CAPTURE AND EXPANSION OF BONE-MARROW-DERIVED MESENCHYMAL PROGENITOR CELLS WITH A TRANSFORMING GROWTH-FACTOR-BETA-1 VON WILLEBRANDS FACTOR FUSION PROTEIN FOR RETROVIRUS-MEDIATED DELIVERY OF COAGULATION-FACTOR-IX

Mesenchymal stem cells give rise to the progenitors of many differenti ated phenotypes, including osteocytes, chrondocytes, myocytes, adipocy tes, fibroblasts, and marrow stromal cells, which are capable of self- renewal and undergo expansion in the presence of transforming growth f actor-beta 1 (TGF-beta 1). The present study was designed to test the concept that mesenchymal progenitor cells could be selected and expand ed by virtue of their intrinsic physiologic responses to TGF-beta 1. H uman bone marrow aspirates were initially cultured, under low serum co nditions, in collagen pads or gels impregnated with a genetically engi neered TGF-beta 1 fusion protein bearing an auxiliary von Willebrand's factor-derived collagen-binding domain (TGF-beta 1-vWF). Histologic e xamination of TGF-beta 1-vWF-supplemented collagen pads from 8-day cul tures revealed the selective survival of a population of mononuclear b lastoid cells. Thee TGF-beta-responsive cells were expanded to form st romal/fibroblastic colonies by serum reconstitution, and further to fo rm osteogenic colonies upon supplementation with osteoinductive factor s. In comparative studies, both marrow-derived progenitor cells and ma ture stromal cells were transduced with a retroviral vector bearing a human factor IX construct. Both the transduced progenitor cells and ma ture stromal cells expressed the factor IX transgene at levels compara ble to those reported for human fibroblasts. Transplantation of murine progenitor cells bearing the human factor IX vector into syngeneic B6 CBA mice resulted in detectable circulating levels of the human factor IX antigen. Taken together, these data demonstrate a novel physiologi c approach for the selection of mesenchymal precursor cells followed b y mitotic expansion, transduction, and transplantation of these progen itor cells with retroviral vectors bearing therapeutic genes.