Time to PBSC engraftment related to flow cytometric quantification of α4 and α6 integrin levels on total CD34⁺ cells and the CD34⁺CD38⁻ stem cell compartment in leukapheresis products. (A) Low expression levels of α4 integrin on total CD34⁺ or on CD34⁺CD38⁻ cells correlate significantly with prolonged transplant engraftment time (P = .0064 and .0035, respectively). Geometric mean values of fluorescence intensity were obtained after flow cytometric α4 (top row) and α6 (bottom row) integrin measurement compared with isotype controls and analyzed by analysis of variance testing. There is no correlation between α6 integrin expression levels on stem and progenitor cells and engraftment velocity. (B) In leukapheresis samples, α4 integrin levels on T cells show no significant variation. (C) Normalization of HSC α4 integrin levels on an internal T-cell control is a reliable analysis strategy and confirms the correlation between α4 integrin levels and transplant engraftment time (P = .0002). (D) After dead cell exclusion by 4,6-diamidino-2-phenylindole versus forward scatter gating (not shown), CD34 and CD3 signals were plotted against side scatter (top quadrants). CD34 cells were further plotted against the CD3 signal, so that residual T cells were excluded (left bottom quadrant) for the comparative gating of CD34⁺ and CD3⁺ cells for α4integrin expression (right bottom quadrant).