Development of a multiplex-based immunoassay for the characterization of diphtheria, tetanus and acellular pertussis antigens in human combined DTaP vaccines

Global availability of critical reagents for biologicals testing - Current status, challenges and possible solutions

On July 2, 2024, the International Alliance for Biological Standardization (IABS) and Humane Society International (HSI) co-hosted a webinar on the global availability and affordability of critical reagents for vaccine and biologics production. Despite growing support for non-animal testing, significant barriers remain, especially in low-income countries facing financial and supply chain challenges. This meeting showcased successful collaborations on reagent production and shared industry and regulatory perspectives. Key barriers included high reagent costs, import complexities, and the limited number of suppliers. Participants stressed the need for tailored risk-based testing, in-house assay validation, and stronger collaboration for standardised testing. The idea of regional hubs in Africa and Southeast Asia for reagent distribution was also discussed to address logistical challenges. A central theme was advocating reliance strategies, which promote shared regulatory assessments and resource optimisation, as demonstrated by the EU/EEA OCABR Network activities and South African-European laboratory collaborations. Difficulties facing smaller national control laboratories in meeting international standards were highlighted, along with the need for further innovation in non-animal-derived reagents to address these challenges. Participants stressed the importance of continued global collaboration and adopting reliance practices to improve access to critical reagents and ensure sustainability in biologics testing.

Development of In Vitro Potency Methods to Replace In Vivo Tests for Enterovirus 71 Inactivated Vaccine (Human Diploid Cell-Based/Vero Cell-Based)

BACKGROUND

The three commercial Enterovirus 71 (EV71) inactivated vaccines which have effectively controlled the EV71 pandemic currently rely on inherent variable in vivo potency methods for batch release. To align with 3R (Replacement, Reduction, Refinement) principles and enhance quality control, this study referred to WHO guidelines and the European Pharmacopoeia to develop in vitro relative potency (IVRP) methods.

METHODS

Working standards tracing to phase 3 clinical vaccines were established. Manufacture-specific IVRP methods were developed and validated per ICH Q14/Q2(R2), utilizing conformational epitope-targeting neutralizing monoclonal antibodies (MAbs). One of the MAbs (CT11F9) recognition sites was clarified with Cryo-EM. Subsequently, the performance of IVRP was assessed using varied concentrations and heat-treated vaccines. The correlation between IVRP and in vivo methods was analyzed, followed by setting IVRP specifications.

RESULTS

The manufacturer-specific working standard exhibited ED50 values comparable to those of related phase 3 clinical vaccines. All IVRP methods achieved a relative bias/precision/total error ≤ 15%. The IVRP methods correlated with in vivo methods (p < 0.05, r > 0.9) can discriminate EV71 antigen concentrations (p < 0.01, r > 0.99) and indicate the stability of the vaccines. Cryo-EM was adopted to identify the epitopes recognized by CT11F9, revealing that this neutralizing antibody recognizes a conformational epitope spanning VP1-3 of the same protomer. Using 31-47 batches of commercial vaccines, IVRP specifications were proposed as 0.56-1.35, 0.58-1.40, and 0.54-1.50.

CONCLUSIONS

Based on conformational epitope-targeting neutralizing MAbs, manufacturer-specific IVRP methods, which were sensitive to process variations and correlated with in vivo results, have been established. IVRP methods provide a reliable, animal-free alternative for EV71 vaccine batch release.

Antibody signatures elicited by potent and subpotent whole-cell pertussis vaccines in mice

Inactivated, whole-cell pertussis (wP) vaccines remain at the frontline in the global fight against the resurgence of whooping cough, especially in low- and middle-income countries. However, the reliance on the intracerebral mouse potency test (ic-MPT or Kendrick assay) as the standard batch release assay is extremely burdensome for commercial wP vaccine production. The ic-MPT is technically challenging, labor intensive, and incongruous with modern animal welfare guidelines. Replacing the ic-MPT with a whole-cell Bordetella pertussis enzyme-linked immunosorbent assay, the so-called pertussis serology potency test, has shown promise but has been difficult to implement in practice. In this report, we tested the hypothesis that potent and subpotent wP vaccines have distinct serological profiles in mice that could be developed as a substitute for the ic-MPT. We established an accelerated decay (thermal stress) protocol in which wP, in the context of diphtheria-tetanus-whole-cell pertussis, was rendered >10-fold less effective than unstressed vaccine when evaluated in a mouse model of B. pertussis lung clearance following intranasal challenge. We then screened immune sera on a limited B. pertussis Tahoma I proteome array and identified >30 antigens whose antibody reactivity profiles either increased, decreased, or were unchanged as a function of wP potency. Moreover, virtually all the "indicator" antigens identified are known virulence factors or reactive with human convalescent sera, thereby establishing a potential link between wP potency and pertussis infection and immunity. These results support the development of a limited B. pertussis antigen array as a stability-indicating surrogate potency assay for the ic-MPT.

IMPORTANCE

Whooping cough (pertussis) is a highly contagious respiratory disease caused by the Gram-negative bacterium, Bordetella pertussis. Globally, tens of millions of whole-cell pertussis (wP) vaccines are administered annually. Whole-cell pertussis vaccines are logistically complex to manufacture and get to market because of the need for each batch of vaccine to be evaluated in a highly laborious and challenging potency test known as the Kendrick assay, which involves mouse intracerebral challenges with B. pertussis. In this report, we describe efforts to develop a serology-based substitute for the Kendrick assay that relies on profiling antibody responses to wP vaccines.

Determination of DTaP vaccine potency by multiplex immunogenicity testing using electrochemiluminescence

Lot release testing of diphtheria, tetanus and acellular pertussis vaccines traditionally relied on in vivo protection models involving challenge of laboratory animals with toxins. Meanwhile, many labs have switched to serological testing of these vaccines, which is often performed in separate in vivo assays, even if all components were formulated into one vaccine product. Here we describe the results of simultaneous serological potency determination of diphtheria (D), tetanus (T) and acellular pertussis (aP) antigens obtained following immunization of guinea pigs with multicomponent pediatric and booster vaccines from different manufacturers. The 4th World Health Organization (WHO) International Standard (IS) for diphtheria toxoid (No. 07/216) and the 4th WHO IS for tetanus toxoid (No. 08/218) were used as reference preparations. For aP, a pediatric vaccine batch containing the antigens pertussis toxoid, filamentous hemagglutinin, pertactin and fimbriae proteins type 2/3 was established as internal control. Quantification of IgG against D, T and aP antigens in guinea pig sera was performed using a hexaplex electrochemiluminescence immunoassay. We further provide proof-of-concept using experimental vaccine samples lacking or containing reduced amounts of diphtheria toxoid in the presence of full amounts of tetanus and pertussis antigens and alum adjuvant. Importantly, the assay confirmed dose-response relationships for all antigens tested and was able to detect diphtheria out-of-specification batches. The results confirmed the suitability of the protocol for combined serology batch release testing of DTaP combination vaccines as first measure towards implementation of full in vitro testing of DTaP vaccines. This report summarizes the data and the protocol used for validation prior to implementation of this method in routine batch release testing of DTaP vaccines, which led to replacement of in vivo challenge experiments in our laboratory following the 3 R (replace, reduce, refine) principle.

Quantitative analysis of pertussis, tetanus, and diphtheria antibodies in sera and breast milk from Tdap vaccinated women using a qualified multiplex assay

Pertussis (whooping cough) is a reemergent, highly contagious respiratory infection of public health concern. Infants prior to initiation of their primary vaccination series are the most vulnerable to severe infection, and even death. Vaccination during pregnancy is an efficacious means of reducing infection in infants. This approach relies on boosting maternal immunity and passive transfer of antibodies to the infant via placenta and breast milk. Similarly, maternal vaccination post-partum can enhance maternal-infant immunity. To support the analysis of pertussis immunity in the context of maternal-infant immunization, we developed a high throughput multiplex assay for simultaneous quantification of serum IgG antibodies against pertussis vaccine antigens: pertussis toxin (PT), filamentous hemagglutinin (FHA), pertactin (PRN), and fimbriae (FIM2/3), and against tetanus (TT) and diphtheria toxoids (DT), using the Meso Scale Discovery (MSD) platform. The assay was qualified, and specificity, sensitivity, accuracy, precision, linearity, and robustness were demonstrated. The assay was subsequently adapted for quantification of IgG and IgA in breast milk. Applied to a serological survey of pregnant women living in the United States and sub-Saharan Africa, this method revealed differences in magnitude and breadth of antibody profile, consistent with history of vaccination. A longitudinal analysis of Tdap responses in women vaccinated post-partum demonstrated a rapid increase in serum IgG that remained elevated for up to 24 months. Likewise, high levels of vaccine-specific IgA and IgG antibodies were present in breast milk, although they exhibited faster decay. This multiplex MSD assay is a reliable and practical tool for quantification of pertussis, tetanus, and diphtheria antibodies in serum and breast milk in serosurveys or vaccine studies.

IMPORTANCE

Pertussis (whooping cough) has reemerged in recent years. Vaccination during pregnancy is an effective approach to prevent illness during the first months of life. We developed a multiplex assay for quantification of pertussis, tetanus, and diphtheria serum antibodies using the Meso Scale Discovery (MSD) platform; the method was qualified, and specificity, precision, accuracy, linearity, and limits of quantification were defined. It was also adapted for quantification of antibodies in breast milk. We successfully determined serostatus in women from different regions and with different vaccination histories, as well as responses to Tdap in blood and breast milk post-partum. This is the first description of a multiplex assay for the quantification of pertussis, tetanus, and diphtheria antibodies in breast milk.