Untersuchungen zur Blockade zellulärer antiviraler Mechanismen durch das Hepatitis A-Virus unter besonderer Berücksichtigung der Proteinkinase R und des Transkriptionsfaktors NF-kappaB

Abstract

Virus infection normally induces expression and secretion of type I interferon (IFN), leading to the establishment of an antiviral state in adjacent cells. In rhesus monkey kidney cells, hepatitis A virus (HAV) suppresses induction of IFN-b on the level of transcription, even after transfection with poly(IC), a dsRNA analogon. Furthermore, HAV is able to establish a persistent infection in cell culture and to suppress apoptosis. The objectives of this work were 1) to transfer the above results into a human cell culture system and 2) to examine possible cellular targets of HAV. Human MRC-5 fibroblasts were infected with HAV, and results show that IFN-b is not induced by infection and that after poly(IC) transfection HAV actively suppresses IFN-b production (determined by plaque reduction assay). Suppression of IFN-b expression occurs on the level of mRNA, as detected by RT-PCR suggesting one or more proteins of dsRNA-dependent signalling pathways to be affected by HAV. Protein kinase R (PKR) is involved in dsRNA-induced apoptosis and furthermore, PKR has been described to be involved in the activation of NF-kB, a transcription factor participating in induction of IFN-b. Autophosphorylation is a hallmark of PKR-activation and therefore, phosphorylation of PKR on Thr-451 was analysed by immunoblotting. In HAV-infected cells, PKR-phosphorylation appeared to be generally reduced, whereas a degradation of PKR could not be detected. When cellular localization of NF-kB was examined by immunofluorescence, poly(IC)-induced nuclear translocation of NF-kB was found to be unaffected and even enhanced by HAV, excluding a role for NF-kB in HAV-mediated suppression of IFN-b. However, HAV-induced activation of anti-apoptotic NF-kB may contribute to survival of HAV-infected cells. The suppression of IFN-b and the influence on PKR, NF-kB and cell vitality may allow HAV to establish a persistent infection in cell culture and might be a prerequisite for successful replication in vivo.