Enrichment of Human Hematopoietic Stem Cell Activity in the CD34+Thy-1+Lin-Subpopulation From Mobilized Peripheral Blood

The number of CD34+cells in the peripheral blood of cancer patients is known to be increased following the administration of high dose chemotherapy and hematopoietic growth factors. These so-called peripheral blood stem cell grafts are now frequently used for autologous transplantation of patients with malignancies. In this report, we address the question of whether true long-term repopulating pluripotent hematopoietic stem cells (PHSC) are mobilized into peripheral blood following chemotherapy plus granulocyte/macrophage colony-stimulating factor (GM-CSF) or granulocyte colony-stimulating factor (G-CSF) mobilization. We have examined the presence of stem cells in mobilized peripheral blood (MPB) by using an antibody to the human Thy-1 molecule to stain the CD34+Lineage-(Lin-) population. The kinetics of mobilization of CD34+Thy-1+Lin-cells into peripheral blood were studied, and the percentage of cells with this phenotype was found tovary widely depending on the day of leukapheresis. A CD34+Thy-1+Lin-cell population, potentially containing PHSCs, was isolated by fluorescence activated cell sorting (FACS) and analyzed for activity. The multilineage differentiative capacity of this candidate stem cell-containing population in MPB was determined using an in vitro long-term culture system, in which cobblestone area formation was used as a means of detecting PHSCs. We also measured repopulating capacity by using two in vivo models in which severe combined immunodeficiency (SCID)-hu mice were implanted with human fetal bone or thymus grafts. Using these assays, we show that the highest frequency of cobblestone area-forming cells (CAFC) after 7 weeks of culture was observed in a subpopulation of CD34+Lin-cells, which expressed low levels of Thy-1. This cell population was capable of producing both B and myeloid cells, and maintaining CD34+Lin-cells in these long term cultures. Moreover, the CD34+Thy-1+Lin-cell subset possessed a higher ability to engraft and to demonstrate multi-lineage differentiative potential at 8 weeks in the SCID-hu bone assay. However, in the SCID-hu thymus model, both Thy-1+and Thy-1-subpopulations were capable of donor T-cell engraftment at 6 weeks, suggesting the presence of cells capable of initiating T lymphopoiesis in both populations. In summary, contained within the CD34+Thy-1+Lin-cell subset is a population that possesses in vitro and in vivo long-term hematopoietic activity, with the ability to give rise to B, T, and myeloid cells. Thus, this population includes cells with features consistent with those of long-term repopulating multilineage potential hematopoietic stem cells. Moreover, from kinetic analysis, high levels of CD34+Thy-1+Lin-cells may be present only transiently in apheresis samples, despite continued presence of total CD34+cells.