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

Abstract

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.