Antigen Delivery Platforms for Next-Generation Coronavirus Vaccines

Abstract

The COVID-19 pandemic, caused by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), is in its sixth year and is being maintained by the inability of current spike-alone-based COVID-19 vaccines to prevent transmission leading to the continuous emergence of variants and sub-variants of concern (VOCs). This underscores the critical need for next-generation broad-spectrum pan-Coronavirus vaccines (pan-CoV vaccine) to break this cycle and end the pandemic. The development of a pan-CoV vaccine offering protection against a wide array of VOCs requires two key elements: (1) identifying protective antigens that are highly conserved between passed, current, and future VOCs; and (2) developing a safe and efficient antigen delivery system for induction of broad-based and long-lasting B- and T-cell immunity. This review will (1) present the current state of antigen delivery platforms involving a multifaceted approach, including bioinformatics, molecular and structural biology, immunology, and advanced computational methods; (2) discuss the challenges facing the development of safe and effective antigen delivery platforms; and (3) highlight the potential of nucleoside-modified mRNA encapsulated in lipid nanoparticles (LNP) as the platform that is well suited to the needs of a next-generation pan-CoV vaccine, such as the ability to induce broad-based immunity and amenable to large-scale manufacturing to safely provide durable protective immunity against current and future Coronavirus threats.

Keywords: LNP; SARS-CoV-2; antigen delivery platform; antigen delivery system; mRNA; pan-Coronavirus vaccine; srRNA.

Figure 1

Peptides to deliver next-generation PanCoVax vaccine candidates: A portion of an antigenic protein such as a peptide or polypeptide from coronavirus is used alone or a combination of peptides from different proteins and/or different variants to make a vaccine, potentially with the addition of an adjuvant. After injection of the vaccine, the peptides can be presented to antigen-presenting cells and loaded onto MHC class I for presentation to CD8+ T-cells or through MHC II for presentation to CD4⁺ T cells. This stimulates some B cells to differentiate into plasma cells, which then release antibodies.

Figure 2

Adenovirus to deliver next-generation PanCoVax vaccine candidates: The adenovirus is non-enveloped and utilizes double-stranded DNA that encodes the antigen of interest from coronavirus. After the construction of the viral vector vaccine, the following steps occur: (1) The vaccine is injected intramuscularly. (2) Muscle cells engulf the cells through endocytosis. (3) The vesicle around the adenovirus breaks down. (4) The adenovirus attaches to the nucleus and transcription begins. (5) The protein is created through translation. After this, the protein peptides can either (a) be loaded onto MHC class I molecules for direct presentation to CD8⁺ T cells or (b) be presented to antigen-presenting cells and loaded onto MHC II for presentation to CD4⁺ T cells, which stimulates some B cells to differentiate into plasma cells, which then release antibodies.

Figure 3

Pan-Coronavirus mRNA-LNP based vaccines: Pan-Coronavirus vaccines involve the use of an mRNA transcript that encodes an antigen and LNP. Linear non-replicating mRNAs include a sequence for antigens of interest (AOI, such as the Spike, Nucleocapsid, and other conserved proteins) flanked by 5′ and 3′ untranslated regions (UTRs), a cap structure at the 5′ end, and a poly(A) tail at the 3′ end. Depending on whether modified or native nucleosides are used during in vitro transcription, either (A) unmodified or (B) modified mRNAs are produced; (C) Self-amplifying RNA (saRNA) has a similar sequence organization but additionally includes (1) a sequence encoding four non-structural proteins (NSP1–4) that form a replicase to amplify the saRNA, and (2) a viral-origin subgenomic promoter (black arrow) that initiates antigen transcription; (D) Circular RNA (circRNA)-based vaccines consists of a covalently closed single-stranded RNA containing the sequence of antigens of interest and an internal ribosome entry site (IRES) to initiate antigen translation. The antigens of interest are produced endogenously by the antigen-presenting cells (APCs) translational machinery (red circles), degraded by proteasomes (pink circles), and presented on major histocompatibility complex (MHC) class I molecules (pink circles), leading to a specific CD8+ cytotoxic T cell response against coronaviruses variant of concern. Additionally, the antigen protein can be exported from the cell, endocytosed by the same or another APC, degraded by endosomal proteases, and presented on MHC II molecules, resulting in a CD4⁺ helper T cell response. Immunization progresses with CD4⁺ helper T cells aid in the activation of B cells to produce anti-coronavirus-specific antibodies and neutralizing antibodies. CD8⁺ cytotoxic T cells specifically target and eliminate coronavirus-infected cells.