The race for a vaccine against the SARS-CoV-2 is on, with 54 different vaccines under development, two of which are being tested in humans, according to the World Health Organization (WTO). And among the various candidates is a new player on the scene—mRNA vaccines.
The concept for the development of an mRNA vaccine is rather intelligible. After the antigen of choice from the pathogen target is identified, the gene is sequenced, synthesized, and cloned into the DNA template plasmid. mRNA is transcribed in vitro, and the vaccine is delivered to the subject. The mRNA vaccine utilizes the host cell machinery for in vivo translation of mRNA into the corresponding antigen, thereby mimicking a viral infection to evoke potent humoral and cellular immune responses. The final cellular location of the antigen is determined by the transmembrane domain and signal peptide. And the antigen can be expressed as intracellular, secreted, or membrane-bound protein. Given its fully synthetic nature, almost any sequence could be designed in silico, synthesized, delivered as an mRNA vaccine, and tested rapidlyin animal models.
An efficient mRNA vaccine demands:
(1) Optimal mRNA stability and cell uptake;
(2) Cytosolic delivery and mRNA expression in target cells;
(3) Elicitation of the desired protective adaptive immune response for vaccines when the correlates of protection are known, such as for the influenza vaccine.
Currently, two forms of mRNA vaccines have been developed: conventional mRNA encoding the antigen of interest flanked by 5' and 3' UTRs, and self-amplifying mRNA derived from the genome of positive-stranded RNA viruses. Self-amplifying mRNA encodes not only the antigen but also the viral replication machinery required for intracellular RNA amplification resulting in high levels of antigen expression.
Figure 1. Schematic representation of mRNA vaccines and mechanism of antigen expression.
mRNA vaccines have gained much interest in vaccinology owing to their numerous advantages. This versatile technology can achieve strong humoral and cellular immune responses, has intrinsic self-adjuvant properties, and results in transitory protein translation in a cell-cycle-independent manner. With the absence of pre-existing vector immunity that can interfere with subsequent vaccinations, as well as a manufacturing process done by an enzymatic/cell-free reaction, this technology offers faster, simpler, and cheaper operations than conventional vaccines do.
Table 1. Advantages and challenges of mRNA vaccines
|Rapid research and development, mRNA vaccine production only takes 40 days||Under physiological conditions, mRNA is unstable and easy to degrade|
|There is no need for any nuclear localization signal and transcription||Trigger an unnecessary immune response|
|It will not be integrated into the genome to avoid possible therapeutic mutations||Safety and effectiveness need to be verified|
Due to the poor stability of mRNA itself, easy degradation by nuclease in tissues, low efficiency of entering cells and low translation efficiency, these defects limit the application of mRNA vaccine. Many technologies are currently used to improve the pharmacological aspects of mRNA. The various mRNA modifications used and their impacts are summarized below.
Efficient in vivo mRNA delivery is crucial to achieving therapeutic relevance. Exogenous mRNA must penetrate the barrier of the lipid membrane in order to reach the cytoplasm to be translated into a functional protein. mRNA uptake mechanisms appear to be cell type dependent, and the physicochemical properties of the mRNA complexes can profoundly influence cellular delivery and organ distribution. So far, there are two basic methods for the delivery of mRNA vaccines that have been described, 1) loading of mRNA into DCs ex vivo, followed by re-infusion of the transfected cells; 2) direct parenteral injection of mRNA with or without a carrier. Ex vivo DC loading enables precise control of the cellular target, transfection efficiency and other cellular conditions; however, as a form of cell therapy, it is an expensive and labor-intensive approach to vaccination. Direct injection of mRNA is comparatively rapid and cost-effective, but it does not yet allow precise and efficient cell-type specific delivery.
Figure 2. Considerations for the effectiveness of a directly injected mRNA vaccine.
mRNA-based vaccines are a promising platform with the potential to be highly versatile, potent, scalable, streamlined, inexpensive, and cold-chain free. More importantly, mRNA-based vaccines may fill the gap between emerging pandemic infectious diseases and a rapid, abundant supply of effective vaccines. The mRNA vaccine technology has a huge potential over conventional vaccines. Nevertheless, it is still too early to fully understand its safety and effectiveness in humans. Further insights into the mechanism of action are needed to understand the impact of innate immune responses generated both by the mRNA and the delivery system, and to determine how learning from animal species will translate to humans.