Inhibition of immature erythroid progenitor cell proliferation by macrophage inflammatory protein-1α by interacting mainly with a C-C chemokine receptor, CCR1

Several lines of evidence indicate that macrophage inflammatory protein- 1α (MIP-1α) modulates the proliferation of hematopoietic progenitor cells, depending on their maturational stages. To clarify the mechanisms for the modulation of hematopoiasis by this chemokine, we examined the expression of a receptor for MIP-1α, CCR1, on bone marrow cells of normal individuals using a specific antibody and explored the effects of MIP-1α on in vitro erythropoiesis driven by stem cell factor (SCF) and erythropoietin (Epo). CCR1 was expressed on glycophorin A-positive erythroblasts in addition to lymphocytes and granulocytes. CCR1⁺ cells, isolated from bone marrow mononuclear cells (BMMNCs) using a cell sorter, comprised virtually all erythroid progenitor cells in the BMMNCs. Moreover, MIP-1α inhibited, in a dose-dependent manner, colony formation by burst-forming unit-erythroid (BFU- E), but not by colony forming unit-erythroid (CFU-E), in a methylcellulose culture of purified human CD34⁺ bone marrow cells. Although reverse- transcription polymerase chain reaction (RT-PCR) showed the presence of CCR1, CCR4, and CCR5 transcripts in CD34⁺ cells in BM, anti-CCR1 antibodies significantly abrogated the inhibitory effects of MIP-1α on BFU-E formation both in a methylcellulose culture and in a single cell proliferation assay of purified CD34⁺ cells. Although the contribution of CCR4 or CCR5 cannot be completely excluded, these results suggest that MIP1α-mediated suppression of the proliferation of immature, but not mature erythroid progenitor cells, is largely mediated by CCR1 expressed on these progenitor cells.