Self-Amplifying Replicon RNA Vaccine Delivery to Dendritic Cells by Synthetic Nanoparticles

Abstract

Dendritic cells (DC) play essential roles determining efficacy of vaccine delivery with respect to immune defence development and regulation. This renders DCs important targets for vaccine delivery, particularly RNA vaccines. While delivery of interfering RNA oligonucleotides to the appropriate intracellular sites for RNA-interference has proven successful, the methodologies are identical for RNA vaccines, which require delivery to RNA translation sites. Delivery of mRNA has benefitted from application of cationic entities; these offer value following endocytosis of RNA, when cationic or amphipathic properties can promote endocytic vesicle membrane perturbation to facilitate cytosolic translocation. The present review presents how such advances are being applied to the delivery of a new form of RNA vaccine, replicons (RepRNA) carrying inserted foreign genes of interest encoding vaccine antigens. Approaches have been developed for delivery to DCs, leading to the translation of the RepRNA and encoded vaccine antigens both in vitro and in vivo. Potential mechanisms favouring efficient delivery leading to translation are discussed with respect to the DC endocytic machinery, showing the importance of cytosolic translocation from acidifying endocytic structures. The review relates the DC endocytic pathways to immune response induction, and the potential advantages for these self-replicating RNA vaccines in the near future.

Keywords: nanoparticle delivery; self-replicating replicon RNA; targeting dendritic cells.

Figure 1

Delivery of RepRNA to DCs by nanoparticulate delivery vehicles. The delivery vehicles can be composed of polysaccharides, lipids, lipoproteins or combinations thereof. The nature of the delivery vehicle composition is to provide encapsulation of the RepRNA to protect against RNases, facilitate delivery to DCs and ensure a level of compaction enabling the RepRNA to interact with the ribosomal translation machinery. The surface of the nanoparticulate delivery vehicle may be coated to enhance stability and/or provide a means of enhance targeting of the DCs.

Figure 2

The basic RepRNA construct ensures efficient translation of the encoded vaccine antigen of interest, as well as replication of the replicon. Insertion of an internal ribosomal entry site (IRES) from, for example, EMC virus ensures that translation of the proteins for replication continues after translation of the vaccine antigen of interest.

Figure 3

Nanoparticulate delivery of the RepRNA is designed to promote efficient uptake into endocytic vesicles, in which the RepRNA can be seen to accumulate. Thereafter, a gradual cytosolic translocation of the RepRNA from the endocytic vesicles, probably promoted by acidification of the vesicles, is essential to ensure delivery of the RepRNA to the intracellular site for RNA translation. It is considered that the acidification process, together with activation of the cellular redox processes facilitated destabilisation of the delivery vehicle and RNA, to permit entry of the ribosomes for translation.