IL-6 interferes with stimulation of HPP-CFC and large CFU-Mk in conjunction with cytokine combinations from primitive murine marrow cells

Abstract

The growth-promoting activities of interleukin 6 (IL-6) in combination with early-acting hematopoietic factors, i.e., stem cell factor (SCF) and interleukin-1α (IL-1α), on primitive hematopoietic and megakaryocyte progenitors (high proliferative potential colony-forming cells [HPP-CFC] and colony-forming units-megakaryocyte [CFU-Mk], respectively) from 5- fluorouracil (5-FU)-treated murine bone marrow cells (BMC) were evaluated in serum-free fibrin clot cultures. IL-6 in combination with SCF and IL-1 induced an irregular and abortive hematopoiesis characterized by a reduction in colony size of at least 50% over those stimulated by SCF + IL-1 + IL-3 and an inability to continue growth to day 12. Moreover, IL-6 in combination with the early-acting factors, SCF and IL-1, had no effect on the formation of HPP-CFC. IL-6 is synergistic with SCF + IL-1 on day 7 CFU-Mk but did not stimulate large day 12 CFU-Mk. Our results suggest that, in the absence of serum, IL-6 prevents the continued proliferation of early hematopoietic and megakaryocytic progenitors initiated by SCF + IL-1 + IL-3. Optimization of cytokine combinations for use in ex vivo expansion of marrow progenitors, either for stem cell transplants or gene therapy, must consider not only the number of colonies but their size, as well as the contributions of serum components.