Alphavirus vectors: from protein production to gene therapy

Abstract

Alphaviruses are enveloped viruses containing a single positive strand RNA molecule as genome. Several vectors derived from alphaviruses have been developed, which include Sindbis virus (SIN), Semliki Forest virus (SFV), and Venezuelan Equine Encephalitis virus (VEE). Alphavirus self-replicating RNA containing heterologous genes can be synthesized in vitro from plasmids having the recombinant alphavirus replicon sequences under the control of a prokaryotic promoter, such as SP6 or T7. High level expression of the heterologous proteins/RNA is obtained in cells transfected with this RNA. Although the system can be used for gene delivery directly as naked RNA, several packaging systems have been developed which allow the encapsidation of the alphaviral recombinant RNA into suicidal viral particles, increasing the efficiency of delivery of the recombinant genome into cells. A more recent variant of the system is based on a DNA/RNA layered alphaviral vector in which the recombinant replicon is transcribed from an RNA polymerase-II promoter, allowing direct delivery of DNA into cells. Alphavirus vectors have been used to express a great number of proteins with many different purposes, including protein production and characterization, functional studies, vaccination, and gene therapy. Both the recombinant alphavirus particles and the alphavirus nucleic acid vectors have shown to be able to induce protective immune responses in animal models. The possible application of these vectors in gene therapy faces, however, two limitations, which are the lack of specific targeting and the transient nature of the vector, due to the induction of apoptosis by the alphavirus replicon. Several strategies have been recently described to improve the targeting of alphavirus vectors and to develop noncytopathic vectors with potential use in gene therapy.