Consistency in AAV analytics doesn't always start with a better assay, but with a better reference.
Two labs can run the same qPCR protocol on the same AAV vector yet report results that differ by an order of magnitude. Primers, instruments, and operators can each add variability across the workflow. The problem may not be the protocol, but the lack of a shared reference standard.
Key takeaways:
- Shared reference material can help contextualize interlaboratory variability in AAV analytics
- Orthogonal characterization shows where measurements agree and diverge across methods
- The rAAV8 reference standard uses standard industry techniques to reflect real-world samples
- Characterization can expand as additional data and analytical methods become available
In this presentation, Igor Alvis Mancila, scientist at Revvity Viral Vector Center of Excellence, explains the production process and orthogonal characterization strategy behind Revvity's new rAAV8 reference standard and how the material is designed to evolve with the field.
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Why variability persists
AAV analytical capabilities have advanced rapidly. Droplet digital PCR has improved on qPCR, cryo-electron microscopy has transformed capsid characterization, and analytical ultracentrifugation is increasingly accessible. Yet interlaboratory variability remains a persistent challenge.
Without shared reference material, improved methods alone cannot show whether two labs are measuring the same thing.
A reference standard built for the real world
The rAAV8 reference standard is an rAAV8-CMV-EGFP vector produced by triple transfection in HEK293-based suspension cells, purified by affinity capture and iodixanol gradient ultracentrifugation to enrich for full capsids, and formulated in DPBS with Poloxamer. It is available as 10,000 vials at 100 µL each to support broad, long-term use.
The material was produced using standard techniques common in the industry, rather than optimized processes intended to create an unusually clean sample. Plasmid backbone contamination, for example, is present at levels representative of a typical AAV preparation.
“This is intentional. A reference standard that looks nothing like a real sample makes relative comparisons difficult and less meaningful. The goal is representativeness, not perfection.”
Orthogonal characterization across methods
A single measurement shows what one method reports under one set of conditions. It does not reveal whether the result is accurate or why it differs from another lab's result.
The rAAV8 reference standard is therefore characterized using multiple independent methods for each attribute, an approach known as orthogonal characterization.
Physical titers, for example, are reported across three platforms and two primer sets:
| Method | Primers |
|---|---|
| qPCR | ITR and eGFP |
| Droplet digital PCR (ddPCR) | ITR and eGFP |
| Nanoplate digital PCR | ITR and eGFP |
Differences among methods are informative. Where results converge or diverge can help identify sources of variability in a workflow.
Revvity is also participating in an interlaboratory study with NIST to further characterize physical titers, with results to be added to the technical note as they become available.
Functional titers
Functional characterization is more complex than physical titer measurement because assays measure different biological activities and labs may not have access to the same cell lines or detection systems.
Revvity uses the eGFP transgene to infect adherent cells, measure the percentage of GFP-positive cells, and calculate transducing units per milliliter. The approach is accessible, reproducible, and easy to implement.
For material that does not express eGFP, ITR qPCR can measure genomic DNA from infected cells, allowing labs to establish a relative infectivity assay linked to the reference standard without a fluorescent reporter.
Impurity characterization
For gene therapy development, impurity profiling is essential. Regulatory expectations are high, and undetected host cell DNA, residual plasmid, or partially filled capsids can have significant consequences.
The rAAV8 reference standard includes an extensive impurity characterization summary covering:
- Protein purity: SDS-PAGE
- DNA purity: Alkaline gel electrophoresis
- Host cell DNA: qPCR and next-generation sequencing
- Host cell protein: ELISA
- Plasmid DNA: ddPCR and NGS
- Partially filled capsids: Cryo-electron microscopy
- Endotoxin and mycoplasma: Standard contamination assays
- Endonuclease: Specific ELISA
Using both qPCR and sequencing-based assays for host cell DNA provides complementary measurements with different assumptions, sensitivities, and sources of variability. Together, they offer a fuller picture of what is in the sample.
A technical note that evolves with the field
Most reference materials come with a fixed certificate of analysis. Revvity's rAAV8 reference standard is accompanied by a technical note that is designed to grow and is already at Version 2.
Planned additions include:
- Results from the NIST interlaboratory study
- Post-translational modification data from mass spectrometry characterization of the AAV capsid
- Data from additional analytical methods: mass photometry, nanoparticle tracking analysis, analytical ultracentrifugation, and transmission electron microscopy
As analytical methods evolve, the technical note will expand with additional characterization data to keep the reference standard relevant and useful.
A shared reference for AAV analytics
A well-characterized reference material can provide a consistent point of comparison across AAV analytical workflows.
The rAAV8 reference standard is designed to support assay validation, instrument calibration, interlaboratory benchmarking, and analytical packages for IND submissions as methods evolve.
