mRNA as a Transformative Technology for Vaccine Development to Control Infectious Diseases

Abstract

In the last two decades, there has been growing interest in mRNA-based technology for the development of prophylactic vaccines against infectious diseases. Technological advancements in RNA biology, chemistry, stability, and delivery systems have accelerated the development of fully synthetic mRNA vaccines. Potent, long-lasting, and safe immune responses observed in animal models, as well as encouraging data from early human clinical trials, make mRNA-based vaccination an attractive alternative to conventional vaccine approaches. Thanks to these data, together with the potential for generic, low-cost manufacturing processes and the completely synthetic nature, the prospects for mRNA vaccines are very promising. In addition, mRNA vaccines have the potential to streamline vaccine discovery and development, and facilitate a rapid response to emerging infectious diseases. In this review, we overview the unique attributes of mRNA vaccine approaches, review the data of mRNA vaccines against infectious diseases, discuss the current challenges, and highlight perspectives about the future of this promising technology.

Keywords: RNA-based vaccine; infectious diseases; self-amplifying mRNA; synthetic vaccine; vaccine on demand.

Figure 1

Schematic Representation of mRNA Vaccines and Mechanism of Antigen Expression Conventional mRNA carries the coding sequence of the antigen of interest (GOI) flanked by 5′ and 3′ UTRs, a terminal 5′ cap structure, and a 3′ poly(A) tail. Once delivered into the cell and released from the endosome into the cytoplasm, the mRNA is translated immediately. The self-amplifying mRNA is often derived from the genome of positive-sense single-stranded RNA viruses, such as alphaviruses. It encodes both the antigen of interest and viral nonstructural proteins (nsPs) required for intracellular RNA amplification and high levels of antigen expression. The self-amplifying mRNA can direct its self-amplification to generate RNA intermediates and many copies of antigen-encoding subgenomic mRNA, producing high levels of the encoded antigen. Both conventional mRNA and self-amplifying mRNA vaccines require a delivery system for cell uptake, usually by endocytosis, which is followed by unloading of mRNA cargo from the endosome into the cytosol, where translation and protein processing for MHC presentation occur. Once delivered in the cell, the mRNA is almost immediately sensed by pattern recognition receptors (PRRs) in the endosome and in the cytoplasm. PRRs such as Toll-like receptors TLR3, TLR7, and TLR8 are localized in the endosome, and cytosolic sensors such as RIG-I, MDA5, PKR, and OAS also recognize double-stranded and single-stranded RNAs in the cytoplasm. GOI, gene of interest; MHC, major histocompatibility complex; nsPs, nonstructural proteins.

Figure 2

Schematic Illustration of mRNA Vaccine Production Once a pathogen is identified or an outbreak declared, the genome of the pathogen and antigen(s) are determined, if not already available, by the combined sequencing, bioinformatics, and computational approach. Candidate vaccine antigen sequences are deposited electronically and available globally for in silico design of mRNA vaccines, followed by construction of plasmid DNA template by molecular cloning or synthesis. Pilot vaccine batches are generated in a cell-free system by in vitro transcription and capping of the mRNA, purification, and formulation with the delivery system. In-process analytic and potency tests are performed to assess the quality of pilot mRNA vaccine batches. If needed, pilot mRNA vaccine batches can be further tested in the immunogenicity and/or disease animal model. The final mRNA vaccine is scaled up and manufactured through a generic process with minimal modifications, rapidly tested, and dispatched for use. GMP, Good Manufacturing Practice.