The chemokine receptor CXCR4 is expressed by CD34+ hematopoietic stem/progenitor cells (HSC/HPC). Several investigators have suggested that expression of CXCR4 may be an important characteristic of HSC/HPC. We studied the dynamic expression of CXCR4 during growth factor-induced mobilization of HSC in a clinically relevant nonhuman primate model, Papio anubis (baboons). We evaluated whether CXCR4 expression in HSC/HPC varies during steady-state hematopoiesis as well as during growth factor-induced mobilization. Peripheral blood stem cells from 5 baboons were mobilized with growth factors. During mobilization, there was a consistent stepwise increase in the proportion of peripheral blood CD34+ cells that were CXCR4-. The highest number of CD34+ CXCR4- cells appeared in the peripheral blood at the same time as the maximum number of assayable colony-forming cells. The cloning efficiency of the CD34+ CXCR4- population was 3-fold greater than that of CD34+ CXCR4+ cells, and the frequency of cobblestone area-forming cells was 6 times higher in the CD34+ CXCR4- population in comparison to CD34+ CXCR4+ cells. Furthermore, the most quiescent CD34+ cells isolated on the basis of low Hoechst 33342 (Ho) and rhodamine 123 (Rho) staining (HoLow/RhoLow) were highly enriched in the CXCR4Low/- cell population. Ex vivo incubation of mobilized peripheral blood CD34+ cells with growth factors for 40 hours resulted in increasing numbers of cells expressing CXCR4. Peripheral blood stem cell grafts containing CD34+ cells that consisted of predominantly CXCR4- cells were able to rapidly engraft lethally irradiated baboons. Because the overwhelming number of CD34+ cells within the mobilized peripheral blood grafts were CXCR4- and were capable of rescuing lethally irradiated baboons, it seems unlikely that the expression of CXCR4 in vitro is an absolute requirement for HSC homing and engraftment. In summary, our data suggest the dynamic nature of CXCR4 expression on CD34+ cells during growth factor-induced HSC/HPC mobilization. In addition, our data indicate that the lack of CXCR4 expression is possibly a characteristic of relatively more primitive HSC/HPC characterized by a higher proliferative capacity.
- Stem cells