Tetanus toxin. The effect of chemical modifications on toxicity, immunogenicity, and conformation

Profound structural conservation of chemically cross-linked HIV-1 envelope glycoprotein experimental vaccine antigens

Chemical cross-linking is used to stabilize protein structures with additional benefits of pathogen and toxin inactivation for vaccine use, but its use has been restricted by the potential for local or global structural distortion. This is of particular importance when the protein in question requires a high degree of structural conservation for inducing a biological outcome such as the elicitation of antibodies to conformationally sensitive epitopes. The HIV-1 envelope glycoprotein (Env) trimer is metastable and shifts between different conformational states, complicating its use as a vaccine antigen. Here we have used the hetero-bifunctional zero-length reagent 1-Ethyl-3-(3-Dimethylaminopropyl)-Carbodiimide (EDC) to cross-link two soluble Env trimers, selected well-folded trimer species using antibody affinity, and transferred this process to good manufacturing practice (GMP) for experimental medicine use. Cross-linking enhanced trimer stability to biophysical and enzyme attack. Cryo-EM analysis revealed that cross-linking retained the overall structure with root-mean-square deviations (RMSDs) between unmodified and cross-linked Env trimers of 0.4-0.5 Å. Despite this negligible distortion of global trimer structure, we identified individual inter-subunit, intra-subunit, and intra-protomer cross-links. Antigenicity and immunogenicity of the trimers were selectively modified by cross-linking, with cross-linked ConS retaining bnAb binding more consistently than ConM. Thus, the EDC cross-linking process improves trimer stability whilst maintaining protein folding, and is readily transferred to GMP, consistent with the more general use of this approach in protein-based vaccine design.

Highly fluorescent, monolithic semiconductor nanorod clusters for ultrasensitive biodetection

Signal amplified, ultrasensitive fluorescence detection of the tetanus toxoidviahighly fluorescent, monolithic semiconductor nanorod clusters.

Brazilian meningococcal C conjugate vaccine: physicochemical, immunological, and thermal stability characteristics

High temperature is known to cause some instability in polysaccharide-protein conjugated vaccines and studies under stress conditions may be useful in determining whether short-term accidental exposure to undesired conditions can compromise product quality. In this study, we examined the structural stability of three industrial batches of Brazilian Meningococcal C conjugate bulk (MPCT) incubated at 4, 37, and 55 °C for 5 weeks. The effect of exposure to the storage temperatures was monitored by HPLC-SEC, CZE, CD and NMR techniques. The immunological significance of any physicochemical changes observed in MPCT was determined by SBA and ELISA assays of serum from immunized mice. Fluorescence emission spectra at 4 and 37 °C were similar among all samples and compatible with the native fold of the carrier protein. Fluorescence spectra of MPCT stored at 55 °C decreased in intensity and had a significant red-shift, indicating conformational changes. Far-UV CD spectra revealed a trend toward loss of structural conformation as storage temperature was increased to 55 °C. The NMR data showed modified signal intensity of the aromatic and aliphatic residues, mainly for samples incubated at 55 °C, suggesting a partial loss of tertiary structure. About 50% free saccharide content was found in bulks stored at 55 °C, but no difference was observed in the IgG or SBA titers. The present study showed physicochemical methods alone are insufficient to predict the biological activity of a MPCT conjugate vaccine without extensive validation against immunological data. However, they provide a sensitive means of detecting changes induced in a vaccine exposed to adverse environmental condition.

Accelerated stability studies for moisture-induced aggregation of tetanus toxoid

PURPOSE

The study was carried out to evaluate the effect of exposing solid tetanus toxoid to moisture in two different ways on the structure and function of the toxoid.

METHODS

Tetanus toxoid was exposed to moisture by (i) the addition of an optimized amount of buffer and (ii) incubation under an environment provided by a saturated solution of K(2)CrO(4.) The changes in the conformational, structural and antigenic properties of tetanus toxoid were measured and compared.

RESULTS

Results show that even at a similar level of moisture-induced aggregation, the amounts of water absorbed by the two preparations of tetanus toxoid are different. Differences in antigenicity and changes in structure of the toxoid at primary, secondary and tertiary structure levels were seen.

CONCLUSION

Although both conditions are used to mimic accelerated stability conditions in the laboratory, the final products are different in the two cases. Thus, conditions for 'accelerated stability studies' for therapeutic proteins need to be selected with care so that they resemble the fate of the actual product.

Characterization of new formalin-detoxified botulinum neurotoxin toxoids

Antigenicities of several formalin-detoxified botulinum neurotoxin preparations were measured by inhibition and sandwich enzyme-linked immunosorbent assay (ELISA), and immunogenicity was studied in mice. The toxoids were derived primarily from the serotype A 150-kDa neurotoxin protein, while one toxoid was derived from the naturally occurring 900-kDa toxin-hemagglutinin complex. Antigenicity was severely compromised in two commercially available toxoids. A variety of new toxoids were synthesized in-house by optimizing formaldehyde reaction conditions. Three of the resulting toxoids were found to be antigenically identical to the native toxin, as measured by inhibition ELISA, in spite of showing a reduction of toxicity by more than 100,000-fold. Sandwich ELISAs indicated that the in-house toxoids were two- to threefold less antigenic than the neurotoxin compared to commercial toxoids, which were about 100-fold less antigenic. Mice were immunized twice, on day 0 and day 14. By day 28, relatively high toxin-specific immunoglobulin G (IgG) titers were detected in animals that had received any of the in-house toxoids, with greater than 99% being IgG1 and the remainder being IgG2. These immunized mice remained asymptomatic after being challenged with 50 to 1,000,000 50% lethal dose (LD(50)) units of the 900-kDa neurotoxin. In contrast, animals immunized with several different batches of commercially available toxoids did not develop measurable toxin-specific antibody titers. However, these mice survived neurotoxin challenges with 2 LD(50) units but died when challenged with 6 LD(50) units. Neutralizing titers measured from pools of sera generated with the in-house toxoid preparations ranged from 2.5 to 5 U/ml. In terms of predicting immunogenicity, inhibition ELISAs comparing each formalin toxoid to the parent toxin provided good insight for screening the new toxoids as well as for estimating their relative in vivo potencies. Inhibition ELISA data indicate that those toxoids that most closely resemble the native toxin are highly immunogenic and protective. The superior quality of these new toxoids makes them useful tools for continued use in ELISA development and for antitoxin production.

Investigation of the detoxification mechanism of formaldehyde-treated tetanus toxin

The tetanus vaccine is based on the extremely potent tetanus neurotoxin (TeNT), which is converted by treatment with formaldehyde and lysine into the non-toxic, but still immunogenic tetanus toxoid (TTd). This formaldehyde-induced detoxification, which to a large extend determines the quality and properties of the vaccine component, occurs through partly unknown chemical modifications of the toxin. The aim of this study was to gain knowledge of the detoxification mechanism in the generation of the tetanus vaccine. Two approaches were chosen: (i) the effect of changes in the concentrations of lysine and formaldehyde in the detoxification process and (ii) characterisation of the chemically detoxified TTd. (i) We examined a number of TTd components that was produced by varying the concentrations of formaldehyde and lysine during the inactivation. Toxicity tests showed that the detoxification failed when the lysine or formaldehyde concentration was < or =1/5 or < or =1/10, respectively, of the standard level. Gel-electrophoretic analyses showed that inter-chain cross-linking was formaldehyde-dependent and, furthermore, revealed that inter-chain cross-linking was not the only requirement for the inactivation. In addition, the measurable amount of tyrosine correlated inversely with the degree of inter-chain cross-linking. (ii) To study the formaldehyde-induced chemical modifications, the TTd was investigated using protein chemical techniques in combination with mass spectrometry (MS). Using off-line liquid chromatography (LC)-MS, the most pronounced chemical modifications were characterised as unstable Schiff-bases (+12 Da) located on lysine residues and the N-termini of peptides throughout the molecule. Several arginine residues were also found with +12 Da modifications due to Schiff-base formation or as a consequence of degenerative fragmentation of lysine/formaldehyde adducts or cross-links during MS. A few tyrosine residues were similarly observed with a mass increase of 12 Da. Even though it cannot be ruled out that this is a residual mass of higher molecule adducts or cross-links to tyrosine, amino acid analysis and MS data indicated that the modification forms a ring structure from a carbon in the aromatic ring to the backbone N(alpha). In addition, several mono-epsilon-methyllysines (+14 Da) were observed as a likely consequence of reductive methylation of the Schiff-bases. A substantial part (87%) of the known TeNT sequence, including the active site, was covered using the off-line LC-MS approach to investigate the tryptic digested TTd. In contrast to the results obtained from the gel-electrophoretic experiments, neither intra/inter-chain cross-links nor cross-links to external lysines were observed in the MS analysis. Instability of the cross-links during separation and/or MS is likely to explain their absence in the analyses. The biological relevance of the observed modifications is discussed in relation to 3D mapping analyses. Proposals for the TeNT detoxification are discussed, although no direct evidence for the exact mechanism could be obtained.

Physico-chemical and immunological examination of the thermal stability of tetanus toxoid conjugate vaccines

The thermal stability of meningococcal C (MenC)- and Haemophilus influenzae b (Hib)-tetanus toxoid (TT) conjugate vaccines was investigated using spectroscopic and chromatographic techniques and immunogenicity assays in animal models. In this stability study, both the bulk concentrate and final fills were incubated at -20, 4, 23, 37 or 55 degrees C for 5 weeks or subjected to cycles of freeze-thawing. The structural stability, hydrodynamic size and molecular integrity of the treated vaccines were monitored by circular dichroism (CD), fluorescence and nuclear magnetic resonance (NMR) spectroscopic techniques, size exclusion chromatography (FPLC-SEC), and high performance anion exchange chromatography coupled with pulsed amperometric detection (HPAEC-PAD). Only storage at 55 degrees C for 5 weeks caused some slight unfolding and modification in the tertiary structure of the carrier protein in the MenC-TT conjugate. Substantial loss of saccharide content from the MenC conjugates was observed at 37 and 55 degrees C. Unexpectedly, the experimental immunogenicity of MenC-TT vaccine adsorbed to Alhydrogel was significantly reduced only by repeated freeze-thawing, but not significantly decreased by thermal denaturation. Neither the molecular integrity nor the immunogenicity of the lyophilised Hib-TT vaccines was significantly affected by freeze-thawing or by storage at high temperature. In conclusion, the MenC- and Hib-TT conjugate vaccines were relatively stable when stored at higher temperatures, though when MenC-TT vaccine was adsorbed to Alhydrogel, it was more vulnerable to repeated freeze-thawing. When compared with CRM(197) conjugate vaccines studied previously using similar techniques, the tetanus toxoid conjugates were found to have higher relative thermal stability in that they retained immunogenicity following storage at elevated temperatures.

Identification and characterization of a neutralizing monoclonal antibody against botulinum neurotoxin serotype F, following vaccination with active toxin

Clostridium botulinum may produce any of seven known serotypes of neurotoxin (BoNT/A-/G), which are the most toxic bacterial proteins known. Efforts to develop a second-generation vaccine to these toxins would benefit from the isolation of hybridomas producing neutralizing monoclonal antibodies (MAbs). We hypothesized that previous efforts to isolate neutralizing MAbs against various BoNTs failed due to use of toxoided, chemically altered antigens. We employed a novel vaccination regimen employing native, active, single-chain BoNT/E (scBoNT/E). A number of the BoNT/E immunized mice were further vaccinated with lethal doses of fully active BoNT/F. MAb 7F8 consistently neutralized BoNT/F in three different assays: in vivo neutralization, passive neutralization, and neutralization of regional paralysis. There was no detectable recognition and essentially no neutralization of scBoNT/E. The epitope recognized by this MAb was denatured when treated with formalin, urea, guanidine chloride, or sodium dodecyl sulfate. Preliminary epitope mapping studies indicate that the MAb bound to a conformational epitope.

Quaternary structure of botulinum and tetanus neurotoxins as probed by chemical cross-linking and native gel electrophoresis

Botulinum and tetanus neurotoxins are water-soluble proteins (mol. wt 150,000) produced by Clostridium botulinum and Clostridium tetani, respectively. It is believed that these neurotoxins, once internalized via receptor-mediated endocytosis, form membrane channels in order to traverse the endosomal membrane and enter the cytoplasm of the nerve terminal. Investigation of the associative properties between neurotoxin molecules could yield an understanding of this channel formation. That is, an association between neurotoxin monomers could result in an oligomeric form of the neurotoxin necessary for assembly of a channel through the hydrophobic interior of the endosomal membrane, thereby allowing passage of the neurotoxin or its active fragment through the resulting pore. Based on the native gel electrophoresis and chemical cross-linking experiments, tetanus neurotoxin exists as a dimer and a trimer, type A botulinum neurotoxin exists as a dimer, trimer, and a larger species, type E botulinum neurotoxin exists as a monomer and dimer, and type B botulinum neurotoxin appears to exist as a dimer in aqueous solution. The results imply that quaternary structures of these neurotoxins may play an important role in their mode of action during neuronal poisoning.

A simplified method for the preparation of tetanus toxin binding fragment for neurobiology

The non-toxic binding fragment of tetanus toxin (fragment C) binds avidly to neural tissue and has a growing number of neurobiological uses. Its current utility is limited by both its high commercial cost and the complex procedure for its preparation requiring highly purified tetanus toxin. We have developed a short procedure which prepares fragments of tetanus toxin from crude C. tetani extracts. The resultant proteins are atoxic with molecular sizes and immunological properties closely resembling fragment C. These proteins undergo retrograde axonal and apparent transneuronal transport in a fashion similar to fragment C.

Molecular structure of tetanus neurotoxin as revealed by Fourier transform infrared and circular dichroic spectroscopy

Secondary structure contents of tetanus neurotoxin have been estimated at neutral and acidic pH using circular dichroism (CD) and Fourier transform infrared (FT-IR) spectroscopy. An analysis of the far-ultraviolet CD spectra of the neurotoxin dissolved in 50 mM citrate-phosphate buffer (pH 7.0) revealed 20.0 +/- 2.1% alpha-helix, 50.5 +/- 2.1% beta-pleated sheets, no beta-turns, and 29.5% random coils, which is at considerable variance with results from an earlier detailed study of tetanus neurotoxin's secondary structures (J.P. Robinson, L.A. Holladay, J.H. Hash and D. Puett, J. Biol. Chem. 257 (1982) 407). However, the alpha-helix content estimated in this study is consistent with the earlier studies of Robinson et al. (J.P. Robinson, L.A. Holladay, J.B. Picklesimer and D. Puett, Mol. Cell. Biochem. 5 (1974) 147; J.P. Robinson, J.B. Picklesimer and D. Puett, J. Biol. Chem. 250 (1975) 7435) and with the study by Lazarovici et al. (P. Lazarovici, P. Yanai and E. Yavin, J. Biol. Chem. 262 (1986) 2645), although other secondary structural features do not agree with those of the previous studies. Secondary structure estimation from Fourier transform infrared spectra in both amide I and amide III frequency regions revealed 22-23% alpha-helix, 49-51% beta-pleated sheets and 27-28% random coils, indicating a good correlation with the secondary structure content estimated from CD analysis. Lowering of the pH of the neurotoxin to 5.5 or 4.0 did not result in any noticeable change in the overall secondary structures. However, there were significant pH-induced variations observed in the individual curve-fitted FT-IR bands in the amide III frequency region. For example, the 1302 cm-1 band (relative area, 4.2%) observed at pH 7.0 was shifted to 1297 cm-1 (relative area, 2.2%) at pH 5.5, and the relative area of the band at 1316-1317 cm-1 (alpha-helix) increased by approx. 40%. This study suggests that contrary to earlier reports, tetanus neurotoxin is a beta-pleated sheet dominated structure, and although lower pH does not change the overall contents of the secondary structures, significant conformational alterations are observed.

Molecular differences between type A botulinum neurotoxin and its toxoid

The neurotoxins (seven serotypes, Mr approximately 150,000) produced by Clostridium botulinum cause the neuroparalytic disease botulism. Prophylaxis, definitive diagnosis and the only effective therapy for botulism depend, at present, on chemically detoxified form(s) of the neurotoxins, i.e. toxoids (immunogens), and the antisera raised with the immunogens. And yet, the toxoids currently used for immunization of humans and animals and for raising antibody are very crude preparations (approximately 90% impure) of the neurotoxins. Hence, the highly heterogenous toxoids were not suitable for physicochemical studies. We have detoxified a pure (greater than 99%) neurotoxin (serotype A) with formaldehyde. The native neurotoxin is composed of two subunit chains (Mr 53,000 and 97,000). The physicochemical properties of the toxoid (immunogenic in rabbits) were analyzed. The chemical modification produced inter- and intrasubunit covalent links at multiple sites and thus extensive aggregation of the neurotoxin. The secondary structure parameters (alpha-helix, beta-sheet, beta-turn and random coil) of the native protein were not significantly altered. Tertiary structure, as measured by exposure of tyrosine residues and fluorescence quantum yield of tryptophan residues, was considerably altered. The data imply that conformational (topographical) antigenic determinants may not contribute significantly to the serological property of the neurotoxin.

Tetanus toxin-[2,3-3H]propionamid. Synthesis, specific activity, immunoreactivity, toxicity and ascent into the spinal cord

Tetanus toxin was tritiated with various amounts of N-succinimidyl-[2,3-3H]propionate. A tenfold molar excess of reagent over toxin was used for the introduction of about 5 nmol of [2,3-3H]propionate into 1 nmol of toxin, corresponding to a specific activity of about 48 MBq/mg protein. The labelling procedure did not change significantly the immunoreactivity of tetanus toxin, but reduced the toxicity in mice from 12 ng/kg to 16 ng/kg. Tetanus toxin-[2,3-3H]propionamid was carried through the peripheral nerves into the spinal cord. It could be used instead of 125I-labelled toxin for the detection of anti-tetanus toxin antibodies with a detection limit of 0.25 mU/ml of serum.

Effect of carrier preimmunization on neutralizing antibody response of rabbits immunized with beta-human chorionic gonadotropin coupled to tetanus toxoid

Antisera to the conjugate of beta-human chorionic gonadotropin and tetanus toxoid (beta hCG-TT) generated in rabbits receiving TT carrier preimmunization were characterized and compared with antisera from rabbits receiving the conjugate without preimmunization. Double immunodiffusion and immunoelectrophoresis were used to identify individual antisera components to the conjugate and its substructures. 125IhCG radioimmunoassay indicated similar antibody titers (P = 0.447) regardless of whether the rabbits were preimmunized with TT. However, bioneutralization of native hCG as measured by inhibition of prepubertal rat uterine weight increase was significantly more effective (P = 0.025) with antisera from rabbits which received carrier presensitization. These results demonstrate that carrier preimmunization provides a method for increasing the efficacy of neutralizing antibody produced against haptenic beta hCG.

A review of the molecular structure of tetanus toxin

A discontinuous preparative polyacrylamide gel electrophoresis system has been developed and used to purify both the nicked and unnicked forms of tetanus toxin. The system was also used to prepare purified H and L chain peptides from the nicked toxin. The results show that the endogenous protease(s), which convert unnicked toxin to the nicked form, produce multiple species of nicked toxin, and heterogeneity in the H and L chains. The major amino termini of the toxins and their peptide components are: extract toxin, proline; filtrate toxin, proline, serine and asparagine; L chain, proline; and H chain, serine and asparagine. The L chain is located in the amino terminal position of the toxin molecule and the H chain the carboxy terminal end. A model is proposed to explain these results. Using the analytical ultracentrifuge, we have determined the molecular weights of extract and filtrate toxins to be 140000 +/- 5000 and 128000 +/- 3000, respectively. Using SDS-polyacrylamide gel electrophoresis we estimate the molecular weights of the H and L chains to be 87000 and 48000 daltons, respectively. Circular dichroic spectra of the toxins and their peptide components indicate that: the major tryptophanyl band in the toxin is contributed almost entirely by the H chain, the microenvironments of all the aromatics and disulfides in the two toxins appear to have small if any differences, the two toxins show little difference in their ordered secondary structure, and the two peptides when separated from one another still retain 80% of the helical structure that is present in the intact toxin but show a considerable loss of beta-structure. The crystalline form of the nicked toxin has a hexagonal symmetry with two dimensional reciprocal lattice constants of 1/150 A-1 and 1/150 A-1. The crystals appear to belong to the two dimensional plane group P6 suggesting that each unit cell contains 6 or a multiple of 6 toxin molecules.

Effect of isoprinosine on in vitro proliferative responses of human lymphocytes stimulated by antigen

Isoprinosine, a synthetic purine derivative, did not significantly interfere with the thymidine uptake of triggered normal lymphocytes, nor exhibit a detectable mitogenic activity. Isoprinosine was able to significantly enhance specific proliferative responses of human lymphocytes from sensitized donors to soluble antigens. Isoprinosine did not alter the early interaction of human mononuclear cells with 125I-labelled antigen, nor the expression of their membrane receptors for immunoglobulin Fc fragments. The agent could not replace the accessory rôle of adherent cells in proliferative responses to antigens. The enhancing effect of isoprinosine on antigen specific responses of T-cells was observed upon adding the compound on any one of the seven days of culture. Isoprinosine partially restored the inhibited proliferation of lymphocytes cultured in the presence of deoxyadenosine and deoxycoformycin. Data suggest that metabolic changes involving purines may account for the effect of isoprinosine on the expression of T-cell responses.