From cellular waste to biomarkers; insights into past, present, and future methods to detect immune cell-derived extracellular vesicles using flow cytometry

Extracellular vesicles (EVs) often possess both ubiquitous and unique tetraspanin molecules that can help elucidate their cell-of-origin. Furthermore, the presence and/or absence of specific tetraspanins can be used to phenotype and identify specific subpopulations of EVs. In the context of immune-related disorders (i.e. multiple sclerosis, rheumatoid arthritis, and various cancers), specific immune cell-derived EVs are now being investigated in the context of biomarker exploration, identifying novel disease mechanisms, and monitoring therapeutic responses in patients. Flow cytometry (FCM) is a technique that uses the light scattering properties of cells and/or subcellular particles (e.g. EVs), while combining fluorescent signals that can detect the presence, absence, or abundance of surface and/or intracellular molecules. To date, however, using FCM to accurately quantify EV populations has been challenging due to their relatively small size and weak light scattering and fluorescence properties compared to intact cells. Historically, the application of calibration beads of known sizes, refractory indices, a violet-side scatter, and standardized methodologies have made positive contributions towards the accurate detection and quantification of EVs while also permitting exploration into their biological properties. This review provides a summary and perspective of current FCM methodologies that are used to assess immune cell-derived EVs within biological fluids and cell supernatants. While acknowledging past and current limitations, as well as the recent successes, improvements, and efficiencies of assays used in EV-related research, the field will inevitably continue to advance through the implementation of standards and guidelines to enhance discovery and reproducibility.