Hematopoietic stem cell interaction with elements ofthe underlying stroma is essential for sustained normal hematopoiesis. Here we have determined that adhesion receptors in the integrin family play a role in promoting adhesion ofhuman hematopoietic stem cells to cultured human marrow stromal cells.

Type-beta transforming growth factors (TGF-beta s) are polypeptides that act hormonally to control proliferation and differentiation of many cell types. Two distinct homodimeric TGF-beta polypeptides, TGF-beta 1 and TGF-beta 2 have been identified which show approximately 70% amino-acid sequence similarity. Despite their structural differences, TGF-beta 1 and TGF-beta 2 are equally potent at inhibiting epithelial cell proliferation and adipogenic differentiation. The recent immunohistochemical localization of high levels of TGF-beta in the bone marrow and haematopoietic progenitors of the fetal liver has raised the possibility that TGF-beta s might be involved in the regulation of haematopoiesis. Here we show that TGF-beta 1, but not TGF-beta 2, is a potent inhibitor of haematopoietic progenitor cell proliferation. TGF-beta 1 inhibited colony formation by murine factor-dependent haematopoietic progenitor cells in response to interleukin-3 (IL-3) or granulocyte-macrophage colony stimulating factor (GM-CSF), as well as colony formation by marrow progenitor cells responding to CSF-1 (M-CSF). The progenitor cell lines examined were approximately 100-fold more sensitive to TGF-beta 1 than TGF-beta 2, and displayed type-I TGF-beta receptors with affinity approximately 20-fold higher for TGF-beta 1 than TGF-beta 2. These results identify TGF-beta 1 as a novel regulator of haematopoiesis that acts through type-I TGF-beta receptors to modulate proliferation of progenitor cells in response to haematopoietic growth factors.

The precursor for transforming growth factor a, pro-TGF-a, is a cell surface glycoprotein that can establish contact with epidermal growth factor (EGF) receptors on adjacent cells. To examine whether the pro-TGF-a/EGF receptor pair can simultaneously mediate cell adhesion and promote cell proliferation, we have expressed pro-TGF-a in a bone marrow stromal cell line. Expression ofpro-TGF-a allows these cells to support long-term attachment of an EGF/ interleukin-3-dependent hematopoietic progenitor cell line that expresses EGF receptors but is unable to adhere to normal stroma. This interaction is inhibited by soluble EGF receptor ligands. Further, the hematopoietic progenitor cells replicate their DNA while they are attached to the stromal cell layer and become foci of sustained cell proliferation. Thus, pro-TGF-a and the EGF receptor can function as mediators ofintercellular adhesion and this interaction may promote a mitogenic response. We propose the term "juxtacrine" to designate this form of stimulation between adjacent cells.

These results argue for the feasibility of intracavitary DCC-E1A administration, provide a clear proof of preclinical concept, and warrant phase II trials to determine the antitumor activity of the E1A gene.

Whether bone marrow stromal cells of donors contribute physiologically to hematopoietic stem cell reconstitution after marrow transplantation is unknown. To determine the transplantability of nonhematopoietic marrow stromal cells, stable clonal stromal cell line (GB1/6) expressing the a isoenzyme of glucose-6-phosphate isomerase (Glu6PI-a, D-glucose-6-phosphate ketol-isomerase; EC 5.3