IgA/IgM chromatographic depletion enables efficient 20-nm virus nanofiltration of mini-pool caprylic-acid IgG

1. Problem Statement

Global shortages of plasma-derived immunoglobulin G (IgG) are critical, particularly in low- and middle-income countries (LMICs). While caprylic acid (CA) precipitation is a simple, cost-effective method to purify IgG from cryo-poor plasma (CPP) in resource-limited settings, it has significant limitations:

• Virus Safety: CA treatment effectively inactivates lipid-enveloped viruses but fails to remove or inactivate small, non-enveloped viruses (e.g., Parvovirus B19, Hepatitis A).
• Impurity Profile: CA-purified IgG intermediates retain substantial amounts of high-molecular-weight immunoglobulins, specifically IgA and IgM.
• Downstream Processing Failure: These residual IgA and IgM molecules cause rapid fouling and clogging of small-pore (20 nm) virus nanofilters. This drastically reduces filtration capacity and throughput, making it difficult to implement the necessary orthogonal virus removal step required for safe therapeutic IgG production.

2. Methodology

The study evaluated a proof-of-concept, laboratory-scale process designed to integrate IgA/IgM depletion prior to nanofiltration. The workflow involved:

• Source Material: Cryo-poor plasma (CPP) derived from pooled human plasma.
• Primary Purification: CA precipitation at pH 5.5 (5% v/v) to isolate IgG, followed by ultrafiltration and buffer exchange.
• Chromatographic Depletion: The CA-IgG intermediate was passed through a Fractogel TMAE (M) anion-exchange column. Under specific conditions (25 mM sodium acetate, pH 6.0), IgA and IgM bind to the resin, while IgG flows through.
• Nanofiltration: The IgG-rich flow-through was subjected to sequential virus nanofiltration using Planova 35N followed by Planova 20N (or S20N) filters (0.001 m² membrane area).
• Comparative Analysis: The study compared the filtration performance of:
  1. Direct nanofiltration of CA-IgG (containing IgA/IgM).
  2. Nanofiltration of CA-IgG after IgA/IgM depletion.
• Characterization: Samples were analyzed for protein concentration (immunoturbidimetry), particle size (Dynamic Light Scattering - DLS, Nanoparticle Tracking Analysis - NTA), and integrity (SDS-PAGE).

3. Key Contributions

• Identification of the Bottleneck: The study definitively identified residual IgA and IgM as the primary cause of nanofilter fouling in CA-based IgG processes.
• Process Optimization: Demonstrated that a simple anion-exchange step to deplete IgA/IgM prior to nanofiltration restores filter performance without compromising IgG yield.
• Scalability for LMICs: Proposed a modular, stepwise manufacturing platform suitable for small-to-medium scale production in resource-constrained environments, utilizing disposable columns and standard nanofilters.
• Enhanced Safety Profile: Validated a dual-virus reduction strategy: CA treatment (enveloped virus inactivation) + 20-nm nanofiltration (non-enveloped virus removal), now made feasible by the pre-treatment step.

4. Key Results

A. Depletion Efficiency and Recovery

• Chromatography: The Fractogel TMAE step reduced IgA from ~1.11 mg/mL to <0.24 mg/mL and IgM from ~0.35 mg/mL to <0.20 mg/mL.
• Yield: IgG recovery in the flow-through was 94.84%.
• Final Product: After sequential nanofiltration (35N → 20N), the overall IgG recovery was 78.05%, with IgA and IgM remaining below the limit of quantification.

B. Filtration Performance (The Critical Finding)

• Without Depletion: Direct nanofiltration of CA-IgG (containing IgA/IgM) resulted in rapid flux decline. Only 16 mL of filtrate was collected in 120 minutes, with an average flux of **8 L·m⁻²·h⁻¹**.
• With Depletion: After IgA/IgM removal, the same filters processed 60 mL in 140 minutes with stable flow.
  • Planova 20N: Average flux increased to 25.7 L·m⁻²·h⁻¹ (>3-fold increase).
  • Planova S20N: Flux increased to ~90 L·m⁻²·h⁻¹ (>10-fold increase compared to non-depleted feed), with a stable flow rate of ~1.5 mL/min.

C. Product Quality and Safety

• Particle Size: DLS and NTA confirmed that the final product consisted of monomeric IgG (Z-average ~11–12 nm) with a significant reduction in subvisible aggregates (11-fold reduction in particle count).
• Integrity: SDS-PAGE showed a dominant ~150 kDa band (intact IgG) with no degradation products.
• Purity: The process effectively removed higher-molecular-weight species and procoagulant contaminants (previously validated in related studies).

5. Significance and Implications

• Enabling Local Production: This study provides a practical roadmap for LMICs to utilize domestic plasma for IgG production. It overcomes the technical barrier that previously prevented the use of robust 20-nm nanofiltration in CA-based processes.
• Virus Safety: By enabling the use of 20-nm filters, the process ensures the removal of small, non-enveloped viruses, addressing a critical safety gap in CA-only purification.
• Therapeutic Quality: The removal of IgA reduces the risk of adverse reactions in patients with anti-IgA antibodies, and the reduction of aggregates improves product homogeneity.
• Flexibility: The modular nature of the process (CA precipitation + Chromatography + Nanofiltration) allows for adaptation to various scales, including the production of hyperimmune or convalescent IgG during outbreaks.

In conclusion, the paper demonstrates that IgA/IgM chromatographic depletion is a critical enabling step that transforms a potentially clogged, inefficient nanofiltration process into a robust, high-throughput method for producing safe, high-quality IgG in resource-limited settings.