Improving the stability of recombinant anthrax protective antigen vaccine

Single Low-Dose Nanovaccine for Long-Term Protection against Anthrax Toxins

Anthrax infections caused by Bacillus anthracis are an ongoing bioterrorism and livestock threat worldwide. Current approaches for management, including extended passive antibody transfusion, antibiotics, and prophylactic vaccination, are often cumbersome and associated with low patient compliance. Here, we report on the development of an adjuvanted nanotoxoid vaccine based on macrophage membrane-coated nanoparticles bound with anthrax toxins. This design leverages the natural binding interaction of protective antigen, a key anthrax toxin, with macrophages. In a murine model, a single low-dose vaccination with the nanotoxoids generates long-lasting immunity that protects against subsequent challenge with anthrax toxins. Overall, this work provides a new approach to address the ongoing threat of anthrax outbreaks and bioterrorism by taking advantage of an emerging biomimetic nanotechnology.

Designing Stable Bacillus anthracis Antigens with a View to Recombinant Anthrax Vaccine Development

Anthrax is a disease caused by Bacillus anthracis that affects mammals, including humans. Recombinant B. anthracis protective antigen (rPA) is the most common basis for modern anthrax vaccine candidates. However, this protein is characterised by low stability due to proteolysis and deamidation. Here, for the first time, two modification variants leading to full-size rPA stabilisation have been implemented simultaneously, through deamidation-prone asparagine residues substitution and by inactivation of proteolysis sites. Obtained modified rPA (rPA83m) has been demonstrated to be stable in various temperature conditions. Additionally, rPA1+2 containing PA domains I and II and rPA3+4 containing domains III and IV, including the same modifications, have been shown to be stable as well. These antigens can serve as the basis for a vaccine, since the protective properties of PA can be attributed to individual PA domains. The stability of each of three modified anthrax antigens has been considerably improved in compositions with tobacco mosaic virus-based spherical particles (SPs). rPA1+2/rPA3+4/rPA83m in compositions with SPs have maintained their antigenic specificity even after 40 days of incubation at +37 °C. Considering previously proven adjuvant properties and safety of SPs, their compositions with rPA83m/rPA1+2/rPA3+4 in any combinations might be suitable as a basis for new-generation anthrax vaccines.

Role of site-directed mutagenesis and adjuvants in the stability and potency of anthrax protective antigen

Anthrax is a zoonotic infection caused by the gram-positive, aerobic, spore-forming bacterium Bacillus anthracis. Depending on the origin of the infection, serious health problems or mortality is possible. The virulence of B. anthracis is reliant on three pathogenic factors, which are secreted upon infection: protective antigen (PA), lethal factor (LF), and edema factor (EF). Systemic illness results from LF and EF entering cells through the formation of a complex with the heptameric form of PA, bound to the membrane of infected cells through its receptor. The currently available anthrax vaccines have multiple drawbacks, and recombinant PA is considered a promising second-generation vaccine candidate. However, the inherent chemical instability of PA through Asn deamidation at multiple sites prevents its use after long-term storage owing to loss of potency. Moreover, there is a distinct possibility of B. anthracis being used as a bioweapon; thus, the developed vaccine should remain efficacious and stable over the long-term. Second-generation anthrax vaccines with appropriate adjuvant formulations for enhanced immunogenicity and safety are desired. In this article, using protein engineering approaches, we have reviewed the stabilization of anthrax vaccine candidates that are currently licensed or under preclinical and clinical trials. We have also proposed a formulation to enhance recombinant PA vaccine potency via adjuvant formulation.

Deciphering deamidation and isomerization in therapeutic proteins: Effect of neighboring residue

Deamidation of asparagine (Asn) and isomerization of aspartic acid (Asp) residues are among the most commonly observed spontaneous post-translational modifications (PTMs) in proteins. Understanding and predicting a protein sequence's propensity for such PTMs can help expedite protein therapeutic discovery and development. In this study, we used proton-affinity calculations with semi-empirical quantum mechanics and microsecond long equilibrium molecular dynamics simulations to investigate mechanistic roles of structural conformation and chemical environment in dictating spontaneous degradation of Asn and Asp residues in 131 clinical-stage therapeutic antibodies. Backbone secondary structure, side-chain rotamer conformation and solvent accessibility were found to be key molecular indicators of Asp isomerization and Asn deamidation. Comparative analysis of backbone dihedral angles along with N-H proton affinity calculations provides a mechanistic explanation for the strong influence of the identity of the n + 1 residue on the rate of Asn/Asp degradation. With these findings, we propose a minimalistic physics-based classification model that can be leveraged to predict deamidation and isomerization propensity of proteins.

Viability evaluation of freeze dried and suspension anthrax spore vaccine formulations stored at different temperatures

Anthrax is endemic in Ethiopia with sporadic outbreaks despite the regular vaccination of domestic livestock. This has raised concerns on the effectiveness of the vaccination strategy which may be associated with breaches in the vaccine cold chain maintenance. This study was aimed at demonstrating the tolerance of anthrax vaccine to cold chain breaches through evaluation of viable spore counts expressed as colony forming units per mL (CFU/mL) of freeze-dried and suspension anthrax vaccines stored at 5 °C, 20 °C and 37 °C for up to 6 months. Both vaccine formulations maintained above the recommended minimum required titre (2 × 10⁶ culturable spores per dose for cattle, buffaloes and horses, and not <1 × 10⁶ for sheep and goats) for up to 6 months at 5 °C storage. In storage at 20 °C, the viability of freeze-dried anthrax vaccine maintained the minimum required titre up to 6 months while up to 90 days in case of the suspension formulation. Both types of vaccine formulations maintained the minimum titre per dose for up to 30 days at 37 °C storage. Generally, both vaccine formulations showed similar trends in titre fall in all of the three storage temperatures (5 °C, 20 °C and 37 °C) as observed in the almost linearly overlapping 95% confidence intervals (CI) up to day 90 at 5 °C and 20 °C storages while up to day 30 at 37 °C storage. However, a significant (P < 0.05) drop in titre was observed after day 90 for storages at 5 °C and 20 °C, and after day 30 for 37 °C storage as observed in the non overlapping 95% CI from the average titres of previous time points. This study showed that if temperature excursion occurs above the recommended temperature range (4-8 °C) during storage or transport, the vaccine should remain effective and can still be used in vaccination programs.

Single-dose combination nanovaccine induces both rapid and durable humoral immunity and toxin neutralizing antibody responses against Bacillus anthracis

Bacillus anthracis, the causative agent of anthrax, continues to be a prominent biological warfare and bioterrorism threat. Vaccination is likely to remain the most effective and user-friendly public health measure to counter this threat in the foreseeable future. The commercially available AVA BioThrax vaccine has a number of shortcomings where improvement would lead to a more practical and effective vaccine for use in the case of an exposure event. Identification of more effective adjuvants and novel delivery platforms is necessary to improve not only the effectiveness of the anthrax vaccine, but also enhance its shelf stability and ease-of-use. Polyanhydride particles have proven to be an effective platform at adjuvanting the vaccine-associated adaptive immune response as well as enhancing stability of encapsulated antigens. Another class of adjuvants, the STING pathway-targeting cyclic dinucleotides, have proven to be uniquely effective at inducing a beneficial inflammatory response that leads to the rapid induction of high titer antibodies post-vaccination capable of providing protection against bacterial pathogens. In this work, we evaluate the individual contributions of cyclic di-GMP (CDG), polyanhydride nanoparticles, and a combination thereof towards inducing neutralizing antibody (nAb) against the secreted protective antigen (PA) from B. anthracis. Our results show that the combination nanovaccine elicited rapid, high titer, and neutralizing IgG anti-PA antibody following single dose immunization that persisted for at least 108 DPI.

Stability of lyophilized and spray dried vaccine formulations

Liquid formulations of vaccines are subject to instabilities that result from degradation processes that proceed via a variety of physical and chemical pathways. In dried formulations, such as those prepared by lyophilization or spray drying, many of these degradation pathways may be avoided or inhibited. Thus, the stability of vaccine formulations can be enhanced significantly in the absence of bulk water. Potential advantages of dry vaccine formulations include extended shelf lives and less stringent cold-chain storage requirements, both of which offer possibilities of reduced vaccine wastage and facilitated distribution to resource-poor areas. Lyophilization and spray drying represent the most common methods of stabilizing vaccines through drying. This article reviews several lyophilized and spray dried vaccines that address a diverse set of pathogens, as well as some of the assays used to quantify their stability. Recent dry vaccine trends include needle-free delivery of dry powder via non-parenteral routes of administration and the incorporation of advanced vaccine adjuvants into formulations, which further contribute to the goal of increasing vaccine distribution to resource-poor areas. Challenges associated with development of these newer technologies are also discussed.

Ndumnego O, Ellis C, Buys A, Beyer W, van Heerden H. Immunogenicity of Non-Living Anthrax Vaccine Candidates in Cattle and Protective Efficacy of Immune Sera in A/J Mouse Model Compared to the Sterne Live Spore Vaccine

The Sterne live spore vaccine (SLSV, Bacillus anthracis strain 34F2) is the veterinary vaccine of choice against anthrax though contra-indicated for use with antimicrobials. However, the use of non-living anthrax vaccine (NLAV) candidates can overcome the SLSV limitation. In this study, cattle were vaccinated with either of the NLAV (purified recombinant PA (PrPA) or crude rPA (CrPA) and formaldehyde-inactivated spores (FIS of B. anthracis strain 34F2) and emulsigen-D^®/alhydrogel^® adjuvants) or SLSV. The immunogenicity of the NLAV and SLSV was assessed and the protective efficacies evaluated using a passive immunization mouse model. Polyclonal IgG (including the IgG1 subset) and IgM responses increased significantly across all vaccination groups after the first vaccination. Individual IgG subsets titres peaked significantly with all vaccines used after the second vaccination at week 5 and remained significant at week 12 when compared to week 0. The toxin neutralization (TNA) titres of the NLAV vaccinated cattle groups showed similar trends to those observed with the ELISA titres, except that the former were lower, but still significant, when compared to week 0. The opsonophagocytic assay indicated good antibody opsonizing responses with 75% (PrPA+FIS), 66% (CrPA+FIS) and 80% (SLSV) phagocytosis following spores opsonization. In the passive protection test, A/J mice transfused with purified IgG from cattle vaccinated with PrPA+FIS+Emulsigen-D^®/Alhydrogel^® and SLSV had 73% and 75% protection from challenge with B. anthracis strain 34F2 spores, respectively, whereas IgG from cattle vaccinated with CrPA+FIS+Emulsigen-D^®/Alhydrogel^® offered insignificant protection of 20%. There was no difference in protective immune response in cattle vaccinated twice with either the PrPA+FIS or SLSV. Moreover, PrPA+FIS did not show any residual side effects in vaccinated cattle. These results suggest that the immunogenicity and protective efficacy induced by the NLAV (PrPA+FIS) in the cattle and passive mouse protection test, respectively, are comparable to that induced by the standard SLSV.

Two approaches for the stabilization of Bacillus anthracis recombinant protective antigen

Anthrax is a zoonotic disease caused by the gram-positive spore-forming bacteria Bacillus anthracis. There is a need for safe, highly effective, long-term storage vaccine formulations for mass vaccination. However, the development of new subunit vaccines based on recombinant protective antigen (rPA) faces the problem of vaccine antigen instability. Here, the potential of simultaneous application of two different approaches to stabilize rPA was demonstrated. Firstly, we employed spherical particles (SPs) obtained from the tobacco mosaic virus (TMV). Previously, we had reported that SPs can serve as an adjuvant and platform for antigen presentation. In the current work, SPs were shown to increase the stability of the full-size rPA without loss of its antigenic properties. The second direction was site-specific mutagenesis of asparagine residues to avoid deamidation that causes partial protein degradation. The modified recombinant protein comprising the PA immunogenic domains 3 and 4 (rPA3 + 4) was stable during storage at 4 and 25°C. rPA3 + 4 interacts with antibodies to rPA83 both individually and as a part of a complex with SPs. The results obtained can underpin the development of a recombinant vaccine with a full-size modified rPA (with similar amino acid substitutions that stabilize the protein) and SPs.

Polysaccharide PCP-I isolated from Poria cocos enhances the immunogenicity and protection of an anthrax protective antigen-based vaccine

Polysaccharides isolated from natural plants may represent a novel source of vaccine adjuvants. In this research, we focused on a natural plant polysaccharide, PCP-I, which is derived from Poria cocos, a Chinese traditional herbal medicine. We chose the anthrax protective antigen (PA) as a model to evaluate the adjuvant ability of PCP-I in enhancing the immunogenicity and protection of a PA-based anthrax vaccine. According to our results, PCP-I could significantly enhance anthrax specific anti-PA antibodies, toxin-neutralizing antibodies, anti-PA antibody affinity, as well as IgG1 and IgG2a levels. Besides, PCP-I increased the frequency of PA-specific memory B cells, increased the proliferation of PA-specific splenocytes, significantly stimulated the secretion of IL-4, and enhanced the activation of Dendritic cells (DCs) in vitro. The combination of PCP-I and CpG significantly enhanced the level of anti-PA antibodies and neutralizing antibodies, particularly PA-specific IgG2a, and shifted the Th2-bias to a Th1/Th2 balanced response. In addition, PCP-I with or without CpG could significantly improve the survival rate of immunized mice following challenge with the anthrax lethal toxin. These findings suggest that PCP-I may be a promising vaccine adjuvant that warrants further study.

Effect of Aluminum Adjuvant and Preservatives on Structural Integrity and Physicochemical Stability Profiles of Three Recombinant Subunit Rotavirus Vaccine Antigens

A nonreplicating rotavirus vaccine (NRRV) containing 3 recombinant fusion proteins adsorbed to aluminum adjuvant (Alhydrogel [AH]) is currently in clinical trials. The compatibility and stability of monovalent NRRV antigen with key components of a multidose vaccine formulation were examined using physicochemical and immunochemical methods. The extent and strength of antigen-adjuvant binding were diminished by increasing phosphate concentration, and acceptable levels were identified along with alternate buffering agents. Addition of the preservative thimerosal destabilized AH-adsorbed P2-VP8-P[8] as measured by differential scanning calorimetry. Over 3 months at 4°C, AH-adsorbed P2-VP8-P[8] was stable, whereas at 25°C and 37°C, instability was observed which was greatly accelerated by thimerosal addition. Loss of antibody binding (enzyme-linked immunosorbent assay) correlated with loss of structural integrity (differential scanning calorimetry, fluorescence spectroscopy) with concomitant nonnative disulfide bond formation (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and Asn deamidation (liquid chromatography -mass spectrometry peptide mapping). An alternative preservative (2-phenoxyethanol) showed similar antigen destabilization. Due to limited availability, only key assays were performed with monovalent P2-VP8-P[4] and P2-VP8-P[6] AH-adsorbed antigens, and varying levels of preservative incompatibility were observed. In summary, monovalent AH-adsorbed NRRV antigens stored at 4°C showed good stability without preservatives; however, future formulation development efforts are required to prepare a stable, preservative-containing, multidose NRRV formulation.

Machine Learning Enables Accurate Prediction of Asparagine Deamidation Probability and Rate

The spontaneous conversion of asparagine residues to aspartic acid or iso-aspartic acid, via deamidation, is a major pathway of protein degradation and is often seriously disruptive to biological systems. Deamidation has been shown to negatively affect both in vitro stability and in vivo biological function of diverse classes of proteins. During protein therapeutics development, deamidation liabilities that are overlooked necessitate expensive and time-consuming remediation strategies, sometimes leading to termination of the project. In this paper, we apply machine learning to a large (n = 776) liquid chromatography-tandem mass spectrometry (LC-MS/MS) dataset of monoclonal antibody peptides to create computational models for the post-translational modification asparagine deamidation, using the random decision forest method. We show that our categorical model predicts antibody deamidation with nearly 5% increased accuracy and 0.2 MCC over the best currently available models. Surprisingly, our model also paces or outperforms advanced and conventional models on an independent non-antibody dataset. In addition to deamidation probability, we are able to accurately predict deamidation rate (R² = 0.963 and Q2 = 0.822), a capability with no peer in current models. This method should enable significant improvement in protein candidate selection, especially in biopharmaceutical development, and can be applied with similar accuracy to enzymes, monoclonal antibodies, next-generation formats, vaccine component antigens, and gene therapy vectors such as adeno-associated virus.

Vaccines against anthrax based on recombinant protective antigen: problems and solutions

Introduction: Anthrax is a dangerous bio-terror agent because Bacillus anthracis spores are highly resilient and can be easily aerosolized and disseminated. There is a threat of deliberate use of anthrax spores aerosol that could lead to serious fatal diseases outbreaks. Existing control measures against inhalation form of the disease are limited. All of this has provided an impetus to the development of new generation vaccines. Areas сovered: This review is devoted to challenges and achievements in the design of vaccines based on the anthrax recombinant protective antigen (rPA). Scientific databases have been searched, focusing on causes of PA instability and solutions to this problem, including new approaches of rPA expression, novel rPA-based vaccines formulations as well as the simultaneous usage of PA with other anthrax antigens. Expert opinion: PA is a central anthrax toxin component, playing a key role in the defense against encapsulated and unencapsulated strains. Subunit rPA-based vaccines have a good safety and protective profile. However, there are problems of PA instability that are greatly enhanced when using aluminum adjuvants. New adjuvant compositions, dry formulations and resistant to proteolysis and deamidation mutant PA forms can help to handle this issue. Devising a modern anthrax vaccine requires huge efforts.

Engineering an anti-CD52 antibody for enhanced deamidation stability

Deamidation evaluation and mitigation is an important aspect of therapeutic antibody developability assessment. We investigated the structure and function of the Asn-Gly deamidation in a human anti-CD52 IgG1 antibody light chain complementarity-determining region 1, and risk mitigation through protein engineering. Antigen binding affinity was found to decrease about 400-fold when Asn³³ was replaced with an Asp residue to mimic the deamidation product, suggesting significant impacts on antibody function. Other variants made at Asn³³ (N33H, N33Q, N33H, N33R) were also found to result in significant loss of antigen binding affinity. The co-crystal structure of the antigen-binding fragment bound to a CD52 peptide mimetic was solved at 2.2Å (PDB code 6OBD), which revealed that Asn³³ directly interacts with the CD52 phosphate group via a hydrogen bond. Gly³⁴, but sits away from the binding interface, rendering it more amendable to mutagenesis without affecting affinity. Saturation mutants at Gly³⁴ were prepared and subjected to forced deamidation by incubation at elevated pH and temperature. Three mutants (G34R, G34K and G34Q) showed increased resistance to deamidation by LC-MS peptide mapping, while maintaining high binding affinity to CD52 antigen measured by Biacore. A complement -dependent cytotoxicity assay indicated that these mutants function by triggering antibody effector function. This study illustrates the importance of structure-based design and extensive mutagenesis to mitigate antibody developability issues.

Single dose of DPX-rPA, an enhanced-delivery anthrax vaccine formulation, protects against a lethal Bacillus anthracis spore inhalation challenge

Anthrax is a serious biological threat caused by pulmonary exposure to aerosolized spores of Bacillus anthracis. Biothrax^® (anthrax vaccine adsorbed (AVA)) is the only Food and Drug Administration-licensed vaccine and requires five administrations over 12 months with annual boosting to maintain pre-exposure prophylaxis. Here we report the evaluation of a single intramuscular injection of recombinant B. anthracis-protective antigen (rPA) formulated in the DPX delivery platform. Immune responses were compared to an alum-based formulation in mice and rabbits. Serological analysis of anti-rPA immunoglobulin G and toxin neutralization activity demonstrated higher responses induced by DPX-rPA when compared to rPA in alum. DPX-rPA was compared to AVA in rabbits and non-human primates (NHPs). In both species, DPX-rPA generated responses after a single immunization, whereas AVA required two immunizations. In rabbits, single injection of DPX-rPA or two injections of AVA conferred 100% protection from anthrax challenge. In NHPs, single-dose DPX-rPA was 100% protective against challenge, whereas one animal in the two-dose AVA group and all saline administered animals succumbed to infection. DPX-rPA was minimally reactogenic in all species tested. These data indicate that DPX-rPA may offer improvement over AVA by reducing the doses needed for protective immune responses and is a promising candidate as a new-generation anthrax vaccine.

A lipid-based anthrax vaccine formulation offers immunity from the first injection. Bacillus anthracis is a lethal pathogen at high risk for use in biological warfare. The only FDA-licensed vaccine for anthrax, AVA, requires multiple doses over six months followed by regular boosters, indicating a need for rapidly immunizing vaccines. Genevieve Weir and Lisa MacDonald, from IMV Inc., with Canadian and US collaborators, here describe a prophylactic consisting of B. anthracis antigens suspended in a lipid-in-oil formulation. Their candidate, DPX-rPA, generated antigen-specific antibodies in rabbits and monkeys after one dose, compared to two for AVA. DPX-rPA also protected both species from B. anthracis spores after one dose. The results indicate that single-dose DPX-rPA is equally protective as two doses of AVA and could serve as pre-exposure and post-exposure prophylaxis. Future studies may confirm its potential as a vaccine for humans.