Marginal expression of CXCR4 on c-kit+Sca-1+Lineage− hematopoietic stem/progenitor cells

Stromal cell-derived factor 1 (SDF-1) and its receptor CXCR4 are the key regulatory molecules of hematopoietic stem cell (HSC) migration and engraftment to the bone marrow (BM) microenvironment. However, the significance of the ligand–receptor complex on HSC in steady-state BM is not clear. There is currently a lack of information as to how CXCR4 is expressed on HSCs. We herein demonstrate that c-kit+Sca-1+Lineage− (KSL) cells freshly isolated from BM expressed very low to undetectable levels of CXCR4. Two hours of incubation at 37°C quickly up-modulated the receptor expression on KSL cells. Protein synthesis was not required for this early stage up-regulation, thus suggesting the emergence of intracellularly pooled receptors to the cell surface. However, protein synthesis was involved at the later stage of up-regulation. The up-regulated CXCR4 was functional, as evidenced by the fact that the incubated KSL cells more efficiently migrated to the SDF-1 gradient in vitro. Therefore, although KSL cells are able to express functional CXCR4, the receptors are only marginally expressed in the steady-state BM microenvironment. These observations therefore indicate the limited role of the SDF-1-CXCR4 axis on HSC functionality in a static BM environment.