CD44-mediated hematopoietic progenitor cell adhesion and its complex role in myelopoiesis.

During hematopoiesis in the bone marrow (BM), hematopoietic progenitor cells (HPC) proliferate and differentiate in intimate contact with BM stromal cells in a highly regulated interplay among cytokines, chemokines, and adhesion molecules. Using the mouse BM stromal cell line MS-5 as immunogen, we raised monoclonal antibodies (mAb) against surface proteins that could participate in interactions between HPC and stroma. Flow cytometry and immunodepletion analysis indicated that two antibodies, MA-8 and MA-2, recognized CD44. Although MA-8 and another anti-CD44 antibody, IM7.8.1, blocked adhesion of hematopoietic cells to hyaluronate, adhesion to BM stroma was notably increased by these mAb, suggesting minor roles for CD44/hyaluronate interaction and the triggering of additional adhesion pathways upon CD44 engagement. The presence of MA-8 in Dexter-type long-term BM cultures (LTBMC) only slightly inhibited production of myeloid cells, and the number of myeloid progenitors (measured as CFU-GM) in the supernatants of LTBMC performed with MA-8 was similar to those carried out with control antibodies. Instead, IM7.8.1 completely blocked myelopoiesis, whereas an anti-alpha 4 integrin mAb increased CFU-GM amounts. In summary, two anti-CD44 antibodies with similar effects in cell adhesion showed notable differences in myelopoietic cultures, indicating the structural and functional complexity of the CD44 molecule.