Distinct and direct synergistic effects of IL-1 and IL-6 on proliferation and differentiation of primitive murine hematopoietic progenitor cells in vitro.

While interleukin-1 (IL-1) and IL-6 have been demonstrated to synergize with colony-stimulating factors (CSFs) and stem cell factor (SCF) to stimulate myeloid colony formation of primitive hematopoietic progenitor cells, it has not yet been established whether these effects are directly mediated. In the present study, direct effects of IL-1 and IL-6 were examined on primitive Lin-Sca-1+ murine bone-marrow progenitor cells that were cultured and plated individually. IL-1 and IL-6 showed not only overlapping, but also distinct, patterns of direct synergy. While IL-1 or IL-6 had no proliferative effects as single growth factors, IL-1, in combination with granulocyte-macrophage CSF (GM-CSF), IL-3, CSF-1, and SCF, but not granulocyte-CSF (G-CSF), enhanced the cloning frequency of Lin-Sca-1+ progenitors three- to five-fold, whereas IL-6 increased the cloning frequency in response to all four CSFs and SCF two- to seven-fold. In all cases, the size of the colonies observed were increased as well. Furthermore, the combined action of IL-1 and IL-6 resulted in additive or synergistic enhancement of CSF- and SCF-stimulated colony formation of Lin-Sca-1+ high proliferative potential colony-forming cells (HPP-CFCs). Finally, IL-6, but not IL-1, enhanced the number of immature blast cells observed in CSF- and SCF-stimulated cultures.