Vaccination with F1-V fusion protein protects black-footed ferrets (Mustela nigripes) against plague upon oral challenge with Yersinia pestis.

Previous studies have established that vaccination of black-footed ferrets (Mustela nigripes) with F1-V fusion protein by subcutaneous (SC) injection protects the animals against plague upon injection of the bacterium Yersinia pestis. This study demonstrates that the F1-V antigen can also protect ferrets against plague contracted via ingestion of a Y. pestis-infected mouse, a probable route for natural infection. Eight black-footed ferret kits were vaccinated with F1-V protein by SC injection at approximately 60 days-of-age. A booster vaccination was administered 3 mo later via SC injection. Four additional ferret kits received placebos. The animals were challenged 6 wk after the boost by feeding each one a Y. pestis-infected mouse. All eight vaccinates survived challenge, while the four controls succumbed to plague within 3 days after exposure. To determine the duration of antibody postvaccination, 18 additional black-footed ferret kits were vaccinated and boosted with F1-V by SC injection at 60 and 120 days-of-age. High titers to both F1 and V (mean reciprocal titers of 18,552 and 99,862, respectively) were found in all vaccinates up to 2 yr postvaccination, whereas seven control animals remained antibody negative throughout the same time period.

Multiple asparagine deamidation of Bacillus anthracis protective antigen causes charge isoforms whose complexity correlates with reduced biological activity.

Protective antigen is essential for the pathology of Bacillus anthracis and is the proposed immunogen for an improved human anthrax vaccine. Known since discovery to comprise differentially charged isoforms, the cause of heterogeneity has eluded specific structural definition until now. Recombinant protective antigen (rPA) contains similar isoforms that appear early in fermentation and are mostly removed through purification. By liquid chromatography-tandem mass spectrometry sequencing of the entire protein and inspection of spectral data for amino acid modifications, pharmaceutical rPA contained measurable deamidation at seven of its 68 asparagine residues. A direct association between isoform complexity and percent deamidation was observed such that each decreased with purity and increased with protein aging. Position N537 consistently showed the highest level of modification, although its predicted rate of deamidation ranked 10th by theoretical calculation, and other asparagines of higher predicted rates were observed to be unmodified. rPA with more isoforms and greater deamidation displayed lower activities for furin cleavage, heptamerization, and holotoxin formation. Lethal factor-mediated macrophage toxicity correlated inversely with deamidation at residues N466 and N408. The described method measures deamidation without employing theoretical isotopic distributions, comparison between differentially treated samples or computational predictions of reactivity rates, and is broadly applicable to the characterization of other deamidated proteins.

Design and testing for a nontagged F1-V fusion protein as vaccine antigen against bubonic and pneumonic plague.

A two-component recombinant fusion protein antigen was re-engineered and tested as a medical counter measure against the possible biological threat of aerosolized Yersinia pestis. The active component of the proposed subunit vaccine combines the F1 capsular protein and V virulence antigen of Y. pestis and improves upon the design of an earlier histidine-tagged fusion protein. In the current study, different production strains were screened for suitable expression and a purification process was optimized to isolate an F1-V fusion protein absent extraneous coding sequences. Soluble F1-V protein was isolated to 99% purity by sequential liquid chromatography including capture and refolding of urea-denatured protein via anion exchange, followed by hydrophobic interaction, concentration, and then transfer into buffered saline for direct use after frozen storage. Protein identity and primary structure were verified by mass spectrometry and Edman sequencing, confirming a purified product of 477 amino acids and removal of the N-terminal methionine. Purity, quality, and higher-order structure were compared between lots using RP-HPLC, intrinsic fluorescence, CD spectroscopy, and multi-angle light scattering spectroscopy, all of which indicated a consistent and properly folded product. As formulated with aluminum hydroxide adjuvant and administered in a single subcutaneous dose, this new F1-V protein also protected mice from wild-type and non-encapsulated Y. pestis challenge strains, modeling prophylaxis against pneumonic and bubonic plague. These findings confirm that the fusion protein architecture provides superior protection over the former licensed product, establish a foundation from which to create a robust production process, and set forth assays for the development of F1-V as the active pharmaceutical ingredient of the next plague vaccine.

Purification and protective efficacy of monomeric and modified Yersinia pestis capsular F1-V antigen fusion proteins for vaccination against plague.

The F1-V vaccine antigen, protective against Yersinia pestis, exhibits a strong tendency to multimerize that affects larger-scale manufacture and characterization. In this work, the sole F1-V cysteine was replaced with serine by site-directed mutagenesis for characterization of F1-V non-covalent multimer interactions and protective potency without participation by disulfide-linkages. F1-V and F1-V(C424S) proteins were overexpressed in Escherichia coli, recovered using mechanical lysis/pH-modulation and purified from urea-solubilized soft inclusion bodies, using successive ion-exchange, ceramic hydroxyapatite, and size-exclusion chromatography. This purification method resulted in up to 2mg/g of cell paste of 95% pure, mono-disperse protein having < or =0.5 endotoxin units per mg by a kinetic chromogenic limulus amoebocyte lysate reactivity assay. Both F1-V and F1-V(C424S) were monomeric at pH 10.0 and progressively self-associated as pH conditions decreased to pH 6.0. Solution additives were screened for their ability to inhibit F1-V self-association at pH 6.5. An L-arginine buffer provided the greatest stabilizing effect. Conversion to >500-kDa multimers occurred between pH 6.0 and 5.0. Conditions for efficient F1-V adsorption to the cGMP-compatible alhydrogel adjuvant were optimized. Side-by-side evaluation for protective potency against subcutaneous plague infection in mice was conducted for F1-V(C424S) monomer; cysteine-capped F1-V monomer; cysteine-capped F1-V multimer; and a F1-V standard reported previously. After a two-dose vaccination with 2 x 20 microg of F1-V, respectively, 100%, 80%, 80%, and 70% of injected mice survived a subcutaneous lethal plague challenge with 10(8) LD(50)Y. pestis CO92. Thus, vaccination with F1-V monomer and multimeric forms resulted in significant, and essentially equivalent, protection.