Short term storage of extracellular vesicle (EV) solutions at +4°C is a common practice, but the stability of EVs during this procedure has not been fully understood yet. Using nanoparticle tracking analysis, we have shown that EVs isolated from the conditioned medium of HT-29 cells exhibit a pronounced concentration decrease when stored in PBS in ordinary polypropylene tubes within the range of (0.5–2.1) × 1010 particles/ml. EV losses reach 51±3% for 0.5 ml of EVs in Eppendorf 2 ml tube at 48 hours of storage at +4°C. Around 2/3 of the observed losses have been attributed to the adsorption of vesicles onto tube walls. This result shows that the lower part (up to at least 2 × 1010 particles/ml) of the practically relevant concentration range for purified EVs is prone to adsorption losses at +4°C. Total particle losses could be reduced to 18–21% at 48 hours by using either Eppendorf Protein LoBind tubes or ordinary tubes with the surface blocked with bovine serum albumin or EVs. Reduction of losses to 15% has been shown for isolated EVs dissolved in the supernatant after 100 000 g centrifugation as a model of conditioned medium. Also, a previously unknown feature of diffusion-controlled adsorption was revealed for EVs. In addition to the decrease in particle count, this process causes the predominant losses of smaller particles.

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Proteins involved in the organizing of lipid rafts can be found in exosomes, as shown for caveolin‐1, and they could contribute to exosomal cargo sorting, as shown for flotillins. Stomatin belongs to the same stomatin/prohibitin/flotillin/HflK/C family of lipid rafts proteins, but it has never been studied in exosomes except for extracellular vesicles (EVs) originating from blood cells. Here we first show the presence of stomatin in exosomes produced by epithelial cancer cells (non–small cell lung cancer, breast, and ovarian cancer cells) as well as in EVs from biological fluids, including blood plasma, ascitic fluids, and uterine flushings. A high abundance of stomatin in EVs of various origins and its enrichment in exosomes make stomatin a promising exosomal marker. Comparison with other lipid raft proteins and exosomal markers showed that the level of stomatin protein in exosomes from different sources corresponds well to that of CD9, while it differs essentially from flotillin‐1 and flotillin‐2 homologs, which in turn are present in exosomes in nearly equal proportions. In contrast, the level of vesicular caveolin‐1 as well as its EV‐to‐cellular ratio vary drastically depending on cell type.

Development of CAR‐T therapy led to immediate success in the treatment of B cell leukemia. Manufacturing of therapy‐competent functional CAR‐T cells needs robust protocols for ex vivo/in vitro expansion of modified T‐cells. This step is challenging, especially if non‐viral low‐efficiency delivery protocols are used to generate CAR‐T cells. Modern protocols for CAR‐T cell expansion are imperfect since non‐specific stimulation results in rapid outgrowth of CAR‐negative T cells, and removal of feeder cells from mixed cultures necessitates additional purification steps. To develop a specific and improved protocol for CAR‐T cell expansion, cell‐derived membrane vesicles are taken advantage of, and the simple structural demands of the CAR‐antigen interaction. This novel approach is to make antigenic microcytospheres from common cell lines stably expressing surface‐bound CAR antigens, and then use them for stimulation and expansion of CAR‐T cells. The data presented in this article clearly demonstrate that this protocol produced antigen‐specific vesicles with the capacity to induce stronger stimulation, proliferation, and functional activity of CAR‐T cells than is possible with existing protocols. It is predicted that this new methodology will significantly advance the ability to obtain improved populations of functional CAR‐T cells for therapy.

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