| Summary: | Classical swine fever virus (CSFV) is a positive-strand RNA virus that belongs to the genus pestivirus ofthe Flaviviridae family. CSFV can replicate in myeloid and endothelial cells without activating type I interferon (IFN) production and is able to inhibit IFN production and apoptosis induced''by double-stranded RNA (dsRNA). The N-terminal protease (NPj of CSFV plays a critical role in this suppression of the host innate immune response and was shown to target IRF-3 for proteasomal degradation, preventing transcriptional activation of the IFN promoter. This thesis aimed to study interactions between Npro and the host cellular machinery to further investigate the role ofNpro in the modulation ofhost antiviral responses by CSFV. In the present study, evidence is provided to show that Npro localises to the cytoplasm and nucleus ofmammalian cells infected with CSFV or expressing Npro. A yeast-two-hybrid screen initially reported that Npro interacts with hilla, the inhibitor of NF-K:B, a nuclear factor involved in the control of apoptosis, immune response and IFN production. This interaction was confirmed and characterised using co-precipitation assays and the functional implication of the interaction investigated. A single peak of NF-lCB activation was reported one hour post-infection in pig kidney cells (PKI5) infected with CSFV, but no modulation of NF-K:B activation was detected later during infection. Interestingly, a significant reduction of NF-lCB relative binding activity was shown to correlate with a transient nuclear' accumulation of IlCBa one hour post-stimulation with TNF-a in PK15 cells stably expressing Npro. In the last part of this thesis, the interaction between CSFV Npro and another cellular protein HAX-l, a mitochondrial anti-apoptotic protein was characterised using coprecipitation assays and immunofluorescence microscopy. The results reported in this thesis suggested that Npro acts as a pleiotropic protein that performs multifunctional roles in order to evade the host's defence mechanisms during CSFV infection.
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