Cell source and biological characteristics of murine bone marrow-derived colony-promoting activity

A murine colony-promoting activity (CPA) was found in the supernatants of Dexter long-term bone marrow cultures (LTBMC). This activity itself failed to stimulate in vitro granulocyte-macrophage colony (CFU-GM) formation but could increase the number of colonies induced by colony-stimulating factors (CSFs). CPA was produced by the adherent stromal cells but not by the nonadherent cells. No CPA could be detected in cultures of pure marrow fibroblasts, nor was it secreted by the stromal cells following macrophage depletion. In contrast, a large amount of CPA was found in cultures of isolated macrophages, suggesting that marrow macrophages may be the main cell source of CPA. Although colony formation was augmented by adding CPA in combination with various CSFs, the colony type induced by CPA plus CSF was no different from that of CSF alone. Preincubation of bone marrow (BM) cells with CPA at 37 degrees C for 24 hours before using in clonal culture assay resulted in a marked colony enhancement. Furthermore, colony formation by 5-fluorouracil (5-FU)-treated marrow cells could be induced by granulocyte-macrophage (GM)-CSF plus CPA but not by GM-CSF alone. These results suggest that CPA may act on early developing hematopoietic stem cells to induce them to differentiate into more mature myeloid progenitor cells capable of responding to CSF stimulation. CPA was nondialyzable and stable under heat (56 degrees C for 30 minutes) and freeze/thawing (3 times). Its activity was acid-labile (pH 2.0) but relatively alkaline-resistant (pH 11.0). When treated with enzymes, CPA was sensitive to trypsin and bacterial protease but not to neuraminidase. In addition, the activity of CPA could be abrogated by anti-CPA antiserum but remained unchanged after treatment with antibodies to other murine hematopoietic synergizing/stimulating factors, including interleukin-1 (IL-1), IL-3, IL-4, IL-6, and stem cell factor (SCF).