Differential short-term and long-term repopulating ability of stem cell subsets in mice

Hematopoietic reconsitution after transplantation requires the presence of cells with early and late repopulating ability. To determine the contribution of relatively immature and mature hematopoietic progenitor cells in engraftment, we used a modified rhodamine staining procedure to purify populations of rhodamine 123 (Rho)bright, Rhodull and Rho- cells from mouse steady-state bone marrow. Following transplantation of these subsets of stem cells in lethally irradiated mice, marrow repopulating cells were mainly present in the small fraction of Rho- cells, indicating that hematopoietic stem cells have a relatively high expression of P-glycoprotein. Similar cell populations could be purified from cytokine-mobilized blood following treatment with cyclophosphamide and G-CSF. The Rho- cell population contained the majority of hematopoietic stem cells with in vivo marrow repopulating ability. In methylcellulose colony assays, the majority of the committed progenitor cells were present in Rhodull and Rhobright fractions. These results indicate that relatively primitive populations of hematopoietic stem cells that lack colony-forming capacity in vitro mediate the early phase of engraftment following syngeneic stem cell transplantation.