Reciprocal expression of CD38 and CD34 by adult murine hematopoietic stem cells

The effects of activation of adult murine stem cells on their expression of CD38 were studied using a murine transplantation model. First, the published finding that the majority of long-term engrafting cells from normal adult steady-state marrow are CD38+ was confirmed. Next, it was determined that the majority of stem cells activated in vivo by injection of 5-fluorouracil (5-FU) or mobilized by granulocyte colony-stimulating factor are CD38−. Stem cells that were activated in culture with interleukin-11 and steel factor were also CD38−. Previous studies have shown that expression of CD34 by adult stem cells is also modulated by in vivo or in vitro activation. To determine whether there is reciprocal expression of CD38 and CD34, 4 populations of post–5-FU marrow cells were analyzed. The majority of the stem cells were in the CD38−CD34+ fraction. However, secondary transplantation experiments indicated that when the bone marrow reaches steady state, the majority of the stem cells become CD38+CD34−. In addition, the minority populations of CD34+ stem cells that occur in steady-state bone marrow are CD38−. This reversible and reciprocal expression of CD38 and CD34 by murine stem cells may have implications for the phenotypes of human stem cells.