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.