Human cytomegalovirus protein US2 interferes with the expression of human HFE, a nonclassical class I major histocompatibility complex molecule that regulates iron homeostasis

Epstein-Barr Virus Infection Is Associated with Elevated Hepcidin Levels

EBV and Helicobacter pylori (H. pylori) cause highly prevalent persistent infections as early as in childhood. Both pathogens are associated with gastric carcinogenesis. H. pylori interferes with iron metabolism, enhancing the synthesis of acute-phase proteins hepcidin, C-reactive protein (CRP), and α-1 glycoprotein (AGP), but we do not know whether EBV does the same. In this study, we correlated the EBV antibody levels and the serum levels of hepcidin, CRP, and AGP in 145 children from boarding schools in Mexico City. We found that children IgG positive to EBV antigens (VCA, EBNA1, and EA) presented hepcidin, AGP, and CRP levels higher than uninfected children. Hepcidin and AGP remained high in children solely infected with EBV, while CRP was only significantly high in coinfected children. We observed positive correlations between hepcidin and EBV IgG antibodies (p < 0.5). Using the TCGA gastric cancer database, we also observed an association between EBV and hepcidin upregulation. The TCGA database also allowed us to analyze the two important pathways controlling hepcidin expression, BMP−SMAD and IL-1β/IL-6. We observed only the IL-1β/IL-6-dependent inflammatory pathway being significantly associated with EBV infection. We showed here for the first time an association between EBV and enhanced levels of hepcidin. Further studies should consider EBV when evaluating iron metabolism and anemia, and whether in the long run this is an important mechanism of undernourishment and EBV gastric carcinogenesis.

Ferroptosis in viral infection: the unexplored possibility

Virus-induced cell death has long been thought of as a double-edged sword in the inhibition or exacerbation of viral infections. The vital role of iron, an essential element for various enzymes in the maintenance of cellular physiology and efficient viral replication, places it at the crossroads and makes it a micronutrient of competition between the viruses and the host. Viruses can interrupt iron uptake and the antioxidant response system, while others can utilize iron transporter proteins as receptors. Interestingly, the unavailability of iron facilitates certain viral infections and causes cell death characterized by lipid peroxide accumulation and malfunction of the antioxidant system. In this review, we discuss how iron uptake, regulation and metabolism, including the redistribution of iron in the host defense system during viral infection, can induce ferroptosis. Fenton reactions, a central characteristic of ferroptosis, are caused by the increased iron content in the cell. Therefore, viral infections that increase cellular iron content or intestinal iron absorption are likely to cause ferroptosis. In addition, we discuss the hijacking of the iron regulatoy pathway and the antioxidant response, both of which are typical in viral infections. Understanding the potential signaling mechanisms of ferroptosis in viral infections will aid in the development of new therapeutic agents.

HFE inhibits type I IFNs signaling by targeting the SQSTM1-mediated MAVS autophagic degradation

Iron metabolism is involved in numerous physiological processes such as erythropoiesis, oxidative metabolism. However, the in vivo physiological functions of the iron metabolism-related gene Hfe in immune response during viral infection remain poorly understood. Here, we identified 5 iron metabolism-associated genes specifically affected during RNA virus infection by a high-throughput assay and further found that HFE was a key negative regulator of RIG-I-like receptors (RLR)-mediated type I interferons (IFNs) signaling. RNA virus infection inhibited the binding of HFE to MAVS (mitochondrial antiviral signaling protein) and blocked MAVS degradation via selective autophagy. HFE mediated MAVS autophagic degradation by binding to SQSTM1/p62. Depletion of Hfe abrogated the autophagic degradation of MAVS, leading to the stronger antiviral immune response. These findings established a novel regulatory role of selective autophagy in innate antiviral immune response by the iron metabolism-related gene Hfe. These data further provided insights into the crosstalk among iron metabolism, autophagy, and innate immune response.Abbreviations: ATG: autophagy-related; BAL: bronchoalveolar lavage fluid; BMDMs: bone marrow-derived macrophages; CGAS: cyclic GMP-AMP synthase; CQ: chloroquine; Dpi: days post-infection; ELISA: enzyme-linked immunosorbent assay; GFP: green fluorescent protein; HAMP: hepcidin antimicrobial peptide; Hpi: hours post-infection; HJV: hemojuvelin BMP co-receptor; IFNs: interferons; IL6: interleukin 6; IRF3: interferon regulatory factor 3; ISRE: interferon-stimulated response element; Lipo: clodronate liposomes; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MAVS: mitochondrial antiviral signaling protein; MEFs: mouse embryonic fibroblasts; SLC40A1/FPN1: solute carrier family 40 (iron-regulated transporter), member 1; flatiron; SQSTM1/p62: sequestosome 1; STAT1: signal transducer and activator of transcription 1; STING1/STING: stimulator of interferon response cGAMP interactor 1; TBK1: TANK-binding kinase 1; TFRC/TfR1: transferrin receptor; TNF/TNFα: tumor necrosis factor; WT: wild type.

Ub to no good: How cytomegaloviruses exploit the ubiquitin proteasome system

Human cytomegalovirus (HCMV) is a ubiquitous member of the Betaherpesvirinae subfamily, causing life-threatening diseases in individuals with impaired, immature, or senescent immunity. Accordingly, HIV-infected AIDS patients, transplant recipients, and congenitally infected neonates frequently suffer from symptomatic episodes of HCMV replication. Like all viruses, HCMV has a split relationship with the host proteome. Efficient virus replication can only be achieved if proteins involved in intrinsic, innate, and adaptive immune responses are sufficiently antagonized. Simultaneously, the abundance and function of proteins involved in the synthesis of chemical building blocks required for virus production, such as nucleotides, amino acids, and fatty acids, must be preserved or even enriched. The ubiquitin (Ub) proteasome system (UPS) constitutes one of the most relevant protein decay systems of eukaryotic cells. In addition to the regulation of the turn-over and abundance of thousands of proteins, the UPS also generates the majority of peptides presented by major histocompatibility complex (MHC) molecules to allow surveillance by T lymphocytes. Cytomegaloviruses exploit the UPS to regulate the abundance of viral proteins and to manipulate the host proteome in favour of viral replication and immune evasion. After summarizing the current knowledge of CMV-mediated misuse of the UPS, we discuss the evolution of viral proteins utilizing the UPS for the degradation of defined target proteins. We propose two alternative routes of adapter protein development and their mechanistic consequences.

Modulation of innate and adaptive immunity by cytomegaloviruses

The coordinated activities of innate and adaptive immunity are critical for effective protection against viruses. To counter this, some viruses have evolved sophisticated strategies to circumvent immune cell recognition. In particular, cytomegaloviruses encode large arsenals of molecules that seek to subvert T cell and natural killer cell function via a remarkable array of mechanisms. Consequently, these 'immunoevasins' play a fundamental role in shaping the nature of the immune system by driving the evolution of new immune receptors and recognition mechanisms. Here, we review the diverse strategies adopted by cytomegaloviruses to target immune pathways and outline the host's response.

Battle between Host Immune Cellular Responses and HCMV Immune Evasion

Human cytomegalovirus (HCMV) is ubiquitously prevalent. HCMV infection is typically asymptomatic and controlled by the immune system in healthy individuals, yet HCMV can be severely pathogenic for the fetus during pregnancy and in immunocompromised persons, such as transplant recipients or HIV infected patients. HCMV has co-evolved with the hosts, developed strategies to hide from immune effector cells and to successfully survive in the human organism. One strategy for evading or delaying the immune response is maintenance of the viral genome to establish the phase of latency. Furthermore, HCMV immune evasion involves the downregulation of human leukocyte antigens (HLA)-Ia molecules to hide infected cells from T-cell recognition. HCMV expresses several proteins that are described for downregulation of the HLA class I pathway via various mechanisms. Here, we review the wide range of immune evasion mechanisms of HCMV. Understanding the mechanisms of HCMV immune evasion will contribute to the development of new customized therapeutic strategies against the virus.

A novel ferritin gene from Procambarus clarkii involved in the immune defense against Aeromonas hydrophila infection and inhibits WSSV replication

Ferritin is a protein related to the storage of iron and widely distributed in animals. It participates in many biological process, including antioxidation, cell activation, angiogenesis, regulation of iron metabolic balance and immune defense. In the present study, a novel ferritin gene was identified from red swamp crayfish Procambarus clarkii, with a cDNA sequence encoding a predicted 221 amino-acid residues. The ferritin protein contains a 19-residue signal peptide and 145-residue classic ferritin domain. The NJ phylogenetic analysis showed PcFer clustered with other crustacean peptides. The recombinant PcFer protein was produced and purified in E. coli, and the anti-rabbit polyclonal antibody was obtained. The rPcFer exhibited iron binding activity at a dose-dependent effect. The qPCR and western blot analysis revealed that PcFer was highly expressed in hemocytes, hepatopancreas, and gills. After challenged with WSSV and Aeromonas hydrophila, the mRNA and protein expression patterns of PcFer were significantly up-regulated in hemocytes and hepatopancreas. dsRNA interfering technique was utilized to silence the expression of PcFer gene. The WSSV copy number in PcFer silenced shrimp was much higher than that in the control group. The present study indicated that PcFer was involved in the immune defense against WSSV and Aeromonas hydrophila, and might inhibit WSSV replication in P. clarkii. These results will provide important data support for further study of the functional role of the ferritin gene.

Human Cytomegalovirus Protein pUL38 Prevents Premature Cell Death by Binding to Ubiquitin-Specific Protease 24 and Regulating Iron Metabolism

Human cytomegalovirus (HCMV) protein pUL38 has been shown to prevent premature cell death by antagonizing cellular stress responses; however, the underlying mechanism remains unknown. In this study, we identified the host protein ubiquitin-specific protease 24 (USP24) as an interaction partner of pUL38. Mutagenesis analysis of pUL38 revealed that amino acids TFV at positions 227 to 230 were critical for its interaction with USP24. Mutant pUL38 TFV/AAA protein did not bind to USP24 and failed to prevent cell death induced by pUL38-deficient HCMV infection. Knockdown of USP24 suppressed the cell death during pUL38-deficient HCMV infection, suggesting that pUL38 achieved its function by antagonizing the function of USP24. We investigated the cellular pathways regulated by USP24 that might be involved in the cell death phenotype by testing several small-molecule compounds known to have a protective effect during stress-induced cell death. The iron chelators ciclopirox olamine and Tiron specifically protected cells from pUL38-deficient HCMV infection-induced cell death, thus identifying deregulated iron homeostasis as a potential mechanism. Protein levels of nuclear receptor coactivator 4 (NCOA4) and lysosomal ferritin degradation, a process called ferritinophagy, were also regulated by pUL38 and USP24 during HCMV infection. Knockdown of USP24 decreased NCOA4 protein stability and ferritin heavy chain degradation in lysosomes. Blockage of ferritinophagy by genetic inhibition of NCOA4 or Atg5/Atg7 prevented pUL38-deficient HCMV infection-induced cell death. Overall, these results support the hypothesis that pUL38 binds to USP24 to reduce ferritinophagy, which may then protect cells from lysosome dysfunction-induced cell death.IMPORTANCE Premature cell death is considered a first line of defense against various pathogens. Human cytomegalovirus (HCMV) is a slow-replicating virus that encodes several cell death inhibitors, such as pUL36 and pUL37x1, which allow it to overcome both extrinsic and intrinsic mitochondrion-mediated apoptosis. We previously identified HCMV protein pUL38 as another virus-encoded cell death inhibitor. In this study, we demonstrated that pUL38 achieved its activity by interacting with and antagonizing the function of the host protein ubiquitin-specific protease 24 (USP24). pUL38 blocked USP24-mediated ferritin degradation in lysosomes, which could otherwise be detrimental to the lysosome and initiate cell death. These novel findings suggest that iron metabolism is finely tuned during HCMV infection to avoid cellular toxicity. The results also provide a solid basis for further investigations of the role of USP24 in regulating iron metabolism during infection and other diseases.

Duck virus enteritis (duck plague) - a comprehensive update

Duck virus enteritis (DVE), also called duck plague, is one of the major contagious and fatal diseases of ducks, geese and swan. It is caused by duck enteritis virus (DEV)/Anatid herpesvirus-1 of the genus Mardivirus, family Herpesviridae, and subfamily Alpha-herpesvirinae. Of note, DVE has worldwide distribution, wherein migratory waterfowl plays a crucial role in its transmission within and between continents. Furthermore, horizontal and/ or vertical transmission plays a significant role in disease spread through oral-fecal discharges. Either of sexes from varying age groups of ducks is vulnerable to DVE. The disease is characterized by sudden death, vascular damage and subsequent internal hemorrhage, lesions in lymphoid organs, digestive mucosal eruptions, severe diarrhea and degenerative lesions in parenchymatous organs. Huge economic losses are connected with acute nature of the disease, increased morbidity and mortality (5%-100%), condemnations of carcasses, decreased egg production and hatchability. Although clinical manifestations and histopathology can provide preliminary diagnosis, the confirmatory diagnosis involves virus isolation and detection using serological and molecular tests. For prophylaxis, both live-attenuated and killed vaccines are being used in broiler and breeder ducks above 2 weeks of age. Since DEV is capable of becoming latent as well as shed intermittently, recombinant subunit and DNA vaccines either alone or in combination (polyvalent) are being targeted for its benign prevention. This review describes DEV, epidemiology, transmission, the disease (DVE), pathogenesis, and advances in diagnosis, vaccination and antiviral agents/therapies along with appropriate prevention and control strategies.

Identifying the ERAD ubiquitin E3 ligases for viral and cellular targeting of MHC class I

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E3 ubiquitin ligases play a central role in viral and cellular degradation of MHC-I.

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HCMV US2 and US11 hijack the mammalian ERAD machinery to induce MHC-I degradation.

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We identified the TRC8 and TMEM129 E3 ligases as crucial for US2/11 function.

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The US2/11 degradation hubs are flexible and enable viral evasion of different immune functions.

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Cellular quality control of MHC-I is controlled by the HRD1/SEL1L E3 ligase complex.

The human cytomegalovirus (HCMV) US2 and US11 gene products hijack mammalian ER-associated degradation (ERAD) to induce rapid degradation of major histocompatibility class I (MHC-I) molecules. The rate-limiting step in this pathway is thought to be the polyubiquitination of MHC-I by distinct host ERAD E3 ubiquitin ligases. TRC8 was identified as the ligase responsible for US2-mediated MHC-I degradation and shown to be required for the cleavage-dependent degradation of some tail-anchored proteins. In addition to MHC-I, plasma membrane profiling identified further immune receptors, which are also substrates for the US2/TRC8 complex. These include at least six α integrins, the coagulation factor thrombomodulin and the NK cell ligand CD112. US2’s use of specific HCMV-encoded adaptors makes it an adaptable viral degradation hub. US11-mediated degradation is MHC-I-specific and genetic screens have identified TMEM129, an uncharacterised RING-C2 E3 ligase, as responsible for US11-mediated degradation. In a unique auto-regulatory loop, US11 readily responds to changes in cellular expression of MHC-I. Free US11 either rebinds more MHC-I or is itself degraded by the HRD1/SEL1L E3 ligase complex. While virally encoded US2 and US11 appropriate mammalian ERAD, the MHC-I complex also undergoes stringent cellular quality control and misfolded MHC-I is degraded by the HRD1/SEL1L complex. We discuss the identification and central role of E3 ubiquitin ligases in ER quality control and viral degradation of the MHC-I chain.

Classical and non-classical MHC I molecule manipulation by human cytomegalovirus: so many targets—but how many arrows in the quiver?

Major mechanisms for the recognition of pathogens by immune cells have evolved to employ classical and non-classical major histocompatibility complex class I (MHC I) molecules. Classical MHC I molecules present antigenic peptide ligands on infected cells to CD8⁺ T cells, whereas a key function for non-classical MHC I molecules is to mediate inhibitory or activating stimuli in natural killer (NK) cells. The structural diversity of MHC I puts immense pressure on persisting viruses, including cytomegaloviruses. The very large coding capacity of the human cytomegalovirus allows it to express a whole arsenal of immunoevasive factors assigned to individual MHC class I targets. This review summarizes achievements from more than two decades of intense research on how human cytomegalovirus manipulates MHC I molecules and escapes elimination by the immune system.

White spot syndrome virus protein kinase 1 defeats the host cell's iron-withholding defense mechanism by interacting with host ferritin

Iron is an essential nutrient for nearly all living organisms, including both hosts and invaders. Proteins such as ferritin regulate the iron levels in a cell, and in the event of a pathogenic invasion, the host can use an iron-withholding mechanism to restrict the availability of this essential nutrient to the invading pathogens. However, pathogens use various strategies to overcome this host defense. In this study, we demonstrated that white spot syndrome virus (WSSV) protein kinase 1 (PK1) interacted with shrimp ferritin in the yeast two-hybrid system. A pulldown assay and 27-MHz quartz crystal microbalance (QCM) analysis confirmed the interaction between PK1 and both ferritin and apoferritin. PK1 did not promote the release of iron ions from ferritin, but it prevented apoferritin from binding ferrous ions. When PK1 was overexpressed in Sf9 cells, the cellular labile iron pool (LIP) levels were elevated significantly. Immunoprecipitation and atomic absorption spectrophotometry (AAS) further showed that the number of iron ions bound by ferritin decreased significantly at 24 h post-WSSV infection. Taken together, these results suggest that PK1 prevents apoferritin from iron loading, and thus stabilizes the cellular LIP levels, and that WSSV uses this novel mechanism to counteract the host cell's iron-withholding defense mechanism.

IMPORTANCE

We show here that white spot syndrome virus (WSSV) ensures the availability of iron by using a previously unreported mechanism to defeat the host cell's iron-withholding defense mechanism. This defense is often implemented by ferritin, which can bind up to 4,500 iron atoms and acts to sequester free iron within the cell. WSSV's novel counterstrategy is mediated by a direct protein-protein interaction between viral protein kinase 1 (PK1) and host ferritin. PK1 interacts with both ferritin and apoferritin, suppresses apoferritin's ability to sequester free iron ions, and maintains the intracellular labile iron pool (LIP), and thus the availability of free iron is increased within cells.

The tug-of-war between dendritic cells and human chronic viruses

The human immune system is under constant challenge from many viruses, some of which the body is successfully able to clear. Other viruses have evolved to escape the host immune responses and thus persist, leading to the development of chronic diseases. Dendritic cells are professional antigen-presenting cells that play a major role in both innate and adaptive immunity against different pathogens. This review focuses on the interaction of different chronic viruses with dendritic cells and the viruses' ability to exploit this critical cell type to their advantage so as to establish persistence within the host.

Beyond hereditary hemochromatosis: new insights into the relationship between iron overload and chronic liver diseases

Following the model of hereditary hemochromatosis, the possible role of iron overload as a cofactor for disease progression in acquired liver diseases has been investigated with controversial results. In recent years, progress has been made in understanding the regulation of iron metabolism, thereby allowing the evaluation of the mechanisms linking liver diseases to excessive iron accumulation. Indeed, deregulation of the transcription of hepcidin, emerging as the master regulator of systemic iron metabolism, has been implicated in the pathogenesis of hepatic iron overload in chronic liver diseases. Whatever the cause, hepatocellular iron deposition promotes liver fibrogenesis, while an emerging possible aggravating factor is represented by the strong link between iron stores and insulin resistance, a recently recognized risk factor for the progression of liver diseases. Overall, these pathogenic mechanisms, together with the known proliferative and mutagenic effect of excess iron, converge in determining an increased susceptibility to hepatocellular carcinoma. Finally, an association between serum ferritin levels and mortality in patients with end-stage liver disease has recently been reported. Prospective, randomized studies are required to evaluate whether iron depletion may reduce fibrosis progression, hepatocellular carcinoma development, and eventually mortality in patients with chronic liver diseases.

ER Stress and Iron Homeostasis: A New Frontier for the UPR

The C282Y mutation of HFE accounts for the majority of cases of the iron overload disease Hereditary Hemochromatosis (HH). The conformational changes introduced by this mutation impair the HFE association with β₂-microglobulin (β₂m) and the cell surface expression of the protein: with two major consequences. From a functional perspective, the ability of HFE to bind to transferrin receptors 1 and 2 is lost in the C282Y mutant, thus affecting hepcidin regulation. Also due to the faulty assembly with β₂m, HFE-C282Y molecules remain in the endoplasmic reticulum (ER) as aggregates that undergo proteasomal degradation and activate an Unfolded Protein Response (UPR). UPR activation, regardless of the ER stress stimuli, was shown to reshape the expression profile of iron-related genes and to decrease MHC-I cell surface expression. The possibility of a HFE-C282Y-mediated interplay between the UPR and iron homeostasis influencing disease progression and the clinical heterogeneity among C282Y carriers is discussed. The responsiveness of the ER chaperone calreticulin to both ER and iron-induced oxidative stresses, and its correlation with HH patients' phenotype, reinforce the interest of dissecting the UPR signaling/iron metabolism crosstalk and points to the potential clinical value of use of pharmacological chaperones in HFE-HH.

The HCMV membrane glycoprotein US10 selectively targets HLA-G for degradation

Human cytomegalovirus (HCMV) encodes an endoplasmic reticulum (ER)-resident transmembrane glycoprotein, US10, expressed early in the replicative cycle of HCMV as part of the same cluster that encodes the known immunoevasins US2, US3, US6, and US11. We show that US10 down-regulates cell surface expression of HLA-G, but not that of classical class I MHC molecules. The unique and short cytoplasmic tail of HLA-G (RKKSSD) is essential in its role as a US10 substrate, and a tri-leucine motif in the cytoplasmic tail of US10 is responsible for down-regulation of HLA-G. Both the kinetics of HLA-G degradation and the mechanisms responsible appear to be distinct from those used by the US2 and US11 pathways, suggesting the existence of a third route of protein dislocation from the ER. We show that US10-mediated degradation of HLA-G interferes with HLA-G-mediated NK cell inhibition. Given the role of HLA-G in protecting the fetus from attack by the maternal immune system and in directing the differentiation of human dendritic cells to promote the evolution of regulatory T cells, HCMV likely targets the HLA-G-dependent axis of immune recognition no less efficiently than it interferes with classical class I MHC-restricted antigen presentation.

Huh-7: a human "hemochromatotic" cell line

Hereditary hemochromatosis (HC) is commonly associated with homozygosity for the cysteine-to-tyrosine substitution at position 282 (C282Y) of the HFE protein. This mutation prevents HFE from binding beta(2)-microglobulin (beta(2)M) and reaching the cell surface. We have discovered that a widely used hepatoma cell line, Huh-7, carries a HFE mutation similar to that associated with human HC. By HFE gene sequencing of Huh-7 genomic DNA, we found a TAC nucleotide deletion (c. 691_693del) responsible for loss of a tyrosine at position 231 (p. Y231del) of the HFE protein. This mutation affects a conserved hydrophobic region in a loop connecting two beta strands that make up the alpha3 domain of HFE, not far from the 282 site. HFE was detected by western blot in HepG2 but not in Huh-7 cell membrane fractions. In WRL-68 cells expressing wild-type HFE, the HFE protein was largely found at the plasma membrane where it colocalizes with beta(2)M. On the contrary, the HFE-Y231del mutant, similarly to an exogenously expressed HFE-C282Y mutant, failed to reach the plasma membrane and did not colocalize with membrane-expressed beta(2)M. C282Y mutant HFE in HC is associated with inadequate hepcidin expression. We found that Huh-7 cells display lower hepcidin messenger RNA levels as compared to HepG2 cells, which carry a wild-type HFE. Interestingly, hepcidin messenger RNA levels increased significantly in Huh-7 cells stably expressing exogenous wild-type HFE at the plasma membrane.

CONCLUSION

Huh-7 cells may represent a novel and valuable tool to investigate the role of altered HFE traffic in iron metabolism and pathogenesis of human HFE HC.

Immune evasion proteins gpUS2 and gpUS11 of human cytomegalovirus incompletely protect infected cells from CD8 T cell recognition

Human cytomegalovirus (HCMV) encodes four glycoproteins, termed gpUS2, gpUS3, gpUS6 and gpUS11 that interfere with MHC class I biosynthesis and antigen presentation. Despite gpUS2-11 expression, however, HCMV infection is efficiently controlled by cytolytic CD8 T lymphocytes (CTL). To address the role of gpUS2 and gpUS11 in antigen presentation during viral infection, HCMV mutants were generated that expressed either gpUS2 or gpUS11 alone without coexpression of the three other proteins. Fibroblasts infected with these viruses showed reduced HLA-A2 and HLA-B7 surface expression. Surprisingly, however, CTL directed against the tegument protein pp65 and the regulatory IE1 protein still recognized and lysed mutant virus infected fibroblasts. Yet, suppression of IE1 derived peptide presentation by gpUS2 or gpUS11 was far more pronounced. The results show that gpUS2 and gpUS11 alone only incompletely protect HCMV infected fibroblasts from CTL recognition and underline the importance of studying infected cells to elucidate HCMV immune evasion.

Dendritic cells in cytomegalovirus infection: viral evasion and host countermeasures

Human cytomegalovirus (HCMV) is a beta-herpesvirus that infects the majority of the population during early childhood and thereafter establishes life-long latency. Primary infection as well as spontaneous reactivation usually remains asymptomatic in healthy hosts but can, in the context of systemic immunosuppression, result in substantial morbidity and mortality. HCMV counteracts the host immune response by interfering with the recognition of infected cells. A growing body of literature has also suggested that the virus evades the immune system by paralyzing the initiators of antiviral immune responses--the dendritic cells (DCs). In the current review, we discuss the effects of CMV (HCMV and murine CMV) on various DC subsets and the ensuing innate and adaptive immune responses. The impact of HCMV on DCs has mainly been investigated using monocyte-derived DCs, which are rendered functionally impaired by infection. In mouse models, DCs are targets of viral evasion as well, but the complex cross-talk between DCs and natural killer cells has, however, demonstrated an instrumental role for DCs in the control and clearance of viral infection. Fewer studies address the role of peripheral blood DC subsets, plasmacytoid DCs and CD11c+ myeloid DCs in the response against HCMV. These DCs, rather than being paralyzed by HCMV, are largely resistant to infection, mount a vigorous first-line defense and induce T-cell responses to the virus. This possibly provides a partial explanation for an intriguing conundrum: the highly efficient control of viral infection and reactivation in immunocompetent hosts in spite of multi-layered viral evasion mechanisms.

In vitro binding of HFE to the cation-independent mannose-6 phosphate receptor

Hereditary hemochromatosis is most frequently associated with mutations in HFE, which encodes a class Ib histocompatibility protein. HFE binds to the transferrin receptor-1 (TfR1) in competition with iron-loaded transferrin (Fe-Tf). HFE is released from TfR1 by increasing concentrations of Fe-Tf, and free HFE may then regulate iron homeostasis by binding other ligands. To search for new HFE ligands we expressed recombinant forms of HFE in the human cell line 293T. HFE protein was purified, biotinylated and made into fluorescently labelled tetramers. HFE tetramers bound to TfR1 in competition with Tf, but in addition we detected a binding activity on some cell types that was not blocked by Fe-Tf or by mutations in HFE that prevent binding to TfR1. We identified this second HFE ligand as the cation independent mannose-6-phosphate receptor (CI-MPR, also known as the insulin-like growth factor-2 receptor, IGF2R). HFE:CI-MPR binding was mediated through phosphorylated mannose residues on HFE. Recombinant murine Hfe also bound to CI-MPR. HFE bound to TfR1 was prevented from binding CI-MPR until released by increasing concentrations of Fe-Tf, a feature consistent with an iron sensing mechanism. However, it remains to be determined whether endogenous HFE in vivo also acquires the mannose-6 phosphate modification and binds to CI-MPR.

Immune evasion during foot-and-mouth disease virus infection of swine

Summary: The interface between successful pathogens and their hosts is often a tenuous balance. In acute viral infections, this balance involves induction and inhibition of innate responses. Foot‐and‐mouth disease virus (FMDV) is considered one of the most contagious viruses known and is characterized by rapid induction of clinical disease in cloven hoofed animals exposed to infection. Viral shedding is extensive before the equally rapid resolution of acute disease. This positive strand RNA virus is an extremely successful pathogen, due in part to the ability to interrupt the innate immune response. Previous reviews have described the inhibition of cellular innate responses in the infected cell both in vitro and in vivo. Here, we present a review of virus inhibition of cells that are a source of antiviral function in swine. Particularly in the case of dendritic cells and natural killer cells, the virus has evolved mechanisms to interrupt the normal function of these important mediators of innate function, even though these cells are not infected by the virus. Understanding how this virus subverts the innate response will provide valuable information for the development of rapidly acting biotherapeutics to use in response to an outbreak of FMDV.

Evidence for a pathogenic determinant in HIV-1 Nef involved in B cell dysfunction in HIV/AIDS

B lymphocyte hyperactivation and elevated immunoglobulin levels (hypergammaglobulinemia) are pathogenic manifestations of HIV-1 infection. Here we provide evidence that these hallmarks are caused by a soluble factor whose production by infected macrophages is induced by the HIV-1 Nef protein. In vitro, HIV-1-infected macrophages or macrophages expressing Nef promoted B cell activation and differentiation to immunoglobulin-secreting cells. Nef-mediated activation of NF-kappaB in macrophages induced secretion of the acute-phase protein ferritin, and ferritin was necessary and sufficient for the observed Nef-dependent B cell changes. The extent of hypergammaglobulinemia in HIV-1-infected individuals correlated directly with plasma ferritin levels and with viral load. Furthermore, the induction of ferritin production and hypergammaglobulinemia was recapitulated when Nef was specifically expressed in macrophages and T cells of transgenic mice. Collectively, these results indicate that the HIV-1 Nef protein carries a pathogenic determinant that governs B cell defects in HIV-1 infection.

Iron overload and unique susceptibility of liver transplant recipients to disseminated disease due to opportunistic pathogens

The propensity of liver transplant recipients to develop more fulminant disease presentation and a higher risk of disseminated disease due to a number of opportunistic infections, including invasive aspergillosis, cryptococcosis, zygomycosis, may be related to iron overload. Abnormalities in iron homeostasis may also be a contributor to severe manifestations due to other pathogens such as cytomegalovirus and Staphylococcus aureus in liver transplant recipients. Iron is essential not only for microbial pathogenesis, but directly impairs pivotal pathogen specific host defenses. Studies to assess iron homeostasis and the mechanisms by which iron overload contributes to the pathogenesis of opportunistic infections in liver transplant recipients are warranted.

Viral infection and iron metabolism

Fundamental cellular operations, including DNA synthesis and the generation of ATP, require iron. Viruses hijack cells in order to replicate, and efficient replication needs an iron-replete host. Some viruses selectively infect iron-acquiring cells by binding to transferrin receptor 1 during cell entry. Other viruses alter the expression of proteins involved in iron homeostasis, such as HFE and hepcidin. In HIV-1 and hepatitis C virus infections, iron overload is associated with poor prognosis and could be partly caused by the viruses themselves. Understanding how iron metabolism and viral infection interact might suggest new methods to control disease.

The unfolded protein response in hereditary haemochromatosis

To cope with the accumulation of unfolded or misfolded proteins the endoplasmic reticulum (ER) has evolved specific signalling pathways collectively called the unfolded protein response (UPR). Elucidation of the mechanisms governing ER stress signallinghas linked this response to the regulation of diverse physiologic processes as well as to the progression of a number of diseases. Interest in hereditary haemochromatosis (HH) has focused on the study of proteins implicated in iron homeostasis and on the identification of new alleles related with the disease. HFE has been amongst the preferred targets of interest, since the discovery that its C282Y mutation was associated with HH. However, the discrepancies between the disease penetrance and the frequency of this mutation have raised the possibility that its contribution to disease progression might go beyond the mere involvement in regulation of cellular iron uptake. Recent findings revealed that activation of the UPR is a feature of HH and that this stress response may be involved in the genesis of immunological anomalies associated with the disease. This review addresses the connection of the UPR with HH, including its role in MHC-I antigen presentation pathway and possible implications for new clinical approaches to HH.

Expression of iron transport proteins divalent metal transporter-1, Ferroportin-1, HFE and transferrin receptor-1 in human monocyte-derived dendritic cells

Iron is essential for cell survival and regulates many cell functions. In the context of the immune response, iron-related metabolism is tightly controlled in activated lymphocytes as well as in cells of the innate immunity. More precisely, for dendritic cells (DCs), which are the key cell type in the development of a specific immune response, the importance of iron absorption was recently unravelled by showing that depletion of iron inhibits the maturation of DCs. On this basis, we studied in detail the expression of iron transport proteins and HFE in DCs. We found that iron uptake in this cell type is mediated by divalent-metal transporter 1 (DMT1) and transferrin receptor-1 (TfR) whereas Ferroportin-1 is very weakly expressed. HFE that regulates TfR's activity is also detected at the mRNA level. The expression of DMT1 and HFE barely varies upon endotoxin-induced maturation but TfR is up-regulated and the iron export molecule Ferroportin-1 is down-regulated. As opposed to MHC class II molecules, the intracellular localization of TfR is not changed during maturation. Our results indicate that the uptake of iron during DCs development and maturation is mediated by a strong expression of iron-uptake molecules such as DMT1 and TfR as well as a down-regulation of iron export molecules such as Ferroportin-1.

In and out of the ER: protein folding, quality control, degradation, and related human diseases

A substantial fraction of eukaryotic gene products are synthesized by ribosomes attached at the cytosolic face of the endoplasmic reticulum (ER) membrane. These polypeptides enter cotranslationally in the ER lumen, which contains resident molecular chaperones and folding factors that assist their maturation. Native proteins are released from the ER lumen and are transported through the secretory pathway to their final intra- or extracellular destination. Folding-defective polypeptides are exported across the ER membrane into the cytosol and destroyed. Cellular and organismal homeostasis relies on a balanced activity of the ER folding, quality control, and degradation machineries as shown by the dozens of human diseases related to defective maturation or disposal of individual polypeptides generated in the ER.

Chemical chaperones reduce endoplasmic reticulum stress and prevent mutant HFE aggregate formation

HFE C282Y, the mutant protein associated with hereditary hemochromatosis (HH), fails to acquire the correct conformation in the endoplasmic reticulum (ER) and is targeted for degradation. We have recently shown that an active unfolded protein response (UPR) is present in the cells of patients with HH. Now, by using HEK 293T cells, we demonstrate that the stability of HFE C282Y is influenced by the UPR signaling pathway that promotes its degradation. Treatment of HFE C282Y-expressing cells with tauroursodeoxycholic acid (TUDCA), a bile acid derivative with chaperone properties, or with the chemical chaperone sodium 4-phenylbutyrate (4PBA) impeded the UPR activation. However, although TUDCA led to an increased stability of the mutant protein, 4PBA contributed to a more efficient disposal of HFE C282Y to the degradation route. Fluorescence microscopy and biochemical analysis of the subcellular localization of HFE revealed that a major portion of the C282Y mutant protein forms intracellular aggregates. Although neither TUDCA nor 4PBA restored the correct folding and intracellular trafficking of HFE C282Y, 4PBA prevented its aggregation. These data suggest that TUDCA hampers the UPR activation by acting directly on its signal transduction pathway, whereas 4PBA suppresses ER stress by chemically enhancing the ER capacity to cope with the expression of misfolded HFE, facilitating its degradation. Together, these data shed light on the molecular mechanisms involved in HFE C282Y-related HH and open new perspectives on the use of orally active chemical chaperones as a therapeutic approach for HH.

Association of hemochromatosis with infectious diseases: expanding spectrum

Withholding iron from potential pathogens is a host defense strategy. There is evidence that iron overload per se compromises the ability of phagocytes to kill microorganisms. Several hypotheses exist to explain the association of hemochromatosis with infection. A combination of mechanisms likely contributes to the increase in susceptibility to infection in these patients. A review of the current literature delineating various pathogens to which patients with hemochromatosis are potentially susceptible, and recent advances in the understanding of the association of hemochromatosis with infection, are discussed.

Stimulation of an unfolded protein response impairs MHC class I expression

HFE C282Y is an example of a mutant protein that does not fold correctly, is retained in the endoplasmic reticulum, and was found previously to diminish surface expression of MHC class I (MHC-I). We now show that its expression in 293T cells triggers an unfolded protein response (UPR), as revealed by the increased levels of H chain binding protein, GRP94, and C/EBP homologous protein. Elevated levels of these proteins were also found in HFE C282Y homozygous PBMCs. Following the UPR induction, a decrease in MHC-I cell surface expression was observed. This defect in MHC-I could be mimicked, however, by overexpression of transcriptionally active isoforms of activating transcription factor-6 and X box-binding protein-1, which induced the UPR, and reversed in HFE C282Y-expressing cells by using dominant-negative constructs that block UPR signaling. The present results provide evidence to the finding that stimulation of an UPR affects MHC-I expression.

Localisation of proteins of iron metabolism in the human placenta and liver

Two anatomical sites that are important in human iron metabolism are the liver and placenta. Liver macrophages recycle iron from erythrocytes, and the placenta transfers iron from the mother to the fetus. The cellular distribution of proteins involved in iron transport in these two sites was studied. Transferrin receptor-1 (TfR1) and Ferroportin (FPN) expression was found on the placental syncytiotrophoblast (STB) and were polarised such that TfR1 was on the apical maternal-facing membrane and FPN was on the basal fetal-facing membrane, consistent with unidirectional iron transport from mother to fetus. Ferritin was strongly expressed in the stroma, suggesting that fetal tissue can store and accumulate iron. HFE was on some parts of the basal STB and, where present, HFE clearly colocalised with FPN but not TfR1. In the stroma, both HFE and FPN were present on CD68+ Hofbauer macrophage cells. In liver, the location of HFE is controversial. Using four mouse monoclonals and two polyclonal sera we showed that the pattern of HFE expression mirrored the distribution of CD68+ macrophage Kupffer cells. FPN was also most strongly expressed by CD68+ Kupffer cells. These findings contribute to understanding how iron is transported and stored in the human placenta and liver.

The Ins and Outs of Intracellular Peptides and Antigen Presentation by MHC Class I Molecules

MHC class I molecules present small intracellular generated fragments to the outside surveying immune system. This is the result of a series of biochemical processes involving biosynthesis, degradation, translocation, intracellular transport, diffusion, and many more. Critical intermediates and end products of this cascade of events are peptides. The peptides are generated by the proteasome, degraded by peptidases unless transported into the ER where another peptidase and MHC class I molecules are waiting. Unless peptides bind to MHC class I molecules, they are released from the ER and enter the cytoplasm by a system resembling the ERAD pathway in many aspects. The cycle of peptides over the ER membrane with the proteasome at the input site and peptidases or MHC class I molecules on the output site are central in the MHC class I antigen presentation pathway and this review.

Transferrin receptor co-localizes and interacts with the hemochromatosis factor (HFE) and the divalent metal transporter-1 (DMT1) in trophoblast cells

Iron uptake and storage are tightly regulated to guarantee sufficient iron for essential cellular processes and to prevent the production of damaging free radicals. The placenta is the entry site for iron, which is delivered to the developing embryo. Iron is taken up by syncytiotrophoblast cells and is transported unidirectionally from mother to fetus against a concentration gradient. Several iron transporters and regulators were recently characterized, including DMT1 and ferroportin/Ireg1 that transport iron through membranes, and HFE that regulates TfR-mediated iron uptake. In this study we demonstrate that in a differentiated choriocarcinoma cell line BeWo, HFE, and TfR are localized mainly in recycling endosomes and a small percentage of these complexes is observed in late endosomes with DMT1 while in trophoblast cells, the level of TfR is significantly lower and it is detected with HFE and DMT1 mainly in late endosomes. Most interestingly, TfR and HFE, as well as TfR and DMT1 interact in placental trophoblast cells. Based on previous and these data we suggest that the level of intracellular iron may regulate both TfR expression (on the post-transcriptional and the post-translational levels) and TfR trafficking/transcytosis in polarized cells.

HFE cross-talks with the MHC class I antigen presentation pathway

HFE is a protein known to be involved in iron metabolism; yet, other than its homology with major histocompatibility complex (MHC) class I molecules, it has not been described as having an immunologic function. Here we report that peripheral blood mononuclear cells (PBMCs) from patients with hereditary hemochromatosis (HH) carrying the C282Y mutation in HFE have reduced cell-surface expression of MHC class I due to an enhanced endocytosis rate of MHC class I molecules caused by premature peptide and β2-microglobulin dissociation. This faster turnover also leads to increased expression levels of cell-surface free class I heavy chains in mutant PBMCs. Biochemical analysis indicates an earlier peptide loading and endoplasmic reticulum maturation of MHC class I molecules in C282Y mutant cells. Thermostability assays further showed that in HFE mutants the MHC class I peptide loading gives rise to low-stability heterotrimers that dissociate prematurely during its intracellular traffic. The present results suggest the existence of an intriguing cross-talk between a particular HFE mutation and the classical MHC class I route. These findings constitute the first description of peptide presentation pathway abnormalities linked to HFE and provide additional evidence for the occurrence of immunologic defects in patients with HH.

HIV-1 Nef down-regulates the hemochromatosis protein HFE, manipulating cellular iron homeostasis

The multifunctional Nef protein of HIV-1 is important for the progression to AIDS. One action of Nef is to down-regulate surface MHC I molecules, helping infected cells to evade immunity. We found that Nef also down-regulates the macrophage-expressed MHC 1b protein HFE, which regulates iron homeostasis and is mutated in the iron-overloading disorder hemochromatosis. In model cell lines, Nef reroutes HFE to a perinuclear structure that overlaps the trans-Golgi network, causing a 90% reduction of surface HFE. This activity requires a Src-kinase-binding proline-rich domain of Nef and a conserved tyrosine-based motif in the cytoplasmic tail of HFE. HIV-1 infection of ex vivo macrophages similarly down-regulates naturally expressed surface HFE in a Nef-dependent manner. The effect of Nef expression on cellular iron was explored; iron and ferritin accumulation were increased in HIV-1-infected ex vivo macrophages expressing wild-type HFE, but this effect was lost with Nef-deleted HIV-1 or when infecting macrophages from hemochromatosis patients expressing mutated HFE. The iron accumulation in HIV-1-infected HFE-expressing macrophages was paralleled by an increase in cellular HIV-1-gag expression. We conclude that, through Nef and HFE, HIV-1 directly regulates cellular iron metabolism, possibly benefiting viral growth.

The non-classical HLA class I molecule HFE does not influence the NK-like activity contained in fresh human PBMCs and does not interact with NK cells

In humans, four beta2-microglobulin-associated non-classical class I molecules are encoded in the MHC: HLA-E, -F, -G and -H. Three of them (HLA-E, -F and -G) were shown to inhibit NK activity. On the contrary, the fourth one, HLA-H, named HFE after it was found to be mutated in patients suffering from inherited hemochromatosis, has been shown to be involved only in the regulation of iron uptake. We tested the capacity of HFE to affect (enhance or reduce) specifically the NK activity contained in non-manipulated fresh human PBMCs. We showed that HFE expression by target cells does not affect their killing by the NK-like activity contained in PBMCs. Moreover, using fluorescent HFE tetramers, we could confirm that blood NK cells as well as blood gammadelta T cells do not bind HFE. Altogether, our data indicate that HFE does not affect the NK activity contained in the PBMCs.

Expression of the hereditary hemochromatosis protein HFE increases ferritin levels by inhibiting iron export in HT29 cells

Iron is essential for life in almost all organisms and, in mammals, is absorbed through the villus cells of the duodenum. Using a human colonic carcinoma cell line that has many duodenal characteristics, HT29, we show that genes involved in intestinal iron transport are endogenously expressed. When stably transfected to express the hereditary hemochromatosis protein HFE these cells have increased ferritin levels. We demonstrate that this is not due to an effect on the transferrin (TF)-mediated iron uptake pathway but rather due to inhibition of iron efflux from the cell. The effect of HFE was independent of its interaction with TF receptor 1 as indicated by similar results using both the wild type HFE and the W81A mutant that binds TF receptor 1 with greatly reduced affinity. HFE expression did not affect the mRNA levels of most of the genes involved in iron absorption that were tested; however, it did correspond to a decrease in hephaestin message levels. These results point to a role for HFE in inhibition of iron efflux in HT29 cells. This is a distinct role from that in HeLa and human embryonic kidney 293 cells where HFE has been shown to inhibit TF-mediated iron uptake resulting in decreased ferritin levels. Such a distinction suggests a multifunctional role for HFE that is dependent upon expression levels of proteins involved in iron transport.

Mouse HFE inhibits Tf-uptake and iron accumulation but induces non-transferrin bound iron (NTBI)-uptake in transformed mouse fibroblasts

Iron-uptake and storage are tightly regulated to guarantee sufficient iron for essential cellular processes and to prevent the production of damaging free radicals. A non-classical class I MHC molecule, the hemochromatosis factor (HFE), has been shown to regulate iron metabolism, potentially via its interaction with the transferrin receptor. Whereas, the effect of human HFE (hHFE) on transferrin/transferrin receptor association, as well as on transferrin receptor recycling and the level of cellular iron pools in various cell lines was analyzed, very little is known about the mouse HFE (mHFE) protein. In the following study, our aim was to analyze in more detail the function of mHFE. Surprisingly, we observed that over-expression of mHFE, but not of hHFE, in a mouse transformed cell line, results in a most significant inhibition of transferrin-uptake which correlated with apoptotic cell death. mHFE inhibited transferrin-uptake immediately following transfection and this inhibition persisted in the surviving stable transfectants. Concomitantly, cellular iron derived from transferrin-iron uptake was dramatically limited. The activation of a non-transferrin bound iron-uptake pathway that functions in the stable mHFE-transfected clones could explain their normal growth curves and survival. The hypothesis that iron starvation can induce iron-uptake by a novel transferrin-independent pathway is discussed.

Tranferrins receptor association and endosomal localization of soluble HFE are not sufficient for regulation of cellular iron homeostasis

Iron uptake and storage are tightly regulated to guarantee sufficient iron for essential cellular processes and to prevent the production of damaging free radicals. A non-classical class I MHC molecule, the hemochromatosis factor HFE, has been shown to regulate iron metabolism, potentially via its direct interaction with the transferrin receptor (TfR). In this study, we demonstrate that a soluble beta2microglobulin-HFE monochain (sHFE) folds with beta2microglobulin (beta2m) and associates with the TfR, indicating that the transmembrane and cytoplasmic domains are not necessary for assembly and trafficking through the ER-Golgi network. We also demonstrate human TfR-specific uptake and accumulation of extracellular sHFE by treated cells. The sHFE localized to the endosomal compartment albeit we observed variation in the time taken for endosomal trafficking between different cell types. The sHFE monochain was effective in reducing Tf uptake into cells, however this did not correlate to any changes in TfR or ferritin synthesis, in contrast to the HFE-induced increase and decrease of TfR and ferritin, respectively. These findings of incongruent sHFE activity suggest that either variation in affinity binding of sHFE to TfR prevents efficient modulation of iron-regulated proteins or that HFE has multiple functions some of which may be independent of TfR but dependent on interactions within the endosomal compartment for effective modulation of iron metabolism.

Human cytomegalovirus US2 causes similar effects on both major histocompatibility complex class I and II proteins in epithelial and glial cells

The human cytomegalovirus (HCMV) glycoprotein US2 specifically binds to major histocompatibility complex (MHC) class I heavy chain (HC) and class II proteins DRalpha and DMalpha, triggering their degradation by proteasomes. Effects of US2 on class II proteins were originally characterized in HCMV- or adenovirus vector-infected U373 astroglioma cells. Here, we have extended characterization of US2-mediated degradation of class II DRalpha to two other cell lines, including biologically relevant epithelial cells. Comparison of the effects of US2 in cells expressing both class I and II proteins demonstrated only a slight preference for class I HC. Moreover, US2 caused degradation of DRalpha and DMalpha when these proteins were expressed by transfection without DRbeta, invariant chain (Ii), or DMbeta. Therefore, US2 binds to alpha chains of DR and DM and triggers endoplasmic reticulum degradation without formation of class II DR alphabeta/Ii or DM alphabeta complexes. Similar levels of degradation of class II alpha were observed in cells expressing vastly different amounts of class II, suggesting that cellular factors, other than class II, were limiting. We concluded that US2 has broad effects in a variety of cells that express both class I and II proteins and is relevant to HCMV infection in vivo.

Evidence for the interaction of the hereditary haemochromatosis protein, HFE, with the transferrin receptor in endocytic compartments

HFE, the protein mutated in hereditary haemochromatosis type 1, is known to interact with the transferrin receptor (TfR) on the cell surface and during endocytosis [Gross, Irrinki, Feder and Enns (1998) J. Biol. Chem. 273, 22068-22074; Roy, Penny, Feder and Enns (1999) J. Biol. Chem. 274, 9022-9028]. However, whether they are capable of interacting with each other once inside the cell is not known. In the present study we present several lines of evidence that they do interact in endosome compartments. Cells expressing a chimaera of HFE protein with the cytoplasmic domain of lysosomal-associated membrane protein 1 (LAMP1) in place of its own (HFE-LAMP) show a decrease in the half-life of the TfR. This implies that the interaction between HFE and TfR in endosomes targets the TfR to lysosomal compartments. The interaction between TfR and HFE-LAMP was confirmed by immunoprecipitation, in addition to immunofluorescence studies. Addition of transferrin (Tf) to HFE-LAMP-expressing cells competes with HFE for binding to the TfR, thereby increasing the half-life of TfR and confirming that the HFE-LAMP-TfR complex reaches the cell surface prior to entering the endosomal vesicles and trafficking to the lysosome. These results raise the possibility that interaction of HFE and TfR in intracellular vesicles may play an important role in determining the function of HFE in iron homoeostasis, which is still unknown. Analysis of endosomal pH and the iron content of internalized Tf indicated that HFE does not appear to alter the unloading of iron from Tf in the endosome.

Human cytomegalovirus US3 chimeras containing US2 cytosolic residues acquire major histocompatibility class I and II protein degradation properties

Human cytomegalovirus (HCMV) glycoprotein US2 increases the proteasome-mediated degradation of major histocompatibility complex (MHC) class I heavy chain (HC), class II DR-alpha and DM-alpha proteins, and HFE, a nonclassical MHC protein. US2-initiated degradation of MHC proteins apparently involves the recruitment of cellular proteins that participate in a process known as endoplasmic reticulum (ER)-associated degradation. ER-associated degradation is a normal process by which misfolded proteins are recognized and translocated into the cytoplasm for degradation by proteasomes. It has been demonstrated that truncated forms of US2, especially those lacking the cytoplasmic domain (CT), can bind MHC proteins but do not cause their degradation. To further assess how the US2 CT domain interacts with the cellular components of the ER-associated degradation pathway, we constructed chimeric proteins in which the US2 CT domain or the CT and transmembrane (TM) domains replaced those of the HCMV glycoprotein US3. US3 also binds both class I and II proteins but does not cause their degradation. Remarkably, chimeras containing the US2 CT domain caused the degradation of both MHC class I and II proteins although this degradation was less than that by wild-type US2. Therefore, the US2 CT and TM domains can confer on US3 the capacity to degrade MHC proteins. We also analyzed complexes containing MHC proteins and US2, US3, US11, or US3/US2 chimeras for the presence of cdc48/p97 ATPase, a protein that binds polyubiquitinated proteins and likely functions in the extraction of substrates from the ER membrane before the substrates meet proteasomes. p97 ATPase was present in immunoprecipitates containing US2, US11, and two chimeras that included the US2 CT domain, but not in US3 complexes. Therefore, it appears that the CT domain of US2 participates in recruiting p97 ATPase into ER-associated degradation complexes.

HFE, the MHC and hemochromatosis: paradigm for an extended function for MHC class I

HFE was discovered as the hereditary hemochromatosis (HH) gene. It is located on chromosome 6 (6p21.3), 4Mb telomeric to the HLA-A locus, and its product has a structure similar to MHC class I molecules. HFE encodes two frequent mutations: C282Y and H63D. One of these (C282Y) is present in a large proportion of Caucasian HH patients. HFE has a tissue distribution compatible with a role in iron absorption (intestine), recycling (macrophages) and transport to the fetus (placenta).

A single viral protein HCMV US2 affects antigen presentation and intracellular iron homeostasis by degradation of classical HLA class I and HFE molecules

HFE is a nonclassical class I molecule that associates with beta 2-microglobulin (beta 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 (MHC) provides HCMV with an efficient tool for altering cellular metabolic functions, as well as supporting the escape of virus-infected cells from cytotoxic T lymphocyte (CTL)-mediated immune responses.

The enigmatic role of the hemochromatosis protein (HFE) in iron absorption

The HFE gene, a member of the class-I transplantation antigen gene family, is responsible for hereditary hemochromatosis, one of the most common inherited diseases in individuals of European descent. Patients exhibit predictable changes in iron homeostasis, including elevations in both transferrin saturation and serum ferritin levels. A subset of patients progress to overt clinical sequelae, resulting from iron overload. A hallmark of the disease is increased absorption of iron by the intestine. Although the HFE protein appears to modulate the function of the transferrin receptor in vitro, its precise role in vivo remains obscure. With multiple cell types involved in iron metabolism, the function of HFE is likely to be complex.

The hemochromatosis protein HFE inhibits iron export from macrophages

Hereditary hemochromatosis (HH) is a disorder of iron metabolism caused by common mutations in the gene HFE. The HFE protein binds to transferrin receptor-1 (TfR1) in competition with transferrin, and in vitro, reduces cellular iron by reducing iron uptake. However, in vivo, HFE is strongly expressed by liver macrophages and intestinal crypt cells, which behave as though they are relatively iron-deficient in HH. These latter observations suggest, paradoxically, that expression of wild-type HFE may lead to iron accumulation in these specialized cell types. Here we show that wild-type HFE protein raises cellular iron by inhibiting iron efflux from the monocytemacrophage cell line THP-1, and extend these results to macrophages derived from healthy individuals and HH patients. In addition, we find that the HH-associated mutant H41D has lost the ability to inhibit iron release despite binding to TfR1 as well as wild-type HFE. Finally, we show that the ability of HFE to block iron release is not competitively inhibited by transferrin. We conclude that HFE has two mutually exclusive functions, binding to TfR1 in competition with Tf, or inhibition of iron release.