Structural characterization of a protein adsorbed on aluminum hydroxide adjuvant in vaccine formulation

Basic Properties and Development Status of Aluminum Adjuvants Used for Vaccines

BACKGROUND

Aluminum adjuvants, renowned for their safety and efficacy, act as excellent adsorbents and vaccine immunogen enhancers, significantly contributing to innate, endogenous, and humoral immunity. An ideal adjuvant not only boosts the immune response but also ensures optimal protective immunity. Aluminum adjuvants are the most widely used vaccine adjuvants and have played a crucial role in both the prevention of existing diseases and the development of new vaccines. With the increasing emergence of new vaccines, traditional immune adjuvants are continually being researched and upgraded. The future of vaccine development lies in the exploration and integration of novel adjuvant technologies that surpass the capabilities of traditional aluminum adjuvants. One promising direction is the incorporation of nanoparticles, which offer precise delivery and controlled release of antigens, thereby enhancing the overall immune response.

CONCLUSIONS

This review summarizes the types, mechanisms, manufacturers, patents, advantages, disadvantages, and future prospects of aluminum adjuvants. Although aluminum adjuvants have certain limitations, their contribution to enhancing vaccine immunity is significant and cannot be ignored. Future research should continue to explore their mechanisms of action and address potential adverse reactions to achieve improved vaccine efficacy.

Combining Solid-State NMR with Structural and Biophysical Techniques to Design Challenging Protein-Drug Conjugates

Several protein-drug conjugates are currently being used in cancer therapy. These conjugates rely on cytotoxic organic compounds that are covalently attached to the carrier proteins or that interact with them via non-covalent interactions. Human transthyretin (TTR), a physiological protein, has already been identified as a possible carrier protein for the delivery of cytotoxic drugs. Here we show the structure-guided development of a new stable cytotoxic molecule based on a known strong binder of TTR and a well-established anticancer drug. This example is used to demonstrate the importance of the integration of multiple biophysical and structural techniques, encompassing microscale thermophoresis, X-ray crystallography and NMR. In particular, we show that solid-state NMR has the ability to reveal effects caused by ligand binding which are more easily relatable to structural and dynamical alterations that impact the stability of macromolecular complexes.

Precise modification of the surface charge of antigen enhances vaccine immunogenicity

Aluminum (alum) adjuvant is the most extensively used protein subunit vaccine adjuvant, and its effectiveness and safety have been widely recognized. The surface charge of the antigen determines its electrostatic adsorption to alum adjuvant, which directly affects the immune efficacy of the protein vaccine. In our study, we precisely modified its surface charge by inserting charged amino acids into the flexible region of the SARS-CoV-2 receptor-binding domain (RBD), achieving electrostatic adsorption and a site-specific anchor between the immunogen and alum adjuvant. This innovative strategy extended the bioavailability of the RBD and directionally displayed the neutralizing epitopes, thereby significantly enhancing humoral and cellular immunity. Furthermore, the required dose of antigen and alum adjuvant was greatly reduced, which improved the safety and accessibility of the protein subunit vaccine. On this basis, the wide applicability of this novel strategy to a series of representative pathogen antigens such as SARS-RBD, MERS-RBD, Mpox-M1, MenB-fHbp, and Tularemia-Tul4 was further confirmed. Charge modification of antigens provides a straightforward approach for antigenicity optimization of alum-adjuvanted vaccines, which has great potential to be adopted as a global defense against infectious diseases.

Aluminum Adjuvants-'Back to the Future'

Aluminum-based adjuvants will continue to be a key component of currently approved and next generation vaccines, including important combination vaccines. The widespread use of aluminum adjuvants is due to their excellent safety profile, which has been established through the use of hundreds of millions of doses in humans over many years. In addition, they are inexpensive, readily available, and are well known and generally accepted by regulatory agencies. Moreover, they offer a very flexible platform, to which many vaccine components can be adsorbed, enabling the preparation of liquid formulations, which typically have a long shelf life under refrigerated conditions. Nevertheless, despite their extensive use, they are perceived as relatively 'weak' vaccine adjuvants. Hence, there have been many attempts to improve their performance, which typically involves co-delivery of immune potentiators, including Toll-like receptor (TLR) agonists. This approach has allowed for the development of improved aluminum adjuvants for inclusion in licensed vaccines against HPV, HBV, and COVID-19, with others likely to follow. This review summarizes the various aluminum salts that are used in vaccines and highlights how they are prepared. We focus on the analytical challenges that remain to allowing the creation of well-characterized formulations, particularly those involving multiple antigens. In addition, we highlight how aluminum is being used to create the next generation of improved adjuvants through the adsorption and delivery of various TLR agonists.

Solid-state NMR methods for the characterization of bioconjugations and protein-material interactions

Protein solid-state NMR has evolved dramatically over the last two decades, with the development of new hardware and sample preparation methodologies. This technique is now ripe for complex applications, among which one can count bioconjugation, protein chemistry and functional biomaterials. In this review, we provide our account on this aspect of protein solid-state NMR.

Epitope Mapping and Binding Assessment by Solid-State NMR Provide a Way for the Development of Biologics under the Quality by Design Paradigm

Multispecific biologics are an emerging class of drugs, in which antibodies and/or proteins designed to bind pharmacological targets are covalently linked or expressed as fusion proteins to increase both therapeutic efficacy and safety. Epitope mapping on the target proteins provides key information to improve the affinity and also to monitor the manufacturing process and drug stability. Solid-state NMR has been here used to identify the pattern of the residues of the programmed cell death ligand 1 (PD-L1) ectodomain that are involved in the interaction with a new multispecific biological drug. This is possible because the large size and the intrinsic flexibility of the complexes are not limiting factors for solid-state NMR.

A Combined Adjuvant TF-Al Consisting of TFPR1 and Aluminum Hydroxide Augments Strong Humoral and Cellular Immune Responses in Both C57BL/6 and BALB/c Mice

TFPR1 is a novel adjuvant for protein and peptide antigens, which has been demonstrated in BALB/c mice in our previous studies; however, its adjuvanticity in mice with different genetic backgrounds remains unknown, and its adjuvanticity needs to be improved to fit the requirements for various vaccines. In this study, we first compared the adjuvanticity of TFPR1 in two commonly used inbred mouse strains, BALB/c and C57BL/6 mice, in vitro and in vivo, and demonstrated that TFPR1 activated TLR2 to exert its immune activity in vivo. Next, to prove the feasibility of TFPR1 acting as a major component of combined adjuvants, we prepared a combined adjuvant, TF-Al, by formulating TFPR1 and alum at a certain ratio and compared its adjuvanticity with that of TFPR1 and alum alone using OVA and recombinant HBsAg as model antigens in both BALB/c and C57BL/6 mice. Results showed that TFPR1 acts as an effective vaccine adjuvant in both BALB/c mice and C57BL/6 mice, and further demonstrated the role of TLR2 in the adjuvanticity of TFPR1 in vivo. In addition, we obtained a novel combined adjuvant, TF-Al, based on TFPR1, which can augment antibody and cellular immune responses in mice with different genetic backgrounds, suggesting its promise for vaccine development in the future.

Interaction of Aluminum-adjuvanted Recombinant P[4] Protein Antigen With Preservatives: Storage Stability and Backbone Flexibility Studies

Eight antimicrobial preservatives used in parenteral multidose formulations (thimerosal, 2-phenoxy ethanol, phenol, benzyl alcohol, m-cresol, chlorobutanol, methyl paraben, propyl paraben) were examined for their effects on the storage stability (4 °C, 25 °C) of an Alhydrogel® (AH) adjuvanted formulation of the non-replicating rotavirus vaccine (NRRV) recombinant P[4] protein antigen. The stability of AH-adsorbed P[4] was monitored for antigen-antibody binding, conformational stability, and antigen-adjuvant interaction via competitive ELISA, DSC, and SDS-PAGE, respectively. There was an unexpected correlation between increasing storage stability of the AH-adsorbed P[4] and preservative hydrophobicity (log P) (e.g., the parabens and chlorobutanol were least destabilizing). We used hydrogen exchange-mass spectrometry (HX-MS) to better understand the destabilizing effects of temperature and preservative on backbone flexibility of AH-adsorbed P[4]. Thimerosal addition immediately increased the backbone flexibility across much of the AH-adsorbed P[4] protein backbone (except the N-terminal P2 region and residues G¹⁷-Y³⁸), and further increase in P[4] backbone flexibility was observed after storage (4 °C, 4 weeks). HX-MS analysis of AH-adsorbed P[4] stored for 4 weeks at 25 °C revealed structural alterations in some regions of the epitope involved in P[4] specific mAb binding. These combined results are discussed in terms of a generalized workflow for multi-dose vaccine formulation development for recombinant protein antigens.

Persistence of vaccine-induced antibody to A/H5N1 influenza after 30 and 36 months of vaccination

Objectives

An A/H5N1 vaccine named IVACFLU-A/ H5N1 was accepted to use in Vietnam, however, antibody persistence after vaccination has not been well characterized yet. We examined the persistence of antibodies after vaccination and related risk factors in individuals enrolled in phase II with 15 mcg dose, 2 injections 21 days apart, IVACFLU-A/ H5N1 vaccine trials in Ninh Hoa, Vietnam.

Methods

We used a longitudinal study to follow 86 participants without control groups. They were tested anti-A/H5N1 IgG seronegative at baseline and received all two doses of the vaccine. Blood was drawn at 30 and 36 months after the full vaccination to assess antibody status. Antibody persistence status is compared by demographics and exposure risk factors using a univariate logistic regression.

Results

Overall incidence of persisting at least 1/10 of A/H5N1 antibodies was 84.9% and 52.3% after 30 months and 36 months of IVACFLU-A/H5N1 vaccination. The odds of antibody persistence were more significant in older people but lower in people who experienced flu-like symptoms in the past 18 months or between two visits. We recorded no differences between A/H5N1 antibodies persistence and exposure risk factors including having poultry farms, contacting with poultry, and slaughtering and processing poultry.

Conclusion

This study demonstrated noteworthy antibody persistence, indicated by seroconversion rate and geometric mean titer at 30 and 36-month post-vaccination, of the IVACFLU-A/H5N1 vaccine. There is a need for further studies on older people, and those who experienced flu-like symptoms to decide the suitable time for the booster shot.

Evaluation of the Higher Order Structure of Biotherapeutics Embedded in Hydrogels for Bioprinting and Drug Release

Biocompatible hydrogels for tissue regeneration/replacement and drug release with specific architectures can be obtained by three-dimensional bioprinting techniques. The preservation of the higher order structure of the proteins embedded in the hydrogels as drugs or modulators is critical for their biological activity. Solution nuclear magnetic resonance (NMR) experiments are currently used to investigate the higher order structure of biotherapeutics in comparability, similarity, and stability studies. However, the size of pores in the gel, protein-matrix interactions, and the size of the embedded proteins often prevent the use of this methodology. The recent advancements of solid-state NMR allow for the comparison of the higher order structure of the matrix-embedded and free isotopically enriched proteins, allowing for the evaluation of the functionality of the material in several steps of hydrogel development. Moreover, the structural information at atomic detail on the matrix-protein interactions paves the way for a structure-based design of these biomaterials.

NMR characterization of a multi-valent conjugate vaccine against Neisseria meningitidis A, C, W, Y and Haemophilus influenzae b infections

Physicochemical technologies are a powerful tool for the structural characterization of vaccine antigens both at bulk level as well as on the final formulation. High-field Nuclear Magnetic Resonance (NMR) spectroscopy has been found to be an extremely and robust tool for tracking the industrial process manufacturing of carbohydrate-based vaccines. I have applied NMR spectroscopy to the characterization of a penta-valent conjugate vaccine against Neisseria meninigitidis group A, C, W, Y (MenACWY) and Haemophilus influenzae type b (Hib) infections, constituted of capsule derived polysaccharide fragments independently conjugated to CRM₁₉₇ protein carrier (CRM-MenA, CRM-MenC, CRM-MenW, CRM-MenY, CRM-Hib). ¹H NMR has been used for the identity testing of the carbohydrate antigens and of the vaccine formulation. The application of NMR-based assays on multivalent conjugate vaccines looks to be a promising approach for identity and stability analyses useful for future vaccines development.

Understanding molecular mechanisms of biologics drug delivery and stability from NMR spectroscopy

Protein therapeutics carry inherent limitations of membrane impermeability and structural instability, despite their predominant role in the modern pharmaceutical market. Effective formulations are needed to overcome physiological and physicochemical barriers, respectively, for improving bioavailability and stability. Knowledge of membrane affinity, cellular internalization, encapsulation, and release of drug-loaded carrier vehicles uncover the structural basis for designing and optimizing biopharmaceuticals with enhanced delivery efficiency and therapeutic efficacy. Understanding stabilizing and destabilizing interactions between protein drugs and formulation excipients provide fundamental mechanisms for ensuring the stability and quality of biological products. This article reviews the molecular studies of biologics using solution and solid-state NMR spectroscopy on structural attributes pivotal to drug delivery and stability. In-depth investigation of the structure-function relationship of drug delivery systems based on cell-penetrating peptides, lipid nanoparticles and polymeric colloidal, and biophysical and biochemical stability of peptide, protein, monoclonal antibody, and vaccine, as the integrative efforts on drug product design, will be elaborated.

Structural Analysis of an Antigen Chemically Coupled on Virus-Like Particles in Vaccine Formulation

Structure determination of adjuvant-coupled antigens is essential for rational vaccine development but has so far been hampered by the relatively low antigen content in vaccine formulations and by their heterogeneous composition. Here we show that magic-angle spinning (MAS) solid-state NMR can be used to assess the structure of the influenza virus hemagglutinin stalk long alpha helix antigen, both in its free, unformulated form and once chemically coupled to the surface of large virus-like particles (VLPs). The sensitivity boost provided by high-field dynamic nuclear polarization (DNP) and proton detection at fast MAS rates allows to overcome the penalty associated with the antigen dilution. Comparison of the MAS NMR fingerprints between the free and VLP-coupled forms of the antigen provides structural evidence of the conservation of its native fold upon bioconjugation. This work demonstrates that high-sensitivity MAS NMR is ripe to play a major role in vaccine design, formulation studies, and manufacturing process development.

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.

Orientation of immobilized antigens on common surfaces by a simple computational model: Exposition of SARS-CoV-2 Spike protein RBD epitopes

The possibility of immobilizing a protein with antigenic properties on a solid support offers significant possibilities in the development of immunosensors and vaccine formulations. For both applications, the orientation of the antigen should ensure ready accessibility of the antibodies to the epitope. However, an experimental assessment of the orientational preferences necessarily proceeds through the preparation/isolation of the antigen, the immobilization on different surfaces and one or more biophysical characterization steps. To predict a priori whether favorable orientations can be achieved or not would allow one to select the most promising experimental routes, partly mitigating the time cost towards the final product. In this manuscript, we apply a simple computational model, based on united-residue modelling, to the prediction of the orientation of the receptor binding domain of the SARS-CoV-2 spike protein on surfaces commonly used in lateral-flow devices. These calculations can account for the experimental observation that direct immobilization on gold gives sufficient exposure of the epitope to obtain a response in immunochemical assays.

Atomic-Scale Description of Interfaces between Antigen and Aluminum-Based Adjuvants Used in Vaccines by Dynamic Nuclear Polarization (DNP) Enhanced NMR Spectroscopy

The addition of aluminum-based adjuvants in vaccines enhances the immune response to antigens. The strength of antigen adsorption on adjuvant gels is known to modulate vaccine efficacy. However, a detailed understanding of the mechanisms of interaction between aluminum gels and antigens is still missing. Herein, a new analytical approach based on dynamic nuclear polarization (DNP) enhanced NMR spectroscopy under magic angle spinning (MAS) is implemented to provide a molecular description of the antigen-adjuvant interface. This approach is demonstrated on hepatitis B surface antigen particles in combination with three aluminum gels obtained from different suppliers. Both noncovalent and covalent interactions between the phospholipids of the antigen particles and the surface of the aluminum gels are identified by using MAS DNP NMR ²⁷ Al and ³¹ P correlation experiments. Although covalent interactions were detected for only one of the formulations, dipolar recoupling rotational echo adiabatic passage double resonance (REAPDOR) experiments reveal significant differences in the strength of weak interactions.

The Integrity of α-β-α Sandwich Conformation Is Essential for a Novel Adjuvant TFPR1 to Maintain Its Adjuvanticity

TFPR1 is a novel peptide vaccine adjuvant we recently discovered. To define the structural basis and optimize its application as an adjuvant, we designed three different truncated fragments that have removed dominant B epitopes on TFPR1, and evaluated their capacity to activate bone marrow-derived dendritic cells and their adjuvanticity. Results demonstrated that the integrity of an α-β-α sandwich conformation is essential for TFPR1 to maintain its immunologic activity and adjuvanticity. We obtained a functional truncated fragment TFPR-ta ranging from 40-168 aa of triflin that has similar adjuvanticity as TFPR1 but with 2-log fold lower immunogenicity. These results demonstrated a novel approach to evaluate and improve the activity of protein-based vaccine adjuvant.

Integrative Approaches in Structural Biology: A More Complete Picture from the Combination of Individual Techniques

With the recent technological and computational advancements, structural biology has begun to tackle more and more difficult questions, including complex biochemical pathways and transient interactions among macromolecules. This has demonstrated that, to approach the complexity of biology, one single technique is largely insufficient and unable to yield thorough answers, whereas integrated approaches have been more and more adopted with successful results. Traditional structural techniques (X-ray crystallography and Nuclear Magnetic Resonance (NMR)) and the emerging ones (cryo-electron microscopy (cryo-EM), Small Angle X-ray Scattering (SAXS)), together with molecular modeling, have pros and cons which very nicely complement one another. In this review, three examples of synergistic approaches chosen from our previous research will be revisited. The first shows how the joint use of both solution and solid-state NMR (SSNMR), X-ray crystallography, and cryo-EM is crucial to elucidate the structure of polyethylene glycol (PEG)ylated asparaginase, which would not be obtainable through any of the techniques taken alone. The second deals with the integrated use of NMR, X-ray crystallography, and SAXS in order to elucidate the catalytic mechanism of an enzyme that is based on the flexibility of the enzyme itself. The third one shows how it is possible to put together experimental data from X-ray crystallography and NMR restraints in order to refine a protein model in order to obtain a structure which simultaneously satisfies both experimental datasets and is therefore closer to the ‘real structure’.