Table of Contents:
In 2003, a new coronavirus was isolated from patients suffering from a severe acute respiratory syndrome (SARS-CoV) characterized by an unusual high mortality rate of 10%.
Within this work the question has been addressed, if the interplay between SARS-CoV and different host defence mechanisms contributes to this unique pathogenesis. The focus was set on the type I interferon (IFN) system and the induction of apoptosis as antiviral mechanisms.
SARS-CoV infection does not induce type I IFNs although dsRNA, known to be a potent inducer of IFNs, is present in infected cells. Therefore, all SARS-CoV proteins should be analyzed for their ability to inhibit the production of type I IFNs. An IFN-sensitive reporter assay revealed that none of the tested ORFs was able to inhibit the induction of type I IFNs. Furthermore, the expression of ORF 7a and ORF 7b led to the induction of type I IFNs as well as apoptosis in transfected cells, whereas a naturally acquired deletion mutant of ORF 7b did not. The inhibition of ORF 7a by peptide-linked phosphorodiamidate morpholino oligomers (PPMO) resulted in a reduction of released SARS-CoV particles, presumably due to an unspecific binding to 5’-regions important for replication and transcription. The inhibition of ORF 7b had no effect.
SARS-CoV-infection induces apoptosis, as shown by cleavage of caspases 3 and 8 as well as cleavage of poly-ADP-ribose polymerase-1 (PARP-1), but did not alter mRNA or protein levels of Bcl2 and Bax.
An important link between IFN system and induction of apoptosis is the antiviral protein kinase PKR. This kinase is transcriptionally induced via IFNs and is able to phosphorylate the eukaryotic translation initiation factor (eIF) 2 leading to the inhibition of translation. Thereby or via direct interactions with pro-apoptotic proteins PKR induces apoptosis. PERK and GCN2 also phosphorylate eIF2 in response to virus-induced stimuli.