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Infection and Immunity, March 2008, p. 1314-1318, Vol. 76, No. 3
0019-9567/08/$08.00+0 doi:10.1128/IAI.01025-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Department of Microbiology and Molecular Genetics, Medical College of Wisconsin, Milwaukee, Wisconsin,1 Food Research Institute, University of Wisconsin—Madison, Madison, Wisconsin2
Received 25 July 2007/ Returned for modification 4 September 2007/ Accepted 19 November 2007
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FIG. 1. Structure-function properties of the BoNT. Ribbon (A) and line (B) diagrams of BoNT/A (Protein Data Bank entry no. 3BTA) are shown. BoNTs are organized into three domains: the N terminus, LC, encodes a zinc protease (red), and the C terminus, HC, encodes an HCT (yellow) and an HCR (blue). (C) Purification of HCR/A to HCR/G. Individual plasmids encoding HCR (serotypes A to G) were expressed as six-His-tag fusion proteins in E. coli. Proteins were purified by affinity- and size-exclusion chromatography. Five micrograms of each HCR was subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The gel was stained with Coomassie blue and is shown. To the left are the migrations of three molecular size markers (kDa).
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A Clostridium-derived, penta-serotype BoNT (BoNT/ABCDE) toxoid vaccine was manufactured by Parke, Davis and Co. and subsequently by the Michigan Department of Public Health (14). The vaccine was prepared by mixing the five serotypes of BoNT with 0.6% formalin (BoNT toxoid) and then with aluminum phosphate as an adjuvant and thimerosal as a preservative. The vaccine from the latter source is currently used to immunize personnel at risk, but it is in limited supply (31). The protective epitopes of the BoNTs are located in the HCR (11, 27, 29), and HCRs have been used for vaccine development. Relative to the Clostridium-derived vaccine immunogens, recombinant HCRs (rHCRs) can be produced in large scale without neurotoxin contamination. Middlebrook and coworkers (11) described an Escherichia coli-derived expression system for HCR serotype A (HCR/A) and showed that the HCR/A produced in E. coli was an effective immunogen. Subsequent studies utilized rHCRs that were expressed in the yeast Pichia pastoris as a heterologous host (6, 7, 31). HCRs expressed in P. pastoris are immunogenic, induce protective immunity in mice, and represent a first generation of vaccine development. However, expression of HCRs in P. pastoris can be a challenge with respect to genetic manipulation and ease of purification (28). Recently, our laboratory and others have expressed HCR/A in E. coli at levels sufficient for vaccine development, addressing strain and culture conditions to optimize HCR expression (4, 34). This promoted the current study to produce a hepta-serotype HCR vaccine to prevent botulism.
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Construction of rHCR expression vectors. DNA encoding the indicated HCRs (Table 1) was synthesized (EZBiolab, Inc., Westfield, IN) with optimal codon usage for expression in E. coli. These DNA fragments were ligated into a TA cloning vector (pGEM-T; Promega), and the sequence was verified. DNA encoding each HCR from pGEM-HCR was subcloned into a modified pET28a (Novagen) expression vector that contained unique KpnI and PstI sites. pET28-HCR was transformed into E. coli BL-21(DE3) (Stratagene) and used for protein expression.
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TABLE 1. Source of BoNT HCR used for protein production
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0.6, and then 0.25 mM of IPTG (isopropyl-β-D-thiogalactopyranoside) was added, followed by overnight culturing at 16°C. Cells (2.4 liters) were harvested and broken with a French press (two to three times) in 40 ml ice-cold buffer (20 mM Tris-HCl [pH 7.9], 0.5 M NaCl, 1 mM dithiothreitol, and 10 mM imidazole) containing an EDTA-free protease inhibitor cocktail (Sigma), 2.5 µg/ml DNase I, and 2.5 µg/ml RNase A. The lysate was clarified by centrifugation at 20,000 x g for 30 min at 4°C and passed through a 0.45-µm filter. The filtered lysate was subjected to Ni2+-nitrilotriacetic acid affinity chromatography (5-ml bed volume; Qiagen). The resin was washed with 40 ml of buffer, and the His-fusion protein was eluted with buffer containing 0.25 M imidazole. Peak fractions were pooled and subjected to gel filtration (Sephacryl S200 high-resolution filtration) (300 ml resin equilibrated at room temperature [RT] in 20 mM Tris-HCl [pH 7.9], 1 mM EDTA, 0.2 M NaCl, and 0.1% Triton X-100). Peak fractions of HCR were pooled and concentrated with a second passage over Ni2+-nitrilotriacetic acid resin. Purified HCRs were dialyzed overnight in 10 mM Tris-HCl (pH 7.9), 200 mM NaCl, and 40% (vol/vol) glycerol. The yields of the HCRs varied from
5 to 20 mg per batch culture. Immunoreactivity and vaccine challenge with the rHCRs (ELISA). Individual rHCRs (0.1 µg per well) were added in 100 µl of coating buffer (50 mM Na2CO3, pH 9.6) to enzyme-linked immunosorbent assay (ELISA) plates (Corning enzyme immunoassay/radioimmunoassay high-binding plate) and incubated overnight at 4°C. The plates were then washed four times with 200 µl of phosphate-buffered saline (PBS) and blocked for 1 h at 37°C with 200 µl per well 2% (wt/vol) bovine serum albumin (BSA) in a coating buffer. The plates were incubated for 1 h at 37°C with serial dilutions of sera (100 µl) from mice immunized with the hepta-serotype HCR vaccine in binding buffer (PBS, 1% [wt/vol] BSA). Controls included wells without antigen or primary antibody or wells with preimmune sera. Following a washing step, the plates were incubated for 1 h at 37°C with goat anti-mouse immunoglobulin G-horseradish peroxidase (IgG-HRP, 1:12,000; Pierce) in binding buffer. Plates were washed four times with 200 µl PBS and then incubated with 100 µl per well tetramethyl benzidine (TMB; Pierce Slow TMB) as the substrate. Reactions were terminated after 30 min with 100 µl of 0.1 M H2SO4, and absorbance was read at 450 nm.
Vaccine challenge. Female ICR mice (18 to 22 g) were immunized intraperitoneally with a pool containing 1.0 µg of each HCR of serotypes A through G (hepta-serotype HCR vaccine) mixed with an equal volume of Alhydrogel as an adjuvant. The mice were vaccinated at 0, 14, 28, and 42 days. Ten days after the final boost, the mice were challenged with 1,000 50% lethal doses (LD50) and monitored for survival; after 3 days, survivors were challenged with 10,000 LD50 and monitored for 4 days for survival. In this experiment, five of five mice immunized with the adjuvant alone did not survive the challenge with 100 LD50 of BoNT/A. Homologous BoNT serotypes were used in the challenge, except for BoNT/E, for which the Alaska subtype was used to challenge mice immunized with the subtype Beluga. Potency of the BoNT serotypes (A to G) was verified with unimmunized mice. These experiments were approved by the animal care and use committee at the University of Wisconsin—Madison.
Solid-phase binding of BoNT HCRs to GT1b. Porcine brain gangliosides (Avanti Polar Lipids) were dissolved in methanol and applied to high-affinity 96-well plates (Costar 9018; Corning) (0.1 µg mixed gangliosides in 100 µl/well). The solvent was evaporated at RT, and the wells were washed with PBS. Nonspecific binding sites were blocked by incubation at RT for 1 h in sodium carbonate buffer, pH 9.6, supplemented with 1% (wt/vol) BSA. Binding assays were performed in binding buffer (10 mM Tris-HCl, 150 mM NaCl, 1% [wt/vol] BSA, pH 7.6; 100 µl/well) for 1 h at RT containing wild-type or mutated HCR/A domains (150 nM final concentration) which had been preincubated for 30 min at 37°C with either preimmune mouse serum or sera from mice immunized with the hepta-serotype HCR vaccine. Unbound protein was removed by washing each well three times with 400 µl of PBS each time. Bound HCRs were detected by incubation with anti-FLAG M2 monoclonal antibody-HRP for 15 min at 4°C. TMB-Ultra served as the substrate for HRP. The reaction was terminated by the addition of 0.2 M H2SO4, and the absorbance at 450 nm was determined using an ELISA plate reader (Victor 3V, PerkinElmer). Controls included wells without antigen (GT1b) or primary antibody or with preimmune sera. The binding of GT1b to the plates was confirmed by probing coated wells with anti-GT1b antibody.
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20% for HCR/B and HCR/C [data not shown]). The two-step purification, utilizing affinity chromatography and gel filtration, was sufficient to yield purifications of each HCR to >90% purity (Fig. 1C). The final yield of soluble HCRs ranged from
5 to 20 mg in batch culture. rHCRs elicit an immune response in mice. Mice were immunized with the hepta-serotype HCR vaccine in an aluminum hydroxide adjuvant (Alhydrogel) as the primary immunogen and then with the vaccine alone as a boost immunogen. Pooled antisera from mice immunized with the heptavalent HCR (serotypes A to G) vaccine demonstrated IgG titers to each of the individual HCRs based on ELISA (Fig. 2). HCR/C, HCR/D, HCR/E, and HCR/F elicited the strongest titer by ELISA, while HCR/B and HCR/A elicited an intermediate titer and HCR/G elicited the lowest titer. Preimmune sera did not show reactivity above that of a nonserum control at the lowest dilution used in the assay (1:50). Immunization with the hepta-serotype HCR vaccine did not elicit distress in the mice.
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FIG. 2. Immunoreactivity of mouse anti-hepta-serotype HCR vaccine in mice. An ELISA was performed, using 100 ng each of the individual rHCRs (serotypes A to G). The plates were incubated for 1 h at 37°C with serial dilutions of sera from mice immunized with the hepta-serotype HCR vaccine obtained prior to BoNT challenge or with preimmunization sera. The plates were then incubated for 1 h at 37°C with goat anti-mouse IgG-HRP (1:12,000; Pierce), washed with PBS, and incubated with TMB as the substrate. Reactions were terminated with 0.2 M H2SO4, and absorbance at 450 nm (A450) was read. The serum titer represents the inverse of the serum dilution used in the analysis. Data presented are representative of two independent immunization challenge experiments.
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1 x 107 to 10 x 107 mouse LD50 per mg of toxin. Mice immunized with the hepta-serotype HCR vaccine were resistant to challenge with each of the homologous BoNT serotypes (A to G) (Table 2) at either a 1,000-LD50 challenge or a 10,000-LD50 challenge. This is the first demonstration of a single vaccine protecting against the seven serotypes of BoNT. |
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TABLE 2. Hepta-serotype HCR vaccine protects against challenge by the seven serotypes of BoNTa
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Preliminary studies observed that among the seven serotypes, HCR/F showed the highest affinity for GT1b (data not shown), while BoNT/D did not show any significant binding under the conditions tested (data not shown). Further analysis of ganglioside binding was carried out using HCR/A and HCR/B due to the significant association of these serotypes with human disease and the availability of structural data (10, 15). Controls using anti-GT1b antibody confirmed the binding of GT1b to the ELISA plate and established conditions in which neither HCR/A nor HCR/B bound to non-GT1b-coated plates (data not shown). Preimmune mouse sera did not interfere with the binding of either HCR/A or HCR/B to GT1b, while the anti-hepta-serotype HCR sera showed dose-dependent inhibition of HCR/A and HCR/B binding to GT1b (Fig. 3). We previously reported the ability of HCR/A alone to generate a serotype-specific neutralizing antibody response (4). Sera from mice immunized with HCR/A alone inhibited the binding of HCR/A, but not HCR/B, to GT1b (data not shown). This indicates that anti-hepta-serotype HCR antibody neutralizes the first step in BoNT intoxication of neurons, the binding of BoNT to gangliosides.
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FIG. 3. Anti-hepta-serotype HCR blocks the binding of HCR/A and HCR/B to GT1b. Gangliosides (0.1 µg in 100 µl methanol) were added to individual wells of an ELISA and incubated overnight at RT. The plates were washed with PBS and blocked for 1 h at RT with 2% (wt/vol) BSA in binding buffer. The plates were then washed and incubated for 1 h at 37°C with the indicated HCR alone ( ), with preimmune sera ( ), or with mouse anti-hepta-serotype HCR sera ( ) in 1% (wt/vol) BSA in Tris-buffered saline. Following a washing step, the plates were incubated with an anti-3 FLAG M2 monoclonal IgG-HRP conjugate (1:12,000) in binding buffer. The plates were washed and then incubated with TMB as the substrate. Reactions were terminated with 0.2 M H2SO4. Absorbance at 450 nm (A450) was read on a Victor 3V plate reader.
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There are two approved therapies against botulism: human botulinum immune globulin (Big-IV), which is used to treat infant botulism in California (2), and horse-derived BoNT antitoxin. Current therapies against botulism are focused on antibody or small-molecule inhibitors of catalysis. Casadevall (8) proposed a strategy for stockpiling antibodies to protect against biological threats such as botulism. Botulism is treated with horse polyclonal antibodies that are associated with a high incidence of systemic reactions. Human monoclonal antibodies are being developed as a safer therapy (23), but these will inevitably be serotype specific and thus a long-term project to produce monoclonal antibodies against each serotype. In addition, subserotype neutralization may also have to be considered in monoclonal antibody therapy. Identifying neutralizing epitopes on BoNTs is also an important step in generating neutralizing monoclonal antibodies and has implications for vaccine development. The development of a hepta-serotype BoNT HCR vaccine provides a mechanism for producing a polyclonal response that will have broad serotype neutralization against BoNTs.
While each HCR serotype elicited a detectable immune response by ELISA, the titer did not correlate with neutralization capacity. This is similar to a recent report by Takeda et al. (33), who observed that serum ELISA titers did not correspond with the neutralization titers of immunized ducks. The study concluded that the neutralizing titer was more accurate than the ELISA titer for establishing levels of BoNT protection. In contrast, Lee et al. (20) reported that the level of protection against BoNT intoxication elicited by immunization with HCR/A in a Venezuelan equine encephalitis virus replicon vector directly correlated with serum ELISA titers against BoNT/A. In addition, using a high-level BoNT/A challenge, Smith and coworkers (7) observed a correlation between the number of doses of vaccine with serum neutralization titers and ELISA titers, while Steinman et al. (32) reported a correlation between protection status and ELISA titers with a BoNT/D challenge. At this stage, there is a need to establish the molecular basis for protection to allow a more accurate interpretation of the neutralization versus ELISA titer data.
While the HCR appears to be the primary domain to elicit a neutralizing immune response against BoNT intoxication, there is limited information on the molecular mechanism for BoNT neutralization by BoNT antisera. Recently, a monoclonal antibody that was specific for the translocation domain of BoNT/B was demonstrated to neutralize the toxin (36). This observation, in conjunction with the data presented in the current study, suggests that numerous steps in the intoxication process can act as targets for neutralization. Levy et al. (21) utilized yeast display of the LCs, HCTs, and HCRs to map the binding sites of neutralizing BoNT/A monoclonal antibodies and reported monoclonal antibodies that bound to the C terminus of the HC, including one neutralizing monoclonal antibody that bound near the ganglioside binding site. In the current study, the neutralizing, anti-hepta-serotype HCR sera blocked the binding of HCR/A and HCR/B to ganglioside GT1b. In addition, Kubota et al. (17) reported the identification of a monoclonal antibody that blocked BoNT/E binding to cells that localized to the C terminus of the toxin. The current study is the first functional determination of how neutralizing antibodies block BoNT intoxication by interfering with ganglioside binding, the initial step in the BoNT intoxication process. Future studies will address whether or not subsequent steps in the intoxication process are neutralized by BoNT-neutralizing antibodies.
The immunogenic potencies of E. coli-derived rHCRs observed in the current study represent tools that allow genetic manipulation to develop the next generation of vaccines and immune therapies against botulism and also represent a malleable platform for responding to the malicious use of these toxins.
Published ahead of print on 10 December 2007. ![]()
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