Checks and balances between human cytomegalovirus replication and indoleamine-2,3-dioxygenase

Despite a rigorous blockade of interferoal (IFN-gamma) signalling in infected fibroblasts as a mechanism of immune evasion by human cytomegalovirus (HCMV), IFN-gamma induced indoleamine-2,3-dioxygenase (IDO) has been proposed to represent the major antiviral restriction factor limiting HCMV replication in epithelial cells. Here we show that HCMV efficiently blocks transcription of IFN-gamma-induced IDO mRNA both in infected fibroblasts and epithelial cells even in the presence of a preexisting IFN-induced antiviral state. This interference results in severe suppression of IDO bioactivity in HCMV-infected cells and restoration of vigorous HCMV replication. Depletion of IDO expression nonetheless substantially alleviated the antiviral impact of IFN-gamma treatment in both cell types. These findings highlight the effectiveness of this IFN-gamma induced effector gene in restricting HCMV productivity, but also the impact of viral counter-measures.