Effect of MVs on the expression of cell-surface markers characteristic of megakaryocytic differentiation of K562 cells. (A) Cells were treated for 72 hours with medium (control, i-ii) or 5 μg/mL membrane MVs (with respect to protein content) obtained from CEM T cells treated with PHA/PMA/act D (iii-iv). Expression of α_IIbβ₃ (ii,iv) was detected by fluorescence microscopy after exposure to antibody to α_IIb and revelation by phycoerythrin-labeled anti–mouse IgG. Panels i and iii show the corresponding phase-contrast images (original magnification ×40). (B-C) Flow cytometry histograms showing MV concentration dependence of the surface expression levels of α_IIbβ₃ and CD42b by K562 cells treated with either medium (control) or 1 to 10 μg/mL membrane MVs obtained from CEM T cells treated with PHA/PMA/act D. Fluorescence (FL2, red fluorescence channel intensity, log scale) is expressed in arbitrary units (au). The respective increases of mean fluorescence intensity (± SEM) reflecting the expression level of α_IIbβ₃ and CD42b after treatment by PHA/PMA/act D or PHA/act D–generated MVs are indicated in Table 1.