Identification of precise phenotypes of hematopoietic stem cells is important for a number of clinical situations including stem cell transplantation and gene therapy. Following the successful hematopoietic reconstitution of baboons using selected CD34+ bone marrow cells, CD34 expression became the hallmark of murine and human hematopoietic stem cells. However, this dogma was recently challenged by the observation that stem cells in the bone marrow of normal adult mice are CD34'. In order to

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... ress this controversy we examined the relationship between CD34 expression and the kinetic state of stem cells by using C57BL16 congenic mice. In most experiments, we used Ly-5.1 as the marker for donor cells and transplanted the test cells together with Ly-5.2 "compromized' marrow cells to lethally irradiated Ly -5.2 mice. We first confirmed that the stem cells in the steady state bone marrow are CD34. We then observed that stem cells stimulated in vivo by 5-flourouracil (5-FU) injection or in vitro by a combination of interleukin4 l and steel factor (SF) may express CD34. We then tested the reversal 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 achieved the steady state, tested the Ly-5.1 cells of the primary recipients for their engraftment capabilities in the Ly-5.2 secondary recipients. The majority of the Ly-5.1 stem cells were in the CD34' cell fraction indicating the reversion of CD34+ to CD34" stem cells. These observations clearly demonstrated that CD34 expression reflects the activation state of the hematopoietic stem cells and that it is reversible.