The type of regimen used might result in\ud mobilization of phenotypically and functionally different CD34+ cells. We compared the phenotype of CD34+ cells in\ud leukapheresis products of three homogeneous groups: I, healthy individuals treated with granulocyte colony-stimulating factor (G-CSF) alone (n = 13); II, patients mobilized\ud with G-CSF following chemotherapy (n = 16); and III, patients mobilized with G-CSF after high-dose chemotherapeutic pretreatment (n = 24). Multiparameter flow cytometry was performed for CD34+ subpopulation analysis and focused on adhesion molecules, differentiation markers and megakaryocytic markers relevant for stem cell homing, with\ud special reference to the importance of L-selectin expression. Regimens I and II led to higher numbers of mobilized CD34+ cells (mean 468 x 106 and 491 x 106 CD341 cells\ud per leukapheresis procedure respectively) than regimen III (mean 41 x 106 CD34+ cells per leukapheresis procedure). Both the expression of L-selectin and CD54 on CD34+ cells was significantly lower in group III, as was the percentage of megakaryocytic (CD41+) progenitors. A higher percentage of primitive (CD38- and/or HLA=DR=) CD34+ cells was found in group III, correlating with a higher clonogenicity of the CD34+ cells. However, when comparing the CD34+ subpopulations that were also positive for L-selectin, there was no significant difference between the three regimens. A\ud similar approach for the megakaryocytic CD34+ population resulted in an even worse quality of regimen III: 5.1% of CD341 being CD41+/L-selectin+ compared with 9.2% and\ud 8.9% in regimens I and II respectively. We concluded that the phenotypes of the CD34+ cells in the G-CSF (group I) and G-CSF-chemotherapy (group II) regimens are similar,\ud whereas the phenotype of the CD34+ cells mobilized in the high-dose regimen (group III) displayed features that might negatively influence homing of the cells. Future studies will be directed towards regimens that will lead to the mobilization of a higher amount of CD34+ cells with a\ud phenotypically favourable phenotype
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