What is mRNA vaccine?
The principle of mRNA vaccines is the translation of messenger RNA into antigens after delivery into the host cell through various pathways. Vaccine development can generally be divided into protein-based and gene-based. The protein-based strategy refer that recombinant or attenuated proteins are delivered directly into cells as immunogens to induce the activation of adaptive and humoral immune responses. The gene-based vaccines contained DNA and RNA are delivered to host cells where they are involved in protein expression to produce the corresponding antigens, thereby inducing the immune response in the host cells. The development of vaccines for COVID-19 has been adopted strategies of both protein-based and gene-based vaccine design.
Advantage of mRNA vaccines
mRNA vaccines have the advantage of fast speed for manufacturing. mRNA vaccines require a transcript that encodes a target antigen or immunogen. The RNA synthesis can immediately be carried out as soon as the sequence encoding the immunogen is available and the process can be easily scalable and cell-free during mRNA formulation and manufacturing. mRNA vaccines have a higher biosafety profile than DNA-based vaccines because translation of the antigen occurs in the cytoplasm rather than the nucleus, therefore mRNA integration into the genome is much less likely than DNA vaccines. While mRNA vaccines require ultralow temperatures for storage and transport, protein-based vaccines can be stored and transported under less stringent conditions. Nevertheless, lipid nanoparticle technologies contribute to the maintenance of stability of mRNA vaccines.
History of mRNA vaccines
As early as the late 1980s, mRNA molecules have been used to deliver into cells to manipulate gene expression or to produce proteins of interest. In early 1990s, by delivering RNA vectors encoding a reporter gene such as luciferase and β-galactosidase into murine muscle cells and transfecting vasopressin mRNA into rats, the therapeutic effects of direct expression of external mRNAs in host animals was first tested. In 1993, reported that an in vitro synthesized mRNA vaccine encoding influenza virus nucleoprotein can activate cytotoxic T lymphocytes in mice. Then, study has shown that in vivo application of mRNA induced both activation of cytotoxic T cells and humoral response of B cells to produce specific antibodies.
Design of mRNA vaccine
mRNAs are intrinsically unstable and tend to degradation because of the ubiquitous of RNases in the serum and plasma. In addition, the cellular machinery recognizes exogenous RNA molecules as immunological mimic of viral infection, which results in an immediate immune response. Therefore, maximizing the stability and translation efficiency of RNA and avoiding the innate immune response of host cells are prerequisites for designing mRNA vaccines. There are seven mRNA candidate vaccines for COVID-19, they are named mRNA-1273, BNT162b (3 LNP-mRNAs), CVnCoV, LUNAR-COV19, LNP-nCoVsaRNA and ARCoV, respectively. Nucleoside modification as one of the optimization strategies for mRNA vaccine design has been applied to the design of the seven vaccines for COVID-19.
RNA recognition by Toll-like receptors (TLRs) can be suppressed by modification of the nucleosides in mRNA molecules. Incorporating 5-methycytidine (m5C), N6-methyladenosine (m6A), 5-methyluridine (m5U), s2U, or pseudouridine (ψ) into mRNAs avoids the activation of TLR-3, TLR-7, and TLR-8, thereby to abrogate the immune response. For all the seven reported vaccines for COVID-19, pseudoUridine was incorporated into the mRNA vaccines in the place of uridine. Four of the mRNA vaccines is known to be used N1-methylpseudoUridine substitution, including mRNA-1273, BNT162b (3 LNP-mRNAs), CVnCoV and LUNAR-COV19. In addition, the substitution with pseudouridine, m6A, and s2U in RNA molecules suppresses the degradation of RNA by RNase L. Thus, the nucleoside modifications not only reduce the innate immune response but also enhance the stability of RNA.