The co-culture supernatants were tested for the secretion of hematopoietic cytokines by bioassays. Except for IL-6, which was already produced constitutively by BM FB, only little amounts of interleukin-1 (IL-1), granulocyte colony-stimulating factor (G-CSF), or granulocyte-macrophage colony-stimulating factor (GM-CSF) could be detected in several co-culture supernatants. It could be shown that, according to cytologic and functional criteria, the myeloid leukemic cell lines ML-2 and PLB-985 differentiate along the monocyte-macrophage pathway after co-culture with BM FB. They revealed a histiocytic phenotype and could be induced to produce reactive oxygen intermediates (ROI) after stimulation with zymosan or phorbol-myristate-acetate (PMA). Additional proof for differentiation was obtained from flow cytometric analysis of surface differentiation antigens and adhesion molecules. The neutralization of IL-6 activity in the co-cultures by antibodies resulted in prevention of differentiation of PLB-985 cells, while differentiation of ML-2 cells in the co-cultures was not affected by addition of anti-IL-6 antibodies. Furthermore, in co-culture experiments with fibroblasts from skin and foreskin, we found a differentiation of PLB-985 cells comparable to that in co-cultures with BM FB, but poor differentiation of ML-2 cells. These data suggest that different mechanisms are involved in the differentiation of ML-2 and PLB-985 cells.