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

Cellular interactions between hematopoietic progenitor cells and bone marro w (BM) stromal cells are mediated by cell adhesion molecules (CAM). In agre ement 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 perip heral 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 e xamined 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 an d PSC CD34(+) cell adhesion to BM stromal cells were observed. However, neu tralizing antibody to VLA-4 significantly inhibited BM CD34+ but not PSC CD 34(+) cell adherence to stromal cells, suggesting a role for alternative CA M in cell binding. Further, in long-term co-cultures of BM CD34(+) cells wi th BM stroma, we observed a significantly higher number of colony-forming u nits granulocyte-macrophage (CFU-GM) released into the media following trea tment with neutralizing antibody to VLA-4 than in untreated control culture s. In contrast, no difference in the frequency of nonadherent CFU-GM betwee n antibody-treated and control long-term co-cultures of PSC CD34(+) cells w ith BM stromal cells was observed. This suggests that VLA-4 expression on m obilized 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 sug gest that the decreased expression of VLA-4 may have a role in the mobiliza tion of progenitor cells, in part, by regulating their adherence to stromal cells, although additional mediators of adhesion are also involved.