The elusive peripheral blood hemopoietic stem cell

Long-term culture initiating cells (LTC-IC) are primitive hemopoietic progenitors that give rise to clonogenic cells when provided with a supportive feeder layer of mesenchymal cells. These LTC-IC possess many of the characteristics expected of marrow-repopulating "stem cells" including high proliferative and multilineage-differentiative capacity and resistance to 4-hydroperoxy-cyclophosphamide (4-HC) killing. In addition, stem cells are known to persist and may proliferate in murine LTC, and human marrow grown in LTC has been successfully used as hemopoietic support for myeloablative therapy. LTC-IC, as well as clonogenic precursors, circulate in normal peripheral blood, and the concentration of both progenitor types can be increased by cytotoxic chemotherapy and/or growth factors. When mobilized peripheral blood cells are used as hemopoietic support for high-dose chemo/radiotherapy, engraftment has often been more rapid than that achieved with autologous marrow. Thus, primitive hemopoietic cells circulate in human blood, which can enable hemopoietic reconstitution following aggressive therapy for malignant disease.