Kunjin virus replicons: an RNA-based, non-cytopathic viral vector system for protein production, vaccine and gene therapy applications

The application of viral vectors for gene expression and delivery is rapidly evolving, with several entering clinical trials. However, a number of issues, including safety, gene expression levels, cell selectivity and antivector immunity, are driving the search for new vector systems. A number of replicon-based vectors derived from positive-strand RNA viruses have recently been developed, and this paper reviews the current knowledge on the first flavivirus replicon system, which is based on the Australian flavivirus Kunjin (KUN). Like most replicon systems, KUN replicons can be delivered as DNA, RNA or virus-like particles, they replicate their RNA in the cytoplasm and direct prolonged high-level gene expression. However, unlike most alphavirus replicon systems, KUN replicons are non-cytopathic, with transfected cells able to divide, allowing the establishment of cell lines stably expressing replicon RNA and heterologous genes. As vaccine vectors KUN replicons can induce potent, long-lived, protective, immunogen-specific CD8+ T cell immunity, a feature potentially related to extended production of antigen and double-stranded RNA-induced 'danger signals'. The identification of KUN replicon mutants that induce increased levels of IFN-alpha/beta has also spawned investigation of KUN replicons for use in cancer gene therapy. The unique characteristics of KUN replicons may thus make them suitable for specific protein production, vaccine and gene therapy applications.