ICP47s

ICP47s refer to proteins encoded by certain viral genomes that interfere with host antigen presentation. The most studied example is the full‑length ICP47 protein from human cytomegalovirus (HCMV) and related herpesviruses, where the acronym stands for “immune evasion protein of the cytomegalovirus." These proteins act as inhibitors of the transporter associated with antigen processing (TAP), a heterodimeric channel that translocates peptides derived from intracellular proteins into the lumen of the endoplasmic reticulum for loading onto major histocompatibility complex (MHC) class I molecules. By blocking TAP function, ICP47 proteins reduce the repertoire of peptides presented to cytotoxic T lymphocytes, thereby obscuring infected cells from immune detection.

The ICP47 molecule is a small, soluble protein typically around 43 to 35 kDa depending on the viral species. Its N‑terminal domain contains a highly charged cluster that directly binds to the cytosolic loops of TAP, mechanically impeding peptide translocation. Structural analyses, including cryo‑electron microscopy, have shown that ICP47’s N‑terminus inserts into the TAP channel, while its C‑terminus remains exposed to the cytoplasm. Some viral homologues possess additional regulatory motifs that enhance their stability or alter their subcellular localization.

Evolutionarily, ICP47s share a common domain architecture with other TAP inhibitors such as UL49.5 from herpes simplex virus and US6 from HCMV, yet they differ markedly in sequence and in the specific residues that contact TAP. Genetic studies suggest that acquisition of ICP47‑like genes during virus evolution is driven by selective pressures to evade cytotoxic T‑cell surveillance.

Clinically, the presence of ICP47 variants in viral strains has been correlated with differential immune evasion capabilities and disease progression. Understanding ICP47 mechanism has inspired therapeutic strategies that aim to block its interaction with TAP, thereby restoring antigen presentation and enhancing antiviral immune clearance. Amid ongoing research, ICP47s remain a key model for studying intracellular immune evasion tactics employed by enveloped DNA viruses.