Antiproliferative effect of bone marrow cells on leukemic cells.

When normal murine bone marrow (BM) cells were cultured with either L1210 lymphoma cells or P815 mastocytoma cells for 24 h, considerable tumor growth suppression without substantial tumor cell lysis was found. Under the same conditions, normal spleen cells also demonstrated the antitumor cytostatic activity, but not as significant as that characteristic of BM cells, whereas both normal thymus and lymph node cells had not any suppressive effect on tumor cell proliferation. The comparable cytostatic effects occurred in both syngeneic and allogeneic BM-tumor cell combinations. The cytostatic BM-effectors were distinct from T and B lymphocytes or mature macrophages. After being separated on a discontinuous Percoll density gradient, the cells active in suppressing tumor growth were recovered predominantly in 1.075 and 1.060 density fractions. The cytostatic BM effectors, at least in their part, were resistant to x-irradiation up to 2000 rad included. Collectively these results suggest that normal BM, being deficient in cell-mediated antitumor cytolytic activity, has a significant leukemia growth inhibitory potential; and that cytostatic BM effectors are similar in their characteristics to natural suppressor cells.