Characterization of CXCR4 and SDF-1 in hematopoietic stem and progenitor cell function

Hematopoietic stem cells (HSC) are defined by a capacity for self-renewal and the ability to form all cells comprising the blood. Chemokines regulate the proliferation and migration of lineage restricted hematopoietic progenitors and are, thus, candidate molecules for regulating HSC function. The studies presented here examined the potential role of the chemokine SDF-1 and its putative receptor, CXCR4, on human hematopoietic progenitors and HSC and in the migration of primitive blood cells to the skeletal muscle hematopoietic site. By screening a panel of chemokine receptors whose ligands have been implicated in the regulation of blood progenitors, CXCR4 was identified as a predominate receptor expressed on rare, stem cell enriched, populations throughout human ontogeny. In vitro and in vivo functional assays demonstrated that CXCR4 is expressed only on a fraction of primitive blood progenitors and candidate human HSC with bone marrow (BM) repopulating capacity (SRC). CXCR4+ and CXCR4- SRC had the identical proliferative potential and multipotent differentiation capacity upon i.v. transplantation into NOD/SCID mice and engrafted similar tissue sites including the BM and skeletal muscle. To gain insight into the potential function of SDF-1 and CXCR4, serum free cultures and transwell assays were used to characterize the effects of this receptor/ligand pair on the proliferation of human blood progenitors and the migration of murine BM cells in response to skeletal myofibers. SDF-1 alone promoted the survival of human hematopoietic progenitors and enhanced their expansion in synergy with the growth factors, stem cell factor (SCF) and Flt-3. In transwell assays, SDF-1 and CXCR4 regulated the migration of hematopoietic progenitors to BM stroma but not towards myofibers. Instead, migration of primitive blood cells towards muscle was inhibited by blocking the hepatocyte growth factor (HGF) receptor, c-met. The work presented here identifies novel CXCR4+ and CXCR4- stem cell subsets in the human and suggests that CXCR4 and SDF-1 are not critical to BM repopulating function but can augment the proliferation and survival of lineage restricted hematopoietic progenitors. In addition, this work demonstrates that HGF induced migration is an alternative mechanism of progenitor cell motility and identifies distinct populations of hematopoietic progenitors in the BM that migrate in response to either SDF-1 or HGF. Taken together the data suggests a relationship between CXCR4 and c-met expression and the migration of primitive blood cells between the BM and skeletal muscle.