Virus-Like Particle Purification Process
VLP vaccine manufacturing — cell lysis, clarification, AEX capture on Capto Q, SEC polishing, and 100 kDa UF/DF formulation for HBsAg, HPV L1, and next-generation vaccines
Process Overview
Virus-like particles (VLPs) are self-assembling protein nanoparticles (20–200 nm) that mimic the structural architecture of viruses without containing genetic material. VLP vaccines (Engerix-B for hepatitis B, Gardasil for HPV, Novavax for COVID-19) elicit strong immune responses. The purification challenge is separating intact VLPs from host cell proteins, nucleic acids, lipids, and disassembled subunits while maintaining particle integrity. Overall yield: 20–50%; purity >90% total protein as VLP.
Process Steps
1
Cell Disruption
Cell Lysis
Lysis method depends on host cell type. Yeast (Pichia pastoris, S. cerevisiae): high-pressure homogenization at 800–1,000 bar, 2–3 passes through a valve homogenizer. Cell disruption >95%; VLPs typically remain intact due to their robust self-assembled structure. Insect cells (Sf9/Hi5): mild detergent lysis (0.1% Triton X-100, 30 min, 4°C) or hypotonic buffer + mechanical disruption. Mammalian cells: freeze-thaw or detergent. Add nuclease (Benzonase, 25 U/mL) to degrade DNA released during lysis.
Yield: 70–90%
Purpose: Release intracellular VLPs
2
Clarification
Centrifugation + Depth Filtration
Remove cell debris and large aggregates by low-speed centrifugation (5,000–10,000 × g, 30 min) or disc-stack centrifuge at scale. The VLP-containing supernatant is then polished by depth filtration (charged depth filter, e.g., Millistak+ C0HC) to remove residual debris, nucleic acid aggregates, and reduce bioburden. Depth filtration also removes some host cell proteins through adsorptive interactions. VLP retention in depth filter: <10% (VLPs pass through).
Yield: 80–90%
Turbidity: <5 NTU
3
Anion Exchange Chromatography
AEX Capture on Capto Q
Load clarified lysate onto Capto Q (strong anion exchanger) equilibrated in 20 mM Tris, pH 7.4. VLPs bind due to their net negative surface charge (nucleic acid contamination contributes negative charge). Most host cell proteins with lower charge density are washed away. Elute VLPs with a linear NaCl gradient (0–500 mM). VLPs typically elute at 150–300 mM NaCl. This step removes >90% of host cell DNA and provides 10–20× purification factor.
Yield: 60–80%
Purity: 70–85%
4
Size Exclusion Chromatography
SEC Polishing — Aggregate Removal
Polish AEX eluate by size exclusion chromatography (Sepharose CL-4B, Superose 6, or CIM monolith) to separate intact VLPs (20–200 nm) from protein aggregates, free subunits, and small molecule impurities. VLPs elute in the void volume of the column due to their large size. Aggregates >500 nm are excluded even faster. Free capsid proteins (<60 kDa) are retained and elute later. SEC is essential for achieving >90% particle integrity by DLS analysis.
Yield: 70–85%
Purity: >90%
5
Ultrafiltration / Diafiltration
100 kDa UF/DF Formulation into PBS
Concentrate and diafilter purified VLPs using a 100 kDa MWCO TFF membrane (hollow fiber or flat sheet). VLPs (20–200 nm) are fully retained; small molecule impurities and process-related contaminants pass through. Perform 5 diavolumes with PBS (pH 7.4) to exchange buffer and reduce process impurities by >99.3%. Concentrate to final bulk drug substance concentration (typically 100–500 μg/mL antigen). Verify particle size and polydispersity by DLS; confirm particle morphology by TEM or cryo-EM.
Yield: >90%
Impurity removal: >99.3%
VLP Key Properties
| Particle Size | 20–200 nm (antigen-dependent) |
| Surface Charge | Typically negative at pH 7 (varies by antigen) |
| Composition | Self-assembled structural proteins; no nucleic acid |
| Examples | HBsAg (22 nm), HPV L1 (55 nm), SARS-CoV-2 spike (80–120 nm on ferritin nanoparticles) |
| Stability | Typically stable 4–25°C in PBS + stabilizer; avoid freeze-thaw without cryoprotectant |
| Analytical methods | DLS (size/PDI), TEM/cryo-EM (morphology), SEC-MALS (MW/aggregation), ELISA (antigen content) |
Cost Considerations
| Step | Key Cost Driver | Relative Cost |
|---|
| Cell Lysis | Homogenizer capital, Benzonase nuclease | Medium |
| Clarification | Depth filter capsules (single-use), centrifuge | Medium |
| AEX Capture | Capto Q resin, buffer salts, column hardware | High |
| SEC Polishing | Large column volume required, low throughput | High |
| UF/DF | 100 kDa TFF membrane, diafiltration buffer | Medium |
SEC is the throughput bottleneck. SEC operates in bind-and-elute mode at low flow rates (~1 column volume/h) and cannot be parallelized easily. Alternative polishing strategies include multimodal chromatography (Capto Core 700) or flow-through AEX, which handle higher loadings. Ultracentrifugation (density gradient) can replace the chromatography train for research-scale but is not scalable to >100 L. Use
untangle.bio to model your VLP process.
Frequently Asked Questions
What is the difference between VLP purification and conventional protein purification?
VLPs are 100–1,000× larger than proteins (20–200 nm vs 2–10 nm), which creates unique challenges. VLPs cannot be filtered through 0.22 μm membranes (would be retained). They are shear-sensitive and can disassemble under high flow rates or turbulence. AEX and HIC resins must operate at lower linear velocities. SEC columns must use large-pore media (Superose 6, Sepharose CL-4B) with fractionation range up to 100 nm. Analytical methods (DLS, TEM, cryo-EM) are required that are not needed for proteins.
Why is ultracentrifugation used in some VLP processes but not others?
Sucrose density gradient ultracentrifugation (25,000 rpm, 2–4 h) separates VLPs by buoyant density, achieving very high purity in a single step. It is used for research and small-scale vaccine production (<1 L). However, it is not scalable beyond 4–6 L per run, is labor-intensive, and zonal rotors are expensive. Industrial VLP production (>100 L batches) uses the chromatography-based platform described here, which is GMP-compatible and scalable.
How is VLP particle integrity assessed during purification?
Dynamic light scattering (DLS) measures particle hydrodynamic diameter and polydispersity index (PDI <0.2 indicates monodisperse intact VLPs). Transmission electron microscopy (TEM) or cryo-EM provides direct visualization of particle morphology. SEC with UV detection shows intact VLPs eluting in the void volume. ELISA quantifies antigen content. Particle concentration can be measured by nanoparticle tracking analysis (NTA). All these methods are used at critical process checkpoints.
What are the residual DNA limits for VLP vaccines?
WHO guidelines require <10 ng residual host cell DNA per vaccine dose. For yeast-derived VLPs, this means reducing yeast genomic DNA from ~1 mg/mL (in lysate) to <10 ng/dose. Benzonase treatment degrades DNA to fragments <200 bp, which are then removed by AEX (DNA fragments bind strongly) and UF/DF. Residual DNA is quantified by qPCR assay specific to the host cell genome.
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