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

Rapid Microfluidic Assay for the Detection of Botulinum Neurotoxin in Animal Sera

Potent Botulinum neurotoxins (BoNTs) represent a threat to public health and safety. Botulism is a disease caused by BoNT intoxication that results in muscle paralysis that can be fatal. Sensitive assays capable of detecting BoNTs from different substrates and settings are essential to limit foodborne contamination and morbidity. In this report, we describe a rapid 96-well microfluidic double sandwich immunoassay for the sensitive detection of BoNT-A from animal sera. This BoNT microfluidic assay requires only 5 μL of serum, provides results in 75 min using a standard fluorescence microplate reader and generates minimal hazardous waste. The assay has a <30 pg·mL⁻¹ limit of detection (LOD) of BoNT-A from spiked human serum. This sensitive microfluidic BoNT-A assay offers a fast and simplified workflow suitable for the detection of BoNT-A from serum samples of limited volume in most laboratory settings.

Atoxic derivative of botulinum neurotoxin A as a prototype molecular vehicle for targeted delivery to the neuronal cytoplasm

We have previously described genetic constructs and expression systems that enable facile production of recombinant derivatives of botulinum neurotoxins (BoNTs) that retain the structural and trafficking properties of wt BoNTs. In this report we describe the properties of one such derivative, BoNT/A ad, which was rendered atoxic by introducing two amino acid mutations to the light chain (LC) of wt BoNT/A, and which is being developed as a molecular vehicle for delivering drugs to the neuronal cytoplasm. The neuronal binding, internalization, and intracellular trafficking of BoNT/A ad in primary hippocampal cultures was evaluated using three complimentary techniques: flow cytometry, immunohistochemistry, and Western blotting. Neuronal binding of BoNT ad was significantly increased when neurons were incubated in depolarizing medium. Flow cytometry demonstrated that BoNT/A ad internalized into neurons but not glia. After 24 hours, the majority of the neuron-bound BoNT/A ad became internalized, as determined by its resistance to pronase E-induced proteolytic degradation of proteins associated with the plasma membrane of intact cells. Significant amounts of the atoxic LC accumulated in a Triton X-100-extractable fraction of the neurons, and persisted as such for at least 11 days with no evidence of degradation. Immunocytochemical analysis demonstrated that the LC of BoNT/A ad was translocated to the neuronal cytoplasm after uptake and was specifically targeted to SNARE proteins. The atoxic LC consistently co-localized with synaptic markers SNAP-25 and VAMP-2, but was rarely co-localized with markers for early or late endosomes. These data demonstrate that BoNT/A ad mimics the trafficking properties of wt BoNT/A, confirming that our platform for designing and expressing BoNT derivatives provides an accessible system for elucidating the molecular details of BoNT trafficking, and can potentially be used to address multiple medical and biodefense needs.

Development and characterization of six monoclonal antibodies to hemagglutinin-70 of Clostridium botulinum and their application in a sandwich ELISA

Botulinum neurotoxins (BoNT) are produced by Clostridium botulinum and cause severe neuroparalytic disease that if not treated quickly is often fatal. The toxin is produced as a 150 kDa precursor protein (holotoxin) that is enzymatically cleaved to form two subunits, heavy and light chains, linked by a single disulfide bond. Seven toxin serotypes are known. BoNT serotypes A1 and B1 are secreted as precursor toxic complexes (PTC) containing of the toxin and non-toxic associated proteins (NAPs) consisting of non-toxic hemagglutinin proteins (HA), designated HA17, HA34, and HA70, and a 120 kDa non-toxin non-hemagglutinin (NTNH) protein. The exact contribution of the NAPs in disease is not known, but it is thought that they protect the toxin as it passes through the harsh environment of the stomach. The structure of the complex is also poorly understood, although recent models suggest that for each molecule of toxin the PTC contains one molecule of the NTNH and multiple copies of each HA. In this paper we describe six monoclonal antibodies that specifically bind the HA70 protein found in the PTC of BoNT/A1 and /B1. Based on these antibodies, we demonstrate a rapid sandwich ELISA assay for detecting HA70.

Detection of botulinum neurotoxin serotypes A and B using a chemiluminescent versus electrochemiluminescent immunoassay in food and serum

Botulinum neurotoxins (BoNTs) are some of the most potent biological toxins. High-affinity monoclonal antibodies (mAbs) have been developed for the detection of BoNT serotypes A and B using a chemiluminescent capture enzyme-linked immunosorbent assay (ELISA). In an effort to improve toxin detection levels in complex matrices such as food and sera, we evaluated the performance of existing antitoxin mAbs using a new electrochemiluminescence (ECL) immunoassay platform developed by Meso Scale Discovery. In side-by-side comparisons, the limits of detection (LODs) observed for ELISA and the ECL immunoassay for BoNT/A were 12 and 3 pg/mL, and for BoNT/B, they were 17 and 13 pg/mL, respectively. Both the ELISA and the ECL method were more sensitive than the "gold standard" mouse bioassay. The ECL assay outperformed ELISA in detection sensitivity in most of the food matrices fortified with BoNT/A and in some foods spiked with BoNT/B. Both the ELISA and the ECL immunoassay platforms are fast, simple alternatives for use in the routine detection of BoNTs in food and animal sera.

Immunity to ricin: fundamental insights into toxin-antibody interactions

Ricin toxin is an extraordinarily potent inducer of cell death and inflammation. Ricin is also a potent provocateur of the humoral immune system, eliciting a mixture of neutralizing, non-neutralizing and even toxin-enhancing antibodies. The characterization of dozens of monoclonal antibodies (mAbs) against the toxin's enzymatic (RTA) and binding (RTB) subunits has begun to reveal fundamental insights into the underlying mechanisms by which antibodies neutralize (or fail to neutralize) ricin in systemic and mucosal compartments. This information has had immediate applications in the design, development and evaluation of ricin subunit vaccines and immunotherapeutics.

Detection of botulinum neurotoxin serotype B at sub mouse LD(50) levels by a sandwich immunoassay and its application to toxin detection in milk

BACKGROUND

Botulinum neurotoxin (BoNT), the causative agent of botulism, a serious neuroparylatic disease, is produced by the anaerobic bacterium Clostridium botulinum and consists of a family of seven serotypes (A-H). We previously reported production of high-affinity monoclonal antibodies to BoNT serotype A.

METHODS AND FINDINGS

Recombinant peptide fragments of the light chain, the transmembrane and receptor-binding domains of the heavy chain of botulinum neurotoxin type B (BoNT/B) were expressed in Escherichia coli as GST-fusion proteins and purified. These proteins were used to immunize BALB/cJ mice for the generation of monoclonal antibodies (mAbs). Antibody-producing hybridomas were detected using either a direct binding ELISA binding to plate-immobilized BoNT/B, or with a capture-capture ELISA whereby the capacity of the antibody to capture BoNT/B from solution was tested. A total of five mAbs were selected, two of which bound the toxin light chain and three bound the receptor-binding domain of BoNT/B heavy chain. MAb MCS6-27 was identified via capture-capture ELISA and was the only mAb able to bind BoNT/B in solution under physiological conditions. MAbs F24-1, F26-16, F27-33 and F29-40 were identified via direct binding ELISA, and were able to capture BoNT/B in solution only in the presence of 0.5-0.9 mM sodium dodecyl sulphate (SDS). MAb MCS6-27 and an anti-BoNT/B polyclonal antibody were incorporated into a sandwich ELISA that did not require SDS.

CONCLUSIONS

We report here the generation of monoclonal antibodies to serotype B and the subsequent development of a sensitive sandwich immunoassay. This immunoassay has a detection limit of 100 fg BoNT/B, fifty times more sensitive than the mouse bioassay detection limit of 5 pg BoNT/B. Additionally, this assay detected as little as 39 pg/mL of toxin in skim, 2% and whole milk.

Characterization of the epitope region of F1-2 and F1-5, two monoclonal antibodies to Botulinum neurotoxin type A

F1-2 and F1-5 are mouse IgG1 monoclonal antibodies that bind the heavy chain of Botulinum neurotoxin serotype A (BoNT/A). To characterize the epitopes of F1-2 and F1-5, three complementary experimental approaches were selected. First, recombinant peptide fragments of BoNT/A heavy-chain were used in Western blots to identify the epitope regions. Second, a peptide phage display library was used to identify specific amino acids bound by F1-2 and F1-5, and these amino acids were mapped onto the three-dimensional structure of BoNT/A. Third, selected amino acids were mutated to alanine and the effects of the mutations on F1-2 and F1-5 binding were evaluated. Data from recombinant peptide fragment binding experiments suggested that the epitopes for antibodies F1-2 and F1-5 are located between amino acids R564 and S793 on the toxin heavy chain. Furthermore, elimination of amino acids from the amino terminus (R564-K595), or from the carboxyl terminus (N759-S793) of this fragment abolished binding of both F1-2 and F1-5, suggesting a conformational epitope for these antibodies. Peptide sequences deduced from antibody binding to the peptide phage display library suggested that tyrosine residues located at positions 748, 750, and 753 might form a significant part of the F1-2 and F1-5 epitope motif. Mutation of Y750 or Y753 to alanine significantly reduced binding of either antibody, while mutation of Y748 to alanine had no effect on antibody binding. The nucleotide and deduced amino acid sequences of the variable regions of the heavy chains of F1-2 and F1-5 are reported. The complementarity determining regions (CDRs) of the heavy chains were found to be 78% identical. It is possible that F1-2 and F1-5 bind the same epitope via the common amino acids within their CDRs.

Antibody protection against botulinum neurotoxin intoxication in mice

Adulteration of food or feed with any of the seven serotypes of botulinum neurotoxin (BoNT) is a potential bioterrorism concern. Currently, there is strong interest in the development of detection reagents, vaccines, therapeutics, and other countermeasures. A sensitive immunoassay for detecting BoNT serotype A (BoNT/A), based on monoclonal antibodies (MAbs) F1-2 and F1-40, has been developed and used in complex matrices. The epitope for F1-2 has been mapped to the heavy chain of BoNT/A, and the epitope of F1-40 has been mapped to the light chain. The ability of these MAbs to provide therapeutic protection against BoNT/A intoxication in mouse intravenous and oral intoxication models was tested. High dosages of individual MAbs protected mice well both pre- and postexposure to BoNT/A holotoxin. A combination therapy consisting of antibodies against both the light and heavy chains of the toxin, however, significantly increased protection, even at a lower MAb dosage. An in vitro peptide assay for measuring toxin activity showed that pretreatment of toxin with these MAbs did not block catalytic activity but instead blocked toxin entry into primary and cultured neuronal cells. The timing of antibody rescue in the mouse intoxication models revealed windows of opportunity for antibody therapeutic treatment that correlated well with the biologic half-life of the toxin in the serum. Knowledge of BoNT intoxication and antibody clearance in these mouse models and understanding of the pharmacokinetics of BoNT are invaluable for future development of antibodies and therapeutics against intoxication by BoNT.