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 publish ed finding that the majority of long-term engrafting cells from normal adul t steady-state marrow are CD38(+) was confirmed. Next, it was determined th at the majority of stem cells activated in vivo by injection of 5-fluoroura cil (5-FU) or mobilized by granulocyte colony-stimulating factor are CD38(- ). Stem cells that were activated in culture with interleukin-II 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. T o determine whether there is reciprocal expression of CD38 and CD34, 4 popu lations of post-5-FU marrow cells were analyzed. The majority of the stem c ells were in the CD38(-)CD34(+) fraction. However, secondary transplantatio n experiments indicated that when the bone marrow reaches steady state, the majority of the stem cells become CD38(+)CD34(-). In addition, the minorit y populations of CD34(+) stem cells that occur in steady-state bone marrow are CD38-. This reversible and reciprocal expression of CD38 and CD34 by mu rine stem cells may have implications for the phenotypes of human stem cell s. (Blood, 2001;97:2618-2624) (C) 2001 by The American Society of Hematolog y.