trOPN interacting via α₉β₁ is potently chemotactic for HSC/HPC, and OPN is critical for the homing of murine HSC/HPC isolated from the central BM, but not those isolated from the endosteal region. (A) FACS isolated CB HSC/HPC were tested in a chemotactic assay for their response to SDF-1 or OPN and compared with medium alone. The red lines indicate the distance migrated by cells stimulated under each condition. (B) A chemotactic response to full-length OPN and trOPN was compared with cell migration in medium alone and SDF-1 for CB HSC (CD34⁺CD38⁻), (C) CB progenitor cells (CD34⁺CD38⁺), (D) human BM HSC (CD34⁺CD38⁻), (E) human BM progenitor cells (CD34⁺CD38⁺), or (F) murine endosteal and central BM HSC/HPC (LSK). Human CD34⁺CD38⁻ and murine LSK cells demonstrated equivalent chemotaxis to SDF-1 compared with trOPN (P < .05). To determine the mechanism of the observed migration of HSC to trOPN, CD34⁺CD38⁻ cells were preincubated in blocking antibodies to either α₉β₁ or α₄. Values are the mean plus the standard error of the mean (SEM) of the number of migrated cells. (G) LSK harvested from the endosteal (RFP⁺ mice) or central (CFSE⁺ labeled C57/B6 mice) BM regions of wild-type mice were transplanted into nonablated wild-type or Opn^−/− recipients. There was a significant decrease in the homing efficiency of LSK isolated from the central BM region homing into an Opn^−/− microenvironment (P < .05). (H) Data are the mean plus SEM from 10 wild-type and 13 Opn^−/− recipients in 3 independent replicates as expressed by different colors.