Interferon (IFN) is a cytokine, inducible by virus infection, that regulates the expression of IFN-stimulated genes including two double-stranded RNA-dependent enzymes: the protein kinase PKR, that mediates inhibition of host protein translation; and the adenosine deaminase ADAR1, that catalyzes deamination of RNA substrates. The roles ADAR1 and PKR in host responses to measles virus (MV) infection were investigated, including IFN-beta induction and stress granule formation. Human HeLa cells deficient in ADAR1 or PKR expression were used together with the Moraten vaccine strain of MV and two isogenic mutants derived from this strain lacking either V or C accessory proteins that alter innate host responses. Our laboratory previously demonstrated that, during MV infection, PKR is proapoptotic and is required for maximal IFN-beta transcription. In contrast, ADAR1 is antiapoptotic, suppresses PKR activation and enhances MV growth.

However, the involvement of ADAR1 in IFN-beta mRNA induction was not known. Furthermore, whether MV induced stress granules and the roles PKR and ADAR1 in this process likewise were not known. The results presented in the first part of this dissertation demonstrate that ADAR1 suppressed IFN-beta mRNA induction by impairment of PKR activation and of IFN-beta transcription factors ATF2, NF-kappaB and IRF3. Additionally, levels of IFN-beta protein production were determined and correlated inversely with PKR activation. The second part of this dissertation describes results pertaining to MV-induced stress granules, inducible aggregates of stalled ribosomes, mRNA and RNA binding proteins, that are hypothesized to regulate host translation patterns and virus replication. I found that stress granule formation required PKR and was impaired by ADAR1 and the MV C protein.

Finally, to shed light on the mechanism of ADAR1 regulation of host responses to MV infection that include SG formation, PKR activation, IFN-beta mRNA induction and enhancement of virus growth, ADAR1-deficient cells were complemented with wildtype and mutant ADAR1 expression plasmids and then tested for functional innate responses following infection. Deaminase activity of ADAR1 was required to modulate these host responses to infection. This work supports the notion that ADAR1 and PKR play opposing roles in the host response to MV infection: PKR is antiviral and ADAR1, remarkably, is proviral despite its IFN inducibility.