Monoclonal antibodies against tetanus toxin and toxoid

High-Throughput IgG Epitope Mapping of Tetanus Neurotoxin: Implications for Immunotherapy and Vaccine Design

Tetanus is an acute, fatal disease caused by exotoxins released from Clostridium tetani during infections. A protective humoral immune response can be induced by vaccinations with pediatric and booster combinatorial vaccines that contain inactivated tetanus neurotoxin (TeNT) as a major antigen. Although some epitopes in TeNT have been described using various approaches, a comprehensive list of its antigenic determinants that are involved with immunity has not been elucidated. To this end, a high-resolution analysis of the linear B-cell epitopes in TeNT was performed using antibodies generated in vaccinated children. Two hundred sixty-four peptides that cover the entire coding sequence of the TeNT protein were prepared in situ on a cellulose membrane through SPOT synthesis and probed with sera from children vaccinated (ChVS) with a triple DTP-vaccine to map continuous B-cell epitopes, which were further characterized and validated using immunoassays. Forty-four IgG epitopes were identified. Four (TT-215-218) were chemically synthesized as multiple antigen peptides (MAPs) and used in peptide ELISAs to screen post-pandemic DTP vaccinations. The assay displayed a high performance with high sensitivity (99.99%) and specificity (100%). The complete map of linear IgG epitopes induced by vaccination with inactivated TeNT highlights three key epitopes involved in the efficacy of the vaccine. Antibodies against epitope TT-8/G can block enzymatic activity, and those against epitopes TT-41/G and TT-43/G can interfere with TeNT binding to neuronal cell receptors. We further show that four of the epitopes identified can be employed in peptide ELISAs to assess vaccine coverage. Overall, the data suggest a set of select epitopes to engineer new, directed vaccines.

Regulation of Clostridium tetani Neurotoxin Expression by Culture Conditions

BACKGROUND

Ensuring consistency of tetanus neurotoxin (TeNT) production by Clostridium tetani could help to ensure consistent product quality in tetanus vaccine manufacturing, ultimately contributing to reduced animal testing. The aim of this study was to identify RNA signatures related to consistent TeNT production using standard and non-standard culture conditions.

METHODS

We applied RNA sequencing (RNA-Seq) to study C. tetani gene expression in small-scale batches under several culture conditions.

RESULTS

We identified 1381 time-dependent differentially expressed genes (DEGs) reflecting, among others, changes in growth rate and metabolism. Comparing non-standard versus standard culture conditions identified 82 condition-dependent DEGs, most of which were specific for one condition. The tetanus neurotoxin gene (tetX) was highly expressed but showed expression changes over time and between culture conditions. The tetX gene showed significant down-regulation at higher pH levels (pH 7.8), which was confirmed by the quantification data obtained with the recently validated targeted LC-MS/MS approach.

CONCLUSIONS

Non-standard culture conditions lead to different gene expression responses. The tetX gene appears to be the best transcriptional biomarker for monitoring TeNT production as part of batch-to-batch consistency testing during tetanus vaccine manufacturing.

Development and validation of a targeted LC-MS/MS quantitation method to monitor cell culture expression of tetanus neurotoxin during vaccine production

The tetanus neurotoxin (TeNT) is one of the most toxic proteins known to man, which prior to the use of the vaccine against the TeNT producing bacteria Clostridium tetani, resulted in a 20% mortality rate upon infection. The clinical detrimental effects of tetanus have decreased immensely since the introduction of global vaccination programs, which depend on sustainable vaccine production. One of the major critical points in the manufacturing of these vaccines is the stable and reproducible production of high levels of toxin by the bacterial seed strains. In order to minimize time loss, the amount of TeNT is often monitored during and at the end of the bacterial culturing. The different methods that are currently available to assess the amount of TeNT in the bacterial medium suffer from variability, lack of sensitivity, and/or require specific antibodies. In accordance with the consistency approach and the three Rs (3Rs), both aiming to reduce the use of animals for testing, in-process monitoring of TeNT production could benefit from animal and antibody-free analytical tools. In this paper, we describe the development and validation of a new and reliable antibody free targeted LC-MS/MS method that is able to identify and quantify the amount of TeNT present in the bacterial medium during the different production time points up to the harvesting of the TeNT just prior to further upstream purification and detoxification. The quantitation method, validated according to ICH guidelines and by the application of the total error approach, was utilized to assess the amount of TeNT present in the cell culture medium of two TeNT production batches during different steps in the vaccine production process prior to the generation of the toxoid. The amount of TeNT generated under different physical stress conditions applied during bacterial culture was also monitored.

Nuclear magnetic resonance immunoassay of tetanus antibodies based on the displacement of magnetic nanoparticles

A nuclear magnetic resonance (NMR) immunoassay based on the application of carbon-coated iron nanoparticles conjugated with recognition molecules was designed. The principle of the assay is that ELISA plates are coated with a capture element, and then an analyte is added and detected by conjugating the magnetic nanoparticles with recognition molecules. Afterwards, the elution solution (0.1-M sodium hydroxide) is added to displace the magnetic nanoparticles from the well surfaces into the solution. The detached magnetic nanoparticles reduce transverse relaxation time (T2) values of protons from the surrounding solution. A portable NMR relaxometer is used to measure the T2. Magnetic nanoparticles conjugated with streptavidin, monoclonal antibodies, and protein G were applied for the detection of biotinylated albumin, prostate-specific antigen, and IgG specific to tetanus toxoid (TT). The limit of detection of anti-TT IgG was 0.08-0.12 mIU/mL. The reproducibility of the assay was within the acceptable range (CV < 7.4%). The key novelty of the immunoassay is that the displacement of the nanoparticles from the solid support by the elution solution allows the advantages of the solid phase assay to be combined with the sensitive detection of the T2 changes in a volume of liquid.

Epitope Mapping of Tetanus Toxin by Monoclonal Antibodies: Implication for Immunotherapy and Vaccine Design

Tetanus as a life-threatening disease is characterized by muscle spasm. The disease is caused by the neurotoxin of Clostridium tetani. Active form of tetanus neurotoxin is composed of the light chain (fragment A) and the heavy chain. Fragment A is a zinc metalloprotease, which cleaves the neuronal soluble N-ethylmaleimide-sensitive attachment receptor (SNARE) protein, leading to the blockade of inhibitory neurotransmitter release and subsequent generalized muscular spasm. Two functional domains of the heavy chain are fragment C, which is required for neuronal cell binding of the toxin and subsequent endocytosis into the vesicles, and fragment B, which is important for fragment A translocation across the vesicular membrane into the neuronal cytosol. Currently, polyclonal immunoglobulins against tetanus neurotoxin obtained from human plasma of hyper-immunized donors are utilized for passive immunotherapy of tetanus; however, these preparations have many disadvantages including high lot-to-lot heterogeneity, possibility of transmitting microbial agents, and the adverse reactions to the other proteins in the plasma. Neutralizing anti-tetanus neurotoxin monoclonal antibodies (MAbs) lack these drawbacks and could be considered as a suitable alternative for passive immunotherapy of tetanus. In this review, we provide an overview of the literature discussing epitope mapping of the published neutralizing MAbs against tetanus toxin.

Comparative in vitro and in vivo assessment of toxin neutralization by anti-tetanus toxin monoclonal antibodies

Tetanus is caused by the tetanus neurotoxin (TeNT), a 150 kDa single polypeptide molecule which is cleaved into an active two-chain molecule composed of a 50 kDa N-terminal light (L) and a 100 kDa C-terminal heavy (H) chains. Recently, extensive effort has focused on characterization of TeNT binding receptors and toxin neutralization by monoclonal antibodies (mAbs). Toxin binding inhibition and neutralization is routinely assessed either in vitro by the ganglioside GT1b binding inhibition assay or in vivo using an animal model. These two assay systems have never been compared. In the present study, we report characterization of eleven mAbs against different parts of TeNT. The toxin inhibitory and neutralization activity of the mAbs was assessed in vitro and in vivo respectively. Our data demonstrated that seven mAbs bind to fragment C of the heavy chain, two mAbs react with the light chain, one mAb recognizes both chains and one mAb reacts with neither light chain nor fragment C. Six fragment C specific mAbs were able to inhibit TeNT binding to GT1b ganglioside in vitro but three failed to neutralize the toxin in vivo. One in vitro inhibitory mAb (1F3E3) was found to synergize with the in vivo neutralizing mAbs to reduce toxin lethal activity in vivo. Sequencing of the immunoglobulin heavy and light chain variable region genes revealed that the three in vivo neutralizing mAbs were derived from a common origin. Altogether, our data suggests that fragment C specific mAbs contribute to toxin neutralization in both systems, though some of the GT1b binding inhibitory mAbs may not be able to neutralize TeNT in vivo.

Synthetic peptide antigens of tetanus toxin

In this study the immunochemical structure of the heavy chain polypeptide from tetanus toxin was studied. Numerous antigenic determinants were identified by probing a set of overlapping peptides derived from the amino acid sequence of tetanus toxin with polyclonal anti-toxoid antibody preparations. Synthetic antigens representing continuous epitopes were prepared and used to immunize mice. The capacity of the resulting anti-peptide antibodies to react with tetanus toxin in vitro and in vivo was determined. The majority of antibodies bound to tetanus toxin and three epitopes capable of eliciting neutralizing antibodies were identified.

Reductive cleavage of tetanus toxin and botulinum neurotoxin A by the thioredoxin system from brain. Evidence for two redox isomers of tetanus toxin

Inhibition of neurotransmitter release by tetanus toxin and botulinum neurotoxin A can be mimicked by intracellular application of the corresponding toxin light chains. The aim of this study was to determine whether the two-chain toxins are reduced by brain preparations to yield free light chains which would represent the ultimate toxins. The interchain disulfide of two-chain tetanus toxin was cleaved by rat cortex homogenate fortified with NADPH. Reduction was promoted further by addition of thioredoxin. Thioredoxin reductase was demonstrated in and purified from porcine brain cortex. The thioredoxin system which consisted of purified enzyme, thioredoxin and NADPH reduced both toxins. The resulting light chains appeared homogeneous in SDS gel electrophoresis. The complementary heavy chain of tetanus but not of botulinum toxin migrated in two bands, the faster one with the velocity of heavy chain obtained by chemical reduction. The major, slower form was converted into the faster by chemical but not by enzymatic reduction. Tetanus toxin, whether in its single-chain or two-chain version also occurred in two forms which differed by their electrophoretic mobility. The two forms of single-chain toxin were interconverted by chemical reduction or oxidation but not by the thioredoxin system. It is concluded that a) a thioredoxin system in brain tissue reduces the interchain disulfide of two-chain tetanus toxin and botulinum neurotoxin A, b) tetanus toxin but not botulinum neurotoxin A consists of two electrophoretically distinct forms which differ by the thiol-disulfide status of their heavy chains, c) the disulfide loop within the heavy chain of tetanus toxin is resistant to the thioredoxin system.

Increase of permeability of synaptosomes and liposomes by the heavy chain of tetanus toxin

In search of a role for the heavy chain of tetanus toxin in poisoning, its actions on natural and artificial membranes have been assessed. The heavy chain increases the permeability of synaptosomes to lactate dehydrogenase and potassium ions, and promotes the outward shift of the lipophilic cation tetraphenylphosphonium which is a particularly sensitive indicator for depolarization. Independent of the assay system the potency of the heavy chain is high, i.e. in the range of about 1 nM, whereas its efficacy is low. Its potency is decreased by the addition of the light chain and by treatment of the synaptosomes with the C-terminal fragment C of the heavy chain, but not with its N-terminal fragment beta 2. Single- or two-chain toxin itself is inactive, and so are the light chain or the two heavy chain fragments beta 2 and C. Liposomes were made from phosphatidylcholine and phosphatidylserine or gangliosides and loaded with calcein. At pH 6 the outflow of calcein is promoted in the order heavy chain greater than toxin much greater than fragment beta 2, and the action of toxin is promoted by ganglioside. At pH 5, fragment beta 2 is nearly as active as the heavy chain and more potent than the toxin. The heavy chain, but neither of the fragments, is strongly adsorbed in hydrophobic interaction chromatography and caused aggregation of polystyrene-divinylbenzene beads. Evidence for polymerization of heavy chains is lacking in zonal centrifugation. It is concluded that both domains of the heavy chain co-operate to exert the membranal events described, and that the heavy chain is partially hidden by the light chain in the complete toxin molecule.

Characteristics of toxin-neutralization by anti-tetanus human monoclonal antibodies directed against the three functional domains [A], [B] and [C] of the tetanus toxin molecule and a reliable method for evaluating the protective effects of monoclonal antibodies

Five anti-tetanus human monoclonal antibodies (MAbs) produced by hybrid cell lines we established previously were characterized. Their abilities to neutralize tetanus toxin in vitro and to protect mice against challenge with toxin were studied by observing the changes in the progress of symptoms in mice. Immunostaining showed that MAbs MAb-G4 and G2 recognized the N-terminal domain, [A] and the C-terminal domain, [C] of the tetanus toxin molecule, respectively, while MAbs MAb-G1, G3 and G6 recognized its middle domain, [B]. Enzyme-linked immunosorbent assay showed that the binding affinity of MAb-G3 was 2.9 x 10(10) M-1 and those of the other MAbs were as high as approximately 10(11) M-1. In in vitro neutralization experiments, at sufficient doses all the MAbs as single reagents protected mice completely against the effect of tetanus toxin. However, at lower doses than those sufficient to rescue mice, the kinetic patterns of progress of symptoms with the individual MAbs differed with each other and, except for MAb-G4, were different from that of anti-tetanus human polyclonal antibody. They suppressed the development and/or slowed the rate of progress of symptoms for over 96 h and delayed death of the mice. We propose that the comparison of the minimum survival dose with that of human polyclonal antibody of known international units is a reliable method for estimating the actual protective activity of a MAb. Intravenous (IV) injection of doses of individual MAbs or their mixtures at over 0.03 IU per mouse protected mice from subsequent challenge with 20 MLD of tetanus toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparative biochemical, pharmacological and immunological study of Clostridium novyi alpha-toxin, C. difficile toxin B and C. sordellii lethal toxin

The three clostridial cytotoxins, i.e. alpha-toxin of C. novyi (Tox alpha-nov), toxin B of C. difficile (ToxB-dif) and lethal toxin of C. sordellii (LT-sor) consist of single peptide chains of about 200,000 (Tox alpha-nov), 250,000 (LT-sor) and 275,000 (ToxB-dif) mol. wts. ToxB-dif and LT-sor but not Tox alpha-nov cross-reacted with rabbit polyclonal antibodies. Toxicity upon i.v. injection in mice was similar (LD50, 100 hr, 50-200 ng/kg) and was characterized by a slowly developing fluid loss into the interstitial space. When injected into the rat paw the toxins caused a delayed local edema lasting for days. In vitro the three toxins provoked a persistent retraction of endothelial cells cultured from pig pulmonary artery. ToxB-dif and Tox alpha-nov triggered the accumulation of F-actin in the perinuclear region at the expense of the tight peripheral bands whereas LT-sor led to a random loss of microfilament structure. The toxins inhibited uridine incorporation into endothelial or chicken embryonic cells whereas T 84 cells responded by an about 10-fold increase of uridine incorporation. Neither toxin ADP-ribosylated actin. The similarities between the three cytotoxins warrant their arrangement into a common group which perturbs the microfilament system.

A solid-phase enzyme immunoassay for the detection of tetanus toxin using human and murine monoclonal antibodies

Three human and three murine monoclonal antibodies were tested for their reactivity to tetanus toxin and toxoid and used to establish an enzyme immunoassay specific for tetanus toxin. The dissociation constants of the monoclonal antibodies were between 3.91 x 10(-9) and 8.48 x 10(-12). Two human monoclonal antibodies recognized conformation determinants on the toxin, whereas the others reacted to the heavy chain. Only a combination of antibodies of the two species allowed the development of an enzyme immunoassay for the detection of tetanus toxin with a lower detection limit of 1.2 micrograms/l.

Expression of tetanus toxin subfragments in vitro and characterization of epitopes

To define epitopes of tetanus toxin, we compared four different in vitro systems in terms of their ability to produce tetanus toxin-specific subfragments from cloned DNA. A transcription-translation system developed from a nontoxigenic strain of Clostridium tetani was found to yield predominantly full-sized peptides. Such peptides were used to map six different epitopes for eight monoclonal antibodies. The toxin-neutralizing properties of the antibodies were determined in an in vitro assay, based on the toxin-mediated inhibition of norepinephrine release from rat brain particles. Two monoclonal antibodies recognizing epitopes within the regions Ser-744 to Ser-864 and Ile-1224 to Asp-1315 could neutralize the toxin. A third nonneutralizing antibody was shown to recognize the synthetic peptide Phe-947 to Glu-967 derived from the tetanus toxin sequence. This peptide contains a human T-cell epitope.

Human monoclonal antibodies with a protective activity against tetanus toxin

The antibodies and their protective activity in response to tetanus toxoid in man were studied by producing human antitetanus monoclonal antibodies after transformation of peripheral blood lymphocytes with Epstein Barr virus. Two human monoclonal IgG that reacted with the heavy chain of the toxin were obtained. One of them binds the COOH-terminal moiety and the other the NH2-terminal moiety. Only the NH2-terminal specific monoclonal antibody neutralized toxin in mouse, but in doses approximately 100-fold higher than those of a polyclonal antiserum. However, the association of these 2 antibodies was protective with doses lower than necessary for the monoclonal antibodies alone. To replace the polyclonal antibodies used, a good protection could be achieved by mixed human monoclonal antibodies against different epitopes of tetanus toxin.

Chains and fragments of tetanus toxin. Separation, reassociation and pharmacological properties

Tetanus toxin, as obtained from bacterial culture filtrates, consists of two chains. Since their roles in poisoning are unknown, we have made a detailed study of their preparation, reassociation and pharmacological activity. 1. Two-chain tetanus toxin (pI 6.0) was subjected to isoelectric focussing under reducing conditions in 2M urea. Both light (pI 4.8) and heavy (pI 7.2) chains separated as nearly homogeneous proteins of low toxicities. Upon removal of urea and reoxidation, partial homodimerization by formation of disulfide bonds took place in the purified fractions. The toxin was reconstituted nearly quantitatively by covalent heterodimerization of the complementary chains, as shown by SDS/gel electrophoresis, toxicity studies, inhibition of evoked [3H]noradrenaline release and binding to rat brain membranes. 2. Accordingly, fragment B (pI 5.6) resulting from papain hydrolysis, was separated into a light chain and the N-terminal moiety of the heavy chain, called fragment beta 2 (pI 7.1 and 6.8, two maxima). Removal of urea and reoxidation led to reconstitution of fragment B. Covalent linkage did not occur between the two parts of the heavy chain, or between the light chain and the C-terminal part of the heavy chain. 3. The heavy chain alone inhibited K+-evoked [3H]noradrenaline release from a rat brain homogenate. However, the concentration-response ratio was flat and 10-100-fold higher concentrations were required than with native or reconstituted two-chain toxin. The light chain was inactive. Purified heavy chain but not light chain decreased the [3H]noradrenaline content, whereas the two-chain toxin increased it. Binding to rat brain membranes was assessed by competition with 125I-labelled two-chain toxin. In hypotonic buffer, the heavy chain, the papain fragment C and native and reconstituted two-chain toxin had comparable affinities to membranes. In isotonic buffer the heavy chain displayed an about 1000-fold lower affinity than native or reconstituted two-chain toxin. The light chain did not bind to membranes in either test. Our data indicate that (a) the light chain and the N-terminal part of the heavy chain are held together not only by one disulfide bond but also by hydrogen bonds and ionic forces to yield a two-chain toxin or fragment B and (b) both chains contribute to the actions of the toxin in vivo and in vitro, and to its binding.

Protection of mice against tetanus toxin by combination of two human monoclonal antibodies recognizing distinct epitopes on the toxin molecule

Human B-lymphocytes were fused with the human lymphoblastoid B-cell line WI-L2-729 HF2. Hybridoma frequencies were in the range of 10(-5) when the mononuclear cells were (a) prestimulated with pokeweed mitogen (PWM), (b) fused with polyethyleneglycol (PEG), and (c) selected in a hypoxanthine-azaserine (HAza) containing medium. To generate monoclonal antibodies (MAb) specific for tetanus toxin (TToxin) human spleen cells were precultured with PWM plus tetanus toxoid (TToxoid) in two separate fusions. Two hybridomas were selected based on high binding activity using an enzyme-linked immunosorbent assay (ELISA) for TToxoid. Both hybridomas, cloned twice and designated anti-TT1 and anti-TT2, exhibited a near tetraploid karyotype and showed stable production of antibody (0.15 micrograms/ml) over several months. Using ELISA for fragments of TToxin and the immunoblotting technique, the two IgG1 monoclonal antibodies were found to bind to the heavy chain portion of the B-fragment (anti-TT1) and on the C-fragment (anti-TT2) of the toxin. When tested in an ELISA with TToxin the combination of anti-TT1 and anti-TT2 showed higher binding activity than either reagent alone. In an in vivo neutralization assay mice were completely protected against TToxin by the combination of the two antibodies while either antibody alone resulted only in a delay of death of the mice. These findings demonstrate that a cocktail of appropriate human monoclonal antibodies can be far superior to a single reagent when used in a therapeutic setting.

Antibodies against the light chain of tetanus toxin in human sera

Tetanus toxoid elicits protective antibodies against tetanus toxin in humans and animals. It has been reported that antitoxin from immunized humans contains no anti-light chain antibodies, based on immunodiffusion and quantitative precipitin analyses. We confirmed the absence of precipitating anti-light chain antibodies in tetanus immune globulin. However, the presence of antibodies against the light chain of the toxin was shown by direct binding and inhibition analyses, using enzyme-linked immunosorbent assays. Using a neutralization inhibition test, we also found that about one-fourth of the neutralizing antibodies in tetanus immune globulin are directed against the light chain. These results suggest that the light chain of tetanus toxin contains immunogenic determinants and that antibodies directed against it may have a role in the prevention of tetanus or treatment of tetanus or both.

Enzymatic hydrolysis of tetanus toxin by intrinsic and extrinsic proteases. Characterization of the fragments by monoclonal antibodies

Enzymatic fragments of tetanus toxin were characterized by immunoblotting using a set of previously characterized antibodies (Ahnert-Hilger et al. (1983) and a set of novel antibodies. The selected antibodies recognized the light chain, fragment C (beta 1), and the complementary piece (beta 2) of the heavy chain when blotted on nitrocellulose. All toxin preparations contained intrinsic esteroprotease activity which became manifest in the presence of urea. The main split product was a fragment (MW 100 000) reacting with anti-fragment C and anti-beta 2 antibodies. Toxicity does not depend on this protease activity. Some crude preparations of tetanus toxin contain another split product (MW 47 000) which resembles fragment C. The main product of papain hydrolysis is fragment C, which appears as a double band under nonreducing conditions but is homogeneous when reduced. Chymotryptic digestion hydrolyses the heavy chain well but leaves the light chain largely intact. Tetanus toxin is very resistant against trypsin as compared with other proteases, although this enzyme splits numerous different links. Our data show the usefulness of immunoblotting with monoclonal antibodies in analytical work with tetanus toxin, and the relevance of intrinsic proteases.

Neutralization of tetanus toxin by distinct monoclonal antibodies binding to multiple epitopes on the toxin molecule

Fifty-seven hybridomas producing antibodies to tetanus toxoid or to the Ibc or B-IIb fragment of the toxin were isolated independently. Competitive inhibition studies demonstrated that monoclonal antibodies from mice immunized with the toxoid bound to at least 20 different epitopes on the toxoid molecule. Similar competitive binding studies revealed eight distinct epitopes on the B-IIb fragment and three to five epitopes on the Ibc fragment of the toxin. Neutralization of toxicity was effected by nine distinct monoclonal antibodies from hybridomas of toxoid-immunized mice and by one monoclonal antibody from B-IIb-immunized mice. Mixtures of two, three, and four different monoclonal antibodies in a variety of combinations exerted a synergistic effect of ca. 200-fold over that observed with individual monoclonal antibodies, indicating that efficient neutralization may involve the simultaneous binding of at least two antibody molecules to different specific regions of the toxin molecule. Only one toxoid-induced monoclonal antibody failed to bind to tetanus toxin. All neutralizing antibodies bound to epitopes on the heavy chain of tetanus toxin. Six of these were directed toward epitopes on the NH2-terminal half, whereas four bound to epitopes on the carboxy-terminal half of the heavy chain. Only one monoclonal antibody bound preferentially to the light chain, but two other monoclonal antibodies appeared to bind to both chains, indicating some homology between these two chains.

Monoclonal antibodies as probes of tetanus toxin structure and function

Monoclonal antibodies specific for fragment B, fragment C, and light chain of tetanus toxin were prepared by fusion of P3X63Ag8 BALB/c myeloma cells with spleen cells from BALB/c mice immunized with tetanus toxoid or fragment B. Hybridoma colonies were assayed for antibody production by an enzyme-linked immunosorbent assay. Fourteen positive clones were identified, cloned by limiting dilution, and injected intraperitoneally into mice to obtain ascites fluids. Thirteen of the monoclonal antibodies were of the immunoglobulin G1 subclass and one was immunoglobulin G2. Two of the antibodies were directed against sites on fragment C, nine were directed against the light chain, and three were directed against the portion of fragment B which does not comprise the light chain of tetanus toxin. At least one antibody in each group exhibited significant toxin neutralization activity. However, only one of these neutralizing antibodies strongly inhibited the binding of 125I-tetanus toxin to ganglioside-coated plates. These data indicate that interference with receptor recognition is not the only means of neutralizing tetanus toxin. Monoclonal antitoxins as potential therapeutic and prophylactic reagents are discussed.