Collaborative study for the calibration of a replacement International Standard for tetanus toxoid for use in flocculation test.

Here we report the results of a study to establish a replacement WHO International Standard (IS) for tetanus toxoid for use in flocculation test. The standard was calibrated in flocculation units (Lf) against the 2nd IS using the Ramon flocculation method. At its 70th meeting in October 2019, WHO ECBS established the material (coded 16/302) as the 3rd WHO IS, with an assigned value of 970 Lf/ampoule from the results of seventeen laboratories across ten different countries. The study also provided an opportunity to assess the use of alternative methods for measuring Lf. Participants were asked to use an in-house Enzyme Linked Immunosorbent Assay (ELISA) developed at NIBSC, or other suitable in-house methods, to determine ELISA-specific Lf values (Lf-eq units are specific only for pre-calibration of antitoxin in the flocculation test) of 16/302 to compare to those of the flocculation test. Nine laboratories participated by performing the NIBSC ELISA, one laboratory performed flocculation by laser light-scattering following an in-house protocol, and three laboratories performed ELISA following in-house protocols. The results intimate that these alternative methods could be useful for monitoring consistency of production at different stages of vaccine manufacturing.

Recommended Immunological Strategies to Screen for Botulinum Neurotoxin-Containing Samples.

Botulinum neurotoxins (BoNTs) cause the life-threatening neurological illness botulism in humans and animals and are divided into seven serotypes (BoNT/A-G), of which serotypes A, B, E, and F cause the disease in humans. BoNTs are classified as "category A" bioterrorism threat agents and are relevant in the context of the Biological Weapons Convention. An international proficiency test (PT) was conducted to evaluate detection, quantification and discrimination capabilities of 23 expert laboratories from the health, food and security areas. Here we describe three immunological strategies that proved to be successful for the detection and quantification of BoNT/A, B, and E considering the restricted sample volume (1 mL) distributed. To analyze the samples qualitatively and quantitatively, the first strategy was based on sensitive immunoenzymatic and immunochromatographic assays for fast qualitative and quantitative analyses. In the second approach, a bead-based suspension array was used for screening followed by conventional ELISA for quantification. In the third approach, an ELISA plate format assay was used for serotype specific immunodetection of BoNT-cleaved substrates, detecting the activity of the light chain, rather than the toxin protein. The results provide guidance for further steps in quality assurance and highlight problems to address in the future.

Development of human-like scFv-Fc antibodies neutralizing Botulinum toxin serotype B.

Botulinum neurotoxins (BoNTs) are responsible for human botulism, a life-threatening disease characterized by flaccid muscle paralysis that occurs naturally by food poisoning or colonization of the gastrointestinal tract by BoNT-producing clostridia. BoNTs have been classified as category A agents by the Centers for Disease Control and Prevention. To date, 7 subtypes of BoNT/B were identified showing that subtypes B1 (16 strains) and B2 (32 strains) constitute the vast majority of BoNT/B strains. Neutralizing antibodies are required for the development of anti-botulism drugs to deal with the potential risk. In this study, macaques (Macaca fascicularis) were immunized with recombinant light chain (LC) or heavy chain (HC) of BoNT/B2, followed by the construction of 2 hyper-immune phage display libraries. The best single-chain variable fragments (scFvs) isolated from each library were selected according to their affinities and cross reactivity with BoNT/B1 toxin subtype. These scFvs against LC and HC were further analyzed by assessing the inhibition of in vitro endopeptidase activity of BoNT/B1 and B2 and neutralization of BoNT/B1 and B2 toxin-induced paralysis in the mouse ex vivo phrenic nerve assay. The antibodies B2-7 (against HC) and BLC3 (against LC) were produced as scFv-Fc, and, when tested individually, neutralized BoNT/B1 and BoNT/B2 in a mouse ex vivo phrenic nerve assay. Whereas only scFv-Fc BLC3 alone protected mice against BoNT/B2-induced paralysis in vivo, when B2-7 and BLC3 were combined they exhibited potent synergistic protection. The present study provided an opportunity to assess the extent of antibody-mediated neutralization of BoNT/B1 and BoNT/B2 subtypes in ex vivo and in vitro assays, and to confirm the benefit of the synergistic effect of antibodies targeting the 2 distinct functional domains of the toxin in vivo. Notably, the framework regions of the most promising antibodies (B2-7 and BLC3) are close to the human germline sequences, which suggest that they may be well tolerated in potential clinical development.

In vitro antigen ELISA for quality control of tetanus vaccines.

Consistency of production is recognised as an important aspect of vaccine manufacture and suitably validated in vitro assays are required for quality control testing of these products. For the manufacture and batch release of tetanus vaccines, antigen content and integrity, and degree of adsorption of antigen to the adjuvant are critical parameters that should be monitored for consistency. Here we describe the development and use of an Enzyme Linked Immunosorbent Assay (ELISA) to quantify tetanus antigen in combined vaccine products and to measure the degree of adsorption of antigen to adjuvant. Whilst the antigen assay cannot be assumed to predict potency for different products, it can be used as part of a panel of in vitro methods to provide a more informative product profile and to monitor trends in production. The antigen assay is particularly valuable for providing quantitative information on every final lot when modifications of in vivo potency tests, such as single dilution assays, are used.

A single-dose cytomegalovirus-based vaccine encoding tetanus toxin fragment C induces sustained levels of protective tetanus toxin antibodies in mice.

The current commercially available vaccine used to prevent tetanus disease following infection with the anaerobic bacterium Clostridium tetani is safe and effective. However, tetanus remains a major source of mortality in developing countries. In 2008, neonatal tetanus was estimated to have caused >59,000 deaths, accounting for 1% of worldwide infant mortality, primarily in poorer nations. The cost of multiple vaccine doses administered by injection necessary to achieve protective levels of anti-tetanus toxoid antibodies is the primary reason for low vaccine coverage. Herein, we show that a novel vaccine strategy using a cytomegalovirus (CMV)-based vaccine platform induces protective levels of anti-tetanus antibodies that are durable (lasting >13 months) in mice following only a single dose. This study demonstrates the ability of a 'single-dose' CMV-based vaccine strategy to induce durable protection, and supports the potential for a tetanus vaccine based on CMV to impact the incidence of tetanus in developing countries.

Collaborative study for the calibration of a replacement International Standard for Tetanus Toxoid Adsorbed.

We present the results of a collaborative study for the establishment of a replacement International Standard (IS) for Tetanus Toxoid Adsorbed. Two candidate preparations were included in the study, one of which was established as the 4th IS for Tetanus Toxoid Adsorbed at the WHO Expert Committee on Biological Standardization meeting in October 2010. This preparation was found to have a unitage of 490 IU/ampoule, based on calibration in guinea pig challenge assays. Results from mouse challenge assays suggest that the relative performance of two candidate preparations may differ significantly between guinea pigs and mice. The authors note that the number of laboratories that performed guinea pig challenge assays, which are used to calibrate and assign IU, is much lower than in previous collaborative studies and this may have implications for calibration of replacement standards in the future. The issue of assigning separate units to the IS for guinea pig and mouse assays is discussed. The study also assessed performance of the replacement standard in serological assays which are used as alternative procedures to challenge assays for tetanus potency testing. Results suggest that the replacement standard is suitable for use as the reference vaccine in serological assays.

Development and use of a novel in vitro assay for testing of diphtheria toxoid in combination vaccines.

Testing of diphtheria vaccines for routine lot release relies heavily on the use of in vivo potency assays. However, consistency of production is also recognised as an important feature of vaccine manufacture, and in vitro assays are superior to in vivo assays for providing this information. In adsorbed vaccines, antigen and adjuvant are the major components contributing to immunogenicity and are therefore critical factors to be evaluated as part of consistency testing. Here we describe a simple and sensitive Enzyme Linked Immunosorbent Assay (ELISA) which has been developed to quantify diphtheria toxoid antigen in combined vaccine products and can also be used to monitor the degree of adsorption. This assay can be applied to a variety of multi-component vaccines and is robust, specific and highly sensitive, with a limit of quantification of approximately 0.005 Lf/ml. The antigen assay is an excellent test to characterise vaccines and monitor trends in production. For well established vaccines, the antigen assay could be used alongside other in vitro methods to provide a more informative product profile, with the ultimate aim of reducing the requirement for in vivo potency assays and therefore the number of animals required for routine batch release testing.

Transcutaneous delivery of tetanus toxin Hc fragment induces superior tetanus toxin neutralizing antibody response compared to tetanus toxoid.

Transcutaneous immunization is a promising vaccination delivery strategy which targets potent immune cells residing in the outer layer of the skin. In this study, the immunogenicity and neutralizing potency of the non-toxic Hc fragment of tetanus toxin (HcWT) and a mutant of Hc lacking ganglioside binding activity were compared with that of tetanus toxoid (TTxd) following transcutaneous immunization (TCI) of mice. Mice immunized with HcWT in the absence of an adjuvant induced highest anti-toxoid and anti-Hc antibody titres, with a significant increase in the toxin neutralizing antibody response compared with TTxd. These results are in contrast to previous studies employing subcutaneous delivery, where TTxd was found to be a more potent immunogen than the Hc fragment of the toxin. We conclude that the HcWT protein is more immunogenic than TTxd when given via the transcutaneous route. Our results suggest that TCI may provide an opportunity for effective delivery of toxin-like antigens which harbor protective epitopes and that traditional toxoid proteins may not be optimal antigens for skin immunization.

Transcutaneous immunization with tetanus toxoid and mutants of Escherichia coli heat-labile enterotoxin as adjuvants elicits strong protective antibody responses.

In this study, the adjuvanticity of 2 nontoxic derivatives (LTK63 and LTR72) of heat-labile enterotoxin of Escherichia coli (LT) was evaluated and was compared with that of a cytosine phosphodiester-guanine (CpG) motif, after transcutaneous immunization with tetanus toxoid (TT). TT plus LTR72 elicited the strongest antibody responses, compared with those elicited by the other vaccines (TT, TT plus LTK63, TT plus CpG, and TT plus LTK63 plus CpG); it neutralized the toxin and conferred full protection after passive transfer in mice. Preexisting immunity to LT mutants did not adversely affect their adjuvant potency. Both LTK63 and LTR72 promoted the induction of IgG1 antibodies. In contrast, mice receiving either CpG motif alone or CpG motif plus LTK63 produced strong IgG2a anti-TT antibody responses. Overall, these findings demonstrate that mutants of enterotoxins with reduced toxicity are effective adjuvants for transcutaneous immunization.