THE EARLY PHASE OF ENGRAFTMENT AFTER MURINE BLOOD-CELL TRANSPLANTATION IS MEDIATED BY HEMATOPOIETIC STEM-CELLS

Blood cell transplantation is largely replacing bone marrow transplant ation because engraftment is more rapid, This accelerated engraftment is thought to be mediated by relatively mature committed hematopoietic progenitor cells. Herein, we have used a modified rhodamine (Rho) sta ining procedure to identify and purify Rho(+/++) (dull/bright) and Rho (-) (negative) subpopulations of hematopoietic progenitor cells in mur ine cytokine-mobilized blood, The Rho(+/++) cell population contained >99% bf committed progenitor cells with in vitro colony-forming abilit y. The Rho(-) cell population contained the majority of hematopoietic stem cells with in vivo marrow repopulating ability. The rate of hemat opoietic reconstitution was identical in recipients of grafts containi ng only purified Rho(-) stem cells or purified Rho(-) stem cells in co mbination with large numbers of Rho(+/++) committed progenitor cells, In contrast, transplantation of 3-fold more hematopoietic stem cells r esulted in accelerated reconstitution, indicating that the reconstitut ion rate was determined by the absolute numbers of Rho(-) stem cells i n the graft. In addition, we observed a 5- to 8-fold reduced frequency of the subset of hematopoietic stem cells with long-term repopulating ability in cytokine-mobilized blood in comparison to steady-state bon e marrow. Our results indicate that hematopoietic stem cells and not c ommitted progenitor cells mediate early hematopoietic reconstitution a fter blood cell transplantation and that relative to bone marrow, the frequency of stem cells with long-term repopulating ability is reduced in mobilized blood.