Mammalian cell substrates (e.g., Vero, MDCK) have become critical production platforms for the modern vaccine industry. Widely applied in rabies and inactivated poliovirus vaccines, as well as influenza and rotavirus vaccines, they deliver inherent advantages in production speed, throughput and antigen consistency.

Nevertheless, residual Host Cell DNA (rHCD) derived from cell substrates remains one of the top quality control concerns for global regulatory authorities. Regulatory requirements set forth by the FDA, WHO and national pharmacopoeias mandate not only compliance with total residual DNA limits, but also link DNA fragment size to potential safety risks. Specifically, continuous cell lines such as Vero and MDCK, while non-tumorigenic, possess certain transformation potential. Large DNA fragments (>200 bp) may carry oncogenes or viral genetic sequences, posing theoretical tumorigenic and infectious hazards.

In response to this evolving regulatory landscape, Huzhou Shenke Biotechnology Co., Ltd. (abbreviated as HZSKBIO^Ⓡ) has developed an integrated dual detection system for "total HCD quantification + fragment size profiling". The suite includes qPCR-based HCD quantification assays, qPCR fragment size analysis assays, and capillary electrophoresis (CE) fragment detection workflows. These tools enable biomanufacturers to systematically and precisely mitigate residual DNA risks during vaccine purification process validation, and guarantee consistent manufacturing performance.

01 Why rHCD from Continuous Cell Lines (Vero, MDCK, etc.) Draws Intense Regulatory Scrutiny

Continuous cell lines offer high yields and stable culture performance, yet their genomic DNA may contain high-copy oncogenes (e.g., ras, myc), integrated latent viral genome fragments, and CpG motifs capable of triggering immune stimulation. The core safety risks associated with rHCD are outlined below:

• Tumorigenicity Risk: Larger DNA fragments are far more likely to harbor intact functional oncogenes. Fragments shorter than 200 bp are generally deemed unable to retain complete functional genetic elements.

• Infectivity Risk: Animal cell substrates may carry latent viruses; insufficient fragmentation of viral genomic sequences retains theoretical infectious potential.

• Immunogenicity Risk: CpG-rich DNA regions can be recognized by TLR9 receptors, eliciting unintended adverse immune responses.

Accordingly, the WHO recommends three core evaluation criteria for residual HCD in vaccines manufactured using engineered cell substrates:

① The extent of total residual HCD reduction across production stages;
② The degree of residual HCD fragment size reduction throughout manufacturing;
③ Chemical inactivation of biological activity of residual HCD during production.

02 Regulation of rHCD Fragment Size: Rationale for the <200 bp Threshold

The FDA guidance document Characterization and Qualification of Cell Substrates and Other Biological Materials Used in the Production of Viral Vaccines for Infectious Disease Indications states that a functional gene requires approximately 200 base pairs of intact DNA. Degradation of DNA fragments to below 200 bp eliminates their capacity to carry complete proto-oncogenes and abolishes intact viral genome architectures, drastically lowering theoretical tumorigenic and infectious risks. As such, controlling rHCD fragment size has become a mandatory metric for regulatory review and process validation of vaccines produced with Vero, MDCK and comparable cell substrates.

Table 1 Consensus Standards from Global Regulatory Authorities

03 Full-process rHCD Control System: From Nucleic Acid Extraction, qPCR Detection, Size Profiling to Risk Assessment

Aligning with customer manufacturing demands and global regulatory standards, HZSKBIO^Ⓡ delivers an end-to-end residual HCD risk assessment solution covering all process intermediates through final finished vaccine products:

• Sample Preprocessing & Optimized Nucleic Acid Extraction

Vaccine production matrices contain abundant inhibitory substances, including high salt concentrations, beta-propiolactone (BPL) viral inactivant, serum proteins, polysaccharides and high concentrations of target antigens.

The SHENTEK^® Residual Host Cell DNA Sample Preprocessing Kit has undergone extensive matrix compatibility testing for Vero, MDCK and other mainstream vaccine production workflows, establishing a high-recovery, low-inhibition nucleic acid extraction workflow.

• HCD Quantification Testing via qPCR (fg-level detection sensitivity)

•  Multi-locus primer/probe design eliminates false-negative detection risks

• Fully validated compendial method compliant with Chinese Pharmacopoeia, USP and EP requirements

• Applicable to purification process validation (TFF, Benzonase digestion, chromatographic purification), process consistency evaluation, in-process testing and finished product release testing

• Dual-platform HCD Fragment Size Analysis: qPCR and Capillary Electrophoresis (CE)

① Commercial qPCR Fragment Size Analysis Kit (independently developed by HZSKBIO^Ⓡ) The kit incorporates 4–5 distinct amplicons spanning fragment ranges: 80–100 bp, 150–200 bp, 200–300 bp and >500 bp. It delivers fg-level quantitative detection across all vaccine manufacturing stages, generating quantitative data directly compatible with regulatory risk modeling. Key applications include:

• Precise quantification of fragment size distribution

• Comparative fragment size profiling pre- and post-purification steps

• Quantification of nuclease digestion and viral inactivation efficiency

• Generation of fragment size risk assessment reports for regulatory filing

② Capillary Electrophoresis (CE) Fragment Size Testing Service Platform CE generates visual electrophoretic peak profiles that intuitively illustrate shifts in residual nucleic acid fragment distribution. Primary use cases:

• R&D and optimization of manufacturing purification processes

• Visualized graphical reporting

• Confirmatory cross-verification of qPCR results

Customers may opt for combined qPCR + CE dual-method testing to strengthen the scientific rigor of regulatory submission dossiers.

• Risk Assessment Modeling Based on Safety Factor (SF)

• The WHO defines Safety Factor as the fold reduction in DNA biological activity, a metric used to monitor clearance and removal of adventitious agents in biological products; higher SF values correspond to superior biological safety.

• The FDA defines Safety Factor as the inverse of risk probability. For example, an SF greater than 10⁷ indicates an expected risk incidence below 1 in 10,000,000.

• In 2005, the Vaccines and Related Biological Products Advisory Committee (VRBPAC) concluded that an SF of ≥10⁷ delivers an acceptable safety margin for inactivated influenza vaccines manufactured using tumorigenic cell substrates.

• Harry Yang et al. proposed a probabilistic safety factor model integrating Benzonase nuclease digestion steps, enabling more accurate quantification of tumorigenic and infectious risks posed by residual host cell DNA in biological products.

Published literature confirms that lower total HCD concentrations and smaller DNA fragment sizes correlate with elevated safety factors for biological products.

Figure 1 Linear correlation between total HCD residual load and Safety Factor; exponential correlation between HCD fragment size and Safety Factor

• Case Study: HCD Fragment Profiling of MDCK Cell-derived Influenza Vaccine Post Benzonase Digestion

The purification workflow for MDCK cell-based influenza vaccines comprises harvest clarification via centrifugation, Benzonase nuclease digestion, tangential flow ultrafiltration, primary chromatography, viral inactivation validation, and ion-exchange polishing chromatography. Combined qPCR and CE testing quantifies the HCD clearance efficiency of Benzonase treatment.

Figure 2 qPCR quantification, CE electrophoretic profiles and HCD clearance rate trends for Benzonase-treated samples

The paradigm of residual HCD control for vaccines has evolved from standalone total DNA quantification to a systematic management framework integrating total DNA quantification, fragment size profiling and statistical risk modeling. HZSKBIO^Ⓡ delivers standardized qPCR quantification workflows, fragment size analytical platforms, visual CE profiling, Safety Factor risk assessment modeling and full process compatibility validation. This integrated HCD control system ensures consistent manufacturing performance from early-stage R&D and pilot production through commercial scale manufacturing, fulfills global regulatory filing requirements, and safeguards final vaccine quality and patient safety.

Figure 3 rHCD Safety Assessment Decision Tree

References

[1] Yang, H.; Zhang, L.; Galinski, M. A probabilistic model for risk assessment of residual host cell DNA in biological products. Vaccine, 2010, 28(19): 3308-3311.