Abstract
Vaccinia virus (VACV) replicates in mouse and human fibroblasts with comparable kinetics and efficiency, yielding similar titers of infectious progeny. Here we demonstrate that gamma interferon (IFN-gamma) but not IFN-alpha or IFN-beta pretreatment of mouse fibroblasts prior to VACV infection induces a long-lasting antiviral state blocking VACV replication. In contrast, high doses of IFN-gamma failed to establish an antiviral state in human fibroblasts. In mouse fibroblasts, IFN-gamma impeded the viral replication cycle at the level of late gene transcription and blocked the multiplication of VACV genomes. The IFN-gamma-induced antiviral state invariably prevented the growth of different VACV strains but was not effective against the replication of ectromelia virus. The IFN-gamma effect required intact IFN-gamma receptor signaling prior to VACV infection through Janus kinase 2 (Jak2) and signal transducer and activator of transcription 1 (STAT1). The permissive state of IFN-gamma-treated human cells was unrelated to the VACV-encoded IFN decoy receptors B8 and B18 and associated with a complete disruption of STAT1 homodimer formation and DNA binding. Unlike human fibroblasts, mouse cells responded with long-lasting STAT1 activation which was preserved after VACV infection. The deletion of the IFN regulatory factor 1 (IRF-1) gene from mouse cells rescued efficient VACV replication, demonstrating that IRF-1 target genes have a critical role in VACV control. These data have implications for the understanding of VACV pathogenesis and identify an incongruent IFN-gamma response between the human host and the mouse model.