Antibody mapping to domains of botulinum neurotoxin serotype A in the complexed and uncomplexed forms

Immunogenicity of Botulinum Toxin A: Insights

BACKGROUND

Botulinum toxin A (BoNT-A) is widely used in treating dystonia and spasticity to managing chronic migraine and cosmetic applications. However, its immunogenic potential presents challenges, such as the development of neutralizing antibodies that lead to diminished therapeutic efficacy over time, known as secondary nonresponse.

OBJECTIVE

This review aims to bridge the knowledge gap regarding the immunogenic mechanisms of BoNT-A and to explore effective management strategies to mitigate these immune responses.

MATERIALS AND METHODS

The authors conducted a systematic search in databases including PubMed, Embase, and Web of Science, using keywords related to BoNT-A's immunogenicity. The selection process refined 157 initial articles down to 23 relevant studies, which underwent analysis to investigate the underlying mechanisms of immunogenicity and the factors influencing it.

RESULTS

The analysis revealed that both the neurotoxin component and the neurotoxin-associated proteins could elicit an immune response. However, only antibodies against the core toxin influence therapeutic outcomes. Various patient-specific factors such as genetic predispositions and prior immune experiences, along with treatment-related factors such as dosage and frequency, play crucial roles in shaping these responses.

CONCLUSION

Understanding the specific immunogenic triggers and responses to BoNT-A is critical for optimizing treatment protocols and improving patient outcomes.

Update on Non-Interchangeability of Botulinum Neurotoxin Products

The growing use of botulinum neurotoxins (BoNTs) for medical and aesthetic purposes has led to the development and marketing of an increasing number of BoNT products. Given that BoNTs are biological medications, their characteristics are heavily influenced by their manufacturing methods, leading to unique products with distinct clinical characteristics. The manufacturing and formulation processes for each BoNT are proprietary, including the potency determination of reference standards and other features of the assays used to measure unit potency. As a result of these differences, units of BoNT products are not interchangeable or convertible using dose ratios. The intrinsic, product-level differences among BoNTs are compounded by differences in the injected tissues, which are innervated by different nerve fiber types (e.g., motor, sensory, and/or autonomic nerves) and require unique dosing and injection sites that are particularly evident when treating complex therapeutic and aesthetic conditions. It is also difficult to compare across studies due to inherent differences in patient populations and trial methods, necessitating attention to study details underlying each outcome reported. Ultimately, each BoNT possesses a unique clinical profile for which unit doses and injection paradigms must be determined individually for each indication. This practice will help minimize unexpected adverse events and maximize efficacy, duration, and patient satisfaction. With this approach, BoNT is poised to continue as a unique tool for achieving individual goals for an increasing number of medical and aesthetic indications.

Incobotulinum toxin A with a one-year long-lasting effect for trapezius contouring and superior efficacy for the treatment of trapezius myalgia

Context:

Based on various Botulinum toxin A products, reports of the lower efficacy of Incobotulinum toxin A compared with Onabotulinum toxin A for muscle contouring were observed. In addition, complications of trapezius myalgia and shoulder contouring treatment from malpractice have been reported.

Aims:

The study aimed at comparing the efficacy between Incobotulinum toxin A and Onabotulinum toxin A; research was conducted on a safe treatment technique for trapezius hypertrophy and trapezius myalgia.

Materials and Methods:

A split-shoulder, double-blind, randomized controlled trial was performed. Twenty volunteers with trapezius hypertrophy and trapezius myalgia were randomly injected with 30 units of Incobotulinum toxin A and Onabotulinum toxin A in each trapezius muscle guided by ultrasound.

Results:

The trapezius thickness among those receiving treatment with Onabotulinum toxin A and Incobotulinum toxin A on day 60 was 7.35 ± 1.11 and 7.33 ± 1.21 mm, respectively, which did not portray a significant difference (P = 0.991). Compared with the muscle size from day 60 to one year, the size of the trapezius muscle that had been treated by Onabotulinum toxin type A regained a significantly larger size compared with that treated by Incobotulinum toxin A (P = 0.027). On comparing the size of the trapezius muscle treated by Incobotulinum toxin A between one year and day 0, it was observed that the trapezius thickness at one year had significantly decreased (P < 0.001). On comparing the pain score from day 60 to day 0, it was observed that the pain scores of trapezius myalgia treated by Onabotulinum toxin A and Incobotulinum toxin A significantly differed (P = 0.003).

Conclusions:

Incobotulinum toxin A had the same efficacy but a longer lasting effect for the trapezius size contouring and a higher efficacy for trapezius myalgia treatment compared with Onabotulinum toxin A.

Tables of Toxicity of Botulinum and Tetanus Neurotoxins

Tetanus and botulinum neurotoxins are the most poisonous substances known, so much so as to be considered for a possible terrorist use. At the same time, botulinum neurotoxin type A1 is successfully used to treat a variety of human syndromes characterized by hyperactive cholinergic nerve terminals. The extreme toxicity of these neurotoxins is due to their neurospecificity and to their metalloprotease activity, which results in the deadly paralysis of tetanus and botulism. Recently, many novel botulinum neurotoxins and some botulinum-like toxins have been discovered. This large number of toxins differs in terms of toxicity and biological activity, providing a potential goldmine for novel therapeutics and for new molecular tools to dissect vesicular trafficking, fusion, and exocytosis. The scattered data on toxicity present in the literature require a systematic organization to be usable by scientists and clinicians. We have assembled here the data available in the literature on the toxicity of these toxins in different animal species. The internal comparison of these data provides insights on the biological activity of these toxins.

Antibodies and Vaccines against Botulinum Toxins: Available Measures and Novel Approaches

Botulinum neurotoxin (BoNT) is produced by the anaerobic, Gram-positive bacterium Clostridium botulinum. As one of the most poisonous toxins known and a potential bioterrosism agent, BoNT is characterized by a complex mode of action comprising: internalization, translocation and proteolytic cleavage of a substrate, which inhibits synaptic exocytotic transmitter release at neuro-muscular nerve endings leading to peripheral neuroparalysis of the skeletal and autonomic nervous systems. There are seven major serologically distinct toxinotypes (A-G) of BoNT which act on different substrates. Human botulism is generally caused by BoNT/A, B and E. Due to its extreme lethality and potential use as biological weapon, botulism remains a global public health concern. Vaccination against BoNT, although an effective strategy, remains undesirable due to the growing expectation around therapeutic use of BoNTs in various pathological conditions. This review focuses on the current approaches for botulism control by immunotherapy, highlighting the future challenges while the molecular underpinnings among subtypes variants and BoNT sequences found in non-clostridial species remain to be elucidated.

Pharmacological differences and clinical implications of various botulinum toxin preparations: a critical appraisal

Three different type A botulinum neurotoxins (BoNTAs) - onabotulinumtoxinA, abobotulinumtoxinA and incobotulinumtoxinA) - are currently marketed in Europe to treat several conditions. Differences between BoNTA preparations, which depend on their specific biotypes and manufacturing processes, lead to clinically relevant pharmacotherapeutic dissimilarities. All three available products are separately recognized and reviewed in American Academy of Neurology guidelines. The neurotoxin load/100U is likewise different among the different BoNTAs, with the result that the specific potency of the 150kD BoNTA neurotoxin is calculated as 137 units/ng for onabotulinumtoxinA, 154 units/ng for abobotulinumtoxinA, and 227 units/ng for incobotulinumtoxinA. It is important for clinicians to have all three BoNTAs available in order to choose the most suitable preparation for the specific indication in the single patient. Commercially available BoNTAs must be recognized as different from one another, and therefore as non-interchangeable. The essential experience of the clinician is of the utmost importance in choosing the most appropriate treatment.

Protein Structure Facilitates High-Resolution Immunological Mapping

Select agents (SA) pose unique challenges for licensing vaccines and therapies. In the case of toxin-mediated diseases, HHS assigns guidelines for SA use, oversees vaccine and therapy development, and approves animal models and approaches to identify mechanisms for toxin neutralization. In this commentary, we discuss next-generation vaccines and therapies against ricin toxin and botulinum toxin, which are regulated SA toxins that utilize structure-based approaches for countermeasures to guide rapid response to future biothreats.

Architecture of the botulinum neurotoxin complex: a molecular machine for protection and delivery

Botulinum neurotoxins (BoNTs) are extremely poisonous protein toxins that cause the fatal paralytic disease botulism. They are naturally produced in bacteria with several nontoxic neurotoxin-associated proteins (NAPs) and together they form a progenitor toxin complex (PTC), the largest bacterial toxin complex known. In foodborne botulism, the PTC functions as a molecular machine that helps BoNT breach the host defense in the gut. Here, we discuss the substantial recent advance in elucidating the atomic structures and assembly of the 14-subunit PTC, including structures of BoNT and four NAPs. These structural studies shed light on the molecular mechanisms by which BoNT is protected against the acidic environment and proteolytic destruction in the gastrointestinal tract, and how it is delivered across the intestinal epithelial barrier.

Botulinum toxin type A products are not interchangeable: a review of the evidence

Botulinum toxin type A (BoNTA) products are injectable biologic medications derived from Clostridium botulinum bacteria. Several different BoNTA products are marketed in various countries, and they are not interchangeable. Differences between products include manufacturing processes, formulations, and the assay methods used to determine units of biological activity. These differences result in a specific set of interactions between each BoNTA product and the tissue injected. Consequently, the products show differences in their in vivo profiles, including preclinical dose response curves and clinical dosing, efficacy, duration, and safety/adverse events. Most, but not all, published studies document these differences, suggesting that individual BoNTA products act differently depending on experimental and clinical conditions, and these differences may not always be predictable. Differentiation through regulatory approvals provides a measure of confidence in safety and efficacy at the specified doses for each approved indication. Moreover, the products differ in the amount of study to which they have been subjected, as evidenced by the number of publications in the peer-reviewed literature and the quantity and quality of clinical studies. Given that BoNTAs are potent biological products that meet important clinical needs, it is critical to recognize that their dosing and product performance are not interchangeable and each product should be used according to manufacturer guidelines.

Assembly and function of the botulinum neurotoxin progenitor complex

Botulinum neurotoxins (BoNTs) are among the most poisonous substances known to man, but paradoxically, BoNT-containing medicines and cosmetics have been used with great success in the clinic. Accidental BoNT poisoning mainly occurs through oral ingestion of food contaminated with Clostridium botulinum. BoNTs are naturally produced in the form of progenitor toxin complexes (PTCs), which are high molecular weight (up to ~900 kDa) multiprotein complexes composed of BoNT and several non-toxic neurotoxin-associated proteins (NAPs). NAPs protect the inherently fragile BoNTs against the hostile environment of the gastrointestinal (GI) tract and help BoNTs pass through the intestinal epithelial barrier before they are released into the general circulation. These events are essential for ingested BoNTs to gain access to motoneurons, where they inhibit neurotransmitter release and cause muscle paralysis. In this review, we discuss the structural basis for assembly of NAPs and BoNT into the PTC that protects BoNT and facilitate its delivery into the bloodstream.

Immunogenicity of botulinum toxins

Botulinum neurotoxins are formulated biologic pharmaceuticals used therapeutically to treat a wide variety of chronic conditions, with varying governmental approvals by country. Some of these disorders include cervical dystonia, post-stroke spasticity, blepharospasm, migraine, and hyperhidrosis. Botulinum neurotoxins also have varying governmental approvals for cosmetic applications. As botulinum neurotoxin therapy is often continued over many years, some patients may develop detectable antibodies that may or may not affect their biological activity. Although botulinum neurotoxins are considered "lower risk" biologics since antibodies that may develop are not likely to cross react with endogenous proteins, it is possible that patients may lose their therapeutic response. Various factors impact the immunogenicity of botulinum neurotoxins, including product-related factors such as the manufacturing process, the antigenic protein load, and the presence of accessory proteins, as well as treatment-related factors such as the overall toxin dose, booster injections, and prior vaccination or exposure. Detection of antibodies by laboratory tests does not necessarily predict the clinical success or failure of treatment. Overall, botulinum neurotoxin type A products exhibit low clinically detectable levels of antibodies when compared with other approved biologic products. This review provides an overview of all current botulinum neurotoxin products available commercially, with respect to the development of neutralizing antibodies and clinical response.

Immunogenicity of botulinum toxins

Botulinum neurotoxins are formulated biologic pharmaceuticals used therapeutically to treat a wide variety of chronic conditions, with varying governmental approvals by country. Some of these disorders include cervical dystonia, post-stroke spasticity, blepharospasm, migraine, and hyperhidrosis. Botulinum neurotoxins also have varying governmental approvals for cosmetic applications. As botulinum neurotoxin therapy is often continued over many years, some patients may develop detectable antibodies that may or may not affect their biological activity. Although botulinum neurotoxins are considered "lower risk" biologics since antibodies that may develop are not likely to cross react with endogenous proteins, it is possible that patients may lose their therapeutic response. Various factors impact the immunogenicity of botulinum neurotoxins, including product-related factors such as the manufacturing process, the antigenic protein load, and the presence of accessory proteins, as well as treatment-related factors such as the overall toxin dose, booster injections, and prior vaccination or exposure. Detection of antibodies by laboratory tests does not necessarily predict the clinical success or failure of treatment. Overall, botulinum neurotoxin type A products exhibit low clinically detectable levels of antibodies when compared with other approved biologic products. This review provides an overview of all current botulinum neurotoxin products available commercially, with respect to the development of neutralizing antibodies and clinical response.

Immunogenicity of botulinum toxins

Botulinum neurotoxins are formulated biologic pharmaceuticals used therapeutically to treat a wide variety of chronic conditions, with varying governmental approvals by country. Some of these disorders include cervical dystonia, post-stroke spasticity, blepharospasm, migraine, and hyperhidrosis. Botulinum neurotoxins also have varying governmental approvals for cosmetic applications. As botulinum neurotoxin therapy is often continued over many years, some patients may develop detectable antibodies that may or may not affect their biological activity. Although botulinum neurotoxins are considered "lower risk" biologics since antibodies that may develop are not likely to cross react with endogenous proteins, it is possible that patients may lose their therapeutic response. Various factors impact the immunogenicity of botulinum neurotoxins, including product-related factors such as the manufacturing process, the antigenic protein load, and the presence of accessory proteins, as well as treatment-related factors such as the overall toxin dose, booster injections, and prior vaccination or exposure. Detection of antibodies by laboratory tests does not necessarily predict the clinical success or failure of treatment. Overall, botulinum neurotoxin type A products exhibit low clinically detectable levels of antibodies when compared with other approved biologic products. This review provides an overview of all current botulinum neurotoxin products available commercially, with respect to the development of neutralizing antibodies and clinical response.

Epitope mapping of botulinum neurotoxins light chains

Botulinum neurotoxins (BoNTs) are listed among the most potent biothreat agents. Simultaneously, two out of seven known serotypes of these toxins are used in medicine and cosmetics. This situation calls for development of detailed epitope maps of these toxins. Such maps will help to develop new ways for decreasing damage caused by these toxins if they were to be used as weapons while retaining the therapeutic effect of these toxins used as medicine. Here, we used a library of random fragments of DNA encoding the catalytic domain of botulinum neurotoxin serotype A to identify short epitope-forming sequences. We demonstrated that knowledge of such sequences in a BoNT of one serotype can be used for identification of epitope-forming sequences in other serotypes of BoNTs. We also demonstrated a serodiagnostic value of identified sequences and their ability to retain epitope-specific structures and trigger production of corresponding antibodies, even when they are transferred into a background of a completely alien carrier protein.

Immunoresistance in cervical dystonia patients after treatment with abobotulinumtoxinA

Formation of antibodies against botulinum toxin type A has been observed following treatment of Cervical Dystonia (CD). We present the immunological findings from two 12-week Phase III prospective, randomized, double-blind, single-dose, placebo-controlled studies (Study 1, n = 116; Study 2, n = 136). Patients in both studies were administered abobotulinumtoxinA 500U or placebo intramuscularly at baseline. Patients could receive up to three or four additional treatments (250-1000U) in an open-label follow-up period. Blood samples were collected at baseline and during treatment to test for antibodies to abobotulinumtoxinA using a radioimmunoprecipitation assay (Study 2 only) and a mouse protection assay. Loss of response was predefined using the Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS) total score at 4 weeks following injection. No subjects in Study 1 and one individual in Study 2 developed neutralizing antibodies (nABs) during the double-blind treatment phase; the individual who developed immunoresistance had received botulinum toxin type A treatment prior to the study and did not respond to treatment. Two subjects demonstrated a change in nAB status during open-label treatment and overall responsiveness was maintained in these patients. In conclusion, the development of immunoresistance was rare and, in the presence of circulating nABs, patients may still gain benefit from intramuscular abobotulinumtoxinA treatment.

Epitope characterization of sero-specific monoclonal antibody to Clostridium botulinum neurotoxin type A

Botulinum neurotoxins (BoNTs) are extremely potent toxins that can contaminate foods and are a public health concern. Anti-BoNT antibodies have been described that are capable of detecting BoNTs; however there still exists a need for accurate and sensitive detection capabilities for BoNTs. Herein, we describe the characterization of a panel of eight monoclonal antibodies (MAbs) generated to the non-toxic receptor-binding domain of BoNT/A (H(C)50/A) developed using a high-throughput screening approach. In two independent hybridoma fusions, two groups of four IgG MAbs were developed against recombinant H(C)50/A. Of these eight, only a single MAb, F90G5-3, bound to the whole BoNT/A protein and was characterized further. The F90G5-3 MAb slightly prolonged time to death in an in vivo mouse bioassay and was mapped by pepscan to a peptide epitope in the N-terminal subdomain of H(C)50/A (H(CN)25/A) comprising amino acid residues (985)WTLQDTQEIKQRVVF(999), an epitope that is highly immunoreactive in humans. Furthermore, we demonstrate that F90G5-3 binds BoNT/A with nanomolar efficiency. Together, our results indicate that F90G5-3 is of potential value as a diagnostic immunoreagent for BoNT/A capture assay development and bio-forensic analysis.

Botulinum neurotoxin is shielded by NTNHA in an interlocked complex

Botulinum neurotoxins (BoNTs) are highly poisonous substances that are also effective medicines. Accidental BoNT poisoning often occurs through ingestion of Clostridium botulinum-contaminated food. Here, we present the crystal structure of a BoNT in complex with a clostridial nontoxic nonhemagglutinin (NTNHA) protein at 2.7 angstroms. Biochemical and functional studies show that NTNHA provides large and multivalent binding interfaces to protect BoNT from gastrointestinal degradation. Moreover, the structure highlights key residues in BoNT that regulate complex assembly in a pH-dependent manner. Collectively, our findings define the molecular mechanisms by which NTNHA shields BoNT in the hostile gastrointestinal environment and releases it upon entry into the circulation. These results will assist in the design of small molecules for inhibiting oral BoNT intoxication and of delivery vehicles for oral administration of biologics.

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.

Identification of the factors that govern the ability of therapeutic antibodies to provide postchallenge protection against botulinum toxin: a model for assessing postchallenge efficacy of medical countermeasures against agents of bioterrorism and biological warfare

Therapeutic antibodies are one of the major classes of medical countermeasures that can provide protection against potential bioweapons such as botulinum toxin. Although a broad array of antibodies are being evaluated for their ability to neutralize the toxin, there is little information that defines the circumstances under which these antibodies can be used. In the present study, an effort was made to quantify the temporal factors that govern therapeutic antibody use in a postchallenge scenario. Experiments were done involving inhalation administration of toxin to mice, intravenous administration to mice, and direct application to murine phrenic nerve-hemidiaphragm preparations. As part of this study, several pharmacokinetic characteristics of botulinum toxin and neutralizing antibodies were measured. The core observation that emerged from the work was that the window of opportunity within which postchallenge administration of antibodies exerted a beneficial effect increased as the challenge dose of toxin decreased. The critical factor in establishing the window of opportunity was the amount of time needed for fractional redistribution of a neuroparalytic quantum of toxin from the extraneuronal space to the intraneuronal space. This redistribution event was a dose-dependent phenomenon. It is likely that the approach used to identify the factors that govern postchallenge efficacy of antibodies against botulinum toxin can be used to assess the factors that govern postchallenge efficacy of medical countermeasures against any agent of bioterrorism or biological warfare.

Efficacy and safety of incobotulinumtoxinA (NT 201, XEOMIN®, botulinum neurotoxin type A, without accessory proteins) in patients with cervical dystonia

OBJECTIVE

IncobotulinumtoxinA differs from available formulations in that it does not have accessory proteins. IncobotulinumtoxinA has previously shown non-inferiority to onabotulinumtoxinA for the treatment of CD with a 1:1 dosing regimen. The objective of this study was to compare the safety and efficacy of incobotulinumtoxinA (120 U, 240 U; Merz Pharmaceuticals) to placebo in subjects with cervical dystonia (CD).

METHODS

This was a prospective, double-blind, randomized, placebo-controlled, multicenter clinical trial in botulinum toxin-treated or toxin-naïve CD patients. The primary outcome measure was change from baseline to Week 4 on the Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS) Total score. Adverse events (AEs) also were evaluated.

RESULTS

Participants (N=233) were mostly women (66%), a mean of 52.8 years old, who had CD for a mean of 51.9 months. Of those, 39% were toxin-naïve. IncobotulinumtoxinA significantly improved TWSTRS-Total scores from baseline to Week 4 compared to placebo (placebo=-2.2; 120 U=-9.9, and 240 U=-10.9; 240 U vs. placebo p<0.001 and 120 U vs. placebo p<0.001). This effect persisted through to the end of the study. The most frequently reported AEs in the incobotulinumtoxinA groups were dysphagia, neck pain, and muscular weakness which were generally mild.

INTERPRETATION

IncobotulinumtoxinA (at doses of 120 U or 240 U) is a safe and effective treatment for CD in previously-treated as well as toxin-naïve subjects.

Immunization of mice with the non-toxic HC50 domain of botulinum neurotoxin presented by rabies virus particles induces a strong immune response affording protection against high-dose botulinum neurotoxin challenge

We previously showed that rabies virus (RABV) virions are excellent vehicles for antigen presentation. Here, a reverse genetic approach was applied to generate recombinant RABV that express a chimeric protein composed of the heavy chain carboxyterminal half (HC50) of botulinum neurotoxin type A (BoNT/A) and RABV glycoprotein (G). To promote surface expression and incorporation of HC50/A into RABV virions, the RABV glycoprotein (G) ER translocation sequence, various fragments of RABV ectodomain (ED) and cytoplasmic domain were fused to HC50/A. The HC50/A chimeric proteins were expressed on the surface of cells infected with all of the recombinant RABVs, however, the highest level of surface expression was detected by utilizing 30 amino acids of the RABV G ED (HV50/A-E30). Our results also indicated that this chimeric protein was effectively incorporated into RABV virions. Immunization of mice with inactivated RABV-HC50/A-E30 virions induced a robust anti-HC50/A IgG antibody response that efficiently neutralized circulating BoNT/A in vivo, and protected mice against 1000 fold the lethal dose of BoNT/A.

Llama single domain antibodies specific for the 7 botulinum neurotoxin serotypes as heptaplex immunoreagents

BACKGROUND

There are currently 7 known serotypes of botulinum neurotoxin (BoNT) classified upon non-cross reactivity of neutralizing immunoglobulins. Non-neutralizing immunoglobulins, however, can exhibit cross-reactivities between 2 or more serotypes, particularly mosaic forms, which can hamper the development of highly specific immunoassays, especially if based on polyclonal antisera. Here we employ facile recombinant antibody technology to subtractively select ligands to each of the 7 BoNT serotypes, resulting in populations with very high specificity for their intended serotype.

METHODS AND FINDINGS

A single llama was immunized with a cocktail of 7 BoNT toxoids to generate a phage display library of single domain antibodies (sdAb, VHH or nanobodies) which were selected on live toxins. Resulting sdAb were capable of detecting both toxin and toxin complex with the best combinations able to detect 100s-10s of pg per 50 microL sample in a liquid bead array. The most sensitive sdAb were combined in a heptaplex assay to identify each of the BoNT serotypes in buffer and milk and to a lesser extent in carrot juice, orange juice and cola. Several anti-A(1) sdAb recognized A2 complex, showing that subtype cross-reactivity within a serotype was evident. Many of our sdAb could act as both captor and tracer for several toxin and toxin complexes suggesting sdAb can be used as architectural probes to indicate BoNT oligomerisation. Six of 14 anti-A clones exhibited inhibition of SNAP-25 cleavage in the neuro-2A assay indicating some sdAb had toxin neutralizing capabilities. Many sdAb were also shown to be refoldable after exposure to high temperatures in contrast to polyclonal antisera, as monitored by circular dichroism.

CONCLUSIONS

Our panel of molecularly flexible antibodies should not only serve as a good starting point for ruggedizing assays and inhibitors, but enable the intricate architectures of BoNT toxins and complexes to be probed more extensively.

Immunological characterization of the subunits of type A botulinum neurotoxin and different components of its associated proteins

Botulinum neurotoxins (BoNTs) constitute a family of seven structurally similar but antigenically distinct proteins produced by different strains of Clostridium botulinum. Type A botulinum neurotoxin (BoNT/A) is produced along with 6 neurotoxin associated proteins (NAPs) including hemagglutinin (Hn-33) through polycistronic expression of a clustered group of genes to form a complex (BoNT/AC). The presence of NAPs enhances the oral toxicity of the neurotoxin significantly. Hn-33 makes up the largest fraction of NAPs in BoNT/AC and strongly protects BoNT/A against proteases of the GI tract. BoNT in its complex form is also used in therapeutic and cosmetic applications to treat several neuromuscular disorders. In this study immunological reactivity of BoNT/A in its purified and complex forms, neurotoxin associated proteins, and Hn-33 have been examined using enzyme-linked immunosorbent assay (ELISA). Antibodies raised against the whole complex reacted 60 times better with the complex and 35 times better with Hn-33 and NAPs compared to the purified neurotoxin suggesting stronger immunogenicity of NAPs over that of purified neurotoxin and a higher potential of BoNT/AC and its associated proteins to induce host immune response. This observation also suggests that Hn-33 and other NAPs could potentially be employed as adjuvants for development of vaccines against botulism and could be a good surrogate for botulinum diagnostics. ELISA binding curves of BoNT/AC and BoNT/A with antibodies raised against BoNT/A indicate that BoNT/A in its purified and complex forms induces equal immunogenic response and a 2.5-fold higher immunogenic response compared to BoNT/A light and heavy chains. We have also discovered a new protein, an intimin analog, present within the complex preparation of BoNT/A which shows dramatically high immunoreactivity.

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.

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.

Use of botulinum toxins in cancer therapy

A recent study has demonstrated for the first time that botulinum neurotoxin (BoNT) briefly opens tumour vessels, allowing more effective destruction of cancer cells by radiotherapy and chemotherapy. This review discusses the implications of BoNTs in cancer treatment. After briefly reviewing the different BoNT serotypes, their pharmacological activities and their general use in medicine, the authors focus on their possible application in cancer and describe how BoNTs have been used so far to treat spasm related to tumour or to therapies. By dissecting the mechanisms of action leading to a potentiation of anticancer therapy, it can be seen that BoNTs act by an effect on the tumour microenvironment rather than by a direct cytotoxic effect on tumour cells.

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.

Comparative Membrane Channel Size and Activity of Botulinum Neurotoxins A and E

In an effort to compare the molecular basis of differential toxic activity of botulinum neurotoxin A (BoNT/A) and BoNT/E, we have analyzed their membrane channel activity by measuring calcein release from liposomes. Both BoNT/A and /E showed a same level of membrane channel activity that was specifically blocked by IgG specific to the neurotoxins. With the use of fluorescein-labeled dextran, we determined that the size of the channel is at least 24.2 A which is appropriate for the translocation of a protein of 50 kDa (the light chain of BoNT). These findings would suggest that the difference in the toxicity level of the two BoNT serotypes might reflect differences in either endopeptidase activity or their binding to receptor(s).

Effect of Nicking the C-terminal Region of the Clostridium botulinum Serotype D Neurotoxin Heavy Chain on its Toxicity and Molecular Properties

A unique strain of Clostridium botulinum serotype D 4947 produces toxin complexes that are composed of un-nicked components, including a neurotoxin (BoNT) and auxiliary proteins. This BoNT showed aberrant elution upon Superdex gel filtration, indicating a much lower molecular weight, due to hydrophobic interaction with the column. Limited trypsin proteolysis of BoNT produces two nicks; first nick yielded a BoNT 50 kDa light chain disulfide linked to a 100 kDa heavy chain (Hc), and a second nick arose in Hc C-terminal 10 kDa. The second nick occurred in the putative binding domain of the BoNT molecule and induced alterations in its secondary structure, leading to a significant reduction of mouse toxicity in comparison with that of the fully-activated singly nicked BoNT. These results help to clarify the role of the C-terminal half of the Hc in the oral toxicity of single-chain and more complex forms of BoNT.

An Initial Assessment of the Systemic Pharmacokinetics of Botulinum Toxin

Botulinum toxin is an extraordinarily potent molecule that has an unusually long duration of action. Despite this, there is little information available on natural mechanisms for metabolism or elimination and virtually no information on pharmacologically induced mechanisms for metabolism and elimination. Therefore, a number of experiments were performed on laboratory animals that addressed two major issues: 1) the effect of blood on the structure, function, and biologic half-life of the toxin, and 2) the effect of neutralizing antibodies on half-life and elimination of circulating toxin. In the first series of studies, the metabolic transformation of toxin was assessed by incubating it in blood for varying lengths of time. At each time point, aliquots were examined to determine the amount of toxin, the structure of toxin, the catalytic activity of toxin, and the neuromuscular blocking activity of toxin. This work demonstrated that blood did not alter any characteristic of the toxin molecule. Experiments were also done in which toxin was administered to mice and rats at doses that produced clinical poisoning. The results demonstrated that the elimination half-life for native (nonmetabolized) toxin in blood and serum was 230 to 260 min. During the second series of studies, the rate of elimination of circulating toxin was studied in the presence of antibodies directed against the carboxyl-terminal half of the toxin molecule. This work demonstrated that neutralizing antibodies 1) enhanced clearance of toxin from the circulation and 2) enhanced tissue accumulation of toxin, particularly in liver and spleen.

Detection of type A, B, E, and F Clostridium botulinum neurotoxins in foods by using an amplified enzyme-linked immunosorbent assay with digoxigenin-labeled antibodies

An amplified enzyme-linked immunosorbent assay (ELISA) for the detection of Clostridium botulinum complex neurotoxins was evaluated for its ability to detect these toxins in food. The assay was found to be suitable for detecting type A, B, E, and F botulinum neurotoxins in a variety of food matrices representing liquids, solid, and semisolid food. Specific foods included broccoli, orange juice, bottled water, cola soft drinks, vanilla extract, oregano, potato salad, apple juice, meat products, and dairy foods. The detection sensitivity of the test for these botulinum complex serotypes was found to be 60 pg/ml (1.9 50% lethal dose [LD50]) for botulinum neurotoxin type A (BoNT/A), 176 pg/ml (1.58 LD50) for BoNT/B, 163 pg/ml for BoNT/E (4.5 LD50), and 117 pg/ml for BoNT/F (less than 1 LD50) in casein buffer. The test could also readily detect 2 ng/ml of neurotoxins type A, B, E, and F in a variety of food samples. For specificity studies, the assay was also used to test a large panel of type A C. botulinum, a smaller panel of proteolytic and nonproteolytic type B, E, and F neurotoxin-producing Clostridia, and nontoxigenic organisms using an overnight incubation of toxin production medium. The assay appears to be an effective tool for large-scale screening of the food supply in the event of a botulinum neurotoxin contamination event.

The evolving field of biodefence: therapeutic developments and diagnostics

Bioweapons are a clear threat to both military and civilian populations. Here, the latest advances in the pursuit of inhibitors against biothreat threat toxins, current therapeutic strategies for treating biodefence related pathogens, and strategies for improving detection and exposure survivability are covered.

There are numerous lead therapeutics that have emerged from drug discovery efforts. However, many of these are toxic and/or fail to possess conventional drug-like properties. One clear advantage of small (non-peptidic) molecules is that they possess scaffolds that are inherently more likely to evolve into real therapeutics.

One of the major obstacles impeding the translation of these lead therapeutics into viable drugs is the lack of involvement of the pharmaceutical industry, which has been discovering leads and translating them into drugs for decades. The expertise of the pharmaceutical industry therefore needs to be more effectively engaged in developing drugs against biothreat agents.

New methods for rapidly detecting and diagnosing biothreat agents are also in development. The detection and diagnosis of biothreats is inherently linked with treatment. The means for detecting the release of bioweapons are being deployed, and new technologies are shortening the timeframe between initial sample collection and conclusive agent determination. However, the organization of this process is imperfect.

At present, a unifying entity that orchestrates the biodefence response is clearly needed to reduce the time-to-drug process and redundancies in drug development efforts. Such a central entity could formulate and implement plans to coordinate all participants, including academic institutions, government agencies and the private sector. This could accelerate the development of countermeasures against high probability biothreat agents.

The threat of bioterrorism and the potential use of biological weapons against both military and civilian populations has become a major concern for governments around the world. For example, in 2001 anthrax-tainted letters resulted in several deaths, caused widespread public panic and exerted a heavy economic toll. If such a small-scale act of bioterrorism could have such a huge impact, then the effects of a large-scale attack would be catastrophic. This review covers recent progress in developing therapeutic countermeasures against, and diagnostics for, such agents.

Characterization of the interaction between subunits of the botulinum toxin complex produced by serotype D through tryptic susceptibility of the isolated components and complex forms

The 650 kDa large toxin complex (L-TC) produced by Clostridium botulinum serotype D strain 4947 (D-4947) has a subunit structure composed of unnicked components, i.e. neurotoxin (NT), non-toxic non-haemagglutinin (NTNHA) and three haemagglutinin subcomponents (HA-70, HA-33 and HA-17). In this study, subunit interactions were investigated through the susceptibilities of the toxin components to limited trypsin proteolysis. Additionally, complex forms were reconstituted in vitro by various combinations of individual components. Trypsin treatment of intact D-4947 L-TC led to the formation of mature L-TC with nicks at specific sites of each component, which is usually observed in other strains of serotype D. NT, NTNHA and HA-17 were cleaved at their specific sites in either the single or complex forms, but HA-33 showed no sign of proteolysis. Unlike the other components, HA-70 was digested into random fragments as a single form, but it was cleaved into two fragments in the complex form. Based on the relative position of exposed or hidden regions of the individual components in the complex derived from their tryptic susceptibilities, an assembly model is proposed for the arrangement of individual subunits in the botulinum L-TC.

Characterization of toxin complex produced by a unique strain of Clostridium botulinum serotype D 4947

A unique strain of Clostridium botulinum, serotype D 4947 (D-4947), produces a considerable amount of a 650 kDa toxin complex (L-TC) and a small amount of a 280 kDa M-TC, a 540 kDa TC, and a 610 kDa TC. The complexes are composed of only un-nicked components, including neurotoxin (NT), nontoxic nonhemagglutinin (NTNHA) and hemagglutinin subcomponents (HA-70, HA-33 and HA-17). Unlike other NTs from all serotype strains, separation of D-4947 NT from L-TC, except for M-TC, during chromatography required highly alkaline conditions around pH 8.8. The separated NT and NTNHA/HAs complex can be reconstituted to L-TC that is indistinguishable from the parent L-TC with respect to toxicity, hemagglutination activity and gel filtration profile. The isoelectric points of NT and NTNHA/HAs were close together depending on the number of HA-33/17 molecules. We have established a new method to separate the unique D-4947 NT from the complex, which will yield valuable information on structure of botulinum toxin.

Recombinant Anti-Botulinum Neurotoxin A Single-Chain Variable Fragment Antibody Generated Using a Phage Display System

A recombinant single-chain fragment variable antibody (scFv) to botulinum A neurotoxin (BoNT/A) was developed. BALB/C mice were immunized with BoNT/A. Splenomic RNA was isolated from the hyperimmune mice and used to prepare a cDNA library, from which the variable regions of the heavy and light chain antibody genes were generated and connected by a DNA linker. The resulting scFv genes were cloned into the phagemid vector pCANTAB5 in order to construct phage display scFv libraries. Individual anti-BoNT/A phage clones were isolated from the phage display libraries by immunoaffinity selection using immobilized BoNT/A and further evaluated by enzyme-linked immunosorbant assay, immunoprecipitation and Western blotting. Forty-eight clones were found to be BoNT/A-reactive. The most reactive clone, designated D12, was selected for further study. The scFv gene of D12 was subcloned into a Pichia pastoris vector, and expression in yeast was evaluated.

Genetic and immunological comparison of anti-botulinum type A antibodies from immune and non-immune human phage libraries

Understanding the antibody response in botulinum intoxication is important for vaccine design and passive prophylaxis. To investigate this activity, we have studied the immune response to BoNT/A (botulinum neurotoxin serotype A) binding domain (HC) at the molecular level using phage display. The scFv antibodies were isolated from V-gene repertoires prepared from (a) human volunteer immunized with pentavalent botulinum toxoid and (b) non-immune human peripheral blood lymphocytes and spleenocytes. A large panel of serotype specific phage expressing botulinum binding scFv could be selected from both libraries. Epitope mapping of immune scFv binders towards BoNT/A HC revealed surprisingly a limited number of scFv recognizing conformational epitopes that corresponded to two distinct groups, clusters I and II. Only scFv from cluster I exhibited neutralizing activity in the mouse hemidiaphragm assay. Anti- BoNT/A HC clones derived from a non-immune library could be conveniently grouped into clusters III-XI and appeared to share no overlapping epitopes with cluster I or II. In addition they showed no neutralization of toxin at biologically significant concentrations. We therefore suggest that a vaccine based on the pentavalent botulinum toxoid directs the humoral immune response to a limited number of immunodominant epitopes exposed on the binding domain HC.

Cloning, expression and evaluation of a recombinant sub-unit vaccine against Clostridium botulinum type F toxin

A synthetic gene encoding the Hc (binding) domain of Clostridium botulinum neurotoxin F (FHc) was expressed in Escherichia coli fused to maltose binding protein (MBP). The purified MBP-FHc and FHc isolated after removal of MBP were evaluated in mice for their ability to protect against toxin challenge. Balb/c mice developed a protective immune response following administration of either protein via the intraperitoneal or intramuscular routes. A comparison of antibody titres and protection following single and multiple vaccinations and the effects of dosage are shown. The long term protection afforded by the vaccines was also investigated. Ten months following vaccination mice were still protected when challenged with 10(4) MLD(50) doses of botulinum toxin F.

Light chain of botulinum A neurotoxin expressed as an inclusion body from a synthetic gene is catalytically and functionally active

Botulinum neurotoxins, the most potent of all toxins, induce lethal neuromuscular paralysis by inhibiting exocytosis at the neuromuscular junction. The light chains (LC) of these dichain neurotoxins are a new class of zinc-endopeptidases that specifically cleave the synaptosomal proteins, SNAP-25, VAMP, or syntaxin at discrete sites. To facilitate the structural and functional characterization of these unique endopeptidases, we constructed a synthetic gene for the LC of the botulinum neurotoxin serotype A (BoNT/A), overexpressed it in Escherichia coli, and purified the gene product from inclusion bodies. Our procedure can provide 1.1 g of the LC from 1 L of culture. The LC product was stable in solution at 4 degrees C for at least 6 months. This rBoNT/A LC was proteolytically active, specifically cleaving the Glu-Arg bond in a 17-residue synthetic peptide of SNAP-25, the reported cleavage site of BoNT/A. Its calculated catalytic efficiency kcat/Km was higher than that reported for the native BoNT/A dichain. Treating the rBoNT/A LC with mercuric compounds completely abolished its activity, most probably by modifying the cysteine-164 residue located in the vicinity of the active site. About 70% activity of the LC was restored by adding Zn2+ to a Zn2+-free, apo-LC preparation. The LC was nontoxic to mice and failed to elicit neutralizing epitope(s) when the animals were vaccinated with this protein. In addition, injecting rBoNT/A LC into sea urchin eggs inhibited exocytosis-dependent plasma membrane resealing. For the first time, results of our study make available a large amount of the biologically active toxin fragment in a soluble and stable form.

Sequence homology and structural analysis of the clostridial neurotoxins

The clostridial neurotoxins (CNTs), comprised of tetanus neurotoxin (TeNT) and the seven serotypes of botulinum neurotoxin (BoNT A-G), specifically bind to neuronal cells and disrupt neurotransmitter release by cleaving proteins involved in synaptic vesicle membrane fusion. In this study, multiple CNT sequences were analyzed within the context of the 1277 residue BoNT/A crystal structure to gain insight into the events of binding, pore formation, translocation, and catalysis that are required for toxicity. A comparison of the TeNT-binding domain structure to that of BoNT/A reveals striking differences in their surface properties. Further, the solvent accessibility of a key tryptophan in the C terminus of the BoNT/A-binding domain refines the location of the ganglioside-binding site. Data collected from a single frozen crystal of BoNT/A are included in this study, revealing slight differences in the binding domain orientation as well as density for a previously unobserved translocation domain loop. This loop and the conservation of charged residues with structural proximity to putative pore-forming sequences lend insight into the CNT mechanism of pore formation and translocation. The sequence analysis of the catalytic domain revealed an area near the active-site likely to account for specificity differences between the CNTs. It revealed also a tertiary structure, highly conserved in primary sequence, which seems critical to catalysis but is 30 A from the active-site zinc ion. This observation, along with an analysis of the 54 residue "belt" from the translocation domain are discussed with respect to the mechanism of catalysis.

Biophysical characterization of the stability of the 150-kilodalton botulinum toxin, the nontoxic component, and the 900-kilodalton botulinum toxin complex species

Botulinum neurotoxin serotype A is initially released from the bacterium Clostridium botulinum as a stable 900-kDa complex. The serotype A 900-kDa complex is one of the forms of the toxin being used as a therapeutic agent for the treatment of various neuromuscular disorders. Previous experiments have demonstrated that the 900-kDa complex form of the toxin protects the toxin from the harsh conditions of the gastrointestinal tract. To provide molecular level details of the stability and equilibrium of the 900-kDa complex, the nontoxic component, and the toxic (botulinum neurotoxin) component, the three species have been investigated with a series of biophysical techniques at the molecular level (dynamic light scattering, proteolysis, circular dichroism, pH incubations, and agglutination assays). These experiments were conducted under harsh conditions which mimic those found along the gastrointestinal tract. Separately, exposure to denaturing and proteolytic conditions degrades both the botulinum neurotoxin and the nontoxic component. In the 900-kDa complex, the botulinum neurotoxin is protected during exposure to the gastrointestinal environment and the nontoxic component is slightly modified. Surprisingly, the toxin protects the ability of the nontoxic component to agglutinate erythrocytes. Contrary to previous reports, the purified 900-kDa complex did not have agglutination ability until after exposure to the proteolytic conditions. These experiments provide new evidence and detail for the theory that the nontoxic component and the toxic component protect one another during exposure to harsh conditions, and a molecular model is presented for the passage of the toxin through the gastrointestinal tract.

Recombinant expression and purification of the botulinum neurotoxin type A translocation domain

Botulinum neurotoxin type A in its fully activated form exists as a dichain protein consisting of a 50-kDa light chain and a 100-kDa heavy chain linked by a disulfide bond (B. R. DasGupta and H. Sugiyama, Biochem. Biophys. Res. Commun. 48, 108-112, 1972). The protein can be further subdivided into three functional domains: a catalytic domain corresponding to the light chain, a translocation domain associated with the N-terminal half of the heavy chain, and a binding domain as the C-terminal half. To facilitate further structural and functional studies on the mechanism of toxin translocation, we report here the recombinant Escherichia coli expression and purification of the isolated translocation domain with a yield of 1 mg pure protein per 1 g cell paste. Circular dichroism, enzyme-linked immunosorbent assays, and preliminary crystallization experiments verify proper protein folding. This reagent should serve as a key tool in elucidating the mechanism of translocation and in determining how the catalytic domain, a large 50-kDa metalloprotease, is delivered to the cytosol.

Molecular characterization of murine humoral immune response to botulinum neurotoxin type A binding domain as assessed by using phage antibody libraries

To produce antibodies capable of neutralizing botulinum neurotoxin type A (BoNT/A), the murine humoral immune response to BoNT/A binding domain (H(C)) was characterized at the molecular level by using phage antibody libraries. Mice were immunized with BoNT/A H(C), the spleens were harvested, and single-chain Fv (scFv) phage antibody libraries were constructed from the immunoglobulin heavy and light chain variable region genes. Phage expressing BoNT/A binding scFv were isolated by selection on immobilized BoNT/A and BoNT/A H(C). Twenty-eight unique BoNT/A H(C) binding scFv were identified by enzyme-linked immunosorbent assay and DNA sequencing. Epitope mapping using surface plasmon resonance in a BIAcore revealed that the 28 scFv bound to only 4 nonoverlapping epitopes with equilibrium constants (Kd) ranging from 7.3 x 10(-8) to 1.1 x 10(-9) M. In a mouse hemidiaphragm assay, scFv binding epitopes 1 and 2 significantly prolonged the time to neuroparalysis, 1.5- and 2.7-fold, respectively, compared to toxin control. scFv binding to epitopes 3 and 4 showed no protection against neuroparalysis. A combination of scFv binding epitopes 1 and 2 had an additive effect on time to neuroparalysis, which increased to 3.9-fold compared to the control. The results suggest that there are two "productive" receptor binding sites on H(C) which lead to toxin internalization and toxicity. Blockade of these two epitopes with monoclonal antibodies may provide effective immunoprophylaxis or therapy against BoNT/A intoxication.