Durable hematopoietic stem cell engraftment requires efficient homing to and seeding in the recipient bone marrow. Dissection of cellular and molecular mechanisms by retrospective analysis of functional engraftment studies imposes severe limitations on the understanding of the early stages of this process. We have established an experimental approach for in vivo functional imaging of labeled cells at the level of recipient bone marrow in real time. The adhesive interaction of hematopoietic cells with the bone marrow stroma evolves as the most important early event. Adhesion to the marrow, rather than the vascular endothelium, determines the efficiency of both homing and seeding, and is absolutely essential to maintain cell viability in the marrow. Seeding and engraftment may be improved either by bypassing homing or by localized transplant of a large number of cells in a relatively small marrow space. There is functional redundancy in the molecular pathways that mediate the cell-stroma interaction, such that blockage of a single pathway has only minor effect on homing and seeding. We hypothesize that successfully seeding-engrafting cells undergo extensive phenotypic changes as a consequence of interaction with the stroma, without engaging in rapid proliferation. Surprisingly, Fas-ligand appears to promote hematopoietic cell engraftment by immunomodulatory and trophic effects
