Organ-Selective Homing Defines Engraftment Kinetics of Murine Hematopoietic Stem Cells and Is Compromised by Ex Vivo Expansion

Hematopoietic reconstitution of ablated recipients requires that intravenously (IV) transplanted stem and progenitor cells “home” to organs that support their proliferation and differentiation. To examine the possible relationship between homing properties and subsequent engraftment potential, murine bone marrow (BM) cells were labeled with fluorescent PKH26 dye and injected into lethally irradiated hosts. PKH26+ cells homing to marrow or spleen were then isolated by fluorescence-activated cell sorting and assayed for in vitro colony-forming cells (CFCs). Progenitors accumulated rapidly in the spleen, but declined to only 6% of input numbers after 24 hours. Although egress from this organ was accompanied by a simultaneous accumulation of CFCs in the BM (plateauing at 6% to 8% of input after 3 hours), spleen cells remained enriched in donor CFCs compared with marrow during this time. To determine whether this differential homing of clonogenic cells to the marrow and spleen influenced their contribution to short-term or long-term hematopoiesis in vivo, PKH26+ cells were sorted from each organ 3 hours after transplantation and injected into lethally irradiated Ly-5 congenic mice. Cells that had homed initially to the spleen regenerated circulating leukocytes (20% of normal counts) approximately 2 weeks faster than cells that had homed to the marrow, or PKH26-labeled cells that had not been selected by a prior homing step. Both primary (17 weeks) and secondary (10 weeks) recipients of “spleen-homed” cells also contained approximately 50% higher numbers of CFCs per femur than recipients of “BM-homed” cells. To examine whether progenitor homing was altered upon ex vivo expansion, highly enriched Sca-1+c-kit+Lin−cells were cultured for 9 days in serum-free medium containing interleukin (IL)-6, IL-11, granulocyte colony-stimulating factor, stem cell factor, flk-2/flt3 ligand, and thrombopoietin. Expanded cells were then stained with PKH26 and assayed as above. Strikingly, CFCs generated in vitro exhibited a 10-fold reduction in homing capacity compared with fresh progenitors. These studies demonstrate that clonogenic cells with differential homing properties contribute variably to early and late hematopoiesis in vivo. The dramatic decline in the homing capacity of progenitors generated in vitro underscores critical qualitative changes that may compromise their biologic function and potential clinical utility, despite their efficient numerical expansion.