Differential response of primitive human CD34− and CD34+ hematopoietic cells to the Notch ligand Jagged-1

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Abstract
Recent reports indicate that activation of the Notch signaling pathway delays the differentiation of hematopoietic progenitors, suggesting that Notch may be used to develop novel ex vivo culture conditions for the expansion of primitive cells to be used in clinical transplantation. Here, we compare Notch expression and the effects of Jagged-1 treatment on highly purified subfractions of primitive CD34+ and CD34- human hematopoietic cells. Unlike response of cultured CD34+ cells, Jagged-1 treatment did not enhance the proliferation of CD34- cells, or promote differentiation of CD34- cells into CD34+ cells. While CD34+ and AC133-CD34- cells were shown to express all known forms of Notch receptors, Notch-3 and Notch-4 were not detected in AC133+CD34- cells. Similarly, CD34+ progeny of differentiated CD34- cells did not upregulate Notch-3 or Notch-4 upon differentiation, although transcripts for these genes were expressed in CD34+ arising from CD34+ CD38- parents, suggesting that the Notch receptor expression is tightly and differentially controlled. Fringe, known to inhibit Notch signaling in response to specific Notch ligands, was expressed in parent CD34- and CD34+ cells as well as their CD34+ progeny. We suggest that the inability of primitive CD34- cells to positively respond to Jagged-1 may be due in part to the absence of Notch-3 and Notch-4. Taken together, our study illustrates functional distinctiveness of the primitive CD34- subsets to CD34+ counterparts in relation to Jagged-1 response, and represents the first demonstration of a molecular difference among de novo isolated CD34+ compared to in vitro generated CD34+ cells arising from primitive CD34- or CD34+ parents.