Innovations in Vaccinology: The Rise of Recombinant Genetic Shields

Immune protection against infectious diseases is a crucial objective in both human and animal health. Currently, the focus of the vaccine landscape is primarily on COVID-19, which has required an unprecedented global effort to develop and deploy vaccines rapidly. Consequently, vaccination has become a dominant subject in scientific literature, and detailed discussions on specific immunization strategies and vaccine-induced immune responses are widespread in the media. This situation has underscored the challenges posed by emerging viral pandemics and the need for a universal approach to vaccine development, regardless of the pathogen-host combination. Traditional vaccine formulations, such as inactivated or attenuated versions of the whole infectious agent, have faced limitations, such as potential safety risks associated with chemical inactivation or the risk of reverting to virulence with attenuated strains. As a result, there is a growing interest in recombinant subunit vaccines. These vaccines offer advantages, including enhanced safety profiles and the ability to bypass the need for high-biosafety facilities and P3 laboratories during manipulation. 

 

Figure 1. Systematic diagram showing adaptive immune activation induced by VLP-based vaccine

By focusing on subunit vaccines, researchers can mitigate the risks associated with working with complete pathogens while still eliciting a protective immune response. This approach is promising and holds the potential to revolutionize vaccine development for a wide range of infectious diseases. Subunit vaccines, an established concept, rely on immune-dominant antigens or a combination of selected antigens purified from the pathogen [1]. While designing effective subunit vaccines poses challenges related to biological efficacy, it also requires addressing issues of speed, cost-effectiveness, biosafety, and scalability in the production process. To overcome these challenges, industrial-scale production of relevant antigens is preferred over extraction from natural sources, which can be costly or impractical. Additionally, the presentation geometry of the selected antigen plays a critical role, particularly in anti-viral vaccines. One successful approach to antigen presentation involves virus-like particles (VLPs), which have been engineered from various pathogenic and non-pathogenic viruses belonging to over 35 families [2]. VLPs spontaneously self-assemble from recombinant versions of capsid proteins upon robust expression of the corresponding genes. These VLPs have shown promise as vaccines and carriers for drugs and imaging agents, demonstrating their potential in diverse fields of precision nanomedicines. The oligomeric presentation of antigens in virus-like structures has contributed to the success of VLP-based vaccines, although regulatory approval has been limited to only a few diseases.

Advancing beyond VLPs, the concept of multiple and repetitive antigen presentation has led to the development of vaccination platforms with versatile antigen presentation systems that feature modular or interchangeable elements. Nanoscale multimeric antigen display on nanostructured materials has been recognized as a promising strategy to enhance the protective immune response, as it mimics the natural structural features of viral particles. These nanoscale vaccines, known as Nano vaccines, offer potential as universal antigen presentation systems. Given their unique and promising characteristics, nanoscale vaccines have gained special attention as a global strategy in the quest for transversal vaccine platforms [3, 4]. By leveraging the benefits of nanotechnology, these Nano vaccines hold the potential to revolutionize vaccination approaches and improve protection against infectious diseases.

The growing structural complexity in formulating subunit vaccines represents a departure from the earlier single soluble antigen approach that emerged in the late 1970s with the beginning of recombinant DNA technologies. Ensuring stability during storage, transportation, and suitability for mass administration are critical considerations in vaccine formulation. To address these challenges, researchers are exploring novel adjuvants and formulation strategies that facilitate simple manipulation of vaccine doses, especially for large-scale administration programs where thermal stability is crucial. In the realm of emerging vaccination technologies, such as DNA or mRNA-based approaches, recombinant antigens have garnered significant interest [5, 6]. Recombinant proteins have been utilized as drugs for many years, demonstrating both clinical safety and scalability in production. The versatile nature of polypeptides allows for the design of multimeric, nanoscale materials that self-assemble through various approaches, making them excellent candidates for new generation vaccines.

Furthermore, recent advancements in biofabrication have introduced natural or engineered cell factories with appealing properties for producing protein drugs, surpassing traditional bacterial, yeast, mammalian, and insect cells. In this article, we explore contemporary approaches in the biological fabrication of recombinant subunit vaccines using alternative cell factories. We discuss how these antigens are adapted to meet the requirements for vaccine effectiveness concerning stability, formulation, and multivalent nanoscale presentation. The immunogenicity of plain subunit vaccines, consisting of non-oligomeric antigens, is often moderate. To overcome this limitation, researchers commonly combine these proteins with immune-stimulant molecules known as adjuvants and nanoparticles, which can enhance the immune response [7]. Over the past few decades, proteins have been displayed in multiple copies on various types of nanoparticles, including polymers and lipids, aiming to prolong their half-life, facilitate targeted delivery, and activate specific immune cells. However, another approach involving protein-only Nano-vaccines resulting from recombinant protein self-assembly has been utilized for some time and offers a means to achieve an ideal vaccine formulation. The study of nano-sized oligomeric protein vaccines gained momentum with the development of virus-like particles (VLPs), where the multimerization of antigens appears crucial for eliciting potent immune responses [8]. The side-by-side presentation of antigens on VLPs is recognized by immune cells as a pathogen-associated molecular pattern, leading to more efficient activation of B-cell receptors. The cross-linking of B-cell receptors is a key and early step in B-cell activation. Furthermore, the dense arrangement of antigens on Nano-vaccines, ranging from 20 to 200 nm in size, facilitates efficient drainage to lymph nodes, enhancing uptake by antigen-presenting cells [2]. This highlights how the size and geometry of the protein assembly play a role in shaping immune responses. The success of the VLP platform is evident by its commercial presence in the market, with two formulations already available for Human Papillomavirus and two others for Hepatitis B Virus. Virus-like particles have proven to be effective immunogens for vaccination. VLPs are produced by recombinant proteins that self-assemble into empty, virus-like structures. For proteins that cannot self-assemble, various tools can modify irrelevant VLP scaffolds to incorporate foreign proteins. This includes genetic fusion of the antigen to coat proteins with self-assembling capabilities or conjugation of antigens to the scaffold using covalent or non-covalent strategies [9]. The VLP platform has been evaluated against various pathogens and has recently shown encouraging results in pre-clinical trials for SARS-CoV-2, the virus responsible for COVID-19. In fact, the COVID-19 pandemic has spurred the expansion of different Nano vaccine platforms, including Novavax's recombinant nanoparticle vaccine based on the SARS-CoV-2 Spike protein, which has demonstrated 89.7% protection against infection and cross-protection against antigenic variants in phase III clinical trials [10]. Its approval marks a significant milestone in the fight against COVID-19. Given the potential advantages of nanostructured subunit vaccines, considerable effort has been invested in identifying and developing alternative non-VLP strategies that enable self-assembly of antigens. Numerous protein oligomerization tags and strategies have been explored and developed, including the use of poly-histidine tails commonly used as purification tags. Across different approaches, multimeric antigen assembly consistently leads to enhanced immune responses. The field of Nano vaccines lies at the intersection of protein engineering and materials sciences [10]. By combining the power and challenges of protein production in heterologous systems with those associated with nanoparticles, these tools offer an exciting opportunity to enhance the efficiency and safety of subunit vaccines significantly.

By: Muhammad Usman Qamar and Areesha Bhatti

References:

1.            Gebre, M.S., et al., Novel approaches for vaccine development. 2021. 184(6): p. 1589-1603.

2.            Zeltins, A.J.M.b., Construction and characterization of virus-like particles: a review. 2013. 53(1): p. 92-107.

3.            Fries, C.N., et al., Advances in nanomaterial vaccine strategies to address infectious diseases impacting global health. 2021. 16(4): p. 1-14.

4.            Lamontagne, F., et al., Vaccination strategies based on bacterial self-assembling proteins as antigen delivery nanoscaffolds. 2022. 10(11): p. 1920.

5.            Wollensack, L., et al., Defossilization of pharmaceutical manufacturing. 2022. 33: p. 100586.

6.            Sanchez-Garcia, L., et al., Recombinant pharmaceuticals from microbial cells: a 2015 update. 2016. 15: p. 1-7.

7.            Dumpa, N., et al., Stability of vaccines. 2019. 20: p. 1-11.

8.            Gomes, A.C., M. Mohsen, and M.F.J.V. Bachmann, Harnessing nanoparticles for immunomodulation and vaccines. 2017. 5(1): p. 6.

9.            Smith, M.T., A.K. Hawes, and B.C.J.C.o.i.b. Bundy, Reengineering viruses and virus-like particles through chemical functionalization strategies. 2013. 24(4): p. 620-626.

10.          Heath, P.T., et al., Safety and efficacy of NVX-CoV2373 Covid-19 vaccine. 2021. 385(13): p. 1172-1183.