MRNA vaccines have been shown to be effective in combating the COVID-19 pandemic. The amount of research on the use of mRNAs as preventive and therapeutic modalities has undergone explosive growth in the last few years. Nonetheless, the issue of the stability of mRNA molecules and their translation efficiency remains incompletely resolved. These characteristics of mRNA directly affect the expression level of a desired protein. Regulatory elements of RNA—5′ and 3′ untranslated regions (UTRs)—are responsible for translation efficiency. An optimal combination of the regulatory sequences allows mRNA to significantly increase the target protein’s expression. We assessed the translation efficiency of mRNA encoding of firefly luciferase with various 5′ and 3′UTRs in vitro on cell lines DC2.4 and THP1. We found that mRNAs containing 5′UTR sequences from eukaryotic genes HBB, HSPA1A, Rabb, or H4C2, or from the adenoviral leader sequence TPL, resulted in higher levels of luciferase bioluminescence 4 h after transfection of DC2.4 cells as compared with 5′UTR sequences used in vaccines mRNA-1273 and BNT162b2 from Moderna and BioNTech. mRNA containing TPL as the 5′UTR also showed higher efficiency (as compared with the 5′UTR from Moderna) at generating a T-cell response in mice immunized with mRNA vaccines encoding a multiepitope antigen. By contrast, no effects of various 5′UTRs and 3′UTRs were detectable in THP1 cells, suggesting that the observed effects are cell type specific. Further analyses enabled us to identify potential cell type-specific RNA-binding proteins that differ in landing sites within mRNAs with various 5′UTRs and 3′UTRs. Taken together, our data indicate high translation efficiency of TPL as a 5′UTR, according to experiments on DC2.4 cells and C57BL/6 mice.

MRNA-based therapeutics have been found to be a promising treatment strategy in immunotherapy, gene therapy, and cancer treatments. Effectiveness of mRNA therapeutics depends on the level and duration of a desired protein’s expression, which is determined by various cis- and trans-regulatory elements of the mRNA. Sequences of 5′ and 3′ untranslated regions (UTRs) are responsible for translational efficiency and stability of mRNA. An optimal combination of the regulatory sequences allows researchers to significantly increase the target protein’s expression. Using both literature data and previously obtained experimental data, we chose six sequences of 5′UTRs (adenoviral tripartite leader [TPL], HBB, rabbit β-globin [Rabb], H4C2, Moderna, and Neo2) and five sequences of 3′UTRs (mtRNR-EMCV, mtRNR-AES, mtRNR-mtRNR, BioNTech, and Moderna). By combining them, we constructed 30 in vitro transcribed RNAs encoding firefly luciferase with various combinations of 5′- and 3′UTRs, and the resultant bioluminescence was assessed in the DC2.4 cell line at 4, 8, 24, and 72 h after transfection. The cellular data enabled us to identify the best seven combinations of 5′- and 3′UTRs, whose translational efficiency was then assessed in BALB/c mice. Two combinations of 5′- and 3′UTRs (5′Rabb-3′mtRNR-EMCV and 5′TPL-3′Biontech) led to the most pronounced increase in the luciferase amount in the in vivo experiment in mice. Subsequent analysis of the stability of the mRNA indicated that the increase in luciferase expression is explained primarily by the efficiency of translation, not by the number of RNA molecules. Altogether, these findings suggest that 5′UTR-and-3′UTR combinations 5′Rabb-3′mtRNR- EMCV and 5′TPL-3′Biontech lead to high expression of target proteins and may be considered for use in preventive and therapeutic modalities based on mRNA.

Self-amplifying RNA (saRNA) is a promising platform for the production of vaccines, anti-tumor therapeutics, and gene therapy solutions. One of the advantages of the saRNA platform is the ability to use small doses of the therapeutic while maintaining prolonged expression of the target protein. However, the presence of auxiliary sequences encoding non-structural alphavirus proteins, which facilitate the replication of saRNA in cells, necessitates a thorough assessment of the biosafety of this platform. In our review, we focus on saRNA functions in the context of its interaction with the innate immune system. Firstly, an analysis is conducted of the side effects of candidate saRNA therapeutics, as observed in preclinical and clinical trials. Then, the mechanisms underlying the function of saRNA products derived from various alphavirus genomes in cell systems are discussed, as well as the reasons for their reactogenicity. The key approaches to optimizing the saRNA platform, which are aimed at reducing the activation of the innate immune response and cytopathic effects, are described. To summarize, this review enables us to systematize our knowledge on the advantages and disadvantages of saRNA, as well as potential approaches to improving this platform in order to develop more effective and safer therapeutics.

The success of mRNA-based vaccine formulations against viral infections motivated many researchers to develop mRNA vaccines against bacterial infections. The development of new anti-tuberculosis vaccine is an urgent task since the only approved BCG vaccine is not effective enough in terms of infection prevention, despite the fact that it reduces the risk of severe disease. The study aimed to compare two anti-tuberculosis mRNA vaccines based on the classic linear mRNA (mRNA-MTBmEp-5-1) and circular RNA (circRNA-MTB-mEp-5-1) by immunogenicity and the capability of protecting I/St mice against M. tuberculosis infection. The efficacy of mRNA vaccines in the formulations with lipid nanoparticles was compared with the BCG efficacy. The findings suggest that immunization with the mRNA vaccine based on the linear mRNA resulted in the cell-based and humoral immune response (OD IgG = 0.36 ± 0.12) that was less pronounced than after BCG vaccination (OD IgG = 0.54 ± 0.14). At the same time, immunization with the mRNA vaccine and BCG ensured comparable reduction of bacterial load in the lung and spleen of experimental mice (CFU in lung tissue for BCG: 4.00 × 105 ± 2.13 × 105, p = 0.0068; mRNA: 4.72 × 105 ± 3.44 × 105, p = 0.0059; LNP: 4.91 × 106 ± 3.89 × 106, ns; PBS: 4.01 × 106 ± 1.69 × 106) and increased survival of mice after getting infected with M. tuberculosis. Immunization with the vaccine based on the circular RNA resulted in developing humoral mmunity only (OD IgG = 0.52 ± 0.13) and did not ensure protection after getting infected with M. tuberculosis (CFU in the lung for circRNA: 2.12 × 106 ± 5.30 × 105, p = 0.85). Thus, in our studies, anti-tuberculosis vaccines based on circular RNAs are inferior in effectiveness to formulations based on linear RNAs.