Sequence variation within botulinum neurotoxin serotypes impacts antibody binding and neutralization

Clostridial toxins

Clostridia produce the highest number of toxins of any type of bacteria and are involved in severe diseases in humans and other animals. Most of the clostridial toxins are pore-forming toxins responsible for gangrenes and gastrointestinal diseases. Among them, perfringolysin has been extensively studied and it is the paradigm of the cholesterol-dependent cytolysins, whereas Clostridium perfringens epsilon-toxin and Clostridium septicum alpha-toxin, which are related to aerolysin, are the prototypes of clostridial toxins that form small pores. Other toxins active on the cell surface possess an enzymatic activity, such as phospholipase C and collagenase, and are involved in the degradation of specific cell-membrane or extracellular-matrix components. Three groups of clostridial toxins have the ability to enter cells: large clostridial glucosylating toxins, binary toxins and neurotoxins. The binary and large clostridial glucosylating toxins alter the actin cytoskeleton by enzymatically modifying the actin monomers and the regulatory proteins from the Rho family, respectively. Clostridial neurotoxins proteolyse key components of neuroexocytosis. Botulinum neurotoxins inhibit neurotransmission at neuromuscular junctions, whereas tetanus toxin targets the inhibitory interneurons of the CNS. The high potency of clostridial toxins results from their specific targets, which have an essential cellular function, and from the type of modification that they induce. In addition, clostridial toxins are useful pharmacological and biological tools.

Sensing the deadliest toxin: technologies for botulinum neurotoxin detection

Sensitive and rapid detection of botulinum neurotoxins (BoNTs), the most poisonous substances known to date, is essential for studies of medical applications of BoNTs and detection of poisoned food, as well as for response to potential bioterrorist threats. Currently, the most common method of BoNT detection is the mouse bioassay. While this assay is sensitive, it is slow, quite expensive, has limited throughput and requires sacrificing animals. Herein, we discuss and compare recently developed alternative in vitro detection methods and assess their ability to supplement or replace the mouse bioassay in the analysis of complex matrix samples.

Recombinant derivatives of botulinum neurotoxin A engineered for trafficking studies and neuronal delivery

Work from multiple laboratories has clarified how the structural domains of botulinum neurotoxin A (BoNT/A) disable neuronal exocytosis, but important questions remain unanswered. Because BoNT/A intoxication disables its own uptake, light chain (LC) does not accumulate in neurons at detectable levels. We have therefore designed, expressed and purified a series of BoNT/A atoxic derivatives (ad) that retain the wild type features required for native trafficking. BoNT/A1ad(ek) and BoNT/A1ad(tev) are full length derivatives rendered atoxic through double point mutations in the LC protease (E(224)>A; Y(366)>A). DeltaLC-peptide-BoNT/A(tev) and DeltaLC-GFP-BoNT/A(tev) are derivatives wherein the catalytic portion of the LC is replaced with a short peptide or with GFP plus the peptide. In all four derivatives, we have fused the S6 peptide sequence GDSLSWLLRLLN to the N-terminus of the proteins to enable site-specific attachment of cargo using Sfp phosphopantetheinyl transferase. Cargo can be attached in a manner that provides a homogeneous derivative population rather than a polydisperse mixture of singly and multiply-labeled molecular species. All four derivatives contain an introduced cleavage site for conversion into disulfide-bonded heterodimers. These constructs were expressed in a baculovirus system and the proteins were secreted into culture medium and purified to homogeneity in yields ranging from 1 to 30 mg per liter. These derivatives provide unique tools to study toxin trafficking in vivo, and to assess how the structure of cargo linked to the heavy chain (HC) influences delivery to the neuronal cytosol. Moreover, they create the potential to engineer BoNT-based molecular vehicles that can target therapeutic agents to the neuronal cytoplasm.

Toxins from bacteria

Bacterial toxins damage the host at the site of bacterial infection or distant from the site. Bacterial toxins can be single proteins or oligomeric protein complexes that are organized with distinct AB structure-function properties. The A domain encodes a catalytic activity. ADP ribosylation of host proteins is the earliest post-translational modification determined to be performed by bacterial toxins; other modifications include glucosylation and proteolysis. Bacterial toxins also catalyze the non-covalent modification of host protein function or can modify host cell properties through direct protein-protein interactions. The B domain includes two functional domains: a receptor-binding domain, which defines the tropism of a toxin for a cell and a translocation domain that delivers the A domain across a lipid bilayer, either on the plasma membrane or the endosome. Bacterial toxins are often characterized based upon the secretion mechanism that delivers the toxin out of the bacterium, termed types I-VII. This review summarizes the major families of bacterial toxins and also describes the specific structure-function properties of the botulinum neurotoxins.

Sensing the deadliest toxin: technologies for botulinum neurotoxin detection

Sensitive and rapid detection of botulinum neurotoxins (BoNTs), the most poisonous substances known to date, is essential for studies of medical applications of BoNTs and detection of poisoned food, as well as for response to potential bioterrorist threats. Currently, the most common method of BoNT detection is the mouse bioassay. While this assay is sensitive, it is slow, quite expensive, has limited throughput and requires sacrificing animals. Herein, we discuss and compare recently developed alternative in vitro detection methods and assess their ability to supplement or replace the mouse bioassay in the analysis of complex matrix samples.

A quadruplex real-time PCR assay for rapid detection and differentiation of the Clostridium botulinum toxin genes A, B, E and F

Clostridium botulinum is the aetiological agent of botulism, a disease marked by flaccid paralysis that can progress to asphyxiation and death. This species is defined by the production of one of the botulinum neurotoxins (BoNTs), which are the most potent toxins known. Because of their potency, these toxins have the potential to be used as biological weapons, and therefore C. botulinum has been classified as a category A select agent. There are four related but antigenically distinct BoNT types that cause disease in humans, A, B, E and F. The mouse bioassay is the current gold standard by which BoNTs are confirmed. However, this method is expensive, slow and labour-intensive. Although PCR-based assays have been used extensively for the detection of BoNT-producing bacteria in food, animals and faecal samples, and recently to help diagnose disease in humans, no real-time quantitative PCR (qPCR) assay has yet been developed that can identify and differentiate all four BoNTs that cause disease in humans. This report describes the development of a qPCR single-tube assay that uniquely identifies these four BoNTs responsible for human disease. A total of 79 C. botulinum isolates with varying toxin types was evaluated in this study, as well as numerous near-neighbours and other bacterial species. The results showed that this quadruplex assay was capable of detecting any of the four toxin genes in a given sample at a sensitivity of about 130–840 fg genomic DNA and could detect the presence of up to all four BoNT genes simultaneously in a given sample. The assay was also functional in the presence of extraneous organic matter commonly found in various environmental samples.

Rapid multiplexed flow cytometric assay for botulinum neurotoxin detection using an automated fluidic microbead-trapping flow cell for enhanced sensitivity

A bead-based sandwich immunoassay for botulinum neurotoxin serotype A (BoNT/A) has been developed and demonstrated using a recombinant 50 kDa fragment (BoNT/A-HC-fragment) of the BoNT/A heavy chain (BoNT/A-HC) as a structurally valid simulant. Three different anti-BoNT/A antibodies were attached to three different fluorescent dye encoded flow cytometry beads for multiplexing. The assay was conducted in two formats: a manual microcentrifuge tube format and an automated fluidic system format. Flow cytometry detection was used for both formats. The fluidic system used a novel microbead-trapping flow cell to capture antibody-coupled beads with subsequent sequential perfusion of sample, wash, dye-labeled reporter antibody, and final wash solutions. After the reaction period, the beads were collected for analysis by flow cytometry. Sandwich assays performed on the fluidic system gave median fluorescence intensity signals on the flow cytometer that were 2-4 times higher than assays performed manually in the same amount of time. Limits of detection were estimated at 1 pM (approximately 50 pg/mL for BoNT/A-HC-fragment) for the 15 min fluidic assay in buffer.

Multiplex PCR for detection of botulinum neurotoxin-producing clostridia in clinical, food, and environmental samples

Botulinum neurotoxin (BoNT), the most toxic substance known, is produced by the spore-forming bacterium Clostridium botulinum and, in rare cases, also by some strains of Clostridium butyricum and Clostridium baratii. The standard procedure for definitive detection of BoNT-producing clostridia is a culture method combined with neurotoxin detection using a standard mouse bioassay (SMB). The SMB is highly sensitive and specific, but it is expensive and time-consuming and there are ethical concerns due to use of laboratory animals. PCR provides a rapid alternative for initial screening for BoNT-producing clostridia. In this study, a previously described multiplex PCR assay was modified to detect all type A, B, E, and F neurotoxin genes in isolated strains and in clinical, food, environmental samples. This assay includes an internal amplification control. The effectiveness of the multiplex PCR method for detecting clostridia possessing type A, B, E, and F neurotoxin genes was evaluated by direct comparison with the SMB. This method showed 100% inclusivity and 100% exclusivity when 182 BoNT-producing clostridia and 21 other bacterial strains were used. The relative accuracy of the multiplex PCR and SMB was evaluated using 532 clinical, food, and environmental samples and was estimated to be 99.2%. The multiplex PCR was also used to investigate 110 freshly collected food and environmental samples, and 4 of the 110 samples (3.6%) were positive for BoNT-encoding genes.

Association of botulinum neurotoxins with synaptic vesicle protein complexes

Botulinum neurotoxins (BoNTs) elicit flaccid paralysis by cleaving SNARE proteins within peripheral neurons. BoNTs are classified into seven serotypes, termed A-G, based on antibody cross-neutralization. Clostridia produce BoNTs as single-chain toxins that are cleaved into a di-chain protein that comprises an N-terminal zinc metalloprotease domain that is linked by a disulfide bond to the C-terminal translocation/receptor-binding domain. BoNT/A and BoNT/B utilize synaptic vesicle protein 2 (SV2) and synaptotagmin, respectively, as receptors for entry into neurons. Using affinity chromatography, BoNT/A and BoNT/B were found to bind a synaptic vesicle protein complex in CHAPS extracts of synaptic vesicles. Mass spectroscopy identified synaptic vesicle protein 2, synaptotagmin I, synaptophysin, vesicle-associated membrane protein 2, and the vacuolar ATPase-proton pump as components of the BoNT-synaptic vesicle protein complex. BoNT/A and BoNT/B possessed unique density-gradient profiles when bound to synaptic vesicle protein complexes. The identification of BoNT/A and BoNT/B bound to synaptic vesicle protein complexes provides insight into the interactions of BoNT and neuronal receptors.

Quantum dot immunoassays in renewable surface column and 96-well plate formats for the fluorescence detection of botulinum neurotoxin using high-affinity antibodies

A fluorescence sandwich immunoassay using high-affinity antibodies and quantum dot (QD) reporters has been developed for detection of botulinum neurotoxin serotype A (BoNT/A) using a nontoxic recombinant fragment of the holotoxin (BoNT/A-H(C)-fragment) as a structurally valid simulant for the full toxin molecule. The antibodies used, AR4 and RAZ1, bind to nonoverlapping epitopes present on both the full toxin and on the recombinant fragment. In one format, the immunoassay is carried out in a 96-well plate with detection in a standard plate reader using AR4 as the capture antibody and QD-coupled RAZ1 as the reporter. Detection to 31 pM with a total incubation time of 3 h was demonstrated. In a second format, the AR4 capture antibody was coupled to Sepharose beads, and the reactions were carried out in microcentrifuge tubes with an incubation time of 1 h. The beads were subsequently captured and concentrated in a rotating rod "renewable surface" flow cell equipped with a fiber optic system for fluorescence measurements. In PBS buffer, the BoNT/A-H(C)-fragment was detected to concentrations as low as 5 pM using the fluidic measurement approach.

Neurotoxin gene clusters in Clostridium botulinum type Ab strains

There is limited knowledge of the neurotoxin gene diversity among Clostridium botulinum type Ab strains. Only the sequences of the bont/A and bont/B genes in C. botulinum type Ab strain CDC1436 and the sequence of the bont/B gene in C. botulinum type Ab strain CDC588 have been reported. In this study, we sequenced the entire bont/A- and bont/B-associated neurotoxin gene clusters of C. botulinum type Ab strain CDC41370 and the bont/A gene of strain CDC588. In addition, we analyzed the organization of the neurotoxin gene clusters in strains CDC588 and CDC1436. The bont/A nucleotide sequence of strain CDC41370 differed from those of the known bont/A subtypes A1 to A4 by 2 to 7%, and the predicted amino acid sequence differed by 4% to 14%. The bont/B nucleotide sequence in strain CDC41370 showed 99.7% identity to the sequence of subtype B1. The bont/A nucleotide sequence of strain CDC588 was 99.9% identical to that of subtype A1. Although all of the C. botulinum type Ab strains analyzed contained the two sets of neurotoxin clusters, similar to what has been found in other bivalent strains, the intergenic spacing of p21-orfX1 and orfX2-orfX3 varied among these strains. The type Ab strains examined in this study had differences in their toxin gene cluster compositions and bont/A and bont/B nucleotide sequences, suggesting that they may have arisen from separate recombination events.

Genetic characterization of Clostridium botulinum associated with type B infant botulism in Japan

The 15 proteolytic Clostridium botulinum type B strains, including 3 isolates associated with infant botulism in Japan, were genetically characterized by phylogenetic analysis of boNT/B gene sequences, genotyping, and determination of the boNT/B gene location by using pulsed-field gel electrophoresis (PFGE) for molecular epidemiological analysis of infant botulism in Japan. Strain Osaka05, isolated from a case in 2005, showed a unique boNT/B gene sequence and was considered to be a new BoNT/B subtype by phylogenetic analysis. Strain Osaka06, isolated from a case in 2006, was classified as the B2 subtype, the same as strain 111, isolated from a case in 1995. The five isolates associated with infant botulism in the United States were classified into the B1 subtype. Isolates from food samples in Japan were divided into the B1 and the B2 subtypes, although no relation with infant botulism was shown by PFGE genotyping. The results of PFGE and Southern blot hybridization with undigested DNA suggested that the boNT/B gene is located on large plasmids (approximately 150 kbp, 260 kbp, 275 kbp, or 280 kbp) in five strains belonging to three BoNT/B subtypes from various sources. The botulinum neurotoxin (BoNT) of Osaka05 was suggested to have an antigenicity different from the antigenicities of BoNT/B1 and BoNT/B2 by a sandwich enzyme-linked immunosorbent assay with the recombinant BoNT/B-C-terminal domain. We established a multiplex PCR assay for BoNT/B subtyping which will be useful for epidemiological studies of type B strains and the infectious diseases that they cause.

Glycosylated SV2 and gangliosides as dual receptors for botulinum neurotoxin serotype F

Botulinum neurotoxin causes rapid flaccid paralysis through the inhibition of acetylcholine release at the neuromuscular junction. The seven BoNT serotypes (A-G) have been proposed to bind motor neurons via ganglioside-protein dual receptors. To date, the structure-function properties of BoNT/F host receptor interactions have not been resolved. Here, we report the crystal structures of the receptor binding domains (HCR) of BoNT/A and BoNT/F and the characterization of the dual receptors for BoNT/F. The overall polypeptide fold of HCR/A is essentially identical to the receptor binding domain of the BoNT/A holotoxin, and the structure of HCR/F is very similar to that of HCR/A, except for two regions implicated in neuronal binding. Solid phase array analysis identified two HCR/F binding glycans: ganglioside GD1a and oligosaccharides containing an N-acetyllactosamine core. Using affinity chromatography, HCR/F bound native synaptic vesicle glycoproteins as part of a protein complex. Deglycosylation of glycoproteins using alpha(1-3,4)-fucosidase, endo-beta-galactosidase, and PNGase F disrupted the interaction with HCR/F, while the binding of HCR/B to its cognate receptor, synaptotagmin I, was unaffected. These data indicate that the HCR/F binds synaptic vesicle glycoproteins through the keratan sulfate moiety of SV2. The interaction of HCR/F with gangliosides was also investigated. HCR/F bound specifically to gangliosides that contain alpha2,3-linked sialic acid on the terminal galactose of a neutral saccharide core (binding order GT1b = GD1a >> GM3; no binding to GD1b and GM1a). Mutations within the putative ganglioside binding pocket of HCR/F decreased binding to gangliosides, synaptic vesicle protein complexes, and primary rat hippocampal neurons. Thus, BoNT/F neuronal discrimination involves the recognition of ganglioside and protein (glycosylated SV2) carbohydrate moieties, providing a structural basis for the high affinity and specificity of BoNT/F for neurons.

Production of catalytically inactive BoNT/A1 holoprotein and comparison with BoNT/A1 subunit vaccines against toxin subtypes A1, A2, and A3

A recombinant, catalytically inactive Clostridium botulinum neurotoxin A1 holoprotein (ciBoNT/A1 HP) was constructed by introducing amino acid substitutions H223A, E224A, and H227A in the active site to ablate proteolytic activity. ciBoNT/A1 HP was produced in the yeast Pichia pastoris and the purified product was evaluated as a vaccine candidate by comparison against recombinant BoNT/A1 LC, LC-belt, LC-H(n), and H(c) antigens and a LC-H(n)+H(c) combination in mouse potency and efficacy bioassays when challenged with BoNT/A subtypes /A1, /A2, and /A3. A single dose of ciBoNT/A1 HP provided equivalent or greater protective immunity, not only against the homologous toxin, but also against two distinct toxin subtypes with significant amino acid divergence. Only the LC-H(n)+H(c) combination provided comparable protection against /A1; however, it was less effective against subtypes /A2 and /A3. Differences in protective immunity diminished after multiple vaccinations with either ciBoNT/A1 HP or BoNT/A1 H(c), and the survival rates were more comparable at the toxin levels used to challenge.

Extraction and inhibition of enzymatic activity of botulinum neurotoxins/A1, /A2, and /A3 by a panel of monoclonal anti-BoNT/A antibodies

Botulinum neurotoxins (BoNTs) are extremely potent toxins that are capable of causing death or respiratory failure leading to long-term intensive care. Treatment includes serotype-specific antitoxins, which must be administered early in the course of the intoxication. Rapidly determining human exposure to BoNT is an important public health goal. In previous work, our laboratory focused on developing Endopep-MS, a mass spectrometry-based endopeptidase method for detecting and differentiating BoNT/A–G serotypes in buffer and BoNT/A, /B, /E, and /F in clinical samples. We have previously reported the effectiveness of antibody-capture to purify and concentrate BoNTs from complex matrices, such as clinical samples. Because some antibodies inhibit or neutralize the activity of BoNT, the choice of antibody with which to extract the toxin is critical. In this work, we evaluated a panel of 16 anti-BoNT/A monoclonal antibodies (mAbs) for their ability to inhibit the in vitro activity of BoNT/A1, /A2, and /A3 complex as well as the recombinant LC of A1. We also evaluated the same antibody panel for the ability to extract BoNT/A1, /A2, and /A3. Among the mAbs, there were significant differences in extraction efficiency, ability to extract BoNT/A subtypes, and inhibitory effect on BoNT catalytic activity. The mAbs binding the C-terminal portion of the BoNT/A heavy chain had optimal properties for use in the Endopep-MS assay.

An assay for botulinum toxin types A, B and F that requires both functional binding and catalytic activities within the neurotoxin

AIM

To develop a novel assay technique for the botulinum neurotoxin family (BoNTs) which is dependent on both the endopeptidase and receptor-binding activities of the BoNTs and which is insensitive to antigenic variation with the toxin family.

METHODS AND RESULTS

An endopeptidase activity, receptor-binding assay (EARB assay) has been developed which captures biologically active toxin from media using brain synaptosomes. After capture, the bound toxin can be incubated with its substrate, and cleavage detected using serotype-specific antibodies raised against the cleaved product of each toxin serotype. The EARB assay was assessed using a range of BoNT serotypes and subtypes. For BoNT/A, detection limits for subtypes A(1), A(2) and A(3) were 0.5, 3 and 10 MLD(50) ml(-1), respectively. The limit of detection for BoNT/B(1) was 5 MLD(50) ml(-1) and a novel antibody-based endopeptidase assay for BoNT/F detected toxin at 0.5 MLD(50) ml(-1). All these BoNTs can be captured from media containing up to 10% serum without loss of sensitivity. BoNT/A(1) could also be detected in dilutions of a lactose- containing formulation similar to that used for clinical preparations of the toxin. Different serotypes were found to possess different optimal cleavage pHs (pH 6.5 for A(1), pH 7.4 for B(1)).

CONCLUSIONS

The EARB assay has been shown to be able to detect a broad range of BoNT serotypes and subtypes from various media.

SIGNIFICANCE AND IMPACT OF THE STUDY

The EARB assay system described is the first convenient in vitro assay system described which is requires multiple functional biological activities with the BoNTs. The assay will have applications in instances where it is essential or desirable to distinguish biologically active from inactive neurotoxin.

Catalytic properties of botulinum neurotoxin subtypes A3 and A4

Botulinum toxins (BoNT) are zinc proteases (serotypes A-G) which cause flaccid paralysis through the cleavage of SNARE proteins within motor neurons. BoNT/A was originally organized into two subtypes, BoNT/A1 and BoNT/A2, which are approximately 95% homologous and possess similar catalytic activities. Subsequently, two additional subtypes were identified, BoNT/A3 (Loch Maree) and BoNT/A4 (657Ba), which are 81 and 88% homologous with BoNT/A1, respectively. Alignment studies predicted that BoNT/A3 and BoNT/A4 were sufficiently different from BoNT/A1 to affect SNAP25 binding and cleavage. Recombinant light chain (LC) of BoNT/A3 (LC/A3) and BoNT/A4 (LC/A4) were subjected to biochemical analysis. LC/A3 cleaved SNAP25 at 50% of the rate of LC/A1 but cleaved SNAPtide at a faster rate than LC/A1, while LC/A4 cleaved SNAP25 and SNAPtide at slower rates than LC/A1. LC/A3 and LC/A4 had similar K(m) values for SNAP25 relative to LC/A1, while the k(cat) for LC/A4 was 10-fold slower than that for LC/A1, suggesting a defect in substrate cleavage. Neither LC/A3 nor LC/A4 possessed autocatalytic activity, a property of LC/A1 and LC/A2. Thus, the four subtypes of BoNT/A bind SNAP25 with similar affinity but have different catalytic capacities for SNAP25 cleavage, SNAPtide cleavage, and autocatalysis. The catalytic properties identified among the subtypes of LC/A may influence strategies for the development of small molecule or peptide inhibitors as therapies against botulism.

Independent evolution of neurotoxin and flagellar genetic loci in proteolytic Clostridium botulinum

BACKGROUND

Proteolytic Clostridium botulinum is the causative agent of botulism, a severe neuroparalytic illness. Given the severity of botulism, surprisingly little is known of the population structure, biology, phylogeny or evolution of C. botulinum. The recent determination of the genome sequence of C. botulinum has allowed comparative genomic indexing using a DNA microarray.

RESULTS

Whole genome microarray analysis revealed that 63% of the coding sequences (CDSs) present in reference strain ATCC 3502 were common to all 61 widely-representative strains of proteolytic C. botulinum and the closely related C. sporogenes tested. This indicates a relatively stable genome. There was, however, evidence for recombination and genetic exchange, in particular within the neurotoxin gene and cluster (including transfer of neurotoxin genes to C. sporogenes), and the flagellar glycosylation island (FGI). These two loci appear to have evolved independently from each other, and from the remainder of the genetic complement. A number of strains were atypical; for example, while 10 out of 14 strains that formed type A1 toxin gave almost identical profiles in whole genome, neurotoxin cluster and FGI analyses, the other four strains showed divergent properties. Furthermore, a new neurotoxin sub-type (A5) has been discovered in strains from heroin-associated wound botulism cases. For the first time, differences in glycosylation profiles of the flagella could be linked to differences in the gene content of the FGI.

CONCLUSION

Proteolytic C. botulinum has a stable genome backbone containing specific regions of genetic heterogeneity. These include the neurotoxin gene cluster and the FGI, each having evolved independently of each other and the remainder of the genetic complement. Analysis of these genetic components provides a high degree of discrimination of strains of proteolytic C. botulinum, and is suitable for clinical and forensic investigations of botulism outbreaks.

Epitope characterization and variable region sequence of f1-40, a high-affinity monoclonal antibody to botulinum neurotoxin type a (Hall strain)

BACKGROUND

Botulism, an often fatal neuroparalytic disease, is caused by botulinum neurotoxins (BoNT) which consist of a family of seven serotypes (A-H) produced by the anaerobic bacterium Clostridium botulinum. BoNT, considered the most potent biological toxin known, is a 150 kDa protein consisting of a 100 kDa heavy-chain (Hc) and a 50 kDa light-chain (Lc). F1-40 is a mouse-derived, IgG1 monoclonal antibody that binds the light chain of BoNT serotype A (BoNT/A) and is used in a sensitive immunoassay for toxin detection. We report the fine epitope mapping of F1-40 and the deduced amino acid sequence of the variable regions of the heavy and light chains of the antibody.

METHODS AND FINDINGS

To characterize the binding epitope of F1-40, three complementary experimental approaches were selected. Firstly, recombinant peptide fragments of BoNT/A light-chain were used in Western blots to identify the epitope domains. Secondly, a peptide phage-display library was used to identify the specific amino acid sequences. Thirdly, the three-dimensional structure of BoNT/A was examined in silico, and the amino acid sequences determined from the phage-display studies were mapped onto the three-dimensional structure in order to visualize the epitope. F1-40 was found to bind a peptide fragment of BoNT/A, designated L1-3, which spans from T125 to L200. The motif QPDRS was identified by phage-display, and was mapped to a region within L1-3. When the three amino acids Q138, P139 and D140 were all mutated to glycine, binding of F1-40 to the recombinant BoNT/A light chain peptide was abolished. Q-138, P-139 and D-140 form a loop on the external surface of BoNT/A, exposed to solvent and accessible to F1-40 binding.

CONCLUSIONS

The epitope of F1-40 was localized to a single exposed loop (ss4, ss5) on the Lc of BoNT. Furthermore amino acids Q138, P139 and D140 forming the tip of the loop appear critical for binding.

Evidence that plasmid-borne botulinum neurotoxin type B genes are widespread among Clostridium botulinum serotype B strains

BACKGROUND

Plasmids that encode certain subtypes of the botulinum neurotoxin type B have recently been detected in some Clostridium botulinum strains. The objective of the present study was to investigate the frequency with which plasmid carriage of the botulinum neurotoxin type B gene (bont/B) occurs in strains of C. botulinum type B, Ab, and A(B), and whether plasmid carriage is bont/B subtype-related.

METHODOLOGY/PRINCIPAL FINDINGS

PCR-Restriction fragment length polymorphism was employed to identify subtypes of the bont/B gene. Pulsed-field gel electrophoresis and Southern blot hybridization with specific probes were performed to analyze the genomic location of the bont/B subtype genes. All five known bont/B subtype genes were detected among the strains; the most frequently detected subtype genes were bont/B1 and /B2. Surprisingly, the bont/B subtype gene was shown to be plasmid-borne in >50% of the total strains. The same bont/B subtype gene was associated with the chromosome in some strains, whereas it was associated with a plasmid in others. All five known bont/B subtype genes were in some cases found to reside on plasmids, though with varying frequency (e.g., most of the bont/B1 subtype genes were located on plasmids, whereas all but one of the bont/B2 subtypes were chromosomally-located). Three bivalent isolates carried both bont/A and /B genes on the same plasmid. The plasmids carrying the bont gene were five different sizes, ranging from approximately 55 kb to approximately 245 kb.

CONCLUSIONS/SIGNIFICANCE

The unexpected finding of the widespread distribution of plasmids harboring the bont/B gene among C. botulinum serotype B strains provides a chance to examine their contribution to the dissemination of the bont genes among heterogeneous clostridia, with potential implications on issues related to pathogenesis and food safety.

Renewable surface fluorescence sandwich immunoassay biosensor for rapid sensitive botulinum toxin detection in an automated fluidic format

A renewable surface biosensor for rapid detection of botulinum neurotoxin serotype A is described based on fluidic automation of a fluorescence sandwich immunoassay, using a recombinant protein fragment of the toxin heavy chain ( approximately 50 kDa) as a structurally valid simulant. Monoclonal antibodies AR4 and RAZ1 bind to separate non-overlapping epitopes of the full botulinum holotoxin ( approximately 150 kDa). Both of the targeted epitopes are located on the recombinant fragment. The AR4 antibody was covalently bound to Sepharose beads and used as the capture antibody. A rotating rod flow cell was used to capture these beads delivered as a suspension by a sequential injection flow system, creating a 3.6 microL column. After perfusing the bead column with sample and washing away the matrix, the column was perfused with Alexa 647 dye-labeled RAZ1 antibody as the reporter. Optical fibers coupled to the rotating rod flow cell at a 90 degrees angle to one another delivered excitation light from a HeNe laser (633 nm) using one fiber and collected fluorescent emission light for detection with the other. After each measurement, the used Sepharose beads are released and replaced with fresh beads. In a rapid screening approach to sample analysis, the toxin simulant was detected to concentrations of 10 pM in less than 20 minutes using this system.

Thermostable llama single domain antibodies for detection of botulinum A neurotoxin complex

Immunoglobulins from animals of the Camelidae family boast unique forms that do not incorporate light chains. Antigen binding in these unconventional heavy-chain homodimers is mediated through a single variable domain. When expressed recombinantly these variable domains are termed single domain antibodies (sdAb) and are among the smallest naturally IgG-derived antigen binding units. SdAb possess good solubility, thermostability, and can refold after heat and chemical denaturation making them promising alternative recognition elements. We have constructed a library of phage-displayed sdAb from a llama immunized with a cocktail of botulinum neurotoxin (BoNT) complex toxoids and panned the library for binders for BoNT A complex toxoid. Six unique binders were isolated and found to specifically bind BoNT A complex in toxoid and untoxoided forms and when used in optimal combinations in buffer and milk could detect 100 pg/mL untoxoided complex. All sdAb retained their ability to specifically bind target after heating to 85 degrees C for 1 h, in contrast to conventional polyclonal sera. All of the sdAb were highly specific for subtype A1 rather than A2 and demonstrated binding to the 33 kDa hemagglutinin, potentially to a somewhat overlapping linear epitope. The unique properties of these sdAb may provide advantages for many diagnostic applications where long-term storage and in-line monitoring require very rugged yet highly specific recognition elements.

Neutralization of botulinum neurotoxin by a human monoclonal antibody specific for the catalytic light chain

BACKGROUND

Botulinum neurotoxins (BoNT) are a family of category A select bioterror agents and the most potent biological toxins known. Cloned antibody therapeutics hold considerable promise as BoNT therapeutics, but the therapeutic utility of antibodies that bind the BoNT light chain domain (LC), a metalloprotease that functions in the cytosol of cholinergic neurons, has not been thoroughly explored.

METHODS AND FINDINGS

We used an optimized hybridoma method to clone a fully human antibody specific for the LC of serotype A BoNT (BoNT/A). The 4LCA antibody demonstrated potent in vivo neutralization when administered alone and collaborated with an antibody specific for the HC. In Neuro-2a neuroblastoma cells, the 4LCA antibody prevented the cleavage of the BoNT/A proteolytic target, SNAP-25. Unlike an antibody specific for the HC, the 4LCA antibody did not block entry of BoNT/A into cultured cells. Instead, it was taken up into synaptic vesicles along with BoNT/A. The 4LCA antibody also directly inhibited BoNT/A catalytic activity in vitro.

CONCLUSIONS

An antibody specific for the BoNT/A LC can potently inhibit BoNT/A in vivo and in vitro, using mechanisms not previously associated with BoNT-neutralizing antibodies. Antibodies specific for BoNT LC may be valuable components of an antibody antidote for BoNT exposure.

A natural human IgM antibody that neutralizes botulinum neurotoxin in vivo

Affinity-matured human antibodies have demonstrated efficacy as countermeasures for exposure to botulinum neurotoxin (BoNT), which is the cause of the disease botulism category A select bioterror agent. Little is known, however, about the potential role of natural (un-mutated) antibodies in the protective immune response to BoNT. Here we describe the cloning of two human IgM antibodies that bind serotype A BoNT. Both are un-mutated IgM antibodies, consistent with an origin in naive B cells. One of the antibodies is able to fully neutralize a lethal dose of serotype A BoNT in vivo. These results suggest that the natural human antibody repertoire may play a role in protection from exposure to biological toxins.

Analysis of epitope information related to Bacillus anthracis and Clostridium botulinum

We have reviewed the information about epitopes of immunological interest from Clostridium botulinum and Bacillus anthracis, by mining the Immune Epitope Database and Analysis Resource. For both pathogens, the vast majority of epitopes reported to date are derived from a single protein: the protective antigen of B. anthracis and the neurotoxin type A of C. botulinum. A detailed analysis of the data was performed to characterize the function, localization and conservancy of epitopes identified as neutralizing and/or protective. In order to broaden the scope of this analysis, we have also included data describing immune responses against defined fragments (over 50 amino acids long) of the relevant antigens. The scarce information on T-cell determinants and on epitopes from other antigens besides the toxins, highlights a gap in our knowledge and identifies areas for future research. Despite this, several distinct structures at the epitope and fragment level are described herein, which could be potential additions to future vaccines or targets of novel immunotherapeutics and diagnostic reagents.

Phylogenetic analysis of Clostridium botulinum type A by multi-locus sequence typing

The genus Clostridium comprises a heterogeneous group of organisms for which the phylogeny and evolutionary relationships are poorly understood. The elucidation of these evolutionary relationships necessitates the use of experimental methods that can distinguish Clostridium lineages that are time and cost effective, and can be accurately and reproducibly employed in different laboratories. Multi-locus sequence typing (MLST) has been successfully used as a reproducible and discriminating system in the study of eukaryotic and prokaryotic evolutionary biology, and for strain typing of various bacteria. In this study, MLST was applied to evaluate the evolutionary lineages in the serotype A group of Clostridium botulinum. C. botulinum type A has recently been shown to produce multiple subtypes, suggesting that it is not monophyletic as previously reported, but comprises distinct lineages. For MLST analysis, we initially evaluated 14 housekeeping genes (gapdh, tuf, sod, oppB, hsp60, dnaE, aroE, pta, 23S rDNA, aceK, rpoB, 16S rDNA, mdh and recA) for amplification and sequence analysis. In the first phase of the analysis, 30 C. botulinum type A strains producing botulinum neurotoxin subtypes A1-A4 were examined. Results of this pilot study suggested that seven of the genes (mdh, aceK, rpoB, aroE, hsp60, oppB and recA) could be used for elucidation of evolutionary lineages and strain typing. These seven housekeeping genes were successfully applied for the elucidation of lineages for 73 C. botulinum type A strains, which resulted in 24 distinct sequence types. This strategy should be applicable to phylogenetic studies and typing of other C. botulinum serotypes and Clostridium species.

Production and characterization of monoclonal antibody to botulinum neurotoxin type B light chain by phage display

A monoclonal antibody to the light chain of botulinum neurotoxin type B (BoNT/B) was generated and its protective activity was evaluated in vivo. A chimeric rabbit/human Fab library was generated using bone marrow and spleen cDNAs of rabbits immunized with the BoNT/B light chain, and three monoclonal antibodies specific to the catalytic domain of BoNT/B were isolated. One of these clones, BCXRH1, was specific to a conformation-dependent epitope, and partially neutralized the BoNT/B complex in vivo.

A human monoclonal antibody that binds serotype A botulinum neurotoxin

Monoclonal antibodies have demonstrated significant potential as therapeutics for botulinum neurotoxin exposures. We previously described a hybridoma method for cloning native human antibodies that uses a murine myeloma cell line that ectopically expresses the human telomerase catalytic subunit gene (hTERT) and the murine interleukin-6 gene (mIL-6). Here we describe a heterohybridoma cell line that ectopically expresses mIL-6 and hTERT and has improved stability of hTERT expression. We fused this cell line to human peripheral blood B cells from a subject who had received the botulinum toxoid vaccine, cloning a high-affinity antibody (13A) specific for serotype A botulinum neurotoxin (BoNT/A). The 13A antibody is an affinity-matured, post-germinal center IgG(1) lambda antibody that has partial neutralization activity in vivo. 13A binds an epitope on BoNT/A that overlaps the binding epitope of an IgG antibody previously shown to fully neutralize a lethal dose of BoNT/A in vivo. The 13A antibody may be useful for diagnostic testing or for incorporation into an oligoclonal therapeutic to counteract BoNT/A exposure.

Analysis of neurotoxin cluster genes in Clostridium botulinum strains producing botulinum neurotoxin serotype A subtypes

Neurotoxin cluster gene sequences and arrangements were elucidated for strains of Clostridium botulinum encoding botulinum neurotoxin (BoNT) subtypes A3, A4, and a unique A1-producing strain (HA(-) Orfx(+) A1). These sequences were compared to the known neurotoxin cluster sequences of C. botulinum strains that produce BoNT/A1 and BoNT/A2 and possess either a hemagglutinin (HA) or an Orfx cluster, respectively. The A3 and HA(-) Orfx(+) A1 strains demonstrated a neurotoxin cluster arrangement similar to that found in A2. The A4 strain analyzed possessed two sets of neurotoxin clusters that were similar to what has been found in the A(B) strains: an HA cluster associated with the BoNT/B gene and an Orfx cluster associated with the BoNT/A4 gene. The nucleotide and amino acid sequences of the neurotoxin cluster-specific genes were determined for each neurotoxin cluster and compared among strains. Additionally, the ntnh gene of each strain was compared on both the nucleotide and amino acid levels. The degree of similarity of the sequences of the ntnh genes and corresponding amino acid sequences correlated with the neurotoxin cluster type to which the ntnh gene was assigned.

Presynaptic neurotoxins with enzymatic activities

Toxins that alter neurotransmitter release from nerve terminals are of considerable scientific and clinical importance. Many advances were recently made in the understanding of their molecular mechanisms of action and use in human therapy. Here, we focus on presynaptic neurotoxins, which are very potent inhibitors of the neurotransmitter release because they are endowed with specific enzymatic activities: (1) clostridial neurotoxins with a metallo-proteolytic activity and (2) snake presynaptic neurotoxins with a phospholipase A2 activity.

Analysis of the neurotoxin complex genes in Clostridium botulinum A1-A4 and B1 strains: BoNT/A3, /Ba4 and /B1 clusters are located within plasmids

BACKGROUND

Clostridium botulinum and related clostridial species express extremely potent neurotoxins known as botulinum neurotoxins (BoNTs) that cause long-lasting, potentially fatal intoxications in humans and other mammals. The amino acid variation within the BoNT is used to categorize the species into seven immunologically distinct BoNT serotypes (A-G) which are further divided into subtypes. The BoNTs are located within two generally conserved gene arrangements known as botulinum progenitor complexes which encode toxin-associated proteins involved in toxin stability and expression.

METHODOLOGY/PRINCIPAL FINDINGS

Because serotype A and B strains are responsible for the vast majority of human botulism cases worldwide, the location, arrangement and sequences of genes from eight different toxin complexes representing four different BoNT/A subtypes (BoNT/A1-Ba4) and one BoNT/B1 strain were examined. The bivalent Ba4 strain contained both the BoNT/A4 and BoNT/bvB toxin clusters. The arrangements of the BoNT/A3 and BoNT/A4 subtypes differed from the BoNT/A1 strains and were similar to those of BoNT/A2. However, unlike the BoNT/A2 subtype, the toxin complex genes of BoNT/A3 and BoNT/A4 were found within large plasmids and not within the chromosome. In the Ba4 strain, both BoNT toxin clusters (A4 and bivalent B) were located within the same 270 kb plasmid, separated by 97 kb. Complete genomic sequencing of the BoNT/B1 strain also revealed that its toxin complex genes were located within a 149 kb plasmid and the BoNT/A3 complex is within a 267 kb plasmid.

CONCLUSIONS/SIGNIFICANCE

Despite their size differences and the BoNT genes they contain, the three plasmids containing these toxin cluster genes share significant sequence identity. The presence of partial insertion sequence (IS) elements, evidence of recombination/gene duplication events, and the discovery of the BoNT/A3, BoNT/Ba4 and BoNT/B1 toxin complex genes within plasmids illustrate the different mechanisms by which these genes move among diverse genetic backgrounds of C. botulinum.

Recombinant holotoxoid vaccine against botulism

The botulinum neurotoxins (BoNT) are the most toxic proteins for humans and designated "Category A Select Agents." The current vaccine against botulism is in limited supply, and there is a need to develop new vaccine strategies. A recombinant BoNT/A toxoid was produced in Clostridium botulinum that contained a double amino acid substitution, R363A Y365F (termed BoNT/A(RYM)). BoNT/A(RYM) was noncatalytic for SNAP25 and nontoxic for mice. Immunization with BoNT/A(RYM) protected mice from challenge at levels that were similar to chemically inactivated BoNT/A toxoid. BoNT/A(RYM) elicited an immune response against the light-chain and heavy-chain components of the toxin. Neutralizing anti-BoNT/A(RYM) sera blocked BoNT toxicity in primary cortical neurons and blocked ganglioside binding by the heavy chain. BoNT/A(RYM) represents a viable vaccine candidate for a holotoxoid against botulism.

Sequencing the botulinum neurotoxin gene and related genes in Clostridium botulinum type E strains reveals orfx3 and a novel type E neurotoxin subtype

Three Clostridium botulinum type E strains were sequenced for the botulinum neurotoxin (BoNT) gene cluster, and 11 type E strains, representing a wide biodiversity, were sequenced for the bont/E gene. The total length of the BoNT/E gene cluster was 12,908 bp, and a novel gene (partial) designated orfx3, together with the complete orfx2 gene, was identified in the three type E strains for the first time. Apart from orfx3, the structure and organization of the neurotoxin gene cluster of the three strains were identical to those of previously published ones. Only minor differences (</=3%) in the nucleotide sequences of the gene cluster components were observed among the three strains and the published BoNT/E-producing clostridia. The orfx3, orfx2, orfx1, and p47 gene sequences of the three type E strains shared homologies of 81%, 67 to 76%, 78 to 79%, and 79 to 85%, respectively, with published sequences for type A1 and A2 C. botulinum. Analysis of bont/E from the 14 type E strains and 19 previously published BoNT/E-producing clostridia revealed six neurotoxin subtypes, with a new distinct subtype consisting of three Finnish isolates alone. The amino acid sequence of the subtype E6 neurotoxin differed 3 to 6% from the other subtypes, suggesting that these subtype E6 neurotoxins may possess specific antigenic or functional properties.

Plasmid encoded neurotoxin genes in Clostridium botulinum serotype A subtypes

Clostridium botulinum, an important pathogen of humans and animals, produces botulinum neurotoxin (BoNT), the most poisonous toxin known. We have determined by pulsed-field gel electrophoresis (PFGE) and Southern hybridizations that the genes encoding BoNTs in strains Loch Maree (subtype A3) and 657Ba (type B and subtype A4) are located on large (approximately 280 kb) plasmids. This is the first demonstration of plasmid-borne neurotoxin genes in Clostridium botulinum serotypes A and B. The finding of BoNT type A and B genes on extrachromosomal elements has important implications for the evolution of neurotoxigenicity in clostridia including the origin, expression, and lateral transfer of botulinum neurotoxin genes.

Botulism diagnostics: from clinical symptoms to in vitro assays

Botulinum neurotoxin (BoNT), which cause the deadly neuroparalytic disease, botulism, is the most toxic substance known to man. BoNT can be used as potential bioterrorism agents, and therefore, pose great threat to national security and public health. Rapid and sensitive detection of BoNTs using molecular and biochemical techniques is an essential component in the diagnosis of botulism, and is yet to be achieved. The most sensitive and widely accepted assay method for BoNTs is mouse bioassay, which takes 4 days to complete. This clearly can not meet the need for clinical diagnosis of botulism, botulinum detection in field conditions, and screening of large scale samples. Consequently, the clinical diagnosis of botulism relies on the clinical symptom development, thus limiting the effectiveness of antitoxin treatment. In response to this critical need, many in vitro methods for BoNT detection are under development. This review is focused on recently developed in vitro detection methods for BoNTs, and emerging new technologies with potential for sensitive and rapid in vitro diagnostics for botulism.

Development and comparison of two immunoassay formats for rapid detection of botulinum neurotoxin type A

We have evaluated two formats of immunoassays for the rapid detection of Clostridium botulinum neurotoxin type A (BoNT/A), in assay buffer and various matrices (human serum and nasal swabs, fresh milk, sugar, flour and talcum). The two formats, a vertical-flow strip immunochromatography (ICT) and a small disposable immunoaffinity column (IAC), were selected because they are both rapid and readily usable in the field without sophisticated equipment. We utilised the same critical reagents to develop and optimise both assays, making it possible to compare the corresponding technologies on the same toxin preparations, without interference due to the properties of the antibodies. Results were interpreted using a standard statistical test (ANOVA) and showed little difference of sensitivity between matrices. Though both assays were completed in 40 min, the sensitivity of the IAC, evaluated at 0.45 pM (5 mouse LD50 units/ml), was 40 to 80 times better than that of the ICT. Furthermore, the sensitivity of the IAC assay was improved to 0.09 pM (1 mouse LD50 unit/ml) when performed on a 5-ml volume of human serum. Thus, the IAC appears to be one of the most sensitive and rapid assays for the detection of BoNT/A reported to date and, because it is also highly transportable, it is amongst the best suited for field diagnosis of BoNT/A poisoning.

Genome sequence of a proteolytic (Group I) Clostridium botulinum strain Hall A and comparative analysis of the clostridial genomes

Clostridium botulinum is a heterogeneous Gram-positive species that comprises four genetically and physiologically distinct groups of bacteria that share the ability to produce botulinum neurotoxin, the most poisonous toxin known to man, and the causative agent of botulism, a severe disease of humans and animals. We report here the complete genome sequence of a representative of Group I (proteolytic) C. botulinum (strain Hall A, ATCC 3502). The genome consists of a chromosome (3,886,916 bp) and a plasmid (16,344 bp), which carry 3650 and 19 predicted genes, respectively. Consistent with the proteolytic phenotype of this strain, the genome harbors a large number of genes encoding secreted proteases and enzymes involved in uptake and metabolism of amino acids. The genome also reveals a hitherto unknown ability of C. botulinum to degrade chitin. There is a significant lack of recently acquired DNA, indicating a stable genomic content, in strong contrast to the fluid genome of Clostridium difficile, which can form longer-term relationships with its host. Overall, the genome indicates that C. botulinum is adapted to a saprophytic lifestyle both in soil and aquatic environments. This pathogen relies on its toxin to rapidly kill a wide range of prey species, and to gain access to nutrient sources, it releases a large number of extracellular enzymes to soften and destroy rotting or decayed tissues.

Trivalent vaccine against botulinum toxin serotypes A, B, and E that can be administered by the mucosal route

Most reports dealing with vaccines against botulinum toxin have focused on the injection route of administration. This is unfortunate, because a mucosal vaccine is likely to be more efficacious for patients and pose fewer risks to health care workers and to the environment. Therefore, efforts were made to generate a mucosal vaccine that provides protection against the botulinum serotypes that typically cause human illness (serotypes A, B, and E). This work demonstrated that carboxy-terminal peptides derived from each of the three serotypes were able to bind to and penetrate human epithelial barriers in vitro, and there was no cross inhibition of membrane binding and transcytosis. The three polypeptides were then tested in vivo as a trivalent vaccine that could be administered to mice by the intranasal route. The results indicated that the mucosal vaccine evoked high secretory titers of immunoglobulin A (IgA), as well as high circulating titers of IgG and IgA, and it also evoked a high level of resistance to challenge with toxin. The immunoglobulin responses and the levels of resistance to challenge were increased by coadministration of adjuvants, such as chitosan and vitamin E. At least three mechanisms were identified to account for the antibody-induced resistance: (i) blockade of toxin absorption across epithelial cells, (ii) enhanced clearance of toxin from the circulation, and (iii) blockade of toxin action at the neuromuscular junction. These results are a compelling demonstration that a mucosal vaccine against multiple serotypes of botulinum toxin has been identified.

Protection with recombinant Clostridium botulinum C1 and D binding domain subunit (Hc) vaccines against C and D neurotoxins

Recombinant botulinum Hc (rBoNT Hc) vaccines for serotypes C1 and D were produced in the yeast Pichia pastoris and used to determine protection against four distinct BoNT C and D toxin subtypes. Mice were vaccinated with rBoNT/C1 Hc, rBoNT/D Hc, or with a combination of both vaccines and challenged with BoNT C1, D, C/D, or D/C toxin. Mice receiving monovalent vaccinations were partially or completely protected against homologous toxin and not protected against heterologous toxin. Bivalent vaccine candidates completely survived challenges from all toxins except D/C toxin. These results indicate the recombinant C1 and D Hc vaccines are not only effective in a monovalent formula but offer complete protection against both parental and C/D mosaic toxin and partial protection against D/C mosaic toxin when delivered as a bivalent vaccine.

Molecular evolution of antibody cross-reactivity for two subtypes of type A botulinum neurotoxin

Broadening antibody specificity without compromising affinity should facilitate detection and neutralization of toxin and viral subtypes. We used yeast display and a co-selection strategy to increase cross-reactivity of a single chain (sc) Fv antibody to botulinum neurotoxin type A (BoNT/A). Starting with a scFv that binds the BoNT/A1 subtype with high affinity (136 pM) and the BoNT/A2 subtype with low affinity (109 nM), we increased its affinity for BoNT/A2 1,250-fold, to 87 pM, while maintaining high-affinity binding to BoNT/A1 (115 pM). To find the molecular basis for improved cross-reactivity, we determined the X-ray co-crystal structures of wild-type and cross-reactive antibodies complexed to BoNT/A1 at resolutions up to 2.6 A, and measured the thermodynamic contribution of BoNT/A1 and A2 amino acids to wild-type and cross-reactive antibody binding. The results show how an antibody can be engineered to bind two different antigens despite structural differences in the antigen-antibody interface and may provide a general strategy for tuning antibody specificity and cross-reactivity.