Thermal stability and epitope integrity of a lyophilized ricin toxin subunit vaccine

Left out in the cold - inequity in infectious disease control due to cold chain disparity

Vaccines and diagnostic tools stand out as among the most influential advancements in public health, credited with averting an estimated 6 million deaths annually and substantially alleviating the burden of infectious disease. Despite this progress, the global imperative to prevent, detect, and combat infectious disease persists. Regrettably, hundreds of thousands of lives are still lost due to inadequate access to vaccines and diagnostics. A critical obstacle in accessibility lies in the requirement of reliable cold chain for their transportation and storage, a resource that remains inadequate in many regions, particularly in the developing world. Various factors, including socio-economic disparities, biological complexities, and manufacturing processes, exert significant influence on the availability and integrity of vaccines and diagnostic materials. This review aims to explore the multifaceted issue of inequality in access to disease control tools, examining the vulnerabilities of vaccines and diagnostics while also investigating recent advancements that offer promising solutions to improve thermal stability.

Silk Fibroin-Coated Nano-MOFs Enhance the Thermal Stability and Immunogenicity of HBsAg

Vaccines are widely regarded as one of the most effective weapons in the fight against infectious diseases. Currently, vaccines must be stored and transported at low temperatures as high temperatures can lead to a loss of vaccine conformation and reduced therapeutic efficacy. Metal-organic frameworks (MOFs), such as zeolitic imidazole framework-8 (ZIF-8), are a new class of hybrid materials with large specific surface areas, high loading rates, and good biocompatibility and are successful systems for vaccine delivery and protection. Silk fibroin (SF) has a good biocompatibility and thermal stability. In this study, the hepatitis B surface antigen (HBsAg) was successfully encapsulated in ZIF-8 to form HBsAg@ZIF-8 (HZ) using a one-step shake and one-pot shake method. Subsequently, the SF coating modifies HZ through hydrophobic interactions to form HBsAg/SF@ZIF-8 (HSZ), which enhanced the thermal stability and immunogenicity of HBsAg. Compared to free HBsAg, HZ and HSZ improved the thermostability of HBsAg, promoted the antigen uptake and lysosomal escape, stimulated dendritic cell maturation and cytokine secretion, formed an antigen reservoir to promote antibody production, and activated CD4+ T and CD8+ T cells to enhance memory T-cell production. Importantly, HSZ induced a strong immune response even after 14 days of storage at 25 °C. Furthermore, the nanoparticles prepared by the one-step shake method exhibited superior properties compared to those prepared by the one-pot shake method. This study highlights the importance of SF-coated ZIF-8, which holds promise for investigating thermostable vaccines and breaking the vaccine cold chain.

Stressed stability and protective efficacy of lead lyophilized formulations of ID93+GLA-SE tuberculosis vaccine

With the recent exception of coronavirus disease 2019 (COVID-19), tuberculosis (TB) causes more deaths globally than any other infectious disease, and approximately 1/3 of the world's population is infected with Mycobacterium tuberculosis (Mtb). However, encouraging progress in TB vaccine development has been reported, with approximately 50% efficacy achieved in Phase 2b clinical testing of an adjuvanted subunit TB vaccine candidate. Nevertheless, current lead vaccine candidates require cold-chain transportation and storage. In addition to temperature stress, vaccines may be subject to several other stresses during storage and transport, including mechanical, photochemical, and oxidative stresses. Optimal formulations should enable vaccine configurations with enhanced stability and decreased sensitivity to physical and chemical stresses, thus reducing reliance on the cold chain and facilitating easier worldwide distribution. In this report, we describe the physicochemical stability performance of three lead thermostable formulations of the ID93 + GLA-SE TB vaccine candidate under various stress conditions. Moreover, we evaluate the impact of thermal stress on the protective efficacy of the vaccine formulations. We find that formulation composition impacts stressed stability performance, and our comprehensive evaluation enables selection of a lead single-vial lyophilized candidate containing the excipient trehalose and Tris buffer for advanced development.

Safety and immunogenicity of a thermostable ID93 + GLA-SE tuberculosis vaccine candidate in healthy adults

Adjuvant-containing subunit vaccines represent a promising approach for protection against tuberculosis (TB), but current candidates require refrigerated storage. Here we present results from a randomized, double-blinded Phase 1 clinical trial (NCT03722472) evaluating the safety, tolerability, and immunogenicity of a thermostable lyophilized single-vial presentation of the ID93 + GLA-SE vaccine candidate compared to the non-thermostable two-vial vaccine presentation in healthy adults. Participants were monitored for primary, secondary, and exploratory endpoints following intramuscular administration of two vaccine doses 56 days apart. Primary endpoints included local and systemic reactogenicity and adverse events. Secondary endpoints included antigen-specific antibody (IgG) and cellular immune responses (cytokine-producing peripheral blood mononuclear cells and T cells). Both vaccine presentations are safe and well tolerated and elicit robust antigen-specific serum antibody and Th1-type cellular immune responses. Compared to the non-thermostable presentation, the thermostable vaccine formulation generates greater serum antibody responses (p < 0.05) and more antibody-secreting cells (p < 0.05). In this work, we show the thermostable ID93 + GLA-SE vaccine candidate is safe and immunogenic in healthy adults.

Serum antibody profiling identifies vaccine-induced correlates of protection against aerosolized ricin toxin in rhesus macaques

Inhalation of the biothreat agent, ricin toxin (RT), provokes a localized inflammatory response associated with pulmonary congestion, edema, neutrophil infiltration, and severe acute respiratory distress. The extreme toxicity of RT is the result of the toxin's B chain (RTB) promoting rapid uptake into alveolar macrophages and lung epithelial cells, coupled with the A chain's (RTA) potent ribosome-inactivating properties. We previously reported that intramuscular vaccination of rhesus macaques with a lyophilized, alum-adsorbed recombinant RTA subunit vaccine (RiVax®) was sufficient to confer protection against a lethal dose of aerosolized RT. That study implicated RT-specific serum IgG, toxin-neutralizing activity (TNA), and epitope-specific responses as being associated with immunity. However, it was not possible to define actual correlates of protection (COP) because all vaccinated animals survived the RT challenge. We addressed the issue of COP in the current study, by vaccinating groups of rhesus macaques with RiVax® following the previously determined protective regimen (100 µg on study days 0, 30 and 60) or one of two anticipated suboptimal regimens (100 µg on study days 30 and 60; 35 µg on study days 0, 30, and 60). Two unvaccinated animals served as controls. The animals were challenged with ~5 × LD50s of aerosolized RT on study day 110. We report that all vaccinated animals seroconverted prior to RT challenge, with the majority also having measurable TNA, although neither antibody levels nor TNA reached statistical significance with regard to a correlation with protection. By contrast, survival correlated with pre-challenge, epitope-specific serum IgG levels, derived from a competitive sandwich ELISA using a panel of toxin-neutralizing monoclonal antibodies directed against distinct epitopes on RiVax®. The identification of a species-neutral, competitive ELISA that correlates with vaccine-induced protection against RT in nonhuman represents an important advance in the development of medical countermeasures (MCM) against a persistent biothreat.

Estimating Vaccine Potency Using Antibody-Based Competition Assays

The issues of vaccine potency and stability constitute formidable challenges associated with the development and readiness of vaccines for biodefense. In most instances, the vaccines will be stockpiled (at considerable cost) for years and used only in the rare event of a public health emergency. It is therefore imperative that there be means to readily monitor overall stability of the stockpiled vaccines, preferably using reliable in vitro assays, without the need for expensive and labor-intensive animal studies. In this chapter, we describe an in vitro monoclonal antibody-based competition ELISA known as RiCoE for assessing the potency of a ricin toxin subunit vaccine. RiCoE can be applied to drug substance and drug products adsorbed to aluminum salts adjuvant. While RiCoE is specific for ricin toxin, the general methodologies and protocols described herein are amenable to virtually any subunit or even virus-like particle-based vaccine. Ultimately, RiCoE-like assays may replace or at least reduce the need for animal studies in vaccine potency determinations.

Durable Immunity to Ricin Toxin Elicited by a Thermostable, Lyophilized Subunit Vaccine

The development of vaccines against biothreat toxins like ricin (RT) is considered an integral component of the U.S. national security efforts. RiVax is a thermostable, lyophilized RT subunit vaccine adsorbed to aluminum salt adjuvant intended for use by military personnel and first responders. Phase 1 studies indicated that RiVax is safe and immunogenic, while a three-dose intramuscular vaccination regimen in nonhuman primates elicited protection against lethal dose RT challenge by aerosol. Here, we investigated, in a mouse model, the durability of RiVax-induced antibody responses and corresponding immunity to lethal dose RT challenge. Groups of mice were subcutaneously administered 3 or 1 μg of RiVax on days 0 and 21 and challenged with 10× 50% lethal dose (LD50) RT by injection at six different intervals over the course of 12 months. Serum antibody titers and epitope-specific competition assays were determined prior to each challenge. We report that the two-dose, 3-μg regimen conferred near-complete protection against RT challenge on day 35 and complete protection thereafter (challenge days 65, 95, 125, 245, and 365). The two-dose, 3-μg regimen was superior to the 1-μg regimen as revealed by slight differences in survival and morbidity scores (e.g., hypoglycemia, weight loss) on challenge days 35 and 365. In separate experiments, a single 3-μg RiVax vaccination proved only marginally effective at eliciting protective immunity to RT, underscoring the necessity of a prime-boost regimen to achieve full and long-lasting protection against RT. IMPORTANCE Ricin toxin (RT) is a notorious biothreat, as exposure to even trace amounts via injection or inhalation can induce organ failure and death within a matter of hours. In this study, we advance the preclinical testing of a candidate RT vaccine known as RiVax. RiVax is a recombinant nontoxic derivative of RT's enzymatic subunit that has been evaluated for safety in phase I clinical trials and efficacy in a variety of animal models. We demonstrate that two doses of RiVax are sufficient to protect mice from lethal dose RT challenge for up to 1 year. We describe kinetics and other immune parameters of the antibody response to RiVax and discuss how these immune factors may translate to humans.

Recombinant protein subunit SARS-CoV-2 vaccines formulated with CoVaccine HT adjuvant induce broad, Th1 biased, humoral and cellular immune responses in mice

The speed at which several COVID-19 vaccines went from conception to receiving FDA and EMA approval for emergency use is an achievement unrivaled in the history of vaccine development. Mass vaccination efforts using the highly effective vaccines are currently underway to generate sufficient herd immunity and reduce transmission of the SARS-CoV-2 virus. Despite the most advanced vaccine technology, global recipient coverage, especially in resource-poor areas remains a challenge as genetic drift in naïve population pockets threatens overall vaccine efficacy. In this study, we described the production of insect-cell expressed SARS-CoV-2 spike protein ectodomain constructs and examined their immunogenicity in mice. We demonstrated that, when formulated with CoVaccine HTTM adjuvant, an oil-in-water nanoemulsion compatible with lyophilization, our vaccine candidates elicit a broad-spectrum IgG response, high neutralizing antibody (NtAb) titers against SARS-CoV-2 prototype and variants of concern, specifically B.1.351 (Beta) and P.1. (Gamma), and an antigen-specific IFN-γ secreting response in outbred mice. Of note, different ectodomain constructs yielded variations in NtAb titers against the prototype strain and some VOC. Dose response experiments indicated that NtAb titers increased with antigen dose, but not adjuvant dose, and may be higher with a lower adjuvant dose. Our findings lay the immunological foundation for the development of a dry-thermostabilized vaccine that is deployable without refrigeration.

Single-vial filovirus glycoprotein vaccines: Biophysical characteristics and immunogenicity after co-lyophilization with adjuvant

Zaire ebolavirus (EBOV), Sudan ebolavirus (SUDV), and Marburg marburgvirus (MARV) are the most prevalent and pathogenic species of filovirus. Previously, we showed that glycoprotein antigens from each virus could be lyophilized to create thermostable monovalent subunit vaccines. However, cross-protection is not expected from the monovalent vaccines and therefore developing a trivalent filovirus vaccine would be desirable. Subunit protein vaccines often require the addition of an adjuvant to sufficiently boost the immunogenicity. Typically, liquid suspensions or emulsions of adjuvants and lyophilized antigens are stored in separate vials to avoid destabilizing interactions and are only mixed immediately before administration. Herein, we describe the development and characterization of monovalent and trivalent filovirus vaccines that are co-lyophilized with a squalane-in-water emulsion adjuvant. We found that the single-vial presentation retained adjuvant particle diameter and zeta potential after lyophilization and reconstitution. Furthermore, the trivalent vaccines elicited high antibody levels against all three antigens in mice and non-human primates. These results advance the prospect of developing a single-vial trivalent filovirus vaccine, which would enable easier distribution and administration of the vaccine to resource-poor areas.

Development of thermostable vaccine adjuvants

INTRODUCTION

The importance of vaccine thermostability has been discussed in the literature. Nevertheless, the challenge of developing thermostable vaccine adjuvants has sometimes not received appropriate emphasis. Adjuvants comprise an expansive range of particulate and molecular compositions, requiring innovative thermostable formulation and process development approaches.

AREAS COVERED

Reports on efforts to develop thermostable adjuvant-containing vaccines have increased in recent years, and substantial progress has been made in enhancing the stability of the major classes of adjuvants. This narrative review summarizes the current status of thermostable vaccine adjuvant development and looks forward to the next potential developments in the field.

EXPERT OPINION

As adjuvant-containing vaccines become more widely used, the unique challenges associated with developing thermostable adjuvant formulations merit increased attention. In particular, more focused efforts are needed to translate promising proof-of-concept technologies and formulations into clinical products.

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.

Recombinant protein subunit SARS-CoV-2 vaccines formulated with CoVaccine HT adjuvant induce broad, Th1 biased, humoral and cellular immune responses in mice

The speed at which several COVID-19 vaccines went from conception to receiving FDA and EMA approval for emergency use is an achievement unrivaled in the history of vaccine development. Mass vaccination efforts using the highly effective vaccines are currently underway to generate sufficient herd immunity and reduce transmission of the SARS-CoV-2 virus. Despite the most advanced vaccine technology, global recipient coverage, especially in resource-poor areas remains a challenge as genetic drift in naïve population pockets threatens overall vaccine efficacy. In this study, we described the production of insect-cell expressed SARS-CoV-2 spike protein ectodomain and examined its immunogenicity in mice. We demonstrated that, when formulated with CoVaccine HT™adjuvant, an oil-in-water nanoemulsion compatible with lyophilization, our vaccine candidates elicit a broad-spectrum IgG response, high neutralizing antibody titers, and a robust, antigen-specific IFN-γ secreting response from immune splenocytes in outbred mice. Our findings lay the foundation for the development of a dry-thermostabilized vaccine that is deployable without refrigeration.

Potency determination of ricin toxin using a monoclonal antibody-based competition assay

Mouse challenge studies with death as an endpoint remain the gold standard in assessing the potency of ricin toxin, a Category B biothreat agent derived from the castor bean (Ricinus communis). However, animal studies are expensive, time consuming and ethically concerning. In an effort to reduce reliance on animals in vaccine development, we developed a monoclonal antibody (MAb)-based ricin competition ELISA (RiCoE) that indicates conformation integrity of ricin toxin. In forced degradation (heat-denaturation) experiments with native ricin holotoxin, we demonstrate a correlation between the decline in MAb reactivity in RiCoE and a corresponding loss of toxin potency in Vero cells (IC₅₀) and mice (LD₅₀). The RiCoE assay was applied to differentially sourced commercial lots of ricin toxin derived from R. communis blends and compared to toxin potency in mice. There was near perfect congruence between RiCoE values with two different MAbs (PB10, SyH7) and ricin potency in the mouse model using morbidity as an endpoint. In conclusion, we propose that RiCoE can serve as a rapid and sensitive substitute to mouse lethal dose challenge studies as a means to determine ricin toxin potency and will be valuable at various stages of vaccine development.

Endpoint and epitope-specific antibody responses as correlates of vaccine-mediated protection of mice against ricin toxin

The successful licensure of vaccines for biodefense is contingent upon the availability of well-established correlates of protection (CoP) in at least two animal species that can be applied to humans, without the need to assess efficacy in the clinic. In this report we describe a multivariate model that combines pre-challenge serum antibody endpoint titers (EPT) and values derived from an epitope profiling immune-competition capture (EPICC) assay as a predictor in mice of vaccine-mediated immunity against ricin toxin (RT), a Category B biothreat. EPICC is a modified competition ELISA in which serum samples from vaccinated mice were assessed for their ability to inhibit the capture of soluble, biotinylated (b)-RT by a panel of immobilized monoclonal antibodies (mAbs) directed against four immunodominant toxin-neutralizing regions on the enzymatic A chain (RTA) of RT. In a test cohort of mice (n = 40) vaccinated with suboptimal doses of the RTA subunit vaccine, RiVax®, we identified two mAbs, PB10 and SyH7, which had EPICC inhibition values in pre-challenge serum samples that correlated with survival following a challenge with 5 × LD₅₀ of RT administered by intraperitoneal (IP) injection. Analysis of a larger cohort of mice (n = 645) revealed that a multivariate model combining endpoint titers and EPICC values for PB10 and SyH7 as predictive variables had significantly higher statistical power than any one of the independent variables alone. Establishing the correlates of vaccine-mediated protection in mice represents an important steppingstone in the development of RiVax® as a medical countermeasure under the United States Food and Drug Administration's "Animal Rule."

Passive immunization with an extended half-life monoclonal antibody protects Rhesus macaques against aerosolized ricin toxin

Inhalation of ricin toxin (RT), a Category B biothreat agent, provokes an acute respiratory distress syndrome marked by pro-inflammatory cytokine and chemokine production, neutrophilic exudate, and pulmonary edema. The severity of RT exposure is attributed to the tropism of the toxin's B subunit (RTB) for alveolar macrophages and airway epithelial cells, coupled with the extraordinarily potent ribosome-inactivating properties of the toxin's enzymatic subunit (RTA). While there are currently no vaccines or treatments approved to prevent RT intoxication, we recently described a humanized anti-RTA IgG1 MAb, huPB10, that was able to rescue non-human primates (NHPs) from lethal dose RT aerosol challenge if administered by intravenous (IV) infusion within hours of toxin exposure. We have now engineered an extended serum half-life variant of that MAb, huPB10-LS, and evaluated it as a pre-exposure prophylactic. Five Rhesus macaques that received a single intravenous infusion (25 mg/kg) of huPB10-LS survived a lethal dose aerosol RT challenge 28 days later, whereas three control animals succumbed to RT intoxication within 48 h. The huPB10-LS treated animals remained clinically normal in the hours and days following toxin insult, suggesting that pre-existing antibody levels were sufficient to neutralize RT locally. Moreover, pro-inflammatory markers in sera and BAL fluids collected 24 h following RT challenge were significantly dampened in huPB10-LS treated animals, as compared to controls. Finally, we found that all five surviving animals, within days after RT exposure, had anti-RT serum IgG titers against epitopes other than huPB10-LS, indicative of active immunization by residual RT and/or RT-immune complexes.

A Collection of Single-Domain Antibodies that Crowd Ricin Toxin's Active Site

In this report, we used hydrogen exchange-mass spectrometry (HX-MS) to identify the epitopes recognized by 21 single-domain camelid antibodies (V_HHs) directed against the ribosome-inactivating subunit (RTA) of ricin toxin, a biothreat agent of concern to military and public health authorities. The V_HHs, which derive from 11 different B-cell lineages, were binned together based on competition ELISAs with IB2, a monoclonal antibody that defines a toxin-neutralizing hotspot ("cluster 3") located in close proximity to RTA's active site. HX-MS analysis revealed that the 21 V_HHs recognized four distinct epitope subclusters (3.1-3.4). Sixteen of the 21 V_HHs grouped within subcluster 3.1 and engage RTA α-helices C and G. Three V_HHs grouped within subcluster 3.2, encompassing a-helices C and G, plus α-helix B. The single V_HH in subcluster 3.3 engaged RTA α-helices B and G, while the epitope of the sole V_HH defining subcluster 3.4 encompassed α-helices C and E, and β-strand h. Modeling these epitopes on the surface of RTA predicts that the 20 V_HHs within subclusters 3.1-3.3 physically occlude RTA's active site cleft, while the single antibody in subcluster 3.4 associates on the active site's upper rim.