The cell surface protein CD34 is frequently used as a marker for positive selection of human hematopoietic stem/progenitor cells in research and in transplantation. However, populations of reconstituting human and murine stem cells that lack cell surface CD34 protein have been identified. In the current studies, we demonstrate that CD34 expression is reversible on human hematopoietic stem/progenitor cells. We identified and functionally characterized a population of human CD45+/CD34- cells that was recovered from the bone marrow of immunodeficient beige/nude/xid (bnx) mice 8 to 12 months after transplantation of highly purified human bone marrow-derived CD34+/CD38- stem/progenitor cells. The human CD45+ cells were devoid of CD34 protein and mRNA when isolated from the mice. However, significantly higher numbers of human colony-forming units and long-term culture-initiating cells per engrafted human CD45+ cell were recovered from the marrow of bnx mice than from the marrow of human stem cell-engrafted nonobese diabetic/severe combined immunodeficient mice, where 24% of the human graft maintained CD34 expression. In addition to their capacity for extensive in vitro generative capacity, the human CD45+/CD34- cells recovered from the bnx bone marrow were determined to have secondary reconstitution capacity and to produce CD34+ progeny following retransplantation. These studies demonstrate that the human CD34+ population can act as a reservoir for generation of CD34- cells. In the current studies we demonstrate that human CD34+/CD38- cells can generate CD45+/CD34- progeny in a long-term xenograft model and that those CD45+/CD34- cells can regenerate CD34+ progeny following secondary transplantation. Therefore, expression of CD34 can be reversible on reconstituting human hematopoietic stem cells.
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