Strategies for Vaccination: Conventional Vaccine Approaches Versus New-Generation Strategies in Combination with Adjuvants

Abstract

The current COVID-19 pandemic, caused by severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2), has raised significant economic, social, and psychological concerns. The rapid spread of the virus, coupled with the absence of vaccines and antiviral treatments for SARS-CoV-2, has galvanized a major global endeavor to develop effective vaccines. Within a matter of just a few months of the initial outbreak, research teams worldwide, adopting a range of different strategies, embarked on a quest to develop effective vaccine that could be effectively used to suppress this virulent pathogen. In this review, we describe conventional approaches to vaccine development, including strategies employing proteins, peptides, and attenuated or inactivated pathogens in combination with adjuvants (including genetic adjuvants). We also present details of the novel strategies that were adopted by different research groups to successfully transfer recombinantly expressed antigens while using viral vectors (adenoviral and retroviral) and non-viral delivery systems, and how recently developed methods have been applied in order to produce vaccines that are based on mRNA, self-amplifying RNA (saRNA), and trans-amplifying RNA (taRNA). Moreover, we discuss the methods that are being used to enhance mRNA stability and protein production, the advantages and disadvantages of different methods, and the challenges that are encountered during the development of effective vaccines.

Keywords: DNA vaccine; RNA vaccine; nanoparticle; vaccine adjuvant; viral vector.

Figure 1

Retrovirus-based vectors. (A) Representation of a retrovirus-based vector, and (B) a SIN lentiviral-based vector. The long terminal repeat (LTR) is divided into three regions (U3, R and U5). The packaging of the viral RNA takes place via interaction between the packaging signal ψ and the viral proteins. The bonding of Rev to the rev response element (RRE) enables the transport of un-spliced or once-spliced RNA from the nucleus to the cytoplasm. (C) A helper plasmid with viral protein gag-pro-pol under the expression of the cauliflower mosaic virus (CMV) promoter. (D) A helper plasmid for the env protein.

Figure 2

Adenovirus-based vectors. (A) A map of the human adenovirus type 5 (HAd5) genome. It consists of early genes (E1–E4) that suppress cell responses against the virus, and are responsible for the replication and regulation of viral transcription. The late genes (L1–L5) encode the structural proteins of the virus. (B) A first-generation adenovirus-based vector (8-kb packaging capacity) in which the E1 and E3 genes have been deleted. (C) A second-generation vector in which the E2 and E4 genes have been deleted to increase packaging capacity (14-kb packaging capacity). For packaging, the plasmid harboring the transgene is transfected with a helper plasmid for the expression of viral genes E1, E2, and E4.

Figure 3

Adeno-associated-based vaccine. (A) The wild-type adeno-associated virus (AAV) genome can be modified by replacing the gene for replication (REP) and structural genes (CAP) with the transgene of interest. (B) A transgene containing promoter and regulatory elements is cloned between the two inverted terminal repeats (ITRs) to generate a recombinant AAV (rAAV) genome. For the production of rAAV particles, the construct carrying the transgene should be co-transfected in permissive cells with plasmids that contain REP and CAP genes (C), and adenovirus helper genes (D,E).

Figure 4

RNA based vaccines. Plasmid DNA carrying replicase genes (for the replication of RNA) and/or the transgene (which encodes the gene of interest) can be transcribed in vitro using a T7 promoter transcription system to generate replicons or positive sense RNAs (Pos sense RNA). (A) The replicon RNA encodes the replicase machinery and the transgene are delivered into the cell using lipofectamine or similar synthetic formulations. Within the cytoplasm, the replicon RNA self-replicates and produce transgene mRNA from the subgenomic promoter, which is translated to protein. (B) The replicon RNA encoding the replicase machinery and the transgene are delivered “in trans”. Within the cytoplasm, the replicon RNA self-replicates and produces transgene mRNA using a subgenomic promoter, which is subsequently translated to protein.