Single-Exosome Profiling by Proximity Barcoding Assay

Single-exosome profiling extends the logic of antibody-oligonucleotide conjugates into extracellular vesicle analysis by converting exosome-associated surface protein phenotypes into sequenceable DNA information. In proximity barcoding assay (PBA), antibody-DNA probes are used to investigate individual exosomes rather than pooled vesicle populations, allowing surface protein signatures to be resolved at single-vesicle level. This is particularly relevant for liquid biopsy, because extracellular vesicles are heterogeneous and bulk measurements can obscure low-abundance but biologically relevant subpopulations.

Why Single-Exosome Profiling Matters

Exosomes are a subclass of membrane-coated extracellular vesicles of roughly 30–100 nm that are released by exocytosis and detected in multiple body fluids. Their surface proteins carry information about tissue of origin, making them attractive as disease-associated biomarkers. The limitation of bulk exosome analysis is that it can report whether a marker is present in the sample overall, but it does not robustly resolve which markers are present on the same individual vesicle. For heterogeneous vesicle populations, this is the major analytical constraint. PBA addresses this by preserving vesicle-level surface protein signatures rather than averaging them away.

PBA Workflow

The workflow uses antibody-DNA conjugates together with rolling circle amplification (RCA)-derived DNA barcodes. Exosomes are first incubated with PBA probes (figure 1) in solution and are then captured in microtiter wells via immobilised cholera toxin subunit B (figure 2), which binds GM1 gangliosides in exosome membranes. RCA products carrying repeated barcode sequences are then introduced (figure 3). Oligonucleotides on antibodies that have bound the same exosome can hybridise to the same RCA product, after which enzymatic extension transfers the barcode sequence (figure 4). The resulting DNA is amplified by PCR, prepared as a sequencing library, and decoded by next-generation sequencing (figure 5).

Proximity barcoding assay (PBA) workflow for single-exosome surface protein profiling. Antibody-DNA probes bind exosome surface proteins (1), exosomes are captured via cholera toxin subunit B (2), RCA products carrying complexTag barcodes are introduced (3), enzymatic extension transfers barcodes to bound antibody oligos (4), and sequencing decodes multi-protein surface phenotypes at single-vesicle resolution (5).

Antibody-DNA Probe Architecture

Each PBA probe is generated by conjugating an antibody to a DNA oligonucleotide that contains an 8-nt proteinTag identifying the target surface protein and an 8-nt random moleculeTag used as a unique molecular identifier after amplification. A second DNA layer is introduced through RCA products generated from circular oligonucleotides containing a 15-nt complexTag. Each RCA product contains many repeated copies of a single complexTag, allowing proteins bound to the same exosome to acquire a common barcode.

What PBA Resolves

The platform was used to profile 38 human proteins on exosomes from 18 different human sources, including serum, seminal fluid, and conditioned media from multiple human cell lines. Exosomes from different sources could be distinguished by specific combinations of surface proteins and by differences in abundance. In practical terms, this means PBA can classify extracellular vesicle subpopulations on the basis of multi-parameter surface phenotypes rather than single-marker readouts alone.

Why the Conjugate Design Matters

From an AOC perspective, assay performance depends on more than antibody specificity. The oligonucleotide must be compatible with barcode transfer, PCR amplification, and sequencing-based decoding while preserving target binding. That is why PBA aligns so closely with AbOliGo's broader content on proximity ligation assays, approaches for making antibody-oligonucleotide conjugates, and the wider survey of scientific techniques using antibody-oligonucleotide conjugates. In all of these formats, the oligo defines how protein recognition is encoded, amplified, and interpreted downstream.

Translational Relevance

For extracellular vesicle research, the main value of PBA is that it enables high-multiplex single-exosome surface profiling by sequencing. That creates a route to identify vesicle subpopulations associated with tissue origin, disease state, or therapeutic response. The same underlying assay logic has also moved into commercial positioning, where single-exosome proximity barcoding is now presented as a high-throughput platform for analysing large numbers of vesicles across multiple sample types. For scientists working in biomarker discovery, oncology, neurology, or liquid biopsy, that progression from assay concept to platform is notable.

Conclusion

PBA is a strong example of where the field is heading. Antibody-oligonucleotide conjugates are no longer being used only as specialist labelling reagents. Increasingly, they are the programmable molecular components that determine how proteins are barcoded, spatially associated, amplified, and read out. In assays such as PBA, the antibody provides the protein-binding layer, while the oligonucleotide defines the analytical readout.

Relevant AbOliGo Products

Reference

• Wu D, Yan J, Shen X, Sun Y, Thulin M, Cai Y, Wik L, Shen Q, Oelrich J, Qian X, Dubois KL, Ronquist KG, Nilsson M, Landegren U, Kamali-Moghaddam M. Profiling surface proteins on individual exosomes using a proximity barcoding assay. Nature Communications. 2019; 10: 3854.