In vivo imaging studies of the effect of recipient conditioning, donor cell phenotype and antigen disparity on homing of haematopoietic cells to the bone marrow.

Homing of transplanted bone marrow cells (BMC) to the host bone marrow (BM) is the first step of engraftment towards durable multilineage haematopoietic reconstitution. We used an in vivo assay to track PKH-labelled cells in the BM of mice after transplantation, using fluorescence microscopy through an optical window placed over the distal femoral epiphysis. Within hours after intravenous injection, the cells moved in and out the femur, and were mobile within the marrow space. One hour after injection of whole BMC into non-conditioned syngeneic and allogeneic recipients, the homing efficiencies (HE) were 1.23 +/- 0.14% and 0.12 +/- 0.02% respectively (P < 0.001). Irrespective of antigen disparity, the number of PKH-labelled cells in the femur decreased by 30% and 50% after 1 and 3 d respectively (P < 0.001). Similar HE of naïve and irradiated cells suggested that the majority of cells (> 80%) were quiescent in the BM during the first 3 d. HE were twofold higher in busulphan-myeloablated recipients (P < 0.001 vs non-conditioned), and allogeneic transplantation resulted in 84 +/- 9% donor chimaerism at 4 weeks. The HE of lin- cells was 16-fold higher than that of lin+ cells (P < 0.001), and the subset of lin- SCA-1+ cells was 4.6-fold higher in the BM-homed cell population (P < 0.001 vs lin- cells). Approximately 1,500 of the BM-homed cells rescued 62-71% secondary syngeneic and allogeneic myeloablated recipients. Strikingly, the HE could be predicted during the first 3 d after transplantation by correcting the measurements performed in vivo for the enrichment of progenitors in donor inoculum, donor-recipient antigen disparity and myeloablative conditioning.