Inhibition of IFN-γ-induced STAT1 tyrosine phosphorylation by human CMV is mediated by SHP2

Human CMV (HCMV) is a ubiquitous beta-herpesvirus which has developed several mechanisms of escape from the immune system. IFN-gamma-induced signaling relies on the integrity of the JAK/STAT pathway which is regulated by phosphorylation steps and leads to nuclear translocation of tyrosine-phosphorylated STAT1 (STAT1-P-Tyr), and its binding to IFN-gamma activation site sequences of IFN-gamma-inducible promoters. Activation of those promoters leads to the expression of genes involved in the immune response and in the antiviral effects of IFN-gamma. Src homology region 2 domain-containing phosphatase 2 (SHP2) is a ubiquitous phosphatase involved in the regulation of IFN-gamma-mediated tyrosine phosphorylation. Several mechanisms account for the inhibition IFN-gamma signaling pathway by HCMV. In this study, we have identified a new mechanism that involved the inhibition of STAT1 tyrosine phosphorylation within 12-24 h postinfection. This defect was dependent on HCMV transcription. Consequences were impaired nuclear translocation of STAT1-P-Tyr, inhibition of IFN-gamma activation site-STAT1 interaction, and inhibition of HLA-DR expression. Expression of indoleamine-2,3-dioxygenase which is involved in the antiviral effects of IFN-gamma was also inhibited. Treatment of cells with sodium orthovanadate rescued STAT1 tyrosine phosphorylation, suggesting that a tyrosine phosphatase was involved in this inhibition. Coimmunoprecipitation of STAT1 and SHP2 was induced by HCMV infection, and SHP2 small interfering RNA restored the expression of STAT1-P-Tyr. Our data suggest that SHP2 activation induced by HCMV infection is responsible for the down-regulation of IFN-gamma-induced STAT1 tyrosine phosphorylation.