A Comprehensive Review of mRNA Vaccines

Abstract

mRNA vaccines have been demonstrated as a powerful alternative to traditional conventional vaccines because of their high potency, safety and efficacy, capacity for rapid clinical development, and potential for rapid, low-cost manufacturing. These vaccines have progressed from being a mere curiosity to emerging as COVID-19 pandemic vaccine front-runners. The advancements in the field of nanotechnology for developing delivery vehicles for mRNA vaccines are highly significant. In this review we have summarized each and every aspect of the mRNA vaccine. The article describes the mRNA structure, its pharmacological function of immunity induction, lipid nanoparticles (LNPs), and the upstream, downstream, and formulation process of mRNA vaccine manufacturing. Additionally, mRNA vaccines in clinical trials are also described. A deep dive into the future perspectives of mRNA vaccines, such as its freeze-drying, delivery systems, and LNPs targeting antigen-presenting cells and dendritic cells, are also summarized.

Keywords: PEGylated lipids; acceptance; adjuvants; antigen presentation; cationic lipids; efficacy; ionizable lipids; lipid nanoparticles (LNPs); lyophilized mRNA vaccines; mRNA structure; mRNA vaccine immune response; mRNA vaccines clinical trials; safety; self-amplifying mRNA vaccines.

Figure 1

mRNA molecule structural components [25].

Figure 2

mRNA lipid nanoparticles’ (mRNA-LNPs) site of intramuscular administration and modes of action of the mRNA-LNPs. mRNA-LNP vaccines can transfect muscle cells and transfect the tissue-resident antigen-presenting cells (APCs) near the injection site. Additionally, mRNA-LNP vaccines can flow into lymph nodes (LNs) and transfect the LN-resident cells, resulting in activation of T and B cells. Adapted with permission from [25].

Figure 3

Pharmacological mechanism of adaptive immune responses induced by mRNA-LNP vaccines. (1) In vitro transcribed mRNA is encapsulated into a lipid nanoparticle (LNP). (2) Transfection of mRNA-LNP vaccine molecules into the host cells, using specialized lipids on the surface of the LNPs. (3) Endocytosis of mRNA-LNP. (4) Endosomal escape of mRNA to the cytosol after endocytosis-mediated internalization. (5) Translation of the mRNA by the host cell ribosomes into the desired antigen protein intracellularly. (6) Antigenic protein released outside the cell, or the antigenic protein is degraded by a proteosome, exposing the antigenic sites. (7) Major histocompatibility complex I (MHC I) epitope presentation of the MHC I to the cell membrane for antigen presentation (APC). MHC I presents the epitope to CD8+ T cells. (9) The exogenous protein released earlier can get degraded and presented via MHC II epitopes. The extracellular antigen can get recognized by B cells, leading to B cell maturation [25].

Figure 4

Components of lipid nanoparticles including ionizable lipids, cholesterol, helper lipids, and PEGylated lipids [35].

Figure 5

The steps and stages of an mRNA vaccine manufacturing process. mRNA vaccine production can be divided into three phases: upstream mRNA manufacturing, downstream mRNA purification, and formulation of mRNA lipid nanoparticles. mRNA production can be performed in a one-step co-transcriptional reaction, where a capping reagent is used, or in a two-step reaction, where the enzymatic capping is performed. mRNA purification process at a smaller lab scale consists of DNase I digestion enzyme followed by LiCl precipitation of the mRNA. Purification of mRNA at a large scale involves utilizing well-established chromatographic methods coupled with tangential flow filtration (TFF). Finally, the formulation of mRNA vaccines consists of mixing mRNA aqueous solution with lipid solution in a non-aqueous phase. This causes self-assembly of the lipid nanoparticles (LNPs) and encapsulates the negatively charged mRNA within the core of the LNPs. The mixing of the mRNA and the lipid molecules in a staggered herringbone micromixer (SHM) occurs in various cycles which results in the formation of the final mRNA-LNP vaccines. Adapted with permission from [86,90].