Growth factor mobilization and modulation of progenitor cell adhesion to stromal cells: Role of VLA-4

Cellular interactions between hematopoietic progenitor cells and bone marrow (BM) stromal cells are mediated by cell adhesion molecules (CAM). In agreement with previous studies, our flow cytometric analysis of isolated CD34⁺ cells showed that VLA-4 expression was significantly (p < 0.001) higher on steady-state BM than on CD34⁺ cells from growth factor-mobilized peripheral stem cell (PSC) products. To determine whether the expression of VLA-4 on progenitor cells plays a role in their adhesion to stromal cells, we examined the binding of isolated CD34⁺ progenitor cells from BM (n = 14) and PSC (n = 10) products to BM stromal cells in the presence or absence of a neutralizing antibody to VLA-4. In these studies, similar kinetics of BM and PSC CD34⁺ cell adhesion to BM stromal cells were observed. However, neutralizing antibody to VLA-4 significantly inhibited BM CD34⁺ but not PSC CD34⁺ cell adherence to stromal cells, suggesting a role for alternative CAM in cell binding. Further, in long-term co-cultures of BM CD34⁺ cells with BM stroma, we observed a significantly higher number of colony-forming units granulocyte-macrophage (CFU-GM) released into the media following treatment with neutralizing antibody to VLA-4 than in untreated control cultures. In contrast, no difference in the frequency of nonadherent CFU-GM between antibody-treated and control long-term co-cultures of PSC CD34⁺ cells with BM stromal cells was observed. This suggests that VLA-4 expression on mobilized PSC versus BM CD34⁺ cells has biologic relevance for at least 2 weeks based on the long-term BM culture results. In summary, these data suggest that the decreased expression of VLA-4 may have a role in the mobilization of progenitor cells, in part, by regulating their adherence to stromal cells, although additional mediators of adhesion are also involved.