Homing and engraftment defects in ex vivo expanded murine hematopoietic cells are associated with downregulation of β1 integrin

Objective Hematopoietic progenitors generated by ex vivo expansion “home” less efficiently to the bone marrow (BM) after intravenous transplantation than fresh cells. To explore the underlying cause of this transplantation defect, we examined the homing and engraftment properties in vivo of fresh and cultured marrow cells differing in β1 integrin expression. Materials and Methods Fresh murine BM cells, or the expanded progeny of enriched Sca-1+ c-kit+Lin− stem cells, were fractionated into β1−/lo and β1+ subpopulations by cell sorting. These populations were assayed for their content of in vitro colony-forming cells (CFCs), cells able to provide radioprotection, and early and long-term multilineage hematopoietic reconstitution following transplantation into myeloablated recipients. These endpoints were correlated with the homing properties of β1−/lo and β1+ cells that were labeled with 5- (and 6-) carboxyfluorescein diacetate succinimidyl ester (CFSE) and tracked to hematopoietic organs 24 hours after injection into lethally irradiated mice. Results Most normal stem and progenitor cells express high levels of β1 integrin. In contrast, most clonogenic cells generated in vitro are β1−/lo. Consequently, expanded β1−/lo progenitors failed to provide radioprotection or repopulate the hematopoietic system following intravenous transplantation. Defective engraftment of expanded cells was associated with reduced homing of β1−/lo cells to the bone marrow. Conclusion Downregulation of β1 integrin on primitive hematopoietic cells during ex vivo expansion reduces their homing efficiency and negatively impacts hematopoietic reconstitution in vivo. Strategies directed at preserving β1 integrin expression during culture may improve the clinical utility of expanded hematopoietic cells.