IL-7 stimulates CSF-induced proliferation of murine bone marrow macrophages and Mac-1+ myeloid progenitors in vitro.

The role of IL-7 as an important stimulator of the growth of B and T cell precursors, as well as mature T cells, is well established. In contrast, the role of IL-7 in myelopoiesis has not been characterized thoroughly, and thus, IL-7 has been regarded as a lymphoid lineage-restricted cytokine. However, we have recently reported that IL-7 enhanced CSF-induced myeloid proliferation of primitive murine hematopoietic (Lin-Sca-1+) progenitors, whereas IL-7 did not affect significantly the proliferation of a population of more mature (Lin-) progenitors. The present study was initiated to investigate further whether IL-7 might affect CSF-induced proliferation of subpopulations of committed myeloid progenitors as well as mature bone marrow macrophages. IL-7 enhanced macrophage colony-stimulating factor (CSF-1)-induced colony formation of single bone marrow macrophages 90%, whereas IL-7 alone had no effect. Furthermore, IL-7, in a concentration-dependent manner, increased the proliferation of mononuclear cells expressing the Mac-1 Ag (Mac-1+ mononuclear cells (MNC); CD11b) up to fivefold in response to CSF-1, granulocyte macrophage-CSF (GM-CSF), or IL-3. In contrast, no effect of IL-7 was observed on Mac-1- MNC. The synergistic effect of IL-7 on Mac-1+ MNC was caused by an increase in macrophage colonies (CFU-M) and mixed granulocyte-macrophage colonies (CFU-GM), whereas the total number of granulocyte colonies (CFU-G) was not affected. This suggests that IL-7 can provide proliferative signals to Mac-1+ progenitors with a macrophage potential, but not to progenitors committed to pure granulocyte differentiation.