Adhesion-modulation of BFU-E and GM-CFC by antibodies against com

Adhesion of hematopoletic progenitors (HPC) to stromal cells induces qulesence (Eaves et at. Stood, 78_: 110,1991; Hurle et al. Jdinlnmst. 3£: 511,1995). Regulation ot adhesion would, thus, indirectly regulate quiesence. We here Investigated the role 01 CD44 in adhesion regulation since positive and negative modulation of adhesiveness has been described to be mediated by this molecule. A panel of 19 monoclonal anntibodws against (mAb) CD which recognized three distinct epitopes: 1. 2, and 3. was studied. MAb were present in an initial z hr adhesion period, whereafter adherent and non-adherent HPC were assessed separately. We found that four group 1 mAb consistently enhanced adhesion of BFU-E and GM-CFC to bone marrow stromal cells 2to 5-1old. In contrast, one mAb from each of the groups Z, and 3 inhibited adhesion. The other 13 mAb did not demonstrate significant adhesion modulatory activity. The mAb, when present for 2 hrs, did not affect growth properties of HPC. since the total recovery of HPC was unchanged. In subsequent experiments, we focused on one particular enhancing mAb, L178. L178 increased adhesion ot CD34+ BFU-E and GM-CFC depended on time. dose, and the kttegrins VLA-4 and VLA-5, but did not involve LFA-1. However, binding of HPC to the common ligand of VLA-4 and -5. tüxonectin was not enhanced by L178. Binding ot untreated and L178-treaied HPC to stromal cells required Mg2, but not the presence of Ca2+. Stroma-adherence of BFU-E was highly sensitive to inhibition by the protein tyrosine klnase (PTK) inhibitor genisteln. whereas that of GMCFC was not. After treatment with L178. both BFU-E and GM-CFC binding was inhibited by genistein, indicating L178 induces PTK activity. In addition, L178-treatment Induced sensitivity to the protein kinase A and C Inhibitors. Thus. L178 activates multiple intracellular signaling pathways to enhance adhesion of BFU-E and GM-CFC. In conclusion, we established a role for CD44 in adhesion-modulation of committed HPC. in which epitope 1 of CD44 is associated with upregulation of adhesion. Since adhesion of HPC controls quiescence, and maybe also proliferation and/or differentiation, the present study points to a key role tor CD44 in hematopoietic regulation.