Molecular immunology of snake toxins

Bothrops fonsecai snake venom activities and cross-reactivity with commercial bothropic venom

In this work, we examined some biochemical and biological activities of Bothrops fonsecai venom, a pitviper endemic to southeastern Brazil, and assessed their neutralization by commercial bothropic antivenom (CAv). Cross-reactivity of venom with CAv was also assessed by immunoblotting and size-exclusion high performance chromatography (SE-HPLC). Bothrops fonsecai venom had PLA₂, proteolytic and esterase activities that were neutralized to varying extents by venom:antivenom ratios of 5:1 and 5:2 (PLA₂ and esterase activities) or not significantly by either venom:antivenom ratio (proteolytic activity). The minimum hemorrhagic dose (69.2μg) was totally neutralized by both ratios. Clotting time in rat citrated plasma was 33±10.5s (mean±SD; n=5) and was completely neutralized by a 5:2 ratio. Edema formation was dose-dependent (1-30μg/site) and significantly inhibited by both ratios. Venom (10-300μg/mL) caused neuromuscular blockade in extensor digitorum longus preparations; this blockade was inhibited best by a 5:2 ratio. Venom caused myonecrosis and creatine kinase release in vivo (gastrocnemius muscle) and in vitro (extensor digitorum longus) that was effectively neutralized by both venom:antivenom ratios. Immunoblotting showed that venom components of ~25-100kDa interacted with CAv. SE-HPLC profiles for venom incubated with CAv or specific anti-B. fonsecai antivenom raised in rabbits (SAv) indicated that CAv had a higher binding capacity than SAv, whereas SAv had higher affinity than CAv. These findings indicate that B. fonsecai venom contains various activities that are neutralized to different extents by CAv and suggest that CAv could be used to treat envenoming by B. fonsecai.

High-throughput immuno-profiling of mamba (Dendroaspis) venom toxin epitopes using high-density peptide microarrays

Snakebite envenoming is a serious condition requiring medical attention and administration of antivenom. Current antivenoms are antibody preparations obtained from the plasma of animals immunised with whole venom(s) and contain antibodies against snake venom toxins, but also against other antigens. In order to better understand the molecular interactions between antivenom antibodies and epitopes on snake venom toxins, a high-throughput immuno-profiling study on all manually curated toxins from Dendroaspis species and selected African Naja species was performed based on custom-made high-density peptide microarrays displaying linear toxin fragments. By detection of binding for three different antivenoms and performing an alanine scan, linear elements of epitopes and the positions important for binding were identified. A strong tendency of antivenom antibodies recognizing and binding to epitopes at the functional sites of toxins was observed. With these results, high-density peptide microarray technology is for the first time introduced in the field of toxinology and molecular details of the evolution of antibody-toxin interactions based on molecular recognition of distinctive toxic motifs are elucidated.

Identification of linear B-cell epitopes on myotoxin II, a Lys49 phospholipase A₂ homologue from Bothrops asper snake venom

Knowledge on toxin immunogenicity at the molecular level can provide valuable information for the improvement of antivenoms, as well as for understanding toxin structure-function relationships. The aims of this study are two-fold: first, to identify the linear B-cell epitopes of myotoxin II from Bothrops asper snake venom, a Lys49 phospholipase A₂ homologue; and second, to use antibodies specifically directed against an epitope having functional relevance in its toxicity, to probe the dimeric assembly mode of this protein in solution. Linear B-cell epitopes were identified using a library of overlapping synthetic peptides spanning its complete sequence. Epitopes recognized by a rabbit antiserum to purified myotoxin II, and by three batches of a polyvalent (Crotalidae) therapeutic antivenom (prepared in horses immunized with a mixture of B. asper, Crotalus simus, and Lachesis stenophrys venoms) were mapped using an enzyme-immunoassay based on the capture of biotinylated peptides by immobilized streptavidin. Some of the epitopes identified were shared between the two species, whereas others were unique. Differences in epitope recognition were observed not only between the two species, but also within the three batches of equine antivenom. Epitope V, located at the C-terminal region of this protein, is known to be relevant for toxicity and neutralization. Affinity-purified rabbit antibodies specific for this site were able to immunoprecipitate myotoxin II, suggesting that the two copies of epitope V are simultaneously available to antibody binding, which would be compatible with the mode of dimerization known as "conventional" dimer.

Cross-neutralization of the neurotoxicity of Crotalus durissus terrificus and Bothrops jararacussu venoms by antisera against crotoxin and phospholipase A2 from Crotalus durissus cascavella venom

We have previously demonstrated that rabbit antisera raised against crotoxin from Crotalus durissus cascavella venom (cdc-crotoxin) and its PLA2 (cdc-PLA2) neutralized the neurotoxicity of this venom and its crotoxin. In this study, we examined the ability of these antisera to neutralize the neurotoxicity of Crotalus durissus terrificus and Bothrops jararacussu venoms and their major toxins, cdt-crotoxin and bothropstoxin-I (BthTX-I), respectively, in mouse isolated phrenic nerve-diaphragm preparations. Immunoblotting showed that antiserum to cdc-crotoxin recognized cdt-crotoxin and BthTX-I, while antiserum to cdc-PLA2 recognized cdt-PLA2 and BthTX-I. ELISA corroborated this cross-reactivity. Antiserum to cdc-crotoxin prevented the neuromuscular blockade caused by C. d. terrificus venom and its crotoxin at a venom/crotoxin:antiserum ratio of 1:3. Antiserum to cdc-PLA2 also neutralized the neuromuscular blockade caused by C. d. terrificus venom or its crotoxin at venom or toxin:antiserum ratios of 1:3 and 1:1, respectively. The neuromuscular blockade caused by B. jararacussu venom and BthTX-I was also neutralized by the antisera to cdc-crotoxin and cdc-PLA2 at a venom/toxin:antiserum ratio of 1:10 for both. Commercial equine antivenom raised against C. d. terrificus venom was effective in preventing the neuromuscular blockade typical of B. jararacussu venom (venom:antivenom ratio of 1:2), whereas for BthTX-I the ratio was 1:10. These results show that antiserum produced against PLA2, the major toxin in C. durissus cascavella venom, efficiently neutralized the neurotoxicity of C. d. terrificus and B. jararacussu venoms and their PLA2 toxins.

The conserved structure of snake venom toxins confers extensive immunological cross-reactivity to toxin-specific antibody

We have demonstrated previously that antisera from mice immunised with DNA encoding the carboxy-terminal domain (JD9) of a potent haemorrhagic metalloproteinase, jararhagin, neutralised over 70% of the haemorrhagic activity of the whole Bothrops jararaca venom. Here, we demonstrate that the JD9-specific antibody possesses extensive immunological reactivity to venom components in snakes of distinct species and genera. The polyspecific immunological reactivity of the antibody showed a correlation with amino acid sequence identity and with predicted antigenic domains of JD9-analogues in venoms of snakes with closest phylogenetic links to B. jararaca. This study further promotes the potential of DNA immunisation to generate toxin-specific antibodies with polyspecific cover. An analysis of the reactivity of the JD9-specific antisera to B. atrox complex venoms that exhibited intraspecific variation in the venom proteome revealed, however, that the toxin-specific approach to antivenom development requires a more in-depth knowledge of the target molecules than is required for conventional antivenoms.

Comparative evaluation of immunological and structural similarities of snake venom C-type lectin proteins

Antibodies raised against denatured and native forms of bothrojaracin were used to analyze the immunological similarities compared to the structural and biological features of five C-type lectin proteins from snake venom (bothrojaracin, botrocetin, Factor IX/X binding protein (FIX/Xbp), convulxin and Bothrops jararaca lectin). Anti-denatured-bothrojaracin antibodies, which recognize mainly linear epitopes, cross-reacted with botrocetin, FIX/Xbp and convulxin, as expected for homologous proteins. On the other hand, anti-native-bothrojaracin antibodies, which mostly interact with conformational epitopes, exhibited a higher degree of selectivity. These results show that differences exist at the surface of these proteins and that they should be related to their different biological activities, while they share a common and similar scaffold.

Neutralization of the pharmacological effects of bothropstoxin-I from Bothrops jararacussu (jararacuçu) venom by crotoxin antiserum and heparin

Bothropstoxin-I (BthTX-I), the principal myotoxin of Bothrops jararacussu venom, is devoid of phospholipase A(2) (PLA(2)) activity but capable of blocking neuromuscular transmission in mouse nerve-muscle preparations. In this study, the ability of crotoxin antiserum and heparin in preventing the neurotoxic and myotoxic effects of BthTX-I was investigated. Phrenic nerve-diaphragm preparations (PND) stimulated indirectly with supramaximal stimuli (0.2 ms, 0.1 Hz) were incubated with BthTX-I (20 microg/ml) alone or with BthTX-I preincubated with antiserum or heparin for 30 min at 37 degrees C prior to testing. Control preparations were incubated with Tyrode solution, antiserum or heparin alone. BthTX-I (20 microg/ml) produced 50% neuromuscular blockade in the PND preparations in 31+/-4min, with complete blockade occurring in 120 min. The antiserum and heparin significantly prevented the neuromuscular blockade caused by BthTX-I (84 +/- 4% and 100% protection, respectively). Light microscopy examination of the muscles at the end of the 120 min incubation showed that BthTX-I damaged 48 +/- 6% of the fibers. Preincubating the toxin with antivenom significantly reduced the extent of this damage (only 15 +/- 4% of fibers affected, corresponding to 69% protection, P<0.01) whereas heparin offered no protection (34 +/- 7% of fibers affected, not significantly different from that seen with toxin alone). These results show that the antivenom was more effective in neutralizing the myotoxic effects of BthTX-I than was heparin.

Plant natural products active against snake bite--the molecular approach

The article surveys the substances identified in plants reputed to neutralize the effects of snake venoms. Protective activity of many of them against the lethal action of the venom of the jararaca (Bothrops jararaca) snake was confirmed by biological assays. It was shown that all belong to chemical classes capable of interacting with macromolecular targets--receptors and enzymes. In a few cases it has been shown that exogenous natural micromolecules can mimic the biological activity of endogenous macromolecules. From the evidence presented, it can be inferred that micromolecules which neutralize the action of snake venoms mechanistically replace endogenous antitoxic serum proteins with venom neutralizing capacity such as produced by some animals.

Experimental evaluation of ovine antisera to Thai cobra (Naja kaouthia) venom and its alpha-neurotoxin

Conventional treatment of Naja kaouthia (Thai cobra) envenoming requires large volumes (up to 600 ml) of equine antivenom, which results in a high incidence of serum reactions. The inefficiency of the antivenom is assumed to be related to the high percentage (approx. 20%) of alpha-neurotoxin, a relatively weak and highly toxic immunogen, present in the native venom. First, antibodies to N. kaouthia venom were raised in sheep, which protected mice against challenge with whole venom. Second, ovine antibodies to the purified neurotoxin and to three different neurotoxin conjugates were developed and their neutralising abilities against either whole venom or neurotoxin were compared using murine ED50 tests. High titre antibodies, assessed by enzyme immunoassay and Western blot, were obtained from all four neurotoxin immunisation regimens. Neurotoxin conjugated to rabbit anti-sheep IgG produced the highest titres against both neurotoxin and whole venom. This antiserum provided protection against neurotoxin challenge but failed to protect against whole venom. Furthermore, the addition of neurotoxin antibodies to whole venom antiserum did not enhance the neutralisation efficacy of the latter. These findings raise the possibility that in mice other toxins apart from the neurotoxin may significantly contribute to the lethal effect of N. kaouthia venom.

CD and FTIR studies of an immunogenic disulphide cyclized octadecapeptide, a fragment of a snake curaremimetic toxin

In a previous paper, the systematic epitope screening of a snake curaremimetic toxin or toxin a was described by this group using a panel of synthetic octadecapeptides. The disulphide cyclized peptide (Cys-23,40)(23-40) corresponding to loop II of the native toxin was found to elicit, with no linkage to a carrier, neutralizing antisera against the toxin. We have now undertaken the conformational study of this immunogenic disulphide cyclized peptide by CD and FTIR. The CD study of the peptide was carried in aqueous solution under various conditions (pH, temperature, presence of micelles) and in trifluoroethanol solution. Low temperature, SDS micelles and trifluoroethanol were found to induce a beta-sheet formation (16 to 39%). FTIR spectra of the peptide in the solid state (dry film) and in D2O solution or deuterated-TFE solution (hydrated film) displayed some characteristic bands indicating the presence of beta-sheet (1623 cm-1) and beta-turn (1637 cm-1; 1694 cm-1). These studies indicate that the immunogenic disulphide cyclized peptide (23-40) can adopt in solution an ordered structure.

Demonstration of membrane-associated phospholipase A2 in cultivated heart muscle cells by immunogold-technique in surface replicas

Recently we produced mAb's to phospholipase A2 (PIA2) from bee venom for different purposes (allergen-identification and -standardization). Obviously one of these monoclonal antibodies "5D5" binds to a conserved (cross reactive) epitope of membrane PIA2 of intact cells. In this study we demonstrate binding of 5D5 to membrane associated phospholipase A2 of rat heart muscle cells by means of two-step immunogold technique in combination with replica technique.

Neutralization of myotoxic phospholipases A2 from the venom of the snake Bothrops asper by monoclonal antibodies

The neutralization of two myotoxic phospholipases A2 from the venom of Bothrops asper, myotoxins I and II, by two murine monoclonal antibodies is reported. The monoclonal antibodies, MAb-3 and Mab-4, recognize different epitopes of the toxin. Both antibodies completely neutralized myotoxic activity of myotoxin I and crude venom. MAb-3 also completely neutralized myotoxic activity of myotoxin II, a lysine-49 phospholipase A2 isoform, whereas MAb-4 neutralized this toxin only partially. MAb-3 neutralized myotoxin II at a molar ratio of 1:1, showing a higher efficiency than affinity-purified polyclonal antibodies. A dissociation of myotoxic and enzymatic activities of myotoxin I was observed with both monoclonal antibodies.

Development of an ELISA to assess the potency of horse therapeutic antivenom against Thai cobra venom

An ELISA for the quantitation of antibodies against Naja naja siamensis venom proteins has been developed for use as a possible replacement for the in vivo neutralization assay of antivenom potency. Comparison was made with three preparations of venom proteins as antigens of ELISA: these were the crude venom, a toxin fraction and the purified principle postsynaptic neurotoxin of the Thai cobra. Eight batches of horse monovalent therapeutic anti-cobra antivenom, one of which served as positive reference, were assayed by the ELISAs and also by the in vivo neutralization assay using mice. When crude venom, the toxin fraction and the pure neurotoxin were used as antigens in the ELISAs, the correlation coefficients between the ELISA antibody titers and in vivo neutralization of the antivenoms were 0.82 (P less than 0.005), 0.94 (P less than 0.001) and 0.95 (P less than 0.001), respectively. Thus, the ELISA which measures only the antibody against the principle toxin of the snake venom should be most suitable for use as an in vitro assay of antivenom potency. The ELISA should also be useful for potency assessment and standardization of antivenoms against other elapid snake venoms whose lethal components are small, poorly immunogenic peptides.

Cross-neutralizations of phospholipase A2 neurotoxins from snake venoms

Rabbit antisera were produced against several purified phospholipase A2 neurotoxins from snake venoms. The neutralizing and cross-neutralizing capacities of these antisera were evaluated using mice. In all except one case, homologous antisera neutralized the lethal effects of the neurotoxins. In several instances, antisera that exhibited strong ELISA cross-reactivity (MIDDLEBROOK and KAISER, Toxicon, 27, 965-977, 1989) were also capable of cross-neutralizations. Homologous antisera neutralized elapid neurotoxins much more effectively than did heterologous antisera. With the crotalid neurotoxins, homologous and heterologous antisera had similar neutralization potentials. These observations further define the immunological relatedness of the phospholipase A2 neurotoxins and suggest that common neutralizing epitopes exist within subgroups of these toxins.

Production and characterization of monoclonal antibodies against Naja naja atra cobrotoxin

Twelve monoclonal antibodies against cobrotoxin from Naja naja atra venom were tested for cross-reactivity with eight different snake toxins, binding to linear epitopes, prevention of cobrotoxin binding to acetylcholine receptor (AchR) in vitro, and protection in mice concomitantly given a lethal dose of cobrotoxin. The antibodies were highly specific, as evidenced by little reactivity with other snake toxins. None of the monoclonal antibodies bound to reduced cobrotoxin or synthesized 8-mer regions spanning the whole molecule, thus suggesting the recognition of conformational epitopes. The in vitro binding of toxin to AchR was competitively inhibited (23-79%) with a 1.66:1 mole ratio of antibody:AchR. Preincubation of monoclonal antibody with toxin before adding AchR (3:1 mole ratio of AchR:antibody) inhibited the in vitro binding of toxin to AchR by 20-80%. Monoclonal antibodies added after the preincubation of toxin with AchR did not dissociate the toxin-AchR complex. An antibody:toxin mole ratio of 2.5:1, with 6 micrograms of cobrotoxin, delayed the time to death of mice 3.7-23.8-fold over control mice. The monoclonal antibodies that most effectively prevented in vitro binding of toxin to AchR also provided the longest delay in time to death in mice.

Topography of toxin-acetylcholine receptor complexes by using photoactivatable toxin derivatives

We have defined the molecular environment of a snake neurotoxin interacting with the high- and low-affinity binding sites of the nicotinic acetylcholine receptor (AcChoR). This was done by photocoupling reactions using three toxin derivatives with photoactivatable moieties on Lys-15, Lys-47, and Lys-51. Competition data showed that Lys-47 belongs to the toxin-AcChoR interacting domain whereas the other two residues are excluded from it. We first tentatively determined the threshold of covalent coupling, indicative of the proximity between the photoactivatable probes and subunits, by quantifying the coupling occurring between the same derivatives and a model compound (i.e., a toxin-specific monoclonal antibody). We then (i) quantified the coupling yields occurring when both binding sites of AcChoR were occupied by the toxin derivatives, (ii) discriminately quantified the coupling yields at the high-affinity binding site, and (iii) deduced the coupling yields at the low-affinity binding site. In the high-affinity site, the probes on Lys-15 and Lys-47 predominantly reacted with the high-affinity site of the AcChoR alpha subunit whereas the probe on Lys-51 reacted with the delta subunit. In the low-affinity site, the probe on Lys-47 predominantly reacted with the low-affinity site of the alpha chain and the beta chain whereas those on Lys-15 and Lys-51 reacted with the gamma and delta chains, respectively. A three-dimensional model showing a unique organization of AcChoR bound to two toxin molecules is presented.

Equine antibodies to Bothrops asper myotoxin II: isolation from polyvalent antivenom and neutralizing ability

Equine antibodies to B. asper myotoxin II were isolated from polyvalent antivenom by affinity chromatography. Purified antibodies were among the most acidic serum immunoglobulins, migrating between the beta- and alpha 2-globulin regions by zone electrophoresis. This might be related to the fact that myotoxin II is a very basic antigen. At an antibody/toxin molar ratio of two or higher, myotoxic effect of myotoxin II was neutralized completely. Antibodies also neutralized myotoxic activity of crude venom in mice, reducing the effect by about 75% when a proportion of 16 mg antibodies/mg venom was tested. Despite the lack of phospholipase A2 activity of myotoxin II, antibodies were able to neutralize this enzymatic activity of myotoxin I, a previously described isoform. This finding is in agreement with the notion that myotoxin II is a phospholipase A2-analog devoid of this enzyme activity.

A monoclonal antibody recognizing a conserved epitope in a group of phospholipases A2

Notexin and nigexine are monomeric phospholipases A2(PLA2s) from the venoms of Notechis scutatus scutatus and Naja nigricollis, respectively. Polyclonal antibodies raised in mice against these antigenic proteins displayed non-reciprocal cross-reactivity; anti-notexin antibodies recognized notexin but not nigexine, whereas anti-nigexine antibodies recognized both antigens. Polyclonal antibodies raised by successive immunization with nigexine and notexin contained cross-reacting antibodies with affinities for both antigens that differed from those of antibodies present in anti-nigexine antiserum. A monoclonal antibody has been obtained from a mouse immunized with both PLA2s. This monoclonal antibody, called MN1, recognized notexin and nigexine with comparable high affinity (Kd = 10(-9) M). It also recognized most purified PLA2s from elapid snake venoms and all PLA2-containing venoms from cobras and sea-snakes. This offers the first demonstration that most PLA2s from cobras and sea-snakes share a fine structure which is not restricted to the common catalytic site.

Differences in distribution of myotoxic proteins in venoms from different Bothrops species

Antigens with high myotoxic activity were isolated from Bothrops jararacussu venom by Sephadex G-75 and SP-Sephadex C-25. These antigens were recognized using western blotting by B. jararacussu, B. moojeni, B. neuwiedi and B. pradoi antivenoms, and weakly by B. jararaca antivenom. B. alternatus, B. atrox, B. cotiara and B. erythromelas antivenoms failed to recognize these antigens. Antisera raised against these antigens recognized bands with mol. wt around 18,000 in the venoms of B. jararacussu, B. moojeni, B. neuwiedi and B. pradoi and reacted in ELISA with non-denaturated B. jararaca venom. However it failed to react in ELISA with nondenatured B. alternatus, B. atrox, B. cotiara and B. erythromelas venoms. The myotoxicity induced by these crude venoms confirmed that these antigens are possibly the only major myotoxin as the levels of creatine phosphokinase activity in mice serum released by intramuscular injection of B. jararacussu, B. moojeni, B. neuwiedi and B. pradoi venoms (myotoxin +) were five to eight-fold higher than those obtained with B. alternatus, B. atrox, B. cotiara, B. erythromelas and B. jararaca venoms. Using the double immunodiffusion technique the myotoxins of B. jararacussu, B. neuwiedi and B. pradoi showed total identity while B. moojeni myotoxin behaved as a partially identical antigen.

Immunological properties of notexin, a potent presynaptic and myotoxic component from venom of the Australian tiger snake Notechis scutatus scutatus

Neutralizing antibodies were raised in mice against notexin, the most toxic phospholipase A2 (PLA2) from Notechis scutatus scutatus venom, without the necessity of detoxifying the toxin prior to immunization. Using a sensitive radioimmunoassay we demonstrated that anti-notexin antibodies recognized (i) the parent antigen, (ii) closely related isoforms of notexin and (iii) venoms from Notechis genus snakes. In contrast, they failed to recognize other purified PLA2 or PLA2-containing venoms from other origins. Substitutions or chemical modifications occurring in the C-terminal part of the polypeptide chain of notexin altered the binding affinity for antibodies, implying that this region constitutes an antigenic domain of notexin.

Immunological relationships of phospholipase A2 neurotoxins from snake venoms

Polyclonal rabbit antisera were raised against ten snake phospholipase A2 neurotoxins and one snake phospholipase A2 cytotoxin. Immunological cross-reactivities between these toxins, two other snake phospholipase A2 enzymes and pancreatic phospholipase A2 were studied using ELISA technology. All snake phospholipase A2 neurotoxins fell into two main antigenic classes. One antigenic class was composed of all the elapid toxins tested (textilotoxin, taipoxin, notexin, pseudexin and beta-bungarotoxin), the cytotoxic phospholipase A2 from Naja naja atra and pancreatic phospholipase A2. beta-Bungarotoxin seemed to be in an immunological subclass of its own compared to the rest of the elapid toxins. The second antigenic class was comprised of crotalid and viperid phospholipase A2 neurotoxins (crotoxin, concolor toxin, Mojave toxin, vegrandis toxin, ammodytoxin and caudoxin). Our data indicated that the viperid toxins, caudoxin and ammodytoxin, were an immunological subclass apart from the crotalid toxins.

Far-u.v. CD-spectroscopy and immunological properties of synthetic sequential peptides derived from cardiotoxin VII1 of Naja nivea venom: an amphipathic alpha-helix formed by sequence 15-25 of a beta-protein

1. Immunological properties of cardiotoxin V(II)1 of Naja nivea were investigated. 2. Polyvalent antiserum raised against the cardiotoxin was tested for its interaction with synthetic peptides of overlapping sequence in order to locate possible sequential epitopes. 3. The conformation of each synthetic peptide in various solvents was determined by circular dichroism spectroscopy for relating immunological to structural properties. 4. It was found that sequential epitopes are absent in this cardiotoxin, but that region 15-25, although part of a beta-structured region, could be a possible T-cell epitope through the formation of an amphipathic helix.

In vitro neutralization by monoclonal antibodies of alpha-bungarotoxin binding to acetylcholine receptor

In order to develop monoclonal antibodies that would neutralize binding of alpha-bungarotoxin to acetylcholine receptor in vitro, mice were hyperimmunized with native toxin. Frequent small doses of toxin were used. Hybridoma supernatants were screened by ELISA and six monoclonal antibodies isolated and tested. The anti-alpha-bungarotoxin monoclonal antibodies consisted of IgM, IgG1 or IgG2a antibodies. In an in vitro neutralization assay measuring the effect of the antibodies on the binding of iodinated alpha-bungarotoxin to BC3H1 and TE671 (mouse and human cell lines bearing acetylcholine receptor), three of the six monoclonal antibodies were able to neutralize toxin binding. These studies demonstrate the feasibility of using native toxin for the generation of hybridomas, and the potential of using in vitro neutralization assays to screen hybridomas for in vivo neutralization.

Reversal of snake neurotoxin binding to mammalian acetylcholine receptor by specific antiserum

Snake curaremimetic toxins are known to bind to the nicotinic acetylcholine receptor (AcChoR) [Changeux et al. (1970) Proc. Natl Acad. Sci. USA, 67, 1241-1247], thus blocking neuromuscular transmission, and producing respiratory failure in mammals. In the present paper we show that the toxic effects of Naja nigricollis toxin alpha to mammals can be efficiently reversed by toxin-alpha-specific antibodies. In vivo we observed that return to normal breathing in toxin-alpha-intoxicated and ventilated rats was 12 times faster after injection of specific antiserum or monoclonal antibody (M-alpha 1) as compared with control animals. Ex vivo we observed that return to normal contraction of a toxin-alpha-blocked phrenic nerve-hemidiaphragm preparation was 14 times more rapid after treatment with specific antiserum than after washings. In vitro we observed that antibodies accelerated the reversal of binding of [3H]toxin alpha to AcChoR prepared from rat diaphragm. The observation made in vitro furthermore indicates that antibodies are capable of destabilizing the [3H]toxin-AcChoR complex. A similar destabilization phenomenon occurs also in vivo, as inferred from measurements of receptor occupancy by [3H]toxin alpha in diaphragm of anaesthetized rats in the presence or absence of antibodies. The property of antibodies to reverse neurotoxin binding to AcChoR may be considered as a critical test for evaluation of the quality of a neurotoxin-specific antisera.

Preparation of a crotoxin neutralizing monoclonal antibody

Crotoxin is a heterodimeric protein composed of an acidic and basic subunit from the venom of Crotalus durissus terrificus and is representative of a number of presynaptically acting neurotoxins found in the venom of rattlesnakes. Four different monoclonal antibodies, typed as IgG1 subclass, were raised against the basic subunit of this toxin. One was a potent neutralizing antibody of intact crotoxin, which could neutralize approximately 1.6 moles of purified crotoxin per mole of antibody. The monoclonal antibody enhanced the neutralizing ability of commercial polyvalent crotalid antivenom against the lethality of crude C. d. terrificus venom four-fold. Paradoxically, this monoclonal antibody by itself was ineffective against the lethality of crude C. d. terrificus venom. Using an enzyme-linked immunosorbent assay, we tested various proteins for competitive inhibition of binding of biotinylated-crotoxin to plates coated with the four individual monoclonal antibodies. Concolor toxin, vegrandis toxin, intact crotoxin, Mojave toxin, and the basic subunit of crotoxin showed increasing effectiveness as displacers of crotoxin from the neutralizing monoclonal antibody. None of the monoclonal antibodies reacted with purified phospholipase A2 enzymes from Crotalus atrox or Crotalus adamanteus, nor any of the components present in the crude venoms from four different elapids known to contain presynaptically acting neurotoxins, which show some sequence identity to crotoxin.

Correlation between the surface hydrophobicities and elution orders of elapid neurotoxins and cardiotoxins on hydrophobic-interaction high-performance liquid chromatography

Hydrophilic and hydrophobic regions were predicted for Elapid neuro- and cardiotoxins. The contribution of these regions to the retention times of neuro- and cardiotoxins on hydrophobic-interaction HPLC was assessed from the known surface accessibilities of amino acid side-chains within these regions. Differences in retention times between neuro- and cardiotoxins on hydrophobic-interaction HPLC could be attributed to differences in hydrophobicity of regions 6-12 and 22-26 between these two types of toxins. Smaller differences in retention times between cardiotoxins were due to the variable hydrophobicities of regions 1-4 and 26-36.

Snake venom hemorrhagins: neutralization by commercial antivenoms

One monovalent (habu-antivenom) and five polyvalent antivenoms (Crotalidae; Orient, North, Central and South Africa) were tested for their ability to neutralize the hemorrhagic activity of 12 snake venoms (Agkistrodon, Bothrops, Crotalus, Sistrurus, Trimeresurus, Bitis, Echis spp.) when mixed prior to injection into the hind leg of mice. Considerable cross-neutralization was observed: antivenoms prepared against African snake venoms were equally or more potent in neutralizing the hemorrhagic activity of Crotalidae venoms. The same applies to Crotalidae antivenom which neutralized the activity of African snake venoms. Anti-hemorrhagic antibodies were isolated from a polyvalent antivenom by affinity chromatography using purified hemorrhagins from Bitis arietans and Crotalus adamanteus venom as ligands. These antibodies neutralized the activity of both hemorrhagins indicating common antigenic determinants in these molecules.