Murine cytomegalovirus paralyzes macrophages by blocking IFNγ-induced promoter assembly

Abstract

Macrophages (M ) are activated by IFN␥ and are important cellular targets for infection by human and murine cytomegalovirus (MCMV), making it advantageous for CMVs to block IFN␥-induced M differentiation. We found that MCMV infection inhibited IFN␥ regulation of many genes in M . MCMV infection blocked IFN␥ responses at the level of transcription without blocking Janus kinase͞signal transducer and activator of transcription pathway activation and targeted IFN response factor 1-and class II transactivator-dependent and independent promoters. MCMV did not alter basal transcription from IFN␥-responsive promoters and left the majority of cellular transcripts unchanged even after 48 h of infection. The effects of MCMV infection were specific to chromosomal rather than transiently transfected promoters. Characterization of the IFN␥-responsive chromosomal class II transactivator promoter revealed that MCMV infection blocked IFN␥-induced promoter assembly, allowing the virus to transcriptionally paralyze infected M responses while allowing basal transcription to proceed.immune evasion ͉ microarray F or the ␤-herpesviruses human cytomegalovirus (HCMV) and murine CMV (MCMV), macrophages (M ) and their progenitors play an important role in pathogenesis by providing vehicles for dissemination and a cellular site for viral latency (1-7). This reliance on M creates a problem for CMVs, because M are activated by antiviral cytokines such as IFN␥ in vivo (8,9) and are critical for control of MCMV infection (2). The M activating cytokine IFN␥ is crucial for controlling persistent MCMV replication in vivo (9) and reversibly inhibits reactivation of MCMV from latency, in part by blocking viral growth (9-11). M are activated to express increased MHC class II in vivo in response to IFN␥ during MCMV infection (8,9,12), and IFN␥ treatment of M decreases HCMV and MCMV growth up to 100-fold (10, 13).Given the importance of M and IFN␥ for control of CMV infection, it is not surprising that these viruses have strategies for altering differentiation of infected cells such as dendritic cells and M (14-18). For example, infection with HCMV or MCMV effectively inhibits IFN␥-induced antigen presentation by MHC class II by inhibiting IFN␥ induction of genes involved in antigen presentation (15,(19)(20)(21).Although HCMV and MCMV inhibit M and DC differentiation, the molecular mechanisms responsible for paralysis of cytokine-driven accessory immune cell differentiation are unknown. Because these viruses rely on cellular proteins during their prolonged replication cycle, it is likely that blockade of differentiation will involve mechanisms that preserve cellular functions critical for viral replication.We report here that MCMV infection inhibits expression of many IFN␥-responsive genes in M at the transcriptional level without affecting proximal signaling machinery or basal transcription. To accomplish this, MCMV inhibits IFN␥-induced chromosomal promoter assembly, providing an explanation for how global blockade of IFN␥-induced gene expression ca...

“Activated” STAT Proteins: A Paradoxical Consequence of Inhibited JAK-STAT Signaling in Cytomegalovirus-Infected Cells

“…1F). These results demonstrate that MCMV abrogates IFN-g-induced gene expression without compromising overall STAT1 protein amounts, receptor-proximal Y701 STAT1 phosphorylation, nuclear translocation of STAT1, or the capability to bind to DNA, largely confirming previous reports (35). In addition, our results reveal that this inhibition is not influenced by pM27, documenting the existence of at least one additional MCMVencoded antagonist of JAK-STAT signal transduction.…”

Cell-type-specific regulation of distinct sets of gene targets by Pax3 and Pax3/FKHR

“…A total of 3 Â 10 6 cells were washed in ice-cold PBS. Chromatin immunoprecipitations were then performed as described previously (Popkin et al, 2003). The predicted promoter regions of the human CNR1, EPHA2 and EPHA4 genes were obtained from Genbank.…”

Human cytomegalovirus interferes with signal transducer and activator of transcription (STAT) 2 protein stability and tyrosine phosphorylation

“…In contrast to the situation in HCMV, expression of STAT1 and IRF-9/p48 is unaffected by MCMV infection (Popkin et al, 2003;Zimmermann et al, 2005). However, we have recently identified that pM27 encoded by MCMV is an antagonist of STAT2 that mediates resistance to type I and type II IFNs.…”

“…This suggests that, during co-evolution of CMVs with their natural host, originally conserved genes like M27/UL27 were selected by divergently evolving immune systems and thus adapted to different functions. Remarkably, STAT1 tyrosine phosphorylation, which is shown to be strongly inhibited in HCMV-infected cells, remains fully intact after MCMV infection (Zimmermann et al, 2005) and is balanced only by an additional downstream mechanism which escapes IFN-c-mediated responses by blocking the assembly of STAT1-driven promoters (Popkin et al, 2003).…”

“…degradation of Jak1 has been proposed (Miller et al, 1998). Additionally, the HCMV IE1 protein pp72 exhibits an IFNantagonistic function, forming a physical complex with STAT2 to inhibit STAT2-dependent gene expression (Paulus et al, 2006).In contrast to the situation in HCMV, expression of STAT1 and IRF-9/p48 is unaffected by MCMV infection (Popkin et al, 2003;Zimmermann et al, 2005). However, we have recently identified that pM27 encoded by MCMV is an antagonist of STAT2 that mediates resistance to type I and type II IFNs.…”