<p>The innate host response to virus infection is largely dominated by the production of type I interferons (IFNs). Fibroblasts, considered nonprofessional immune cells, respond to virus infection after recognition of viral components such as double-stranded (ds)RNA. The constitutively expressed transcription factor IFN regulatory factor 3 (IRF3) is rapidly activated and type I IFNs are produced. In the absence of IRF3, it was found that IFNs are still produced. This thesis identifies IRF9 as the transcription factor responsible for IFN production in the absence of IRF3 based on its ability to bind the murine (m)IFNβ promoter determined via oligonucleotide pull-down assays.</p> <p>In the absence of both IRF3 and IRF9, primary fibroblasts are deficient for IFN signalling. Surprisingly, significant inhibition of virus replication following dsRNA treatment of cells deficient for IRF3 and IFN signalling was recently observed with the large DNA virus herpes simplex virus type 1 (HSV-1) being more susceptible to inhibition than the small RNA virus vesicular stomatitis virus (VSV). As nitric oxide is known for its nonspecific antiviral effects against DNA viruses, involvement of this molecule in the antiviral response to HSV-1 in the absence of IRF3 and type I IFN induction and signalling was investigated. Here it is shown that in the absence of IRF3 and IFN, nitric oxide constitutes a major component of the innate response against HSV-1 in response to dsRNA in primary fibroblasts. In these cells, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and IRF1 regulate inducible nitric oxide synthase (iNOS) expression, subsequently producing nitric oxide. As most viruses encode strategies to render their environment IRF3 and/or IFN deficient, it appears that IRF9 and nitric oxide serve as secondary responses to protect the host against viral infection. These data emphasize the importance and requirement of the host to employ multiple strategies to overcome infection.</p> / Master of Science (MSc)