Tailoring mRNA Vaccine to Balance Innate/Adaptive Immune Response

mRNA-based vaccines enable the well-controlled design of on-demand transcript sequences and could be adapted to any pandemic crisis. In vitro-transcribed (IVT) mRNA used for formulation and subsequent administration has to be highly homogeneous with no DNA, dsRNA, or 5′ triphosphate transcripts in order to avoid any overstimulation of innate immunity. During the formulation process, mRNA–cargo interactions could unmask sequences leading to unexpected side effects. The intricate mechanisms of RNA sensing by PRRs, accounts for the unpredictability of in vitro assays. To date, there is no animal model for mRNA vaccines that matches perfectly human immune responses, underlying the importance of clinical trials. Moreover, innate immune activation by mRNA molecules is dependent on the route of administration and the species, hampering the interpretation of in vitro or preclinical data. mRNA vaccine platforms present numerous advantages, such as versatility, rapid production, and induction of cellular and humoral responses. Moreover, mRNAs have inherent adjuvant properties due to their complex interaction with pattern recognition receptors (PRRs). This recognition can be either beneficial in activating antigen-presenting cells (APCs) or detrimental by indirectly blocking mRNA translation. To decipher this Janus effect, we describe the different innate response mechanisms triggered by mRNA molecules and how each element from the 5′ cap to the poly-A tail interferes with innate/adaptive immune responses. Then, we emphasize the importance of some critical steps such as production, purification, and formulation as key events to further improve the quality of immune responses and balance innate and adaptive immunity. mRNA vaccine platforms present numerous advantages, such as versatility, rapid production, and induction of cellular and humoral responses. Moreover, mRNAs have inherent adjuvant properties due to their complex interaction with pattern recognition receptors (PRRs). This recognition can be either beneficial in activating antigen-presenting cells (APCs) or detrimental by indirectly blocking mRNA translation. To decipher this Janus effect, we describe the different innate response mechanisms triggered by mRNA molecules and how each element from the 5′ cap to the poly-A tail interferes with innate/adaptive immune responses. Then, we emphasize the importance of some critical steps such as production, purification, and formulation as key events to further improve the quality of immune responses and balance innate and adaptive immunity. nonprotein coding region important for protein expression and mRNA stability. nonprotein coding region important for ribosome–mRNA recognition and translation initiation, protein expression, and mRNA stability. the main players in mediating the adaptive immune response by processing and presenting antigens to T lymphocytes. APCs examples are DCs, macrophages, and B cells. these molecules are recognized as PAMPs in the cytoplasm of mammalian cells by different PRRs. dsRNA molecules are normally the result of a viral infection, although some endogenous dsRNA can be found mainly in the cell nucleus. RNA molecules of 19–22 nucleotides in length. They can act as post-transcriptional regulators, inhibiting the translation of RNAs. method to produce mRNA using DNA as a template. It is produced by an enzymatic reaction where the DNA contains the promoter for a DNA-dependent RNA polymerase and all ad hoc sequences. Later, the reaction is treated with DNases, that degrade the DNA template, and purified to obtain pure mRNA. corresponds to the sequence that encodes the antigen, begins with a start codon (AUG) and finishes with a stop codon. For optimal translation initiation, the ORF should contain an optimal Kozak sequence (GCC-(A/G)-CCAUGG) surrounding the initiation AUG codon. molecules that are found in different pathogens and are not naturally found in the host. Examples are dsRNA, 5′-triphosphate and uncapped RNAs, LPS, and flagellin. part of the cellular innate immune system. These receptors are invariable and encoded in the germline. PRRs are located in the cytosol (RIG-like receptor, Nod-like receptor) or in the extracellular membrane and intracellular vesicles (TLRs, C-type lectin receptors). Their function is to recognize and sense PAMPs to initiate the immune response. confers mRNA stability and its length has been related to positively increasing the half-life of mRNA. represented in the cell by mRNA, rRNA, small interfering (si)RNA, long noncoding RNA, and tRNA. ssRNA can form intramolecular dsRNA secondary structures such as a helix, stem-loop, or pseudoknots and trigger PRRs. group of cytokines that gather the most important interferon IFN-α, IFN-β, but also IFN-ε, -κ, -τ, -δ, -ζ, -ω, -ν. They are produced as an antiviral response and are responsible for the activation of hundreds of ISGs, providing an antiviral cell status of own cells (autocrine) and surrounding cells (paracrine), which includes mRNA translation blockade and its degradation.