Virus-based vectors for gene expression in mammalian cells: Coronavirus

The coronavirus and the torovirus genera form the Coronaviridae family, which is closely related to the Arteriviridae family. Both families are included in the Nidovirales order. Recently, a new group of invertebrate viruses, the Roniviridae, with a genetic structure and replication strategy similar to those of coronaviruses, has been described. This new virus family has been included within the Nidovirales. Coronaviruses have several advantages as vectors over other viral expression systems: (1) coronaviruses are single-stranded RNA viruses that replicate within the cytoplasm without a DNA intermediary, making integration of the virus genome into the host cell chromosome unlikely, (2) these viruses have the largest RNA virus genome and, in principle, have room for the insertion of large foreign genes, (3) a pleiotropic secretory immune response is best induced by the stimulation of gut-associated lymphoid tissues, (4) the tropism of coronaviruses may be modified by manipulation of the spike (S) protein allowing engineering of the tropism of the vector, (5) non-pathogenic coronavirus strains infecting most species of interest (human, porcine, bovine, canine, feline, and avian) are available to develop expression systems, and (6) infectious coronavirus cDNA clones are available to design expression systems. Within the coronavirus two types of expression vectors have been developed: one requires two components (helper–dependent expression system) and the other a single genome that is modified either by targeted recombination or by engineering a cDNA encoding an infectious RNA. This chapter focuses on the advantages and limitations of these coronavirus expression systems, the attempts to increase their expression levels by studying the transcription-regulating sequences (TRSs), and the proven possibility of modifying their tissue and species-specificity.