Differential effects of stromal derived factor-1α (SDF-1α) on early and late stages of human megakaryocytic development

Stromal derived factor-1α (SDF-1α), the high-affinity ligand of CXC-chemokine receptor 4 (CXCR4), was added to human CD34⁺ hematopoietic progenitor cells that can be induced to differentiate along the monocytic or megakaryocytic lineages. In control liquid cell cultures supplemented with two different cytokine cocktails: stem cell factor (SCF), interleukin-3 (IL-3), macrophage-colony stimulating factor (M-CSF), and 10% fetal calf serum (FCS), or, SCF and thrombopoietin (TPO), the expression of surface CXCR4 progressively increased in both the CD14⁺ monocytic and CD41⁺ megakaryocytic lineages. While SDF-1α caused only modest effects on cells of the monocytic lineage, it induced profound down-regulation of CXCR4 in megakaryocytic cells at all stages of differentiation. Moreover, while SDF-1α initially up-regulated the early megakaryocytic antigen CD41, at later time points (days 12-16) it induced down-regulation of the late megakaryocytic antigen CD42b. Consistently, at day 16, the number of mature megakaryocytes was significantly decreased in cultures supplemented with SDF-1α. These findings indicate that, besides its primary role in regulating the retention of precursor cells in hematopoietic tissues, the SDF-1α/CXCR4 system participates in the regulation of megakaryocytic development by stimulating the formation of immature megakaryoblasts and inhibiting the formation of mature megakaryocytes. (C) 2000 Wiley-Liss, Inc.