Considerations for MESF-bead based assignment of absolute fluorescence values to nanoparticles and extracellular vesicles by flow cytometry
Flow cytometry is commonly used to characterize nanoparticles (NPs) and extracellular vesicles (EVs) but results are often expressed in arbitrary units to indicate fluorescence intensity. This hampers interlaboratory and inter-platform comparisons. We investigated the use of molecules of equivalent soluble fluorophores (MESF)-beads for assignment of fluorescence values to NPs and EVs by comparing two FITC-MESF bead sets as calibrators on different flow cytometry platforms (BD Influx, CytoFLEX LX and SORP BD FACSCelesta). Next, fluorescence signals of NPs and EVs were calibrated using different sets of FITC and PE-MESF beads. Fluorescence calibration using beads designed for cellular flow cytometry allowed inter-platform comparison. However, the intrinsic uncertainty in the fluorescence assignment to these MESF beads impacts the reliable assignment of MESF values to NPs and EVs based on extrapolation into the dim fluorescence range. Our findings demonstrate that the use of the same set of calibration materials (vendor and lot number) and the same number of calibration points, greatly improves robust interlaboratory and inter-platform comparison of fluorescent submicron sized particles.