Review An Update on mRNA-Based Viral Vaccines Subbiah Jeeva, Ki-Hye Kim, Chong Hyun Shin , Bao-Zhong Wang and Sang-Moo Kang * Center for Inflammation, Immunity & Infection, Institute for Biomedical Sciences, Georgia State University, Atlanta, GA 30303, USA; [email protected] (S.J.); [email protected] (K.-H.K.); [email protected] (C.H.S.); [email protected] (B.-Z.W.) * Correspondence: [email protected] Abstract: With the success of COVID-19 vaccines, newly created mRNA vaccines against other infectious diseases are beginning to emerge. Here, we review the structural elements required for designing mRNA vaccine constructs for effective in vitro synthetic transcription reactions. The unprecedently speedy development of mRNA vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was enabled with previous innovations in nucleoside modifications during in vitro transcription and lipid nanoparticle delivery materials of mRNA. Recent updates are briefly described in the status of mRNA vaccines against SARS-CoV-2, influenza virus, and other viral pathogens. Unique features of mRNA vaccine platforms and future perspectives are discussed. Keywords: mRNA vaccines; SARS-CoV-2; influenza 1. Introduction Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was identified as the Citation: Jeeva, S.; Kim, K.-H.; Shin, C.H.; Wang, B.-Z.; Kang, S.-M. An causative agent for the rapidly transmitting pandemic coronavirus disease 2019 (COVID-19) Update on mRNA-Based Viral across the globe since the first outbreak in December 2019 [1,2]. With a strong consen- Vaccines. Vaccines 2021, 9, 965. sus, there were unprecedented efforts and global coordination in developing COVID-19 https://doi.org/10.3390/ vaccines to control the pandemic sustainably. The vaccine platforms in a developmen- vaccines9090965 tal race included conventional approaches such as adjuvanted whole inactivated virus, subunits, and viral vectors, as well as genetic vaccines resulting in a renaissance of RNA Academic Editor: E. Diane vaccines despite unproven mRNA vaccine technologies on the market [3,4]. Just 64 days Williamson after the sequence information of novel SARS-CoV-2 RNA was available [1], the first USA clinical trial started with volunteers receiving the Moderna mRNA-1273 vaccine candi- Received: 3 July 2021 date [5–7]. With promising early safety and efficacy results from Moderna (mRNA-1273) Accepted: 25 August 2021 and Pfizer/BioNTech mRNA (BNT162b2) vaccine candidates, phase 3 trials were initiated Published: 29 August 2021 on 27 July 2020 [8]. As a consequence of successful completion of phase 3 clinical stud- ies, these two mRNA-based SARS-CoV-2 vaccines were proven to be safe and 94 to 95% Publisher’s Note: MDPI stays neutral efficacious in both healthy adults and elderly populations [9–11]. with regard to jurisdictional claims in It is a miraculous scientific triumph to develop novel mRNA vaccines, licensed first published maps and institutional affil- in the United Kingdom and then Canada and the USA, for emergency use authorization iations. for humans in an actual pandemic situation within a year with an unprecedented shorter time and higher efficacy than traditional vaccine platforms. COVID-19 mRNA vaccines are not the whole story of the RNA vaccine field. Scientists and companies had been developing, innovating, and improving RNA technologies for almost three decades prior Copyright: © 2021 by the authors. to COVID-19. In the 1990s, the injection of mRNA into mouse muscle resulted in protein Licensee MDPI, Basel, Switzerland. expression [12], and antigen-specific T cell responses were induced by immunization with This article is an open access article mRNA encoding influenza virus nucleoprotein [13]. However, inflammation and toxicity distributed under the terms and associated with unmodified mRNA and delivery vehicles were recognized as challenging conditions of the Creative Commons problems in translating mRNA vaccines to humans since RNA itself can be a reactogenic Attribution (CC BY) license (https:// inflammatory molecule. In 2005, the inclusion of modified nucleosides in mRNA transcripts creativecommons.org/licenses/by/ produced significantly lower reactogenic and inflammatory responses [14], improving the 4.0/). Vaccines 2021, 9, 965. https://doi.org/10.3390/vaccines9090965 https://www.mdpi.com/journal/vaccines Vaccines 2021, 9, x FOR PEER REVIEW 2 of 17 Vaccines 2021, 9, 965 2 of 17 improving the vaccine safety and enhanced mRNA translation efficacy in later studies [15–18]. Naked mRNA is ineffective in entering the cells, unstable, and easily destroyed. The development of lipid nanoparticles (LNP) to facilitate the delivery of RNA molecules vaccine safety and enhanced mRNA translation efficacy in later studies [15–18]. Naked into the cells in vivo has become a major step in innovating RNA technologies [19,20]. The mRNA is ineffective in entering the cells, unstable, and easily destroyed. The development first clinical phase 1 studies using modified mRNA vaccines in LNP were against influ- of lipid nanoparticles (LNP) to facilitate the delivery of RNA molecules into the cells enza virus H10 and H7 hemagglutinin (HA) during the years between 2015 and 2018, re- in vivo has become a major step in innovating RNA technologies [19,20]. The first clinical phasesulting 1 studiesin 100% using seroconversion modified mRNA [21,22]. vaccines The success in LNP of were COVID-19 against mRNA influenza vaccines virus H10 has andproven H7 that hemagglutinin RNA technology, (HA) during as a new the platform, years between is safe 2015and effective and 2018, for resulting commercial in 100% pro- seroconversionduction. [21,22]. The success of COVID-19 mRNA vaccines has proven that RNA technology,Here, we as areview new platform, the structural is safe elements and effective in designing for commercial mRNA production.vaccine constructs, the parametersHere, we to reviewbe optimized, the structural the current elements updates in designing of mRNA-based mRNA vaccine vaccines constructs, against viral the parametersdiseases, and to the be optimized,unique features the current of mRNA updates vaccine of mRNA-basedtechnology in vaccinesan era of againsta pandemic. viral diseases,New perspectives and the unique and future features applications of mRNA of vaccinemRNA technologytechnology inare an discussed era of a pandemic.to address Newthe development perspectives of and novel future mRNA applications vaccines of against mRNA challenging technology and are recurring discussed viruses to address and thethe developmentantigenic variants of novel of viral mRNA pathogens. vaccines against challenging and recurring viruses and the antigenic variants of viral pathogens. 2. Structural Elements in Constructing DNA Template for In Vitro mRNA Synthesis 2. StructuralFor efficient Elements translation, in Constructing as detailed DNA in prev Templateious review for In articles Vitro mRNA[23,24], Synthesisin vitro tran- scribedFor (IVT) efficient mRNA translation, products as should detailed have in previouscritical structural review articleselements, [23 which,24], in include vitro tran- the scribed5′ cap, untranslated (IVT) mRNA regions products (UTR) should on have both critical ends, structuralopen reading elements, frames which (ORF) include encoding the 5proteins,0 cap, untranslated and a poly-A regions tail (UTR)(Figure on 1). both The ends, 5′ cap open at the reading 5′end frames of the (ORF)mRNA encoding strand is pro- re- teins,quired and for aprotection poly-A tail from (Figure exonuclease1). The 5 0attack,cap at recognition the 50 end of by the eukaryotic mRNA strand translation is required initi- foration protection factor 4E from (eIF4E), exonuclease and to promote attack, recognitionthe translation by eukaryoticinitiation complex translation of ribosomes initiation factor[25,26]. 4E The (eIF4E), UTRs and of mRNAs to promote contribute the translation to stability, initiation translation complex efficiency, of ribosomes and recruiting [25,26]. ThemRNAs UTRs to of the mRNAs ribosomes contribute [27,28]. to stability,ORF in translationIVT mRNA efficiency, contains and coding recruiting sequences mRNAs to tobe thetranslated ribosomes into [ 27proteins.,28]. ORF Current in IVT COVID-19 mRNA contains mRNA coding vaccines sequences are in tovitro be translatedmanufactured, into proteins.replacing Current uridine COVID-19bases with mRNAN1-methylpseudouridine vaccines are in vitro (m1manufactured,ψ) to improve replacing safety and uridine trans- baseslation withefficacy N1-methylpseudouridine (Figure 1) [23,29–32]. The (m1 poly-A ) to improve tail also safetycontributes and translation to mRNA stability efficacy (Figureand is required1)[ 23,29 for–32 ].recognition The poly-A by tail poly-A also contributesbinding protein to mRNA (PABP), stability which and in return is required inter- foracts recognition with ribosome by poly-A initiation binding complex protein for (PABP), effective which translation in return [33–35]. interacts The with Pfizer/BioN- ribosome initiationTech mRNA complex vaccines for effectivewere designed translation to contain [33–35 two]. The segmented Pfizer/BioNTech poly-A tails mRNA with vaccines a short werespacer designed [23]. Essential to contain roles two of IVT segmented mRNA struct poly-Aural tails elements with a are short detailed spacer in [ 23the]. following Essential rolessections. of IVT mRNA structural elements are detailed in the following sections. Initiation complex Ribosome m1ψ m1ψ m1ψ m1ψ m1ψ m1ψ eIF4E m1ψ m1ψ PABP PABP 5’- -3’ Kozak Cap 5’UTR ORF