Vaccinia virus (VACV) expresses many proteins that subvert the host innate immune
response and this thesis describes the characterisation of C4, a VACV protein
unstudied hitherto, which was hypothesised to have such a role.
C4 is conserved in six orthopoxvirus species and shares 43.7% amino acid identity to
VACV protein C16, a known virulence factor. C4 is an intracellular protein that is
expressed early during infection and localises to both the cytoplasm and nucleus.
Functional screens revealed C4 is an inhibitor of IFNβ promoter activation via the
inhibition of NF-κB activation. Mechanistically, this inhibition is attributable to the
prevention of IκBα phosphorylation. Consistent with this mode of action, C4 coprecipitated
with the canonical IKK proteins, IKKβ and NEMO. Protein C16 and the
icIL-1Ra were also found to inhibit NF-κB activation. Furthermore, C4 is an inhibitor
of cytoplasmic DNA sensing in MEFs and interacts with Ku70 and Ku80 (Ku), two
components of the DNA sensor, DNA-PK.
Another novel discovery is that VACV infection leads to the stabilisation and
translocation of HIF-1α. This process is largely dependent on C16, however C4 is
also required for maximal HIF-1α stabilisation and transactivation. The role of HIF-1α during infection and the mechanism used by C4 to contribute to its stabilisation
process require further investigation.
Whilst C4 is non-essential for virus replication and spread in cell culture, its deletion
does result in reduced virus virulence in a murine intranasal model of infection.
Finally, the deletion of both C16 and C4 lead to a synergistic attenuation of virus
virulence in both the intranasal and intradermal models of infection.