Immune-activating mechanisms of replicase-based DNA and RNA vaccines and their role in immune-apoptosis

Abstract

DNA vaccines have revolutionized the field of vaccinology and hold enormous promise due to their versatility, safety and simplicity. Although naked, non-adjuvanted DNA vaccines have proven their ability to prevent or treat a very broad range of diseases in animal models, their immunogenicity - not only in certain animal models but also when used in non-human primates or humans in general - has been unsatisfactory. Numerous strategies have been developed to improve the immunogenicity and efficacy of DNA vaccine. These range from modifications of the plasmid to improvements in the delivery method, the addition of immunostimulatory molecules (both conventional and molecular adjuvants) and the design of heterologous prime-boost regimens in which a DNA prime is followed by a booster immunization with another vaccine platform such as a recombinant virus. One of the more recent innovations is the incorporation of gene sequences for the alphaviral replicase complex into a naked DNA plasmid. The original objective was to significantly increased antigen production with the help of a viral enzyme which replicates (vaccine encoded-) mRNA in transfected cells. Although this mechanism continues to be cited in the literature as the reason for the higher immunogenicity and efficacy of replicase-based vaccines, antigen-levels induced by such vectors do not necessarily correlate with their immunogenicity. Instead, cells transfected with replicase-based vectors detect dsRNA generated by the RNA replicase through various RNA sensors and interpret the presence of those nucleic acid species as evidence of a viral infection. The role each of the currently known dsRNA sensors plays in this event is still not well known and the analysis is complicated by a high degree of redundancy in their functions. While the 2′,5′-oligoadenylate synthetase/RNase L pathway has been shown to be essential, the removal of another dsRNA sensing molecule that is triggered by replicase-based vaccines (toll like receptor (TLR) 3) did not affect the immunogenicity of these vaccines.