Homing and trafficking of hemopoietic progenitor cells.

Investigations of the homing of transplanted hemopoietic cells into preconditioned recipients have in most studies been referable to parameters that determine engraftment. The principles, however, that govern their early traffic into the host's blood and tissues have remained much less explored. Early studies and experiments from our own laboratory suggest that intravenously administered hemopoietic cells, including progenitors, are not selectively taken up by bone marrow, as they are distributed widely in several tissues (liver, lung, kidney, spleen, bone marrow). As the hemopoietic cells are later on found almost exclusively in the bone marrow (and spleen in the mouse), the data argue for either preferential retention in these tissues or just only preferential survival and proliferation. Our recent studies showing modulation of cells lodged to the bone marrow, before proliferation ensues (i.e. 3 h after infusion), would favor preferential retainment and/or survival of these cells within the bone marrow. Furthermore we established that the VLA4/VCAM-1 adhesion pathway plays a significant role in this process, thus defining VCAM-1 as the dominant bone marrow endothelial addressin in hemopoietic cell homing. Since homing likely represents a cascade of adhesive interactions between hemopoietic cells and bone marrow stroma and/or its extracellular matrix, other adhesion pathways are likely to be involved and remain to be defined. Finally, our data on mobilization of hemopoietic progenitors from normal individuals, induced by blocking the VLA4/VCAM-1 adhesion pathway, suggest that the molecular pathways involved in homing are also of importance in governing hemopoietic progenitor cell trafficking in and out of the bone marrow.