Extraction of BoNT/A, /B, /E, and /F with a single, high affinity monoclonal antibody for detection of botulinum neurotoxin by Endopep-MS

Validation of a clinical assay for botulinum neurotoxins through mass spectrometric detection

Botulism is a paralytic disease due to the inhibition of acetylcholine exocytosis at the neuromuscular junction, which can be lethal if left untreated. Botulinum neurotoxins (BoNTs) are produced by some spore-forming Clostridium bacteria. The current confirmatory assay to test for BoNTs in clinical specimens is the gold-standard mouse bioassay. However, an Endopep-MS assay method has been developed to detect BoNTs in clinical samples using benchtop mass spectrometric detection. This work demonstrates the validation of the Endopep-MS method for clinical specimens with the intent of method distribution in public health laboratories. The Endopep-MS assay was validated by assessing the sensitivity, robustness, selectivity, specificity, and reproducibility. The limit of detection was found to be equivalent to or more sensitive than the mouse bioassay. Specificity studies determined no cross-reactivity between the different serotypes and no false positives from an exclusivity panel of culture supernatants of enteric disease organisms and non-toxigenic strains of Clostridium. Inter-serotype specificity testing with 19 BoNT subtypes was 100% concordant with the expected results, accurately determining the presence of the correct serotype and the absence of incorrect serotypes. Additionally, a panel of potential interfering substances was used to test selectivity. Finally, clinical studies included clinical specimen stability and reproducibility, which was found to be 99.9% from a multicenter evaluation study. The multicenter validation study also included a clinical validation study, which yielded a 99.4% correct determination rate. Use of the Endopep-MS method will improve the capacity and response time for laboratory confirmation of botulism in public health laboratories.

Single-Domain Antibody Multimers for Detection of Botulinum Neurotoxin Serotypes C, D, and Their Mosaics in Endopep-MS

Botulinum neurotoxins (BoNTs) are highly toxic proteins that require high-affinity immunocapture reagents for use in endopeptidase-based assays. Here, 30 novel and 2 earlier published llama single-domain antibodies (VHHs) against the veterinary-relevant BoNT serotypes C and D were yeast-produced. These VHHs recognized 10 independent antigenic sites, and many cross-reacted with the BoNT/DC and CD mosaic variants. As VHHs are highly suitable for genetically linking to increase antigen-binding affinity, 52 VHH multimers were produced and their affinity for BoNT/C, D, DC, and CD was determined. A selection of 15 multimers with high affinity (KD < 0.1 nM) was further shown to be resilient to a high salt wash that is used for samples from complex matrices and bound native BoNTs from culture supernatants as shown by Endopep-MS. High-affinity multimers suitable for further development of a highly sensitive Endopep-MS assay include four multimers that bind both BoNT/D and CD with KD of 14-99 pM, one multimer for BoNT/DC (65 pM) that also binds BoNT/C (75 pM), and seven multimers for BoNT/C (<1-19 pM), six of which also bind BoNT/DC with lower affinity (93-508 pM). In addition to application in diagnostic tests, these VHHs could be used for the development of novel therapeutics for animals or humans.

A Three-Monoclonal Antibody Combination Potently Neutralizes BoNT/G Toxin in Mice

Equine-derived antitoxin (BAT^®) is the only treatment for botulism from botulinum neurotoxin serotype G (BoNT/G). BAT^® is a foreign protein with potentially severe adverse effects and is not renewable. To develop a safe, more potent, and renewable antitoxin, humanized monoclonal antibodies (mAbs) were generated. Yeast displayed single chain Fv (scFv) libraries were prepared from mice immunized with BoNT/G and BoNT/G domains and screened with BoNT/G using fluorescence-activated cell sorting (FACS). Fourteen scFv-binding BoNT/G were isolated with K_D values ranging from 3.86 nM to 103 nM (median K_D 20.9 nM). Five mAb-binding non-overlapping epitopes were humanized and affinity matured to create antibodies hu6G6.2, hu6G7.2, hu6G9.1, hu6G10, and hu6G11.2, with IgG K_D values ranging from 51 pM to 8 pM. Three IgG combinations completely protected mice challenged with 10,000 LD₅₀s of BoNT/G at a total mAb dose of 6.25 μg per mouse. The mAb combinations have the potential for use in the diagnosis and treatment of botulism due to serotype G and, along with antibody combinations to BoNT/A, B, C, D, E, and F, provide the basis for a fully recombinant heptavalent botulinum antitoxin to replace the legacy equine product.

Resolving the Molecular Steps in Clostridial Neurotoxin Light Chain Translocation

The clostridial neurotoxins (CNTs), botulinum toxin and tetanus toxin, are the most toxic proteins for humans. Neurotoxicity is based upon the specificity of the CNTs for neural host receptors and substrates. CNTs are organized into three domains, a Light Chain (LC) that is a metalloprotease and a Heavy Chain (HC) that has two domains, an N-terminal LC translocation domain (HCN) and a C-terminal receptor binding domain (HCC). While catalysis and receptor binding functions of the CNTs have been developed, our understanding of LC translocation is limited. This is due to the intrinsic complexity of the translocation process and limited tools to assess the step-by-step events in LC translocation. Recently, we developed a novel, cell-based TT-reporter to measure LC translocation as the translocation of a β-lactamase reporter across a vesicle membrane in neurons. Using this approach, we identified a role for a cis-Loop, located within the HCN, in LC translocation. In this commentary, we describe our current understanding of how CNTs mediate LC translocation and place the role of the cis-Loop in the LC translocation process relative to other independent functions that have been implicated in LC translocation. Understanding the basis for LC translocation will enhance the use of CNTs in vaccine development and as human therapies.

Metody stosowane do wykrywania i identyfikacji toksyn botulinowych w próbkach klinicznych i żywności*

Botulism is a severe neuroparalytic illness, which affects the nervous system. It is caused by botulinum neurotoxins (BoNTs), produced by anaerobic gram-positive bacteria Clostridium botulinum. There are 7 serotypes of BoNT A-G, but BoNT A/B/D/E plays a major role in botulism affecting humans. Foodborne botulism (classic botulism) is the most frequent clinical manifestation occurring after consumption of food containing botulinum neurotoxins. The diagnosis of botulism is based on clinical symptoms; however, recommended and alternative laboratory methods are used to confirm the etiology of symptoms and the identification of BoNT toxin type. The aim of this work was to present the epidemiology of foodborne botulism in Poland and to gather and analyze the available diagnostic methods that allow us to detect BoNT in clinical samples. Using the epidemiological reports of National Institute of Hygiene in Poland and findings presented in the Przegląd Epidemiologiczny, the incidence of classical botulism in Poland has been presented over a period of recent 18 years. Searching for the optimal diagnostic method for BoNT identification in various samples, we have confronted the sensitivity and specificity of recently available alternative methods with classical biological assay.

De novo protein design, a retrospective

Proteins are molecular machines whose function depends on their ability to achieve complex folds with precisely defined structural and dynamic properties. The rational design of proteins from first-principles, or de novo, was once considered to be impossible, but today proteins with a variety of folds and functions have been realized. We review the evolution of the field from its earliest days, placing particular emphasis on how this endeavor has illuminated our understanding of the principles underlying the folding and function of natural proteins, and is informing the design of macromolecules with unprecedented structures and properties. An initial set of milestones in de novo protein design focused on the construction of sequences that folded in water and membranes to adopt folded conformations. The first proteins were designed from first-principles using very simple physical models. As computers became more powerful, the use of the rotamer approximation allowed one to discover amino acid sequences that stabilize the desired fold. As the crystallographic database of protein structures expanded in subsequent years, it became possible to construct proteins by assembling short backbone fragments that frequently recur in Nature. The second set of milestones in de novo design involves the discovery of complex functions. Proteins have been designed to bind a variety of metals, porphyrins, and other cofactors. The design of proteins that catalyze hydrolysis and oxygen-dependent reactions has progressed significantly. However, de novo design of catalysts for energetically demanding reactions, or even proteins that bind with high affinity and specificity to highly functionalized complex polar molecules remains an importnant challenge that is now being achieved. Finally, the protein design contributed significantly to our understanding of membrane protein folding and transport of ions across membranes. The area of membrane protein design, or more generally of biomimetic polymers that function in mixed or non-aqueous environments, is now becoming increasingly possible.

Detecting Biothreat Agents: From Current Diagnostics to Developing Sensor Technologies

Although a fundamental understanding of the pathogenicity of most biothreat agents has been elucidated and available treatments have increased substantially over the past decades, they still represent a significant public health threat in this age of (bio)terrorism, indiscriminate warfare, pollution, climate change, unchecked population growth, and globalization. The key step to almost all prevention, protection, prophylaxis, post-exposure treatment, and mitigation of any bioagent is early detection. Here, we review available methods for detecting bioagents including pathogenic bacteria and viruses along with their toxins. An introduction placing this subject in the historical context of previous naturally occurring outbreaks and efforts to weaponize selected agents is first provided along with definitions and relevant considerations. An overview of the detection technologies that find use in this endeavor along with how they provide data or transduce signal within a sensing configuration follows. Current "gold" standards for biothreat detection/diagnostics along with a listing of relevant FDA approved in vitro diagnostic devices is then discussed to provide an overview of the current state of the art. Given the 2014 outbreak of Ebola virus in Western Africa and the recent 2016 spread of Zika virus in the Americas, discussion of what constitutes a public health emergency and how new in vitro diagnostic devices are authorized for emergency use in the U.S. are also included. The majority of the Review is then subdivided around the sensing of bacterial, viral, and toxin biothreats with each including an overview of the major agents in that class, a detailed cross-section of different sensing methods in development based on assay format or analytical technique, and some discussion of related microfluidic lab-on-a-chip/point-of-care devices. Finally, an outlook is given on how this field will develop from the perspective of the biosensing technology itself and the new emerging threats they may face.

Selection of Antibody Fragments by Yeast Display

The critical need for renewable, high-quality affinity reagents in biological research, as well as for diagnostic and therapeutic applications, has required the development of new platforms of discovery. Yeast display is one of the main methods of in vitro display technology with phage display. Yeast display has been chosen by numerous groups to refine both affinity and specificity of antibodies because it enables fine discrimination between mutant clones of similar affinity. In addition, the construction of display libraries of antibody fragments in yeast permits to sample the immune antibody repertoire more fully than using phage. This chapter gives an updated overview of the available systems of yeast display platforms and libraries, followed up by technical descriptions of selection methods of antibody fragments by yeast display.

Characterization of Hemagglutinin Negative Botulinum Progenitor Toxins

Botulism is a disease involving intoxication with botulinum neurotoxins (BoNTs), toxic proteins produced by Clostridium botulinum and other clostridia. The 150 kDa neurotoxin is produced in conjunction with other proteins to form the botulinum progenitor toxin complex (PTC), alternating in size from 300 kDa to 500 kDa. These progenitor complexes can be classified into hemagglutinin positive or hemagglutinin negative, depending on the ability of some of the neurotoxin-associated proteins (NAPs) to cause hemagglutination. The hemagglutinin positive progenitor toxin complex consists of BoNT, nontoxic non-hemagglutinin (NTNH), and three hemagglutinin proteins; HA-70, HA-33, and HA-17. Hemagglutinin negative progenitor toxin complexes contain BoNT and NTNH as the minimally functional PTC (M-PTC), but not the three hemagglutinin proteins. Interestingly, the genome of hemagglutinin negative progenitor toxin complexes comprises open reading frames (orfs) which encode for three proteins, but the existence of these proteins has not yet been extensively demonstrated. In this work, we demonstrate that these three proteins exist and form part of the PTC for hemagglutinin negative complexes. Several hemagglutinin negative strains producing BoNT/A, /E, and /F were found to contain the three open reading frame proteins. Additionally, several BoNT/A-containing bivalent strains were examined, and NAPs from both genes, including the open reading frame proteins, were associated with BoNT/A. The open reading frame encoded proteins are more easily removed from the botulinum complex than the hemagglutinin proteins, but are present in several BoNT/A and /F toxin preparations. These are not easily removed from the BoNT/E complex, however, and are present even in commercially-available purified BoNT/E complex.

A three monoclonal antibody combination potently neutralizes multiple botulinum neurotoxin serotype F subtypes

Human botulism is primarily caused by botulinum neurotoxin (BoNT) serotypes A, B and E, with around 1% caused by serotype F (BoNT/F). BoNT/F comprises at least seven different subtypes with the amino acid sequence difference between subtypes as high as 36%. The sequence differences present a significant challenge for generating monoclonal antibodies (mAbs) that can bind, detect and neutralize all BoNT/F subtypes. We used repertoire cloning of immune mouse antibody variable (V) regions and yeast display to generate a panel of 33 lead single chain Fv (scFv) mAbs that bound one or more BoNT/F subtypes with a median equilibrium dissociation constant (KD) of 4.06 × 10-9 M. By diversifying the V-regions of the lead mAbs and selecting for cross reactivity we generated five mAbs that bound each of the seven subtypes. Three scFv binding non-overlapping epitopes were converted to IgG that had KD for the different BoNT/F subtypes ranging from 2.2×10-8 M to 1.47×10-12 pM. An equimolar combination of the mAbs was able to potently neutralize BoNT/F1, F2, F4 and F7 in the mouse neutralization assay. The mAbs have potential utility as diagnostics capable of recognizing the known BoNT/F subtypes and could be developed as antitoxins to prevent and treat type F botulism.

Implementing the Bruker MALDI Biotyper in the Public Health Laboratory for C. botulinum Neurotoxin Detection

Currently, the gold standard method for active botulinum neurotoxin (BoNT) detection is the mouse bioassay (MBA). A Centers for Disease Control and Prevention-developed mass spectrometry (MS)-based assay that detects active BoNT was successfully validated and implemented in a public health laboratory in clinical matrices using the Bruker MALDI-TOF MS (Matrix-assisted laser desorption ionization–time of flight mass spectrometry) Biotyper. For the first time, a direct comparison with the MBA was performed to determine MS-based assay sensitivity using the Bruker MALDI Biotyper. Mice were injected with BoNT/A, /B, /E, and /F at concentrations surrounding the established MS assay limit of detection (LOD) and analyzed simultaneously. For BoNT/B, /E, and /F, MS assay sensitivity was equivalent or better than the MBA at 25, 0.3, and 8.8 mLD₅₀, respectively. BoNT/A was detected by the MBA between 1.8 and 18 mLD₅₀, somewhat more sensitive than the MS method of 18 mLD₅₀. Studies were performed to compare assay performance in clinical specimens. For all tested specimens, the MS method rapidly detected BoNT activity and serotype in agreement with, or in the absence of, results from the MBA. We demonstrate that the MS assay can generate reliable, rapid results while eliminating the need for animal testing.

Immunological Characterization and Neutralizing Ability of Monoclonal Antibodies Directed Against Botulinum Neurotoxin Type H

BACKGROUND

Only Clostridium botulinum strain IBCA10-7060 produces the recently described novel botulinum neurotoxin type H (BoNT/H). BoNT/H (N-terminal two-thirds most homologous to BoNT/F and C-terminal one-third most homologous to BoNT/A) requires antitoxin to toxin ratios ≥1190:1 for neutralization by existing antitoxins. Hence, more potent and safer antitoxins against BoNT/H are needed.

METHODS

We therefore evaluated our existing monoclonal antibodies (mAbs) to BoNT/A and BoNT/F for BoNT/H binding, created yeast-displayed mutants to select for higher-affinity-binding mAbs by using flow cytometry, and evaluated the mAbs' ability to neutralize BoNT/H in the standard mouse bioassay.

RESULTS

Anti-BoNT/A HCC-binding mAbs RAZ1 and CR2 bound BoNT/H with high affinity. However, only 1 of 6 BoNT/F mAbs (4E17.2A) bound BoNT/H but with an affinity >800-fold lower (equilibrium dissociation binding constant [KD] = 7.56 × 10(-8)M) than its BoNT/F affinity (KD= 9.1 × 10(-11)M), indicating that the N-terminal two-thirds of BoNT/H is immunologically unique. The affinity of 4E17.2A for BoNT/H was increased >500-fold to KD= 1.48 × 10(-10)M (mAb 4E17.2D). A combination of mAbs RAZ1, CR2, and 4E17.2D completely protected mice challenged with 280 mouse median lethal doses of BoNT/H at a mAb dose as low as 5 µg of total antibody.

CONCLUSIONS

This 3-mAb combination potently neutralized BoNT/H and represents a potential human antitoxin that could be developed for the prevention and treatment of type H botulism.

Ultrasensitive detection of protease activity of anthrax and botulinum toxins by a new PCR-based assay

Anthrax and botulism are dangerous infectious diseases that can be fatal unless detected and treated quickly. Fatalities from these diseases are primarily due to endopeptidase toxins secreted by the pathogens. Rapid and sensitive detection of the presence of active toxins is the key element for protection from natural outbreaks of anthrax and botulism, as well as from the threat of bioterrorism. We describe an ultrasensitive polymerase chain reaction (PCR)-based assay for detecting proteolytic activity of anthrax and botulinum toxins using composite probes consisting of covalent peptide-DNA conjugate for the detection of anthrax, and noncovalent protein-aptamer assembly to assay botulinum toxin activity. Probes immobilized on the solid-phase support are cleaved by toxins to release DNA, which is detected by real-time PCR. Both assays can detect subpicogram quantities of active toxins isolated from composite matrices. Special procedures were developed to isolate intact toxins from the matrices under mild conditions. The assay is rapid, uses proven technologies, and can be modified to detect other proteolytic and biopolymer-degrading enzymes.

Generation and Characterization of Six Recombinant Botulinum Neurotoxins as Reference Material to Serve in an International Proficiency Test

The detection and identification of botulinum neurotoxins (BoNT) is complex due to the existence of seven serotypes, derived mosaic toxins and more than 40 subtypes. Expert laboratories currently use different technical approaches to detect, identify and quantify BoNT, but due to the lack of (certified) reference materials, analytical results can hardly be compared. In this study, the six BoNT/A1–F1 prototypes were successfully produced by recombinant techniques, facilitating handling, as well as improving purity, yield, reproducibility and biosafety. All six BoNTs were quantitatively nicked into active di-chain toxins linked by a disulfide bridge. The materials were thoroughly characterized with respect to purity, identity, protein concentration, catalytic and biological activities. For BoNT/A₁, B₁ and E₁, serotypes pathogenic to humans, the catalytic activity and the precise protein concentration were determined by Endopep-mass spectrometry and validated amino acid analysis, respectively. In addition, BoNT/A₁, B₁, E₁ and F₁ were successfully detected by immunological assays, unambiguously identified by mass spectrometric-based methods, and their specific activities were assigned by the mouse LD₅₀ bioassay. The potencies of all six BoNT/A1–F1 were quantified by the ex vivo mouse phrenic nerve hemidiaphragm assay, allowing a direct comparison. In conclusion, highly pure recombinant BoNT reference materials were produced, thoroughly characterized and employed as spiking material in a worldwide BoNT proficiency test organized by the EQuATox consortium.

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.

Mass Spectrometric Detection of Bacterial Protein Toxins and Their Enzymatic Activity

Mass spectrometry has recently become a powerful technique for bacterial identification. Mass spectrometry approaches generally rely upon introduction of the bacteria into a matrix-assisted laser-desorption time-of-flight (MALDI-TOF) mass spectrometer with mass spectrometric recognition of proteins specific to that organism that form a reliable fingerprint. With some bacteria, such as Bacillus anthracis and Clostridium botulinum, the health threat posed by these organisms is not the organism itself, but rather the protein toxins produced by the organisms. One such example is botulinum neurotoxin (BoNT), a potent neurotoxin produced by C. botulinum. There are seven known serotypes of BoNT, A–G, and many of the serotypes can be further differentiated into toxin variants, which are up to 99.9% identical in some cases. Mass spectrometric proteomic techniques have been established to differentiate the serotype or toxin variant of BoNT produced by varied strains of C. botulinum. Detection of potent biological toxins requires high analytical sensitivity and mass spectrometry based methods have been developed to determine the enzymatic activity of BoNT and the anthrax lethal toxins produced by B. anthracis. This enzymatic activity, unique for each toxin, is assessed with detection of the toxin-induced cleavage of strategically designed peptide substrates by MALDI-TOF mass spectrometry offering unparalleled specificity. Furthermore, activity assays allow for the assessment of the biological activity of a toxin and its potential health risk. Such methods have become important diagnostics for botulism and anthrax. Here, we review mass spectrometry based methods for the enzymatic activity of BoNT and the anthrax lethal factor toxin.

Recommended mass spectrometry-based strategies to identify botulinum neurotoxin-containing samples

Botulinum neurotoxins (BoNTs) cause the disease called botulism, which can be lethal. BoNTs are proteins secreted by some species of clostridia and are known to cause paralysis by interfering with nerve impulse transmission. Although the human lethal dose of BoNT is not accurately known, it is estimated to be between 0.1 μg to 70 μg, so it is important to enable detection of small amounts of these toxins. Our laboratory previously reported on the development of Endopep-MS, a mass-spectrometric‑based endopeptidase method to detect, differentiate, and quantify BoNT immunoaffinity purified from complex matrices. In this work, we describe the application of Endopep-MS for the analysis of thirteen blinded samples supplied as part of the EQuATox proficiency test. This method successfully identified the presence or absence of BoNT in all thirteen samples and was able to successfully differentiate the serotype of BoNT present in the samples, which included matrices such as buffer, milk, meat extract, and serum. Furthermore, the method yielded quantitative results which had z-scores in the range of -3 to +3 for quantification of BoNT/A containing samples. These results indicate that Endopep-MS is an excellent technique for detection, differentiation, and quantification of BoNT in complex matrices.

Isolation and functional characterization of the novel Clostridium botulinum neurotoxin A8 subtype

Botulism is a severe neurological disease caused by the complex family of botulinum neurotoxins (BoNT). Based on the different serotypes known today, a classification of serotype variants termed subtypes has been proposed according to sequence diversity and immunological properties. However, the relevance of BoNT subtypes is currently not well understood. Here we describe the isolation of a novel Clostridium botulinum strain from a food-borne botulism outbreak near Chemnitz, Germany. Comparison of its botulinum neurotoxin gene sequence with published sequences identified it to be a novel subtype within the BoNT/A serotype designated BoNT/A8. The neurotoxin gene is located within an ha-orfX+ cluster and showed highest homology to BoNT/A1, A2, A5, and A6. Unexpectedly, we found an arginine insertion located in the HC domain of the heavy chain, which is unique compared to all other BoNT/A subtypes known so far. Functional characterization revealed that the binding characteristics to its main neuronal protein receptor SV2C seemed unaffected, whereas binding to membrane-incorporated gangliosides was reduced in comparison to BoNT/A1. Moreover, we found significantly lower enzymatic activity of the natural, full-length neurotoxin and the recombinant light chain of BoNT/A8 compared to BoNT/A1 in different endopeptidase assays. Both reduced ganglioside binding and enzymatic activity may contribute to the considerably lower biological activity of BoNT/A8 as measured in a mouse phrenic nerve hemidiaphragm assay. Despite its reduced activity the novel BoNT/A8 subtype caused severe botulism in a 63-year-old male. To our knowledge, this is the first description and a comprehensive characterization of a novel BoNT/A subtype which combines genetic information on the neurotoxin gene cluster with an in-depth functional analysis using different technical approaches. Our results show that subtyping of BoNT is highly relevant and that understanding of the detailed toxin function might pave the way for the development of novel therapeutics and tailor-made antitoxins.

Detection of Botulinum Toxins A, B, E, and F in Foods by Endopep-MS

Botulism is caused by exposure to botulinum neurotoxins (BoNTs). BoNTs are proteins secreted by some species of clostridia; these neurotoxins are known to interfere with nerve impulse transmission, thus causing paralysis. Botulism may be contracted through consumption of food either naturally or intentionally contaminated with BoNT. The human lethal dose of BoNT is not known but is estimated to be between 0.1 and 70 μg; thus, it is important to be able to detect small amounts of this toxin in foods to ensure food safety and to identify the source of an outbreak. Our laboratory previously reported on the development of Endopep-MS, a mass-spectrometric-based endopeptidase method for the detection and differentiation of BoNT. This method can detect BoNT at levels below the historic standard mouse bioassay in clinical samples such as serum, stool, and culture supernatants. We have now expanded this assay to detect BoNT in over 50 foods including representative products that were involved in actual botulism investigations. The foods tested by the Endopep-MS included those with various acidities, viscosities, and fat levels. Dairy and culturally diverse products were also included. This work demonstrates that the Endopep-MS method can be used to detect BoNT/A, /B, /E, and /F in foods at levels spiked below that of the limit of detection of the mouse bioassay. Furthermore, we successfully applied this method to investigate several foods associated with botulism outbreaks.

Validation of the Endopep-MS method for qualitative detection of active botulinum neurotoxins in human and chicken serum

Botulinum neurotoxins (BoNTs) are highly toxic proteases produced by anaerobic bacteria. Traditionally, a mouse bioassay (MBA) has been used for detection of BoNTs, but for a long time, laboratories have worked with alternative methods for their detection. One of the most promising in vitro methods is a combination of an enzymatic and mass spectrometric assay called Endopep-MS. However, no comprehensive validation of the method has been presented. The main purpose of this work was to perform a validation for the qualitative analysis of BoNT-A, B, C, C/D, D, D/C, and F in serum. The limit of detection (LOD), selectivity, precision, stability in matrix and solution, and correlation with the MBA were evaluated. The LOD was equal to or even better than that of the MBA for BoNT-A, B, D/C, E, and F. Furthermore, Endopep-MS was for the first time successfully used to differentiate between BoNT-C and D and their mosaics C/D and D/C by different combinations of antibodies and target peptides. In addition, sequential antibody capture was presented as a new way to multiplex the method when only a small sample volume is available. In the comparison with the MBA, all the samples analyzed were positive for BoNT-C/D with both methods. These results indicate that the Endopep-MS method is a valid alternative to the MBA as the gold standard for BoNT detection based on its sensitivity, selectivity, and speed and that it does not require experimental animals.

Optimization of peptide substrates for botulinum neurotoxin E improves detection sensitivity in the Endopep-MS assay

Botulinum neurotoxins (BoNTs) produced by Clostridium botulinum are the most poisonous substances known to humankind. It is essential to have a simple, quick, and sensitive method for the detection and quantification of botulinum toxin in various media, including complex biological matrices. Our laboratory has developed a mass spectrometry-based Endopep-MS assay that is able to rapidly detect and differentiate all types of BoNTs by extracting the toxin with specific antibodies and detecting the unique cleavage products of peptide substrates. Botulinum neurotoxin type E (BoNT/E) is a member of a family of seven distinctive BoNT serotypes (A-G) and is the causative agent of botulism in both humans and animals. To improve the sensitivity of the Endopep-MS assay, we report here the development of novel peptide substrates for the detection of BoNT/E activity through systematic and comprehensive approaches. Our data demonstrate that several optimal peptides could accomplish 500-fold improvement in sensitivity compared with the current substrate for the detection of both not-trypsin-activated and trypsin-activated BoNT/E toxin complexes. A limit of detection of 0.1 mouse LD50/ml was achieved using the novel peptide substrate in the assay to detect not-trypsin-activated BoNT/E complex spiked in serum, stool, and food samples.

Improved detection of botulinum type E by rational design of a new peptide substrate for endopeptidase-mass spectrometry assay

Botulinum neurotoxins (BoNTs) are the most toxic substances known to humans. Endopeptidase-mass spectrometry (Endopep-MS) is used as a specific and rapid in vitro assay to detect BoNTs. In this assay, immunocaptured toxin cleaves a serotype-specific peptide substrate, and the cleavage products are then detected by MS. To further improve the sensitivity of the assay, we report here the rational design of a new substrate peptide for the detection of botulinum neurotoxin type E (BoNT/E). Our strategy was based on previously reported structural interactions integrated with analysis method efficiency considerations. Integration of the newly designed substrate has led to a more than one order of magnitude increased sensitivity of the assay.

Three enzymatically active neurotoxins of Clostridium botulinum strain Af84: BoNT/A2, /F4, and /F5

Botulinum neurotoxins (BoNTs) are produced by various species of clostridia and are potent neurotoxins which cause the disease botulism, by cleaving proteins needed for successful nerve transmission. There are currently seven confirmed serotypes of BoNTs, labeled A-G, and toxin-producing clostridia typically only produce one serotype of BoNT. There are a few strains (bivalent strains) which are known to produce more than one serotype of BoNT, producing either both BoNT/A and /B, BoNT/A and /F, or BoNT/B and /F, designated as Ab, Ba, Af, or Bf. Recently, it was reported that Clostridium botulinum strain Af84 has three neurotoxin gene clusters: bont/A2, bont/F4, and bont/F5. This was the first report of a clostridial organism containing more than two neurotoxin gene clusters. Using a mass spectrometry based proteomics approach, we report here that all three neurotoxins, BoNT/A2, /F4, and /F5, are produced by C. botulinum Af84. Label free MS(E) quantification of the three toxins indicated that toxin composition is 88% BoNT/A2, 1% BoNT/F4, and 11% BoNT/F5. The enzymatic activity of all three neurotoxins was assessed by examining the enzymatic activity of the neurotoxins upon peptide substrates, which mimic the toxins' natural targets, and monitoring cleavage of the substrates by mass spectrometry. We determined that all three neurotoxins are enzymatically active. This is the first report of three enzymatically active neurotoxins produced in a single strain of Clostridium botulinum.

RNA aptasensor for rapid detection of natively folded type A botulinum neurotoxin

A surface plasmon resonance based RNA aptasensor for rapid detection of natively folded type A botulinum neurotoxin is reported. Using detoxified recombinant type A botulinum neurotoxin as the surrogate, the aptasensor detects active toxin within 90 min. The detection limit of the aptasensor in phosphate buffered saline, carrot juice, and fat free milk is 5.8 ng/ml, 20.3 ng/ml and 23.4 ng/ml, respectively, while that in 5-fold diluted human serum is 22.5 ng/ml. Recovery of toxin from disparate sample matrices are within 91-116%. Most significant is the ability of this aptasensor to effectively differentiate the natively folded toxin from denatured, inactive toxin, which is important for homeland security surveillance and threat assessment. The aptasensor is stable for more than 30 days and over 400 injections/regeneration cycles. Such an aptasensor holds great promise for rapid detection of active botulinum neurotoxin for field surveillance due to its robustness, stability and reusability.

Mass Spectrometric Identification and Differentiation of Botulinum Neurotoxins through Toxin Proteomics

Botulinum neurotoxins (BoNTs) cause the disease botulism, which can be lethal if untreated. There are seven known serotypes of BoNT, A-G, defined by their response to antisera. Many serotypes are distinguished into differing subtypes based on amino acid sequence and immunogenic properties, and some subtypes are further differentiated into toxin variants. Toxin characterization is important as different types of BoNT can respond differently to medical countermeasures for botulism, and characterization of the toxin can aid in epidemiologic and forensic investigations. Proteomic techniques have been established to determine the serotype, subtype, or toxin variant of BoNT. These techniques involve digestion of the toxin into peptides, tandem mass spectrometric (MS/MS) analysis of the peptides, and database searching to identify the BoNT protein. These techniques demonstrate the capability to detect BoNT and its neurotoxin-associated proteins, and differentiate the toxin from other toxins which are up to 99.9% identical in some cases. This differentiation can be accomplished from toxins present in a complex matrix such as stool, food, or bacterial cultures and no DNA is required.

Development of a fusion protein SNVP as substrate for assaying multi-serotype botulinum neurotoxins

The SNARE super family has three core members, namely SNAP-25, VAMP-2, and syntaxin. SNAP-25 is cleaved by botulinum toxins (BoNTs)/A, /C, and /E, whereas VAMP-2 is the substrate for proteolytic BoNTs/B, /D, /F, and /G. In this study, we constructed a hybrid gene encoding the fusion protein SNVP that encompasses SNAP-25 residues Met1 to Gly206 and VAMP-2 residues Met1 to Lys94. The hybrid gene was cloned in a prokaryotic vector carrying an N-terminal pelB signal sequence and overexpressed in Escherichia coli BL21(DE3) Rosetta. To easily purify the protein, 6× His double-affinity tags were designed as the linker and C terminus of the fusion protein. SNVP was purified to homogeneity by affinity chromatography on a HisTrap FF column and determined to be more than 97% pure by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. N-terminal sequencing of the purified protein showed that signal peptide was successfully removed. The fusion protein SNVP contained the protease cleavage sites of all seven serotypes of BoNTs. SNVP was also proved to be recognized and cleaved by the endopeptidase of BoNTs (BoNT/A-LC, BoNT/B-LC, BoNT/E-LC, and BoNT/G-LC). The novel fusion substrate SNVP exhibited high biological activity under the optimal conditions, suggesting its potential use as a reagent for BoNT assay.

High sensitivity ELISA for detection of botulinum neurotoxin serotype F

Botulinum neurotoxins (BoNTs) are classified as category A biological threat agents by the Centers for Disease Control and Prevention (CDC) in the United States for its hazardous and potential bioterrorist threat to the public. About 1% naturally occurring botulisms are caused by Botulinum neurotoxin serotype F (BoNT/F). Most of the immunoassays for detecting BoNTs focus on the serotypes A and B, but few methods have been established for the detection of BoNT/F. Recently, the recombinant Hc subunit of botulinum neurotoxin type F (rFHc) was expressed as an effective vaccine against BoNT/F, indicating that this rFHc could be an effective immunogen to raise monoclonal antibodies (MAbs) for the detection and neutralization of BoNT/F. Here we present a novel sandwich enzyme-linked immunosorbent assay (ELISA) based on two MAbs against rFHc, which were FMMU-BTF-8 and FMMU-BTF-29 as capture antibody and detection antibody, respectively. The limit of detection (LOD) of this ELISA reached 12.09 pg/mL, much less than that of the other reported immunoassays. A simple, sensitive ELISA for detecting and quantifying BoNT/F was established, which can be used as a valuable method to detect and quantify BoNT/F.

Analysis of a unique Clostridium botulinum strain from the Southern hemisphere producing a novel type E botulinum neurotoxin subtype

Background

Clostridium botulinum strains that produce botulinum neurotoxin type E (BoNT/E) are most commonly isolated from botulism cases, marine environments, and animals in regions of high latitude in the Northern hemisphere. A strain of C. botulinum type E (CDC66177) was isolated from soil in Chubut, Argentina. Previous studies showed that the amino acid sequences of BoNT/E produced by various strains differ by < 6% and that the type E neurotoxin gene cluster inserts into the rarA operon.

Results

Genetic and mass spectral analysis demonstrated that the BoNT/E produced by CDC66177 is a novel toxin subtype (E9). Toxin gene sequencing indicated that BoNT/E9 differed by nearly 11% at the amino acid level compared to BoNT/E1. Mass spectrometric analysis of BoNT/E9 revealed that its endopeptidase substrate cleavage site was identical to other BoNT/E subtypes. Further analysis of this strain demonstrated that its 16S rRNA sequence clustered with other Group II C. botulinum (producing BoNT types B, E, and F) strains. Genomic DNA isolated from strain CDC66177 hybridized with fewer probes using a Group II C. botulinum subtyping microarray compared to other type E strains examined. Whole genome shotgun sequencing of strain CDC66177 revealed that while the toxin gene cluster inserted into the rarA operon similar to other type E strains, its overall genome content shared greater similarity with a Group II C. botulinum type B strain (17B).

Conclusions

These results expand our understanding of the global distribution of C. botulinum type E strains and suggest that the type E toxin gene cluster may be able to insert into C. botulinum strains with a more diverse genetic background than previously recognized.

Improved detection of botulinum neurotoxin serotype A by Endopep-MS through peptide substrate modification

Botulinum neurotoxins (BoNTs) are a family of seven toxin serotypes that are the most toxic substances known to humans. Intoxication with BoNT causes flaccid paralysis and can lead to death if untreated with serotype-specific antibodies. Supportive care, including ventilation, may be necessary. Rapid and sensitive detection of BoNT is necessary for timely clinical confirmation of clinical botulism. Previously, our laboratory developed a fast and sensitive mass spectrometry (MS) method termed the Endopep-MS assay. The BoNT serotypes are rapidly detected and differentiated by extracting the toxin with serotype-specific antibodies and detecting the unique and serotype-specific cleavage products of peptide substrates that mimic the sequence of the BoNT native targets. To further improve the sensitivity of the Endopep-MS assay, we report here the optimization of the substrate peptide for the detection of BoNT/A. Modifications on the terminal groups of the original peptide substrate with acetylation and amidation significantly improved the detection of BoNT/A cleavage products. The replacement of some internal amino acid residues with single or multiple substitutions led to further improvement. An optimized peptide increased assay sensitivity 5-fold with toxin spiked into buffer solution or different biological matrices.

Simultaneous and sensitive detection of six serotypes of botulinum neurotoxin using enzyme-linked immunosorbent assay-based protein antibody microarrays

Botulinum neurotoxins (BoNTs), produced by Clostridium botulinum, are a group of seven (A-G) immunologically distinct proteins and cause the paralytic disease botulism. These toxins are the most poisonous substances known to humans and are potential bioweapon agents. Therefore, it is necessary to develop highly sensitive assays for the detection of BoNTs in both clinical and environmental samples. In the current study, we have developed an enzyme-linked immunosorbent assay (ELISA)-based protein antibody microarray for the sensitive and simultaneous detection of BoNT serotypes A, B, C, D, E, and F. With engineered high-affinity antibodies, the BoNT assays have sensitivities in buffer ranging from 1.3fM (0.2pg/ml) to 14.7fM (2.2pg/ml). Using clinical and food matrices (serum and milk), the microarray is capable of detecting BoNT serotypes A to F to similar levels as in standard buffer. Cross-reactivity between assays for individual serotype was also analyzed. These simultaneous, rapid, and sensitive assays have the potential to measure botulinum toxins in a high-throughput manner in complex clinical, food, and environmental samples.

Botulinum neurotoxin: where are we with detection technologies?

Because of its high toxicity, botulinum neurotoxin (BoNT) poses a significant risk to humans and it represents a possible biological warfare agent. Nevertheless, BoNT serotypes A and B are considered an effective treatment for a variety of neurological disorders. The growing applicability of BoNT as a drug, and its potential use as a biological threat agent, highlight the urgent need to develop sensitive detection assays and therapeutic counter measures. In the last decade, significant progress has been made in BoNT detection technologies but none have fully replaced the mouse lethality assay, the current "gold standard". Recently, new advances in robotics and the availability of new reagents have allowed development of methods for rapid toxin analysis. These technologies while promising need further refinement.

Selection of antibody fragments by yeast display

The critical need for renewable, high-quality affinity reagents in biological research, as well as for diagnostic and therapeutic applications, has required the development of new platforms of discovery. Yeast display is one of the main methods of in vitro display technology with phage display. Yeast display has been chosen by numerous groups to refine both affinity and specificity of antibodies because it enables fine discrimination between mutant clones of similar affinity. In addition, the construction of display libraries of antibody fragments in yeast permit to sample the immune antibody repertoire more fully than using phage. This chapter gives an updated overview of the available systems of yeast display platforms and libraries, followed up by technical descriptions of selection methods of antibody fragments by yeast display.

Discovery of a novel enzymatic cleavage site for botulinum neurotoxin F5

Botulinum neurotoxins (BoNTs) cause botulism by cleaving proteins necessary for nerve transmission. There are seven serotypes of BoNT, A-G, characterized by their response to antisera. Many serotypes are further distinguished into differing subtypes based on amino acid sequence, some of which result in functional differences. Our laboratory previously reported that all tested subtypes within each serotype have the same site of enzymatic activity. Recently, three new subtypes of BoNT/F; /F3, /F4, and /F5, were reported. Here, we report that BoNT/F5 cleaves substrate synaptobrevin-2 in a different location than the other BoNT/F subtypes, between (54)L and (55)E. This is the first report of cleavage of synaptobrevin-2 in this location.

Extraction and inhibition of enzymatic activity of botulinum neurotoxins /B1, /B2, /B3, /B4, and /B5 by a panel of monoclonal anti-BoNT/B antibodies

Background

Botulism is caused by botulinum neurotoxins (BoNTs), extremely toxic proteins which can induce respiratory failure leading to long-term intensive care or death. Treatment for botulism includes administration of antitoxins, which must be administered early in the course of the intoxication; therefore, rapid determination of human exposure to BoNT is an important public health goal. In previous work, our laboratory reported on Endopep-MS, a mass spectrometry-based activity method for detecting and differentiating BoNT/A, /B, /E, and /F in clinical samples. We also demonstrated that antibody-capture is effective for purification and concentration of BoNTs from complex matrices such as clinical samples. However, some antibodies inhibit or neutralize the enzymatic activity of BoNT, so the choice of antibody for toxin extraction is critical.

Results

In this work, we evaluated 24 anti-BoNT/B monoclonal antibodies (mAbs) for their ability to inhibit the in vitro activity of BoNT/B1, /B2, /B3, /B4, and /B5 and to extract those toxins. Among the mAbs, there were significant differences in ability to extract BoNT/B subtypes and inhibitory effect on BoNT catalytic activity. Some of the mAbs tested enhanced the in vitro light chain activity of BoNT/B, suggesting that BoNT/B may undergo conformational change upon binding some mAbs.

Conclusions

In addition to determining in vitro inhibition abilities of a panel of mAbs against BoNT/B1-/B5, this work has determined B12.2 and 2B18.2 to be the best mAbs for sample preparation before Endopep-MS. These mAb characterizations also have the potential to assist with mechanistic studies of BoNT/B protection and treatment, which is important for studying alternative therapeutics for botulism.

Synergistic capture of Clostridium botulinum type A neurotoxin by scFv antibodies to novel epitopes

A non-immune library of human single chain fragment variable (scFv) antibodies displayed on Saccharomyces cerevisiae was screened for binding to the Clostridium botulinum neurotoxin serotype A binding domain [BoNT/A (Hc)] with the goal of identifying scFv to novel epitopes. To do this, an antibody-mediated labeling strategy was used in which antigen-binding yeast clones were selected after labeling with previously characterized monoclonal antibodies (MAbs) specific to the Hc. Twenty unique scFv clones were isolated that bound Hc. Of these, 3 also bound to full-length BoNT/A toxin complex with affinities ranging from 5 to 48 nM. Epitope binning showed that the three unique clones recognized at least two epitopes distinct from one another as well as from the detection MAbs. After production in E. coli, scFv were coupled to magnetic particles and tested for their ability to capture BoNT/A holotoxin using an Endopep-MS assay. In this assay, toxin captured by scFv coated magnetic particles was detected by incubation of the complex with a peptide containing a BoNT/A-specific cleavage sequence. Mass spectrometry was used to detect the ratio of intact peptide to cleavage products as evidence for toxin capture. When tested individually, each of the scFv showed a weak positive Endopep-MS result. However, when the particles were coated with all three scFv simultaneously, they exhibited significantly higher Endopep-MS activity, consistent with synergistic binding. These results demonstrate novel approaches toward the isolation and characterization of scFv antibodies specific to unlabeled antigens. They also provide evidence that distinct scFv antibodies can work synergistically to increase the efficiency of antigen capture onto a solid support.

Quantitative mass spectrometry for bacterial protein toxins--a sensitive, specific, high-throughput tool for detection and diagnosis

Matrix-assisted laser-desorption time-of-flight (MALDI-TOF) mass spectrometry (MS) is a valuable high-throughput tool for peptide analysis. Liquid chromatography electrospray ionization (LC-ESI) tandem-MS provides sensitive and specific quantification of small molecules and peptides. The high analytic power of MS coupled with high-specificity substrates is ideally suited for detection and quantification of bacterial enzymatic activities. As specific examples of the MS applications in disease diagnosis and select agent detection, we describe recent advances in the analyses of two high profile protein toxin groups, the Bacillus anthracis toxins and the Clostridium botulinum neurotoxins. The two binary toxins produced by B. anthracis consist of protective antigen (PA) which combines with lethal factor (LF) and edema factor (EF), forming lethal toxin and edema toxin respectively. LF is a zinc-dependent endoprotease which hydrolyzes specific proteins involved in inflammation and immunity. EF is an adenylyl cyclase which converts ATP to cyclic-AMP. Toxin-specific enzyme activity for a strategically designed substrate, amplifies reaction products which are detected by MALDI-TOF-MS and LC-ESI-MS/MS. Pre-concentration/purification with toxin specific monoclonal antibodies provides additional specificity. These combined technologies have achieved high specificity, ultrasensitive detection and quantification of the anthrax toxins. We also describe potential applications to diseases of high public health impact, including Clostridium difficile glucosylating toxins and the Bordetella pertussis adenylyl cyclase.

Beyond natural antibodies: the power of in vitro display technologies

In vitro display technologies, best exemplified by phage and yeast display, were first described for the selection of antibodies some 20 years ago. Since then, many antibodies have been selected and improved upon using these methods. Although it is not widely recognized, many of the antibodies derived using in vitro display methods have properties that would be extremely difficult, if not impossible, to obtain by immunizing animals. The first antibodies derived using in vitro display methods are now in the clinic, with many more waiting in the wings. Unlike immunization, in vitro display permits the use of defined selection conditions and provides immediate availability of the sequence encoding the antibody. The amenability of in vitro display to high-throughput applications broadens the prospects for their wider use in basic and applied research.

Neutralizing human monoclonal antibodies binding multiple serotypes of botulinum neurotoxin

Botulism, a disease of humans characterized by prolonged paralysis, is caused by botulinum neurotoxins (BoNTs), the most poisonous substances known. There are seven serotypes of BoNT (A-G) which differ from each other by 34-64% at the amino acid level. Each serotype is uniquely recognized by polyclonal antibodies, which originally were used to classify serotypes. To determine if there existed monoclonal antibodies (mAbs) capable of binding two or more serotypes, we evaluated the ability of 35 yeast-displayed single-chain variable fragment antibodies generated from vaccinated humans or mice for their ability to bind multiple BoNT serotypes. Two such clonally related human mAbs (1B18 and 4E17) were identified that bound BoNT serotype A (BoNT/A) and B or BoNT/A, B, E and F, respectively, with high affinity. Using molecular evolution techniques, it proved possible to both increase affinity and maintain cross-serotype reactivity for the 4E17 mAb. Both 1B18 and 4E17 bound to a relatively conserved epitope at the tip of the BoNT translocation domain. Immunoglobulin G constructed from affinity matured variants of 1B18 and 4E17 were evaluated for their ability to neutralize BoNT/B and E, respectively, in vivo. Both antibodies potently neutralized BoNT in vivo demonstrating that this epitope is functionally important in the intoxication pathway. Such cross-serotype binding and neutralizing mAbs should simplify the development of antibody-based BoNT diagnostics and therapeutics.