Use of Monoclonal Antibodies in the Sensitive Detection and Neutralization of Botulinum Neurotoxin Serotype B

Neutralization mechanism of human monoclonal antibodies against type B botulinum neurotoxin

Botulism is a deadly neuroparalytic condition caused by the botulinum neurotoxin (BoNT) produced by Clostridium botulinum and related species. Toxin-neutralizing antibodies are the most effective treatments for BoNT intoxication. We generated human monoclonal antibodies neutralizing type B botulinum neurotoxin (BoNT/B), designated M2 and M4. The combination of these antibodies exhibited a strong neutralizing effect against BoNT/B toxicity. In this study, we analyzed the mechanisms of action of these antibodies in vitro. M4 binds to the C-terminus of the heavy chain (the receptor-binding domain) and inhibits BoNT/B binding to neuronal PC12 cells. Although M2 recognized the light (L) chain (the metalloprotease domain), it did not inhibit substrate (VAMP2) cleavage in the cleavage assay. M2 increased the surface localization of BoNT/B in PC12 cells at a later time point, suggesting that M2 inhibits the translocation of the L chain from synaptic vesicles to the cytosol. These results indicate that M2 and M4 inhibit the different processes of BoNT/B individually and that multistep inhibition is important for the synergistic effect of the combination of monoclonal antibodies. Our findings may facilitate the development of effective therapeutic antibodies against BoNTs.

Recombinant L-HN Fusion Antigen Derived from the L and HN Domains of Botulinum Neurotoxin B Stimulates a Protective Antibody Response Against Active Neurotoxin

Botulinum neurotoxin (BoNT) is a neurotoxin produced by Clostridium botulinum in an anaerobic environment. BoNT is the most toxic protein among bacteria, animals, plants, and chemical substances reported to date. BoNTs are 150 kDa proteins composed of three major functional domains: catalytic (L domain, 50 kDa), translocation (HN domain, 50 kDa), and receptor-binding (Hc domain, 50 kDa) domains. Most studies have focused on the use of the Hc domain as an antigen because it is capable of generating robust protective immunity and contains some functional neutralizing epitopes. In the present study, we produced and characterized a recombinant L-HN fusion fragment of the parent BoNT/B (BL-HN) composed of L and HN domains with a deletion in the Hc domain (BHc). When the BL-HN protein was expressed in E. coli, it retained its stable structure and antigenicity. As a vaccine antigen, the recombinant BL-HN protein was found to induce sufficient protection against native BoNT/B in a mouse model. The BL-HN subunit vaccine could also induce a strong humoral immune response and generate sufficient neutralizing antibodies in immunized mice. Therefore, BL-HN may retain the native neurotoxin structure and critical epitopes responsible for inducing serum neutralizing antibodies. Studies of the dose-dependent immunoprotective effects further confirmed that the BL-HN antigen could provide potent protective immunity. This finding suggests that BL-HN can play an important role in immune protection against BoNT/B. Therefore, the BL-HN fusion fragment provides an excellent platform for the design of recombinant botulinum vaccines and neutralizing antibodies.

Fully Human Monoclonal Antibodies Effectively Neutralizing Botulinum Neurotoxin Serotype B

Botulinum neurotoxin (BoNT) is the most potent natural toxin known. Of the seven BoNT serotypes (A to G), types A, B, E, and F cause human botulism. Treatment of human botulism requires the development of effective toxin-neutralizing antibodies without side effects such as serum sickness and anaphylaxis. In this study, we generated fully human monoclonal antibodies (HuMAbs) against serotype B BoNT (BoNT/B1) using a murine–human chimera fusion partner cell line named SPYMEG. Of these HuMAbs, M2, which specifically binds to the light chain of BoNT/B1, showed neutralization activity in a mouse bioassay (approximately 10 i.p. LD₅₀/100 µg of antibody), and M4, which binds to the C-terminal of heavy chain, showed partial protection. The combination of two HuMAbs, M2 (1.25 µg) and M4 (1.25 µg), was able to completely neutralize BoNT/B1 (80 i.p. LD₅₀) with a potency greater than 80 i.p. LD₅₀/2.5 µg of antibodies, and was effective both prophylactically and therapeutically in the mouse model of botulism. Moreover, this combination showed broad neutralization activity against three type B subtypes, namely BoNT/B1, BoNT/B2, and BoNT/B6. These data demonstrate that the combination of M2 and M4 is promising in terms of a foundation for new human therapeutics for BoNT/B intoxication.

A simple and fast protocol for the synthesis of 2-amino-4-(4-formylphenyl)-4H-chromene-3-carbonitrile to develop an optical immunoassay for the quantification of botulinum neurotoxin type F

In this study, a novel optical immunoassay platform using (S)-2-amino-4-(4-formylphenyl)-4H-chromene-3-carbonitrile, which was synthesized by an ultra-sonication method, as an optical probe.

A Novel Rabbit Spirometry Model of Type E Botulism and Its Use for the Evaluation of Postsymptom Antitoxin Efficacy

Botulinum neurotoxins (BoNTs), the most poisonous substances known in nature, pose significant concern to health authorities. The only approved therapeutic for botulism is antitoxin. While administered to patients only after symptom onset, antitoxin efficacy is evaluated in animals mostly in relation to time postintoxication regardless of symptoms. This is most likely due to the difficulty in measuring early symptoms of botulism in animals. In this study, a rabbit spirometry model was developed to quantify early respiratory symptoms of type E botulism that were further used as a trigger for treatment. Impaired respiration, in the form of a reduced minute volume, was detected as early as 18.1 ± 2.9 h after intramuscular exposure to 2 rabbit 50% lethal doses (LD₅₀) of BoNT serotype E (BoNT/E), preceding any visible symptoms. All rabbits treated with antitoxin immediately following symptom onset survived. Postsymptom antitoxin efficacy was further evaluated in relation to toxin and antitoxin dosages as well as delayed antitoxin administration. Our system enabled us to demonstrate, for the first time, full antitoxin protection of animals treated with antitoxin after the onset of objective and quantitative type E botulism symptoms. This model may be utilized to evaluate the efficacy of antitoxins for additional serotypes of BoNT as well as that of next-generation anti-BoNT drugs that enter affected cells and act when antitoxin is no longer effective.

Highly Stable Lyophilized Homogeneous Bead-Based Immunoassays for On-Site Detection of Bio Warfare Agents from Complex Matrices

This study shows the development of dry, highly stable immunoassays for the detection of bio warfare agents in complex matrices. Thermal stability was achieved by the lyophilization of the complete, homogeneous, bead-based immunoassay in a special stabilizing buffer, resulting in a ready-to-use, simple assay, which exhibited long shelf and high-temperature endurance (up to 1 week at 100 °C). The developed methodology was successfully implemented for the preservation of time-resolved fluorescence, Alexa-fluorophores, and horse radish peroxidase-based bead assays, enabling multiplexed detection. The multiplexed assay was successfully implemented for the detection of Bacillus anthracis, botulinum B, and tularemia in complex matrices.