Lipid-based mRNA vaccine delivery systems Expert Rev. Vaccines Early online, 1–14 (2014) Patrick Midoux* and Chantal Pichon* Centre de Biophysique Mole ´ culaire, CNRS UPR4301, Inserm and Universite ´ d’Orle ´ ans, Orle ´ans, 45071, cedex 02, France *Authors for correspondence: Patrick.midoux@cnrs-orleans.fr Chantal.pichon@cnrs-orleans.fr Synthetic mRNAs can become biopharmaceutics allowing vaccination against cancer, bacterial and virus infections. Clinical trials with direct administration of synthetic mRNAs encoding tumor antigens demonstrated safety and induction of tumor-specific immune responses. Although immune responses are generated by naked mRNAs, their formulations with chemical carriers are expected to provide more specificity and internalization in dendritic cells (DCs) for better immune responses and dose reduction. This review reports lipid-based formulations (LBFs) that have proved preclinical efficacy. The selective delivery of mRNA LBFs to favor intracellular accumulation in DCs and reduction of the effective doses is discussed, notably to decorate LBFs with carbohydrates or glycomimetics allowing endocytosis in DCs. We also report how smart intracellular delivery is achieved using pH-sensitive lipids or polymers for an efficient mRNA escape from endosomes and limitations regarding cytosolic mRNA location for translation. KEYWORDS: dendritic cells • glycomimetics • glycotargeting • histidine • lipid-based formulations • lipoplexes • lipopolyplexes • mannose receptors • pH-sensitive delivery • RNA vaccines Dendritic cells (DCs) play a critical role in antigen presentation to elicit a specific immune response against antigens which can be proteins, peptides and nucleic acids like plasmid DNA or mRNA encoding the anti- gens. In this context, the transfection of DCs with synthetic mRNA encoding antigen is becoming increasingly important for the design of innovative vaccines [1,2]. One major challenge to be tackled for their development is their targeting to DCs. It is crucial to develop delivery systems that in vivo protect mRNAs from degradation and help internali- zation in DCs and favor intracellular delivery in the right compartment. Since mRNA trans- lation occurs in the cytosol, the transfection of DCs with synthetic mRNA avoids the neces- sity of mRNA to pass the selective barrier of the nuclear envelope. Therefore, the delivery of synthetic mRNA into the cytosol of non- dividing cells looks easier than that of plasmid DNA in the nucleus. When the synthetic mRNA encoding tumor antigen is expressed in DCs, it allows the antigen expression in its native form. When processed by the protea- some, all generated peptide epitopes of the antigen enter the endoplasmic reticulum (ER), bind to the major histocompatibility complex (MHC) class I molecules and then are expressed on the surface of DCs. The antigen presentation by the MHC class I complex leads to the induction of tumor-specific CD8 + cytotoxic T lymphocytes that in turn kill tumor cells (FIGURE 1). The MHC class II anti- gen presentation is induced after the uptake of the antigen released in the extracellular medium or the transfection with mRNA encoding the antigen fused with the lysosomal sorting signal of LAMP1 protein targeting the MHC class II compartments (FIGURE 1). Natural mRNAs comprise a 5´ untranslated region, an open-reading frame consisting of the antigen nucleic sequence and a 3´ untranslated region. They are also ended by a cap structure at the 5´ terminus and a Poly(A) tail of at least 100–250 adenosine residues at the 3´ termi- nus. Synthetic mRNAs obtained by in vitro transcription have a similar structure (FIGURE 2). They are capped either with 3´-O-methyl- m7 5´ Gppp5´G (ARCA) or 2´-O-methyl- m7 5´ GppSp5´G (b-S-ARCA) anti-reverse cap ana- log allowing cap incorporation in the right ori- entation [3] and bear a Poly(A) tail comprising 64 or about 100 adenosine residues [4,5]. In contrast to plasmid DNAs, synthetic mRNAs cannot be integrated in the genome, do not bear promoter, terminator, enhancer and antibiotic-resistance nucleic sequences. There is informahealthcare.com 10.1586/14760584.2015.986104 Ó 2014 Informa UK Ltd ISSN 1476-0584 1 Review Expert Review of Vaccines Downloaded from informahealthcare.com by Nandini Loganathan on 01/06/15 For personal use only.