Quantitative measurements of the number, size, and cargo of extracellular vesicles (EVs) are essential to both basic research on how EVs are produced and function, and to application of this knowledge to the development of EV-based biomarkers and therapeutics. Flow cytometry is a popular method for analyzing EVs, but their small size and dim signals have made this a challenge using the conventional flow cytometry approaches developed for analysis of cells (1). Moreover, established flow cytometry calibrators, standards, and experimental design considerations for cell studies are not regularly used in EV studies. As a result, there is significant variation in instrument set up, sample preparation, and data reporting for flow cytometric measurements of EVs. These issues are increasingly appreciated (1-5), but much needs to be done to develop consensus on best practices. To address these issues, members of the International Society for Extracellular Vesicles (ISEV), the International Society for Advancement of Cytometry (ISAC), and the International Society on Thrombosis and Hemostasis (ISTH) are participating in a tri-Society Working Group, which includes several ERCC members, to improve the reporting of methods and results for FC-based EV measurements.
Reporting of EV Measurement Methods
A flow cytometer is an instrument, not a method. An EV analysis method that uses a flow cytometer involves many instrument setup, sample preparation, and data analysis decisions, including: 1) what signal to use for EV detection (light scatter or fluorescence); 2) how to resolve single EVs from the simultaneous occurrence of many EVs in the laser at the same time (aka coincidence or “swarm”); 3) how to gate the data to focus on EVs versus background events (without introducing artifacts or mis-representing the data); 4) how to estimate the size of the particles detected; 5) how to estimate the brightness of the particles detected; 6) how to verify that the particles detected are EVs and not other particles present in the sample, to name just some of the many decisions involved.
Several years ago, ISAC developed and introduced the Minimum Information about a Flow Cytometry Experiment (MIFlowCyt) (6), a set of guidelines to promote the sharing, reproducibility, and proper interpretation of flow cytometry data. These guidelines were developed with cell analysis, and particularly high parameter immunophenotyping, in mind, but they also apply to multiparameter EV analysis. However, there are several additional details about an EV measurement that are essential to include. The ISEV-ISAC-ISTH EV FC Working Group has been conducting a series of standardization studies to develop a consensus on the essential elements of an FC-based EV measurement that should be reported. These studies will be reported, along with the consensus reporting guidelines, in a paper planned for the coming year.
Standards and Calibrators for EV Analysis
Standards and calibration are essential components of any analytical method. These standards, and their use, are well established for flow cytometry and include 1) counting beads that can be used to calibrate sample flow rates for reporting of absolute particle concentrations, 2) fluorescence intensity standards that enable particle brightness to be expressed in NIST-traceable absolute units of mean equivalent soluble fluorochromes (MESF) (7) or equivalent reference fluorochromes (ERF) (8); 3) antibody-capture standards that can be used to estimate antibody binding in immunofluorescence measurements; and 4) NIST-traceable particle size standards.
Most of these standards and calibrators, and their methods of use, can be applied to EV measurements, with some caveats and cautions. Particle size standards, in particular, are often mis-used in FC-based EV measurements due to a lack of understanding of the effect on light scatter of refractive index (RI), which is different for polystyrene, silica, and lipids. With care, however, these differences can be used in conjunction with Mie scattering theory to enable estimates of EV size based on FC light scatter measurements. Commercially available fluorescence intensity and antibody-capture standards are generally designed for cell measurements, and tend to be brighter than EVs, but still have value for facilitating comparison of measurements between labs or instruments. EV-scaled intensity and antibody-binding standards will be a useful addition to the EV analysis toolbox, and are in development by several groups and companies.
A major unmet need is for EV standards, which will have use not only in FC-based EV measurements, but across the EV field. This is a challenging prospect, as an ideal EV standard will reflect not only the size and number of EVs, but also cargo, including surface molecules (for immunophenotyping) and intra-vesicular cargo, including nucleic acids, soluble proteins, and small molecules. Moreover, EVs are themselves quite diverse, raising the question of what type of EV, if any, might represent a universal standard. EV preparations for various cultured cell lines are commercially available from a number of sources but, in general, these have not been subjected to rigorous, independent characterization of these essential features or their uniformity, stability, or reproducibility. Such characterization is essential for validation of any putative standard and may be the subject of future activities by the ISEV-ISAC-ISTH EV FC Working Group.
Conclusions and Prospects
As EV research expands to impact every area of biology, issues with rigor and reproducibility are front and center. Translating observations made in the basic research lab into mechanistic understanding of EV actions and clinically actionable knowledge requires robust and validated analytical methods. Careful attention to the description of methods, standardization and calibration of analytical instrument and methods, and reporting of results are essential. Community efforts by the ERCC and relevant international societies will be key to helping researchers maximize the value of their work to the broader community.
In future blog posts we will discuss the controversial issue of whether to use light scatter or fluorescence to detect EVs, as well as new EV detection methods we’ve developed using fluorogenic membrane probes.
References
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