Exosome Detection by Flow Cytometry: Protocol, Markers & Gating Strategy

Exosomes are nanoscale extracellular vesicles (EVs) secreted by cells, typically ranging from 30–150 nm in diameter. Their role in intercellular communication, disease biomarker discovery, and therapeutic delivery has made them one of the most actively studied topics in cell biology. However, their small size pushes the detection limits of conventional flow cytometry — specialized strategies are required for reliable analysis. This guide covers the principles, protocols, and practical tips for exosome detection by flow cytometry.

1. What Are Exosomes?

Exosomes are nanoscale extracellular vesicles (EVs) secreted by cells, typically ranging from 30–150 nm in diameter. Their size is comparable to certain viruses, although they lack autonomous replication capability. In 2013, the Nobel Prize in Physiology or Medicine was awarded to James Rothman, Randy Schekman, and Thomas Südhof for their seminal work on vesicle trafficking mechanisms, which brought exosome biology into the research mainstream.

Exosomes originate from most cell types as intraluminal vesicles formed by inward budding within multivesicular bodies (MVBs). When MVBs fuse with the plasma membrane, these vesicles are released as exosomes into the extracellular space. Their cargo includes proteins, lipids, mRNA, and microRNA from the parent cell, enabling intercellular communication and modulation of recipient cell behavior.

Exosomes are widely present in cell culture supernatants and various biological fluids such as blood, lymph, saliva, urine, semen, and breast milk. They are also found in tissue-derived samples, including tumor microenvironment fluid. Key surface markers include the tetraspanins CD9, CD63, and CD81, which serve as the primary identification markers for exosome characterization.

Figure 1. Structure of exosomes (DOI:10.3390/biology10040285)

2. Flow Cytometry Approaches for Exosome Detection

The nanoscale dimensions of exosomes exceed the optical detection limits of conventional flow cytometers. Therefore, specific strategies are required for reliable analysis. Current mainstream approaches include:

Bead-Based Amplification Detection

Exosome-specific antibodies (e.g., anti-CD9, anti-CD63, anti-CD81) are pre-coated onto microspheres. These beads capture exosomes, which are then detected using fluorochrome-labelled secondary antibodies (targeting a different tetraspanin) via standard flow cytometry. This approach converts nanoscale exosome signals into micron-scale bead signals detectable by any conventional flow cytometer.

Figure 2. Principle of bead-based exosome detection (DOI:10.1016/j.jddst.2021.102526)

Nano-Flow Cytometry

High-sensitivity instruments (e.g., Beckman CytoFLEX nano, ApogeeFlow MicroPLUS/Micro) allow direct detection of exosomes. These cytometers feature optimised optics, detectors, and software, enabling single-vesicle resolution for particles as small as 40–100 nm. This approach provides true single-exosome analysis without the need for bead capture, but requires specialized instrumentation.

Figure 3. Flow cytometric characterisation of exosome size and surface markers (image source: Beckman website)

A. Standardised microspheres for calibration. B. Exosome size distribution. C. Size distribution after Triton X-100 treatment. D. Expression analysis of PE-CD81 or APC-CD63-labelled exosomes.

3. Practical Considerations for Flow Cytometric Detection of Exosomes

1. Thorough removal of impurities: Prior to acquisition, remove cell debris and large particles by low-speed centrifugation or filtration to reduce background noise.

2. Use bright fluorochromes: Employ antibodies conjugated with high-intensity fluorochromes such as PE or APC, and use bright membrane-binding dyes (e.g., PKH, DiI) to enhance signal from these tiny particles. For fluorochrome selection guidance, see our dedicated guide.

3. Sample dilution: Exosomes are prone to the “swarm effect”, where multiple small particles are recorded as a single event. Proper dilution and assessment of signal linearity are essential for accurate quantification.

4. Appropriate controls:

• Unstained control: for voltage adjustment and defining negative populations

• Isotype control: to discriminate specific from non-specific binding

• FMO control: essential for multicolour experiments

• Triton X-100 lysis control: disruption of the exosomal membrane should significantly reduce or abolish true positive signals, providing critical evidence of signal specificity

5. Instrument calibration and standardisation: Fluorescent calibration beads of known size and concentration should be used to standardise cytometer performance, allowing more reproducible comparisons across experiments and instruments.

4. Alternative Methods for Exosome Detection

In addition to flow cytometry, exosomes can be analysed by several other techniques:

Fluorochrome labelling and tracking: Lipophilic dyes (e.g., PKH67, PKH26, DiI, DiD, DiO) stably incorporate into the lipid bilayer, enabling visualisation of exosome uptake, biodistribution, and cellular interactions by fluorescence microscopy or immunofluorescence.

Electron microscopy (TEM/SEM): Direct observation of vesicle morphology, size, and structural features.

Western blotting: Detection of exosomal marker proteins (e.g., CD9, CD63, CD81, TSG101, Alix).

ELISA: Quantitative detection using exosome-specific capture antibodies, suitable for targeted analysis.

5. Recommended Exosome Antibodies for Flow Cytometry

References

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3. Kowal J, et al. Proteomic comparison defines novel markers to characterize heterogeneous populations of extracellular vesicle subtypes. PNAS. 2016;113(8):E968-E977. doi: 10.1073/pnas.1521230113

4. van der Pol E, et al. Classification, functions, and clinical relevance of extracellular vesicles. Pharmacol Rev. 2012;64(3):676-705. doi: 10.1124/pr.112.005983

5. Raposo G, Stoorvogel W. Extracellular vesicles: exosomes, microvesicles, and friends. J Cell Biol. 2013;200(4):373-383. doi: 10.1083/jcb.201211138

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7. Kadbhane A, et al. Perspective insights and application of exosomes as a novel tool against neurodegenerative disorders. J Drug Deliv Sci Technol. 2021;66:102526. doi: 10.1016/j.jddst.2021.102526

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