Self-Replicating RNA Viruses for Vaccine Development against Infectious Diseases and Cancer

Abstract

Alphaviruses, flaviviruses, measles viruses and rhabdoviruses are enveloped single-stranded RNA viruses, which have been engineered for recombinant protein expression and vaccine development. Due to the presence of RNA-dependent RNA polymerase activity, subgenomic RNA can replicate close to 10⁶ copies per cell for translation in the cytoplasm providing extreme transgene expression levels, which is why they are named self-replicating RNA viruses. Expression of surface proteins of pathogens causing infectious disease and tumor antigens provide the basis for vaccine development against infectious diseases and cancer. Self-replicating RNA viral vectors can be administered as replicon RNA at significantly lower doses than conventional mRNA, recombinant particles, or DNA plasmids. Self-replicating RNA viral vectors have been applied for vaccine development against influenza virus, HIV, hepatitis B virus, human papilloma virus, Ebola virus, etc., showing robust immune response and protection in animal models. Recently, paramyxovirus and rhabdovirus vector-based SARS-CoV-2 vaccines as well as RNA vaccines based on self-amplifying alphaviruses have been evaluated in clinical settings. Vaccines against various cancers such as brain, breast, lung, ovarian, prostate cancer and melanoma have also been developed. Clinical trials have shown good safety and target-specific immune responses. Ervebo, the VSV-based vaccine against Ebola virus disease has been approved for human use.

Keywords: approval; cancer; immune response; infectious diseases; protection; self-replicating RNA viruses; tumor regression; vaccines.

Figure 1

Schematic illustration of self-replicating RNA alphavirus-based expression systems. Alphavirus-based delivery and expression systems comprise of infection of recombinant viral particles, electroporation/lipid-based transfection of in vitro transcribed RNA or transfection of plasmid DNA. Recombinant protein expression can be obtained as follows. In vitro transcribed RNA carrying the replicase gene and the gene of interest is electroporated/transfected into mammalian host cells (A). Alternatively, the replicon RNA can be delivered to host cells by infection with recombinant alphavirus particles (B). The third option is to transfect alphavirus DNA replicons (C), which after DNA delivery to the nucleus RNA is in vivo transcribed. The replicase complex will amplify RNA molecules (self-replication) and recombinant protein will be expressed from the 26S subgenomic promoter. 5′ cap, 5′ end cap analogue; 26S, alphavirus subgenomic promoter; CMV, cytomegalovirus promoter; GoI, gene of interest; pA, poly A signal; SP6, bacteriophage SP6 RNA polymerase promoter.

Figure 2

Self-replicating RNA viral vectors of negative polarity. (A) VSV vector for replacement of VSV G protein. (B) Rabies virus and (C). Measles virus expression vectors. CMV, cytomegalovirus promoter; Fu, fusion protein, G, glycoprotein; GoI, gene of interest; H, hemagglutinin; L, large protein; M, matrix protein; P. phosphoprotein; T7, T7 RNA polymerase promoter; T7T, T7 terminal sequence.

Figure 3

Replication strategy of positive- and negative-strand self-replicating RNA viruses. For positive-strand RNA viruses, the viral RNA is directly translated in the cytoplasm and replication of new positive-strand RNA copies require transcription of a negative-strand RNA template. In contrast, negative-strand RNA viruses rely on mRNA transcription before translation can take place.