Reversible expression of CD34 by murine hematopoietic stem cells

We used a mouse transplantation model to address the recent controversy abo ut CD34 expression by hematopoietic stem cells. Cells from Ly-5.1 C57BL/6 m ice were used as donor cells and Ly-5.2 mice were the recipients, The test cells were transplanted together with compromised marrow cells of Ly-5.2 mi ce. First, we confirmed that the majority of the stem cells with long-term engraftment capabilities of normal adult mice are CD34(-). We then observed that, after the injection of 150 mg/kg 5-fluorouracil (5-FU), stem cells m ay be found in both CD34(-) and CD34(+) cell populations. These results ind icated that activated stem cells express CD34. We tested this hypothesis al so by using in vitro expansion with interleukin-11 and steel factor of line age(-) c-kit(+) Sca-1(+) CD34(-) bone marrow cells of normal mice. When the cells expanded for 1 week were separated into CD34(-) and CD34(+) cell pop ulations and tested for their engraftment capabilities, only CD34(+) cells were capable of 2 to 5 months of engraftment. Finally, we tested reversion of CD34(+) stem cells to CD34(-) state. We transplanted Ly-5.1 CD34(+) post -5-FU marrow cells into Ly-5.2 primary recipients and, after the marrow ach ieved steady state, tested the Ly-5.1 cells of the primary recipients for t heir engraftment capabilities in Ly-5.2 secondary recipients. The majority of the Ly-5.1 stem cells with long-term engraftment capability were in the CD34(-) cell fraction, indicating the reversion of CD34(+) to CD34(-) stem cells. These observations clearly demonstrated that CD34 expression reflect s the activation state of hematopoietic stem cells and that this is reversi ble. (C) 1999 by The American Society of Hematology.