For these studies, 10 μl of supernatant MPs were incubated with 10 μl of specific antibody (1 μg/ml; FITC-conjugated; BioLegend, San Diego, CA). After 15 min of incubation at + 4°C, samples were diluted in 500 μl of 0.9% saline salt solution. Then, 10 μl of Flowcount beads were added to each sample and analyzed in a flow cytometer (Beckman Coulter coulter epics XL-MCL). Sample analysis was stopped after the count of 10,000 events.