The phenotypic profile of CD34-positive peripheral blood stem cells in different mobilization regimens

The type of regimen used might result in mobilization of phenotypically and functionally different CD34⁺ cells. We compared the phenotype of CD34⁺ cells in leukapheresis products of three homogeneous groups: I, healthy individuals treated with granulocyte colony-stimulating factor (G-CSF) alone (n = 13); II, patients mobilized 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 special reference to the importance of L-selectin expression. Regimens I and II led to higher numbers of mobilized CD34⁺ cells (mean 468 × 10⁶ and 491 × 10⁶ CD34⁺ cells per leukapheresis procedure respectively) than regimen III (mean 41 × 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 similar approach for the megakaryocytic CD34⁺ population resulted in an even worse quality of regimen III: 5.1% of CD34⁺ being CD41⁺/L-selectin⁺ compared with 9.2% and 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, 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 phenotypically favourable phenotype.