Characterization of Extracellular Vesicles From Fresh vs. Frozen Human Milk Including the Vesicular microRNA Cargo

Human milk is rich in extracellular vesicles (EV) that may contribute to shaping neonatal immunity. Here, we evaluated whether freezing, and the addition of sodium citrate (SC), affect the characteristics of human milk EVs and their miRNAs. Freezing may compromise the milk EV population and their miRNA profile by creating artificial vesicles due to cell lysis. Furthermore, SC can be added to clear the EV fraction of micelles, that is, protein aggregates that co-isolate with milk EVs, and may affect certain downstream analyses. To investigate potential differences between milk EV and their miRNA cargo when isolated from fresh and frozen samples, mature milk samples were collected from 10 women and subjected to four different treatments: fresh and frozen; freshSC and frozenSC. Ultracentrifugation was used for EV isolation, and subsequently characterized by Nanoparticle tracking analysis, flow cytometry, Western blot and electron microscopy. While freezing without SC has no impact on the evaluated EV parameters, freezing with SC significantly altered particle mean size as measured by NTA and protein levels as studied by MACSPlex flow cytometry. Importantly, neither freezing nor SC had an impact on the EV miRNA cargo, measured by qPCR. These findings also suggest that EV isolates from frozen samples, in comparison to freshly isolated ones, can produce valid results concerning morphology, size, surface markers and the EV miRNA profile.