Medium conditioned by activated T lymphocytes stimulates the in vitro proliferation of pluripotent hematopoietic stem cells (spleen colony-forming units [CFU-S]) but the factors involved have not been identified. Because the lymphokine interleukin 3 (IL-3) enhances in vitro colony formation by committed hematopoietic progenitor cells, we examined the effect of IL-3 on the in vitro proliferation of CFU-S using an 11-d spleen colony assay. When mouse marrow cells were placed in liquid culture, CFU-S content declined progressively and by 96 h only 13% of the CFU-S remained. By contrast, after 96 h in the presence of 20 U/ml of IL-3, the number of CFU-S were the same as that in the initial inoculum. Although the number of CFU-S eventually declined, they could still be recovered after 264 h of culture. In the absence of IL-3, the number of CFU-S synthesizing DNA was negligible; in its presence, greater than 20% of the CFU-S were in cycle. IL-3 stimulated CFU-S proliferation at a concentration of 0.2 U/ml. The dose-response curve was similar to that observed for other biologic effects of the lymphokine, and as little as 1 h of exposure to IL-3 enhanced the survival of CFU-S in vitro. Treatment of marrow cells with anti-Thy 1.2 antibody and complement before exposure to IL-3 did not inhibit spleen colony formation, but treatment of the cells with anti-Thy 1.2 antibody and complement after exposure to IL-3 reduced CFU-S recovery after 96 h of culture by 45%. The cell composition of day 11 spleen colonies formed by IL-3-treated marrow cells was similar to that of colonies formed by untreated marrow cells. Finally, day 11 CFU-S persisting in the marrow of mice treated with 5-fluorouracil required IL-3 for proliferation in vitro. Taken together, these data indicate that IL-3 promotes the proliferation of CFU-S in vitro, increases the number of CFU-S synthesizing DNA, but does not alter their commitment program, and the target cell population includes CFU-S with self-renewal and marrow-repopulating ability.
J L Spivak, R R Smith, J N Ihle