Background and objectives: The maintenance of adequate interactions with the bone marrow (BM) microenvironment is critical to ensure efficient homing of ex vivo-expanded hematopoietic cells. This study was intended to assess adhesion and migration properties of long-term culture-initiating cells (LTC-IC) harvested after self-renewal division in ex vivo culture and to determine their susceptibility to growth-inhibitory signals mediated by adhesion to BM stromal ligands.
Design and methods: We used cell tracking to isolate primitive LTC-IC that had accomplished 1 or 2 divisions ex vivo. Adhesion, migration and growth inhibition of divided LTC-IC were determined in the presence of purified BM ligands, and compared to the properties of uncultured LTC-IC.
Results: As compared to undivided LTC-IC, adhesion and migration mediated by very late antigen (VLA)-4 integrin across both vascular cell adhesion molecule-1 (VCAM-1) and fibronectin (Fn) were downregulated in post-mitotic LTC-IC. Conversely, binding and motility via VLA-5 across Fn were stimulated. No changes occurred in LTC-IC interactions with intercellular adhesion molecule-1 (ICAM-1) or with E- or P-selectin. Proliferation of uncultured LTC-IC was inhibited by VLA-4-mediated binding to VCAM-1 and the CS-1 domain of Fn, as well as binding to P-selectin. Growth of ex vivo-generated LTC-IC became unresponsive to these 3 ligands but was suppressed through VLA-5 engagement by the cell binding domain of Fn.
Interpretation and conclusions: The generation of LTC-IC in expansion culture is associated with functional alterations of adhesion receptors, modulating not only binding and migration in the BM but also responsiveness to adhesion-mediated growth inhibitory signals. Such changes may limit homing and engraftment of expanded primitive stem/progenitor cells.