HFE is a nonclassical class I major histocompatibility complex (MHC) molecule that is mutated in the autosomal recessive iron overload disease hereditary hemochromatosis. There is evidence linking HFE with reduced iron uptake by the transferrin receptor (TfR). Using a panel of HFE and TfR monoclonal antibodies to examine human HFE (hHFE)-expressing cell lines, we demonstrate the expression of stable and fully glycosylated TfR-free and TfR-associated hHFE/␤2m complexes. We show that both the stability and assembly of hHFE complexes can be modified by the human cytomegalovirus (HCMV) viral protein US2, known to interfere with the expression of classical class I MHC molecules. HCMV US2, but not US11, targets HFE molecules for degradation by the proteasome. Whether this interference with the regulation of iron metabolism by a viral protein is a means of potentiating viral replication remains to be determined. The reduced expression of classical class I MHC and HFE complexes provides the virus with an efficient tool for altering cellular metabolism and escaping certain immune responses.

HFE is a nonclassical class I molecule that associates with ␤2-microglobulin (␤2m) and with the transferrin receptor. HFE accumulates in transferrin-containing endosomes, and its overexpression in human cell lines correlates with decreased transferrin receptor (TFR)-mediated iron uptake and decreased intracellular iron pools. A mutation that interferes with proper folding and assembly of HFE complexes results in a severe iron-overload disease hereditary hemochromatosis. We previously suggested that viruses could also interfere with iron metabolism through the production of proteins that inactivate HFE, similarly to classical class I proteins. In particular, we demonstrated in a transient expression system that human cytomegalovirus (HCMV) US2 targeted HFE for proteasomal degradation. Here we demonstrate that the stable expression of HCMV US2 in HEK 293 cells constitutively expressing HFE leads to loss of HFE expression both intracellularly and on the cell surface, and the significant reduction of classical class I expression. Both HFE and classical class I molecules are targeted to degradation via a similar pathway. This HCMV US2-mediated degradation of HFE leads to increased intracellular iron pools as indicated by reduced synthesis of TfR and increased ferritin synthesis. Whether this interference with regulation of iron metabolism potentiates viral replication and/or promotes damage of HCMV-infected tissues remains to be determined. Nevertheless, the deleterious effect of US2 on the expression of HFE and classical class I major histo-compatibility complexes ( IntroductionHuman cytomegalovirus (HCMV) down-regulates the surface expression of major histocompatibility complex (MHC) class I molecules. 1-3 Analysis of HCMV deletion mutants led to the identification of genes within the short segment of the HCMV genome, ie, US2, US3, US6, and US11 that independently mediate this effect. [4][5][6][7][8][9][10][11] Two of these gene products, US2 and US11, target class I heavy chains for dislocation from the endoplasmic reticulum (ER) to the cytosol. 6,7 The heavy chains are deglycosylated by N-glycanase and, subsequently, degraded by the proteasome. The general mode of action of the 2 proteins seem to be similar, but they differ in their ability to attack allelic class I heavy chain gene products. 12 It has been suggested that US2 and US11 specifically down-regulate molecules that are associated with classical antigen presentation pathways because they preferentially promote the degradation of HLA-A and -B loci products relative to those encoded by Supporting the association of HCMV US2 with reduced antigen presentation by HCMV-infected cells and their escape from specific immune responses are data showing that US2 also down-regulates class II MHC DM␣ and DR␣ molecules. 16,17 Most effects of US2 were demonstrated using an astrocytoma cell line; however, recently both US2 and US11 have been shown to down-regulate class I, but not class II, MHC molecules in human dendritic cells. 18 We have previously observed that US...

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