Human cytomegalovirus (HCMV), which infects the majority of the population worldwide, causes few, if any, symptoms in otherwise healthy people but is responsible for considerable morbidity and mortality in immunocompromised patients and in congenitally infected newborns. The evolutionary success of HCMV depends in part on its ability to evade host defense systems. Here we review recent progress in elucidating the remarkable assortment of mechanisms employed by HCMV and the related beta-herpesviruses, murine cytomegaloviruses (MCMV) and rhesus cytomegaloviruses (RhCMV), for counteracting the host
interferon (IFN) response. Very early after
infection, cellular membrane sensors such as the
lymphotoxin beta receptor initiate the production of
antiviral cytokines including type I IFNs. However, virion factors, such as pp65 (ppUL83) and
viral proteins made soon after
infection including the immediate early gene 2
protein (pUL122), repress this response by interfering with steps in the activation of
IFN regulatory factor 3 and
NF-kappaB. CMVs then exert a multi-pronged attack on downstream IFN signaling. HCMV
infection results in decreased accumulation and phosphorylation of the IFN signaling
kinases Jak1 and Stat2, and the MCMV
protein pM27 mediates Stat2 down-regulation, blocking both type I and type II IFN signaling. The HCMV immediate early gene 1
protein (pUL123) interacts with Stat2 and inhibits transcriptional activation of IFN-regulated genes.
Infection also causes reduction in the abundance of p48/IRF9, a component of the
ISGF3 transcription factor complex. Furthermore, CMVs have multiple genes involved in blocking the function of IFN-induced effectors. For example, viral
double-stranded RNA-binding proteins are required to prevent the shutoff of
protein synthesis by
protein kinase R, further demonstrating the vital importance of evading the IFN response at multiple levels during
infection.