mRNA vaccines for infectious diseases: principles, delivery and clinical translation

Abstract

Over the past several decades, messenger RNA (mRNA) vaccines have progressed from a scepticism-inducing idea to clinical reality. In 2020, the COVID-19 pandemic catalysed the most rapid vaccine development in history, with mRNA vaccines at the forefront of those efforts. Although it is now clear that mRNA vaccines can rapidly and safely protect patients from infectious disease, additional research is required to optimize mRNA design, intracellular delivery and applications beyond SARS-CoV-2 prophylaxis. In this Review, we describe the technologies that underlie mRNA vaccines, with an emphasis on lipid nanoparticles and other non-viral delivery vehicles. We also overview the pipeline of mRNA vaccines against various infectious disease pathogens and discuss key questions for the future application of this breakthrough vaccine platform.

Fig. 1. IVT mRNA is formulated into lipid nanoparticle vaccines using a cell-free production pipeline.

a | In vitro-transcribed (IVT) mRNA contains five structural elements: a 5′ cap containing 7-methylguanosine linked through a triphosphate bridge to a 2′-O-methylated nucleoside, flanking 5′ and 3′ untranslated regions (UTRs), an open reading frame (ORF) and a poly(A) tail. b | The mRNA is synthetically produced and formulated into vaccines. (1) Once the genome of a pathogen has been sequenced, a sequence for the target antigen is designed and inserted into a plasmid DNA construct. (2) Plasmid DNA is transcribed into mRNA by bacteriophage polymerases in vitro and (3) mRNA transcripts are purified by high performance liquid chromatography (HPLC) to remove contaminants and reactants. (4) Purified mRNA is mixed with lipids in a microfluidic mixer to form lipid nanoparticles. Rapid mixing causes the lipids to encapsulate mRNA instantaneously and precipitate as self-assembled nanoparticles. (5) The nanoparticle solution is dialysed or filtered to remove non-aqueous solvents and any unencapsulated mRNA and (6) the filtered mRNA vaccine solution is stored in sterilized vials.

Fig. 2. All mRNA delivery vehicles contain cationic or ionizable molecules.

a | Lipid nanoparticles encapsulate mRNA in their core. They consist of four components: ionizable lipids, such as DLin-MC3-DMA, SM-102 (ref.), ALC-0315 (ref.), A18-Iso5-2DC18 (ref.), A6 (ref.) and 306O_i10 (ref.); cholesterol or its variants, β-sitosterol and 20α-hydroxycholesterol; helper lipids, such as DSPC and DOPE; and PEGylated lipids, such as ALC-0159 (ref.) and PEG-DMG. b | Polymers, such as PEI, PBAE, PEG-PAsp(DET) and CART form polymer–mRNA complexes. c | Cationic nanoemulsions contain a squalene core surrounded by an outer shell made of cationic lipid (for example, DOTAP) and surfactants, such as Tween 80 and Span 85. The mRNA adsorbs to the surface via electrostatic binding.

Fig. 3. Messenger RNA vaccines elicit immunity through transfection of antigen-presenting cells.

(1) Injected mRNA vaccines are endocytosed by antigen-presenting cells. (2) After escaping the endosome and entering the cytosol, mRNA is translated into protein by the ribosome. The translated antigenic protein can stimulate the immune system in several ways. (3) Intracellular antigen is broken down into smaller fragments by the proteasome complex, and the fragments are displayed on the cell surface to cytotoxic T cells by major histocompatibility complex (MHC) class I proteins. (4) Activated cytotoxic T cells kill infected cells by secreting cytolytic molecules, such as perforin and granzyme. (5) Additionally, secreted antigens can be taken up by cells, degraded inside endosomes and presented on the cell surface to helper T cells by MHC class II proteins. (6) Helper T cells facilitate the clearance of circulating pathogens by stimulating B cells to produce neutralizing antibodies, and by activating phagocytes, such as macrophages, through inflammatory cytokines. BCR, B cell receptor; ER, endoplasmic reticulum; TCR, T cell receptor.

Fig. 4. mRNA vaccines in development protect against an array of common pathogens using disease-specific targeting strategies.

Surface proteins that enable cell entry are commonly used by mRNA vaccines to target viruses, for example, spike protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), haemagglutinin protein of influenza viruses, membrane and envelope protein (prM-E) of Zika virus, fusion protein of respiratory syncytial virus (RSV) and surface glycoproteins of human immunodeficiency virus (HIV), Ebola virus and rabies virus. Additionally, complex pathogens such as Plasmodium can be targeted using non-surface antigens such as Plasmodium macrophage migratory inhibiting factor (PMIF) or Plasmodium falciparum glutamic-acid-rich protein (PfGARP). Each pathogen poses a unique set of challenges, including high lethality, rapid mutations, immune evasion, new strains and variants^–. Depending on the challenges, mRNA vaccines encoding conformation-specific proteins, conserved regions of antigens or monoclonal antibodies can be safely delivered to healthy adults, children, elderly people and pregnant people. VAED, vaccine-associated enhanced disease.