Functional activity of murine CD34+and CD34− hematopoietic stem cell populations

The transmembrane glycoprotein CD34 is expressed on human hematopoietic stem cells and committed progenitors in the bone marrow, and CD34-positive selection currently is used to isolate bone marrow repopulating cells in clinical transplantation protocols. Recently, CD34− hematopoietic stem cells were described in both humans and mice, and it was suggested that CD34+ murine bone marrow cells may lack long-term reconstituting ability. In this study, the long-term repopulating ability of CD34+Lin− vs CD34−Lin− cells was compared directly using syngeneic murine bone marrow transplantation. Highly purified populations of CD34+Lin− and CD34−Lin− cells each are able to reconstitute bone marrow, confirming that both populations contain hematopoietic stem cells; however, the number of hematopoietic stem cells in the CD34+Lin− fraction is approximately 100-fold greater than the number in the CD34−Lin− fraction. In competitive repopulation experiments, CD34+ stem cells are better able to engraft the bone marrow than are CD34− cells. CD34+Lin− cells provide both short- and long-term engraftment , but the CD34−Lin− cells are capable of only long-term engraftment. Ex vivo, the CD34+Lin− stem cells expand over 3 days in culture and maintain the ability to durably engraft animals in a serial transplant model. In contrast, when CD34−Lin− cells are cultured using the same conditions ex vivo, the cell number decreases, and the cells do not retain the ability to repopulate the bone marrow. Thus, the CD34+Lin− and CD34−Lin− cells constitute two functionally distinct populations that are capable of long-term bone marrow reconstitution.