VLA-5 IS EXPRESSED BY MOUSE AND HUMAN LONG-TERM REPOPULATING HEMATOPOIETIC-CELLS AND MEDIATES ADHESION TO EXTRACELLULAR-MATRIX PROTEIN FIBRONECTIN

Fibronectin (FN), an extracellular matrix protein, is involved in the adhesion and migration of hematopoietic cells and has been shown to en hance retroviral gene transfer into primitive hematopoietic cells by c o-localization of target cells and retrovirus when used as a substrate in vitro. We have previously found that mouse hematopoietic stem cell s could be transduced on a FN fragment that included the recognition s equence Arg-Gly-Asp (RGD), suggesting that stem cells may express the integrin very late antigen (VLA)-5, To address this, we investigated t he binding of mouse and human hematopoietic cells to recombinant pepti des that contained one or a combination of the three principle cell-bi nding domains of FN, These domains included the VLA-5-binding sequence RGD, the VLA-4-binding site CS1, and the high affinity heparin-bindin g domain. Here we show that mouse long-term in vivo repopulating stem cells, as well as primitive human NOD/SCID mouse repopulating cells, c an bind extracellular matrix protein FN by using integrin VLA-5 in vit ro. This binding is specific and can be inhibited by antibodies to VLA -5, In addition, preincubation of BM cells with peptide CH-296, which contains all three primary FN-binding domains, decreased the engraftme nt of cells in the bone marrow in vivo, while intravenous injection of the same peptide induced an increase of progenitor cells in the splee n. In summary, our data demonstrate that VLA-5 is expressed on primiti ve mouse and human hematopoietic cells and suggest that there may be s ignificant cooperation between integrin receptors and proteoglycan mol ecules in the engraftment of bone marrow cells and hematopoietic cell adhesion in vivo.