Poxviruses are large dsDNA viruses that replicate in the cytosol. Upon entry, the virus core expands allowing early gene expression, followed by core disassembly, then genome release and replication within viral factories. Some poxviruses fail to grow in mammalian cell culture; however receptor specificity does not determine this. All poxviruses should enter cells and generate early mRNA. Besides mediating core disassembly and genome release, early gene expression is also crucial to subvert host defences. In this thesis the hypothesis that the early events in poxvirus infection are a method of preventing innate immune sensing were investigated alongside whether these events could be exploited to generate novel detection methods.

No quantitative reporter cell line exists for poxviruses. Here, a fluorescent reporter cell line is presented for poxvirus detection based on the use of reverse transcriptase (RT) to create cDNA from an mRNA reporter cassette. Stable cell lines expressing the reporter cassette and RT showed detectable fluorescence upon infection with several poxvirus species. The dynamics of activation correlated with virus multiplicity and replicative cycle. This cell line offers distinct advantages over alternative detection methods, such as detecting infectious virus, offering the possibility of propagating the virus and being a rapid pan-poxvirus detection method.

Viruses lacking the cGAMP nuclease (Poxin/vSlfn) activate an antiviral response that depends on the cellular DNA sensor cGAS. Here, these viruses are used to show poxvirus sensing occurs prior to virus genome replication, but this is prevented by Poxin/vSlfn expression during uncoating. Unexpectedly, despite sensing replicated genomes, cGAS is excluded from viral factories and accumulates in the nucleus. These data indicate that despite replication in the hostile cytosolic environment, poxviruses have evolved complex processes preventing immune recognition and activation. Whether these functions are conserved in all poxviruses or how they co-operate with Poxin/vSlfn remains unknown.