Differential role for very late antigen-5 in mobilization and homing of hematopoietic stem cells

The role of very late antigen-5 (VLA-5) in homing and mobilization of hematopoietic stem cells from normal bone marrow (NBM) and bone marrow (MBM) and peripheral blood (MPB) from mobilized mice was investigated. We found a decreased number of VLA-5-expressing cells in the lineage-negative fraction of MPB. However, virtually all stem/progenitor cells were present in the VLA- 5(+) fraction and hence mobilization of hematopoietic stem cell subsets does not coincide with a downregulation of VLA-5. Stem/progenitor cells from MPB and MBM demonstrated enhanced stromal-derived factor-alpha-induced migration. This enhanced migration correlates with an improved hematopoietic reconstitution potential, with the migrated MPB cells showing the fastest reconstitution. Interestingly, homing of MPB, MBM and NBM stem/progenitor cells in bone marrow and spleen did not differ and is therefore not responsible for the differences in hematopoietic reconstitution. The observed increase in VLA- 5(+) cells in the recipients after transplantation can most probably be attributed to selective homing of VLA- 5(+) cells instead of an upregulation of VLA-5. Treatment with an antibody to VLA-5 partially inhibited bone marrow homing of progenitor cells, whereas homing in the spleen was hardly affected. These data indicate a differential role for VLA- 5 in the movement of stem cells from and toward bone marrow.