Homing of human cells in the fetal sheep model: modulation by antibodies activating or inhibiting very late activation antigen-4-dependent function.

The mechanisms by which intravenously (IV)-administered hematopoietic cells home to the bone marrow (BM) are poorly defined. Although insightful information has been obtained in mice, our knowledge about homing of human cells is very limited. In the present study, we investigated the importance of very late activation antigen (VLA)-4 in the early phases of lodgment of human CD34(+) progenitors into the sheep hematopoietic compartment after in utero transplantation. We have found that preincubation of donor cells with anti-VLA-4 blocking antibodies resulted in a profound reduction of human cell lodgment in the fetal BM at 24 and 48 hours after transplantation, with a corresponding increase of human cells in the peripheral circulation. Furthermore, IV infusion of the anti-VLA-4 antibody at later times (posttransplantation days 21 to 24) resulted in redistribution or mobilization of human progenitors from the BM to the peripheral blood. In an attempt to positively modulate homing, we also pretreated human donor cells with an activating antibody to beta1 integrins. This treatment resulted in increased lodgment of donor cells in the fetal liver, presumably for hemodynamic reasons, at the expense of the BM. Given previous involvement of the VLA-4/vascular cell adhesion molecule (VCAM)-1 adhesion pathway in homing and mobilization in the murine system, our present data suggest that cross-reacting ligands (likely VCAM-1) for human VLA-4 exist in sheep BM, thereby implicating conservation of molecular mechanisms of homing and mobilization across disparate species barriers. Thus, information from xenogeneic models of human hematopoiesis and specifically, the human/sheep model of in utero transplantation, may provide valuable insights into human hematopoietic transplantation biology.