Inhibition of MHC Class II Expression and Immune Responses by c-MIR

Role of MARCH E3 ubiquitin ligases in cancer development

Membrane-associated RING-CH (MARCH) E3 ubiquitin ligases, a family of RING-type E3 ubiquitin ligases, have garnered increased attention for their indispensable roles in immune regulation, inflammation, mitochondrial dynamics, and lipid metabolism. The MARCH E3 ligase family consists of eleven distinct members, and the dysregulation of many of these members has been documented in several human malignancies. Over the past two decades, extensive research has revealed that MARCH E3 ligases play pivotal roles in cancer progression by ubiquitinating key oncogenes and tumor suppressors and orchestrating various signaling pathways. Some MARCH E3s act as oncogenes, while others act as tumor suppressors, and the majority of MARCH E3s play both oncogenic and tumor suppressive roles in a context-dependent manner. Notably, there is special emphasis on the sole mitochondrial MARCH E3 ligase MARCH5, which regulates mitochondrial homeostasis within cancer cells. In this review, we delve into the diverse functions of MARCH E3 ligases across different cancer types, shedding light on the underlying molecular mechanisms mediating their effects, their regulatory effects on cancer and their potential as therapeutic targets.

Further Characterization of the Antiviral Transmembrane Protein MARCH8

The cellular transmembrane protein MARCH8 impedes the incorporation of various viral envelope glycoproteins, such as the HIV-1 envelope glycoprotein (Env) and vesicular stomatitis virus G-glycoprotein (VSV-G), into virions by downregulating them from the surface of virus-producing cells. This downregulation significantly reduces the efficiency of virus infection. In this study, we aimed to further characterize this host protein by investigating its species specificity and the domains responsible for its antiviral activity, as well as its ability to inhibit cell-to-cell HIV-1 infection. We found that the antiviral function of MARCH8 is well conserved in the rhesus macaque, mouse, and bovine versions. The RING-CH domains of these versions are functionally important for inhibiting HIV-1 Env and VSV-G-pseudovirus infection, whereas tyrosine motifs are crucial for the former only, consistent with findings in human MARCH8. Through analysis of chimeric proteins between MARCH8 and non-antiviral MARCH3, we determined that both the N-terminal and C-terminal cytoplasmic tails, as well as presumably the N-terminal transmembrane domain, of MARCH8 are critical for its antiviral activity. Notably, we found that MARCH8 is unable to block cell-to-cell HIV-1 infection, likely due to its insufficient downregulation of Env. These findings offer further insights into understanding the biology of this antiviral transmembrane protein.

Human MARCH1, 2, and 8 block Ebola virus envelope glycoprotein cleavage via targeting furin P domain

Membrane-associated RING-CH (MARCH) family proteins were recently reported to inhibit viral replication through multiple modes. Previous work showed that human MARCH8 blocked Ebola virus (EBOV) glycoprotein (GP) maturation. Our study here demonstrates that human MARCH1 and MARCH2 share a similar pattern to MARCH8 in restricting EBOV GP-pseudotyped viral infection. Human MARCH1 and MARCH2 retain EBOV GP at the trans-Golgi network, reduce its cell surface display, and impair EBOV GP-pseudotyped virions infectivity. Furthermore, we uncover that the host proprotein convertase furin could interact with human MARCH1/2 and EBOV GP intracellularly. Importantly, the furin P domain is verified to be recognized by MARCH1/2/8, which is critical for their blocking activities. Besides, bovine MARCH2 and murine MARCH1 also impair EBOV GP proteolytic processing. Altogether, our findings confirm that MARCH1/2 proteins of different mammalian origins showed a relatively conserved feature in blocking EBOV GP cleavage, which could provide clues for subsequent MARCHs antiviral studies and may facilitate the development of novel strategies to antagonize enveloped virus infection.

HPV upregulates MARCHF8 ubiquitin ligase and inhibits apoptosis by degrading the death receptors in head and neck cancer

The membrane-associated RING-CH-type finger ubiquitin ligase MARCHF8 is a human homolog of the viral ubiquitin ligases Kaposi's sarcoma herpesvirus K3 and K5 that promote host immune evasion. Previous studies have shown that MARCHF8 ubiquitinates several immune receptors, such as the major histocompatibility complex II and CD86. While human papillomavirus (HPV) does not encode any ubiquitin ligase, the viral oncoproteins E6 and E7 are known to regulate host ubiquitin ligases. Here, we report that MARCHF8 expression is upregulated in HPV-positive head and neck cancer (HNC) patients but not in HPV-negative HNC patients compared to normal individuals. The MARCHF8 promoter is highly activated by HPV oncoprotein E6-induced MYC/MAX transcriptional activation. The knockdown of MARCHF8 expression in human HPV-positive HNC cells restores cell surface expression of the tumor necrosis factor receptor superfamily (TNFRSF) death receptors, FAS, TRAIL-R1, and TRAIL-R2, and enhances apoptosis. MARCHF8 protein directly interacts with and ubiquitinates the TNFRSF death receptors. Further, MARCHF8 knockout in mouse oral cancer cells expressing HPV16 E6 and E7 augments cancer cell apoptosis and suppresses tumor growth in vivo. Our findings suggest that HPV inhibits host cell apoptosis by upregulating MARCHF8 and degrading TNFRSF death receptors in HPV-positive HNC cells.

MARCH8 attenuates cGAS-mediated innate immune responses through ubiquitylation

Cyclic GMP-AMP synthase (cGAS) binds to microbial and self-DNA in the cytosol and synthesizes cyclic GMP-AMP (cGAMP), which activates stimulator of interferon genes (STING) and downstream mediators to elicit an innate immune response. Regulation of cGAS activity is essential for immune homeostasis. Here, we identified the E3 ubiquitin ligase MARCH8 (also known as MARCHF8, c-MIR, and RNF178) as a negative regulator of cGAS-mediated signaling. The immune response to double-stranded DNA was attenuated by overexpression of MARCH8 and enhanced by knockdown or knockout of MARCH8. MARCH8 interacted with the enzymatically active core of cGAS through its conserved RING-CH domain and catalyzed the lysine-63 (K63)-linked polyubiquitylation of cGAS at Lys⁴¹¹. This polyubiquitylation event inhibited the DNA binding ability of cGAS, impaired cGAMP production, and attenuated the downstream innate immune response. Furthermore, March8-deficient mice were less susceptible than their wild-type counterparts to herpes simplex virus 1 (HSV-1) infection. Together, our findings reveal a mechanism underlying the functional regulation of cGAS and the fine-tuning of the innate immune response.

Advances in HIV-1 Assembly

The assembly of HIV-1 particles is a concerted and dynamic process that takes place on the plasma membrane of infected cells. An abundance of recent discoveries has advanced our understanding of the complex sequence of events leading to HIV-1 particle assembly, budding, and release. Structural studies have illuminated key features of assembly and maturation, including the dramatic structural transition that occurs between the immature Gag lattice and the formation of the mature viral capsid core. The critical role of inositol hexakisphosphate (IP6) in the assembly of both the immature and mature Gag lattice has been elucidated. The structural basis for selective packaging of genomic RNA into virions has been revealed. This review will provide an overview of the HIV-1 assembly process, with a focus on recent advances in the field, and will point out areas where questions remain that can benefit from future investigation.

The immune-specific E3 ubiquitin ligase MARCH1 is upregulated during HCMV infection to regulate iron levels

Human cytomegalovirus (HCMV) modulates numerous cellular pathways to facilitate infection. Iron is essential to many cellular processes and is often incorporated into proteins and enzymes involved in oxidative phosphorylation and DNA synthesis and repair, among others. Despite its prominent role in the cell, little is known about the regulation of iron metabolism during HCMV infection. Herein, we observe modulation of the transferrin receptor (TfR) during infection and a corresponding change in the cellular labile iron pool. TfR and the iron pool are increased early during infection and then return to mock levels at the late stages of infection. We identified the cellular ubiquitin ligase MARCH1 as an important regulator of TfR. MARCH1 plays a proviral role during infection, as its knockdown leads to a decrease in infectious titers. Knockdown of MARCH1 also leads to an increase in ROS, lipid peroxidation and mitochondrial dysfunction. Inhibiting an early increase in TfR expression during infection also decreases virus production. These findings indicate the importance of tightly regulating iron metabolism during HCMV infection to facilitate efficient virus production. Importance Iron is essential for cells, playing important roles in energy generation, DNA replication, and gene expression. During infection, HCMV alters many cellular processes to aid its replication. We found that iron levels are tightly regulated during infection and that dysregulation of iron levels alters the ability to produce infectious virions. We also found that HCMV inactivates many of the cellular safeguards put in place to deal with excess iron. Thus, infected cells become more susceptible to variations in iron levels, which could be exploited as a therapeutic strategy for dealing with HCMV infections.

MARCH8 Targets Cytoplasmic Lysine Residues of Various Viral Envelope Glycoproteins

The host transmembrane protein MARCH8 is a RING finger E3 ubiquitin ligase that downregulates various host transmembrane proteins, such as MHC-II. We have recently reported that MARCH8 expression in virus-producing cells impairs viral infectivity by reducing virion incorporation of not only HIV-1 envelope glycoprotein but also vesicular stomatitis virus G-glycoprotein through two different pathways. However, the MARCH8 inhibition spectrum remains largely unknown. Here, we show the antiviral spectrum of MARCH8 using viruses pseudotyped with a variety of viral envelope glycoproteins. Infection experiments revealed that viral envelope glycoproteins derived from the rhabdovirus, arenavirus, coronavirus, and togavirus (alphavirus) families were sensitive to MARCH8-mediated inhibition. Lysine mutations at the cytoplasmic tails of rabies virus-G, lymphocytic choriomeningitis virus glycoproteins, SARS-CoV and SARS-CoV-2 spike proteins, and Chikungunya virus and Ross River virus E2 proteins conferred resistance to MARCH8. Immunofluorescence showed impaired downregulation of the mutants of these viral envelope glycoproteins by MARCH8, followed by lysosomal degradation, suggesting that MARCH8-mediated ubiquitination leads to intracellular degradation of these envelopes. Indeed, rabies virus-G and Chikungunya virus E2 proteins proved to be clearly ubiquitinated. We conclude that MARCH8 has inhibitory activity on a variety of viral envelope glycoproteins whose cytoplasmic lysine residues are targeted by this antiviral factor.

IMPORTANCE A member of the MARCH E3 ubiquitin ligase family, MARCH8, downregulates many different kinds of host transmembrane proteins, resulting in the regulation of cellular homeostasis. On the other hands, MARCH8 acts as an antiviral factor when it binds to and downregulates HIV-1 envelope glycoprotein and vesicular stomatitis virus G-glycoprotein that are viral transmembrane proteins. This study reveals that, as in the case of cellular membrane proteins, MARCH8 shows broad-spectrum inhibition against various viral envelope glycoproteins by recognizing their cytoplasmic lysine residues, resulting in lysosomal degradation.

Potential of E3 Ubiquitin Ligases in Cancer Immunity: Opportunities and Challenges

Cancer immunotherapies, including immune checkpoint inhibitors and immune pathway–targeted therapies, are promising clinical strategies for treating cancer. However, drug resistance and adverse reactions remain the main challenges for immunotherapy management. The future direction of immunotherapy is mainly to reduce side effects and improve the treatment response rate by finding new targets and new methods of combination therapy. Ubiquitination plays a crucial role in regulating the degradation of immune checkpoints and the activation of immune-related pathways. Some drugs that target E3 ubiquitin ligases have exhibited beneficial effects in preclinical and clinical antitumor treatments. In this review, we discuss mechanisms through which E3 ligases regulate tumor immune checkpoints and immune-related pathways as well as the opportunities and challenges for integrating E3 ligases targeting drugs into cancer immunotherapy.

Deubiquitylating enzymes: potential target in autoimmune diseases

The ubiquitin-proteasome pathway is responsible for the turnover of different cellular proteins, such as transport proteins, presentation of antigens to the immune system, control of the cell cycle, and activities that promote cancer. The enzymes which remove ubiquitin, deubiquitylating enzymes (DUBs), play a critical role in central and peripheral immune tolerance to prevent the development of autoimmune diseases and thus present a potential therapeutic target for the treatment of autoimmune diseases. DUBs function by removing ubiquitin(s) from target protein and block ubiquitin chain elongation. The addition and removal of ubiquitin molecules have a significant impact on immune responses. DUBs and E3 ligases both specifically cleave target protein and modulate protein activity and expression. The balance between ubiquitylation and deubiquitylation modulates protein levels and also protein interactions. Dysregulation of the ubiquitin-proteasome pathway results in the development of various autoimmune diseases such as inflammatory bowel diseases (IBD), psoriasis, multiple sclerosis (MS), systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). This review summarizes the current understanding of ubiquitination in autoimmune diseases and focuses on various DUBs responsible for the progression of autoimmune diseases.

MHC Class II Ubiquitination Regulates Dendritic Cell Function and Immunity

MHC class II (MHC II) Ag presentation by dendritic cells (DCs) is critical for CD4⁺ T cell immunity. Cell surface levels of MHC II loaded with peptide is controlled by ubiquitination. In this study, we have examined how MHC II ubiquitination impacts immunity using MHC IIKR^KI/KI mice expressing mutant MHC II molecules that are unable to be ubiquitinated. Numbers of conventional DC (cDC) 1, cDC2 and plasmacytoid DCs were significantly reduced in MHC IIKR^KI/KI spleen, with the remaining MHC IIKR^KI/KI DCs expressing an altered surface phenotype. Whereas Ag uptake, endosomal pH, and cathepsin protease activity were unaltered, MHC IIKR^KI/KI cDC1 produced increased inflammatory cytokines and possessed defects in Ag proteolysis. Immunization of MHC IIKR^KI/KI mice identified impairments in MHC II and MHC class I presentation of soluble, cell-associated and/or DC-targeted OVA via mAb specific for DC surface receptor Clec9A (anti-Clec9A-OVA mAb). Reduced T cell responses and impaired CTL killing was observed in MHC IIKR^KI/KI mice following immunization with cell-associated and anti-Clec9A-OVA. Immunization of MHC IIKR^KI/KI mice failed to elicit follicular Th cell responses and generated barely detectable Ab to anti-Clec9A mAb-targeted Ag. In summary, MHC II ubiquitination in DCs impacts the homeostasis, phenotype, cytokine production, and Ag proteolysis by DCs with consequences for Ag presentation and T cell and Ab-mediated immunity.

HSC70 Inhibits Spring Viremia of Carp Virus Replication by Inducing MARCH8-Mediated Lysosomal Degradation of G Protein

As a fierce pathogen, spring viremia of carp virus (SVCV) can cause high mortality in the common carp, and its glycoprotein (G protein) is a component of the viral structure on the surface of virion, which is crucial in viral life cycle. This report adopted tandem affinity purification (TAP), mass spectrometry analysis (LC-MS/MS), immunoprecipitation, and confocal microscopy assays to identify Heat shock cognate protein 70 (HSC70) as an interaction partner of SVCV G protein. It was found that HSC70 overexpression dramatically inhibited SVCV replication, whereas its loss of functions elicited opposing effects on SVCV replication. Mechanistic studies indicate that HSC70 induces lysosomal degradation of ubiquitinated-SVCV G protein. This study further demonstrates that Membrane-associated RING-CH 8 (MARCH8), an E3 ubiquitin ligase, is critical for SVCV G protein ubiquitylation and leads to its lysosomal degradation. Furthermore, the MARCH8 mediated ubiquitylation of SVCV G protein required the participation of HSC70 through forming a multicomponent complex. Taken together, these results demonstrate that HSC70 serves as a scaffold for MARCH8 and SVCV G, which leads to the ubiquitylation and degradation of SVCV G protein and thus inhibits viral replication. These findings have established a novel host defense mechanism against SVCV.

Rutaecarpine Increases Anticancer Drug Sensitivity in Drug-Resistant Cells through MARCH8-Dependent ABCB1 Degradation

The overexpression of adenosine triphosphate (ATP)-binding cassette (ABC) subfamily B member 1 (ABCB1; P-glycoprotein; MDR1) in some types of cancer cells is one of the mechanisms responsible for the development of multidrug resistance (MDR), which leads to the failure of chemotherapy. Therefore, it is important to inhibit the activity or reduce the expression level of ABCB1 to maintain an effective intracellular level of chemotherapeutic drugs. In this study, we found that rutaecarpine, a bioactive alkaloid isolated from Evodia Rutaecarpa, has the capacity to reverse ABCB1-mediated MDR. Our data indicated that the reversal effect of rutaecarpine was related to the attenuation of the protein level of ABCB1. Mechanistically, we demonstrated that ABCB1 is a newly discovered substrate of E3 ubiquitin ligase membrane-associated RING-CH 8 (MARCH8). MARCH8 can interact with ABCB1 and promote its ubiquitination and degradation. In short, rutaecarpine increased the degradation of ABCB1 protein by upregulating the protein level of MARCH8, thereby antagonizing ABCB1-mediated MDR. Notably, the treatment of rutaecarpine combined with other anticancer drugs exhibits a therapeutic effect on transplanted tumors. Therefore, our study provides a potential chemotherapeutic strategy of co-administrating rutaecarpine with other conventional chemotherapeutic agents to overcome MDR and improve therapeutic effect.

MARCH8 inhibits influenza A virus infection by targeting viral M2 protein for ubiquitination-dependent degradation in lysosomes

The membrane-associated RING-CH (MARCH) proteins are E3 ligases that regulate the stability of various cellular membrane proteins. MARCH8 has been reported to inhibit the infection of HIV-1 and a few other viruses, thus plays an important role in host antiviral defense. However, the antiviral spectrum and the underlying mechanisms of MARCH8 are incompletely defined. Here, we demonstrate that MARCH8 profoundly inhibits influenza A virus (IAV) replication both in vitro and in mice. Mechanistically, MARCH8 suppresses IAV release through redirecting viral M2 protein from the plasma membrane to lysosomes for degradation. Specifically, MARCH8 catalyzes the K63-linked polyubiquitination of M2 at lysine residue 78 (K78). A recombinant A/Puerto Rico/8/34 virus carrying the K78R M2 protein shows greater replication and more severe pathogenicity in cells and mice. More importantly, we found that the M2 protein of the H1N1 IAV has evolved to acquire non-lysine amino acids at positions 78/79 to resist MARCH8-mediated ubiquitination and degradation. Together, our data support the important role of MARCH8 in host anti-IAV intrinsic immune defense by targeting M2, and suggest the inhibitory pressure of MARCH8 on H1N1 IAV transmission in the human population.

MARCH8: the tie that binds to viruses

Membrane-associated RING-CH (MARCH) family member proteins are RING-finger E3 ubiquitin ligases that are known to downregulate cellular transmembrane proteins. MARCH8 is a novel antiviral factor that inhibits HIV-1 envelope glycoprotein and vesicular stomatitis virus G by downregulating these envelope glycoproteins from the cell surface, resulting in their reduced incorporation into virions. More recently, we have found that MARCH8 reduces viral infectivity via two different mechanisms. Additionally, several groups have reported further antiviral or virus-supportive functions of the MARCH8 protein and its other cellular mechanisms. In this review, we summarize the current knowledge about the molecular mechanisms by which MARCH8 can regulate cellular homeostasis and inhibit and occasionally support enveloped virus infection.

When MARCH family proteins meet viral infections

Membrane-associated RING-CH (MARCH) ubiquitin ligases belong to a RING finger domain E3 ligases family. Recent studies have demonstrated that MARCH proteins play critical roles during various viral infections. MARCH proteins can directly antagonize different steps of the viral life cycle and promote individual viral infection. This mini-review will focus on the latest advances of MARCH family proteins' emerging roles during viral infections.

Elucidating the Antiviral Mechanism of Different MARCH Factors

The membrane-associated RING-CH (MARCH) proteins belong to a family of E3 ubiquitin ligases, whose main function is to remove transmembrane proteins from the plasma membrane. Recent work has shown that the human MARCH1, 2, and 8 are antiretroviral factors that target the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins by reducing their incorporation in the budding virions. Nevertheless, the dearth of information regarding the antiviral mechanism of this family of proteins necessitates further examination. In this study, using both the human MARCH proteins and their mouse homologues, we provide a comprehensive analysis of the antiretroviral mechanism of this family of proteins. Moreover, we show that human MARCH proteins restrict to various degrees the envelope glycoproteins of a diverse number of viruses. This report sheds light on the important antiviral function of MARCH proteins and their significance in cell intrinsic immunity.IMPORTANCE This study examines the mechanism utilized by different MARCH proteins to restrict retrovirus infection. MARCH proteins block the incorporation of envelope glycoproteins to the budding virions. In this report, by comparing the human and mouse MARCH genes and using murine leukemia virus (MLV) and HIV-1, we identify differences in the mechanism of restriction among MARCH proteins. Furthermore, we perform a comprehensive analysis on a number of envelope glycoproteins and show that MARCH proteins have broad antiviral functions.

iTRAQ-based analysis for the identification of MARCH8 targets in human esophageal squamous cell carcinoma

MARCH8 is an E3 ligase, primarily involved in immune-modulation. Recently, we reported its aberrant expression in human esophageal squamous cell carcinoma. However, exact mechanisms by which it regulates cancer have been poorly understood. We applied high-throughput quantitative proteomics approach to identify downstream protein targets of MARCH8. Silencing of endogenous MARCH8 in ESCC cells followed by LC-MS/MS analysis led to identification of 1,029 unique proteins showing altered expression post MARCH8 knockdown. Several previously reported MARCH8 target proteins viz. TFR1, syntaxin-4, e-cadherin and CD44 were found to be upregulated. Furthermore, new putative targets of MARCH8, including β2M, were identified in the present study. We demonstrated that MARCH8 interacts with and ubiquitinates CDH1 and β2M. Inhibiting proteasome activity with MG132 prevented CDH1 and β2M degradation, indicating that MARCH8 might be targeting CDH1 and β2M for proteasomal degradation. Further, loss of β2M and CDH1 expression significantly and inversely correlated with MARCH8 expression in ESCC tissues (r =  -0.737 and  - 0.651, respectively; p < 0.01). In conclusion, our present study has led to identification of new targets of MARCH8 and suggests the role of MARCH8 in regulating CDH1 and β2M turnover in esophageal cancer cells. SIGNIFICANCE: The use of quantitative proteomics carried out has led to the recognition of new targets of MARCH8. The present study gives a broad understanding of the molecular remodeling arising in the ESCC after MARCH8 knockdown. The study also solidifies the idea that role of MARCH8 is not just limited to immunomodulation as silencing of MARCH8 affects various other processes such as protein processing and localization. This study might help in understanding the regulation of MARCH8 in ESCCs and the mechanism by which MARCH8 might be facilitating cancer cells to evade immune surveillance.

Targeting the ubiquitination/deubiquitination process to regulate immune checkpoint pathways

The immune system initiates robust immune responses to defend against invading pathogens or tumor cells and protect the body from damage, thus acting as a fortress of the body. However, excessive responses cause detrimental effects, such as inflammation and autoimmune diseases. To balance the immune responses and maintain immune homeostasis, there are immune checkpoints to terminate overwhelmed immune responses. Pathogens and tumor cells can also exploit immune checkpoint pathways to suppress immune responses, thus escaping immune surveillance. As a consequence, therapeutic antibodies that target immune checkpoints have made great breakthroughs, in particular for cancer treatment. While the overall efficacy of immune checkpoint blockade (ICB) is unsatisfactory since only a small group of patients benefited from ICB treatment. Hence, there is a strong need to search for other targets that improve the efficacy of ICB. Ubiquitination is a highly conserved process which participates in numerous biological activities, including innate and adaptive immunity. A growing body of evidence emphasizes the importance of ubiquitination and its reverse process, deubiquitination, on the regulation of immune responses, providing the rational of simultaneous targeting of immune checkpoints and ubiquitination/deubiquitination pathways to enhance the therapeutic efficacy. Our review will summarize the latest findings of ubiquitination/deubiquitination pathways for anti-tumor immunity, and discuss therapeutic significance of targeting ubiquitination/deubiquitination pathways in the future of immunotherapy.

Negative regulation of NEMO signaling by the ubiquitin E3 ligase MARCH2

NF‐κB essential modulator (NEMO) is a key regulatory protein that functions during NF‐κB‐ and interferon‐mediated signaling in response to extracellular stimuli and pathogen infections. Tight regulation of NEMO is essential for host innate immune responses and for maintenance of homeostasis. Here, we report that the E3 ligase MARCH2 is a novel negative regulator of NEMO‐mediated signaling upon bacterial or viral infection. MARCH2 interacted directly with NEMO during the late phase of infection and catalyzed K‐48‐linked ubiquitination of Lys326 on NEMO, which resulted in its degradation. Deletion of MARCH2 resulted in marked resistance to bacterial/viral infection, along with increased innate immune responses both in vitro and in vivo. In addition, MARCH2^−/− mice were more susceptible to LPS challenge due to massive production of cytokines. Taken together, these findings provide new insight into the molecular regulation of NEMO and suggest an important role for MARCH2 in homeostatic control of innate immune responses.

MARCH8 inhibits viral infection by two different mechanisms

Membrane-associated RING-CH 8 (MARCH8) inhibits infection with both HIV-1 and vesicular stomatitis virus G-glycoprotein (VSV-G)-pseudotyped viruses by reducing virion incorporation of envelope glycoproteins. The molecular mechanisms by which MARCH8 targets envelope glycoproteins remain unknown. Here, we show two different mechanisms by which MARCH8 inhibits viral infection. Viruses pseudotyped with the VSV-G mutant, in which cytoplasmic lysine residues were mutated, were insensitive to the inhibitory effect of MARCH8, whereas those with a similar lysine mutant of HIV-1 Env remained sensitive to it. Indeed, the wild-type VSV-G, but not its lysine mutant, was ubiquitinated by MARCH8. Furthermore, the MARCH8 mutant, which had a disrupted cytoplasmic tyrosine motif that is critical for intracellular protein sorting, did not inhibit HIV-1 Env-mediated infection, while it still impaired infection by VSV-G-pseudotyped viruses. Overall, we conclude that MARCH8 reduces viral infectivity by downregulating envelope glycoproteins through two different mechanisms mediated by a ubiquitination-dependent or tyrosine motif-dependent pathway.

Ubiquitination of MHC Class II Is Required for Development of Regulatory but Not Conventional CD4+ T Cells

MHC class II (MHC II) displays peptides at the cell surface, a process critical for CD4⁺ T cell development and priming. Ubiquitination is a mechanism that dictates surface MHC II with the attachment of a polyubiquitin chain to peptide-loaded MHC II, promoting its traffic away from the plasma membrane. In this study, we have examined how MHC II ubiquitination impacts the composition and function of both conventional CD4⁺ T cell and regulatory T cell (T_reg) compartments. Responses were examined in two models of altered MHC II ubiquitination: MHCIIKR^{KI /KI} mice that express a mutant MHC II unable to be ubiquitinated or mice that lack membrane-associated RING-CH 8 (MARCH8), the E3 ubiquitin ligase responsible for MHC II ubiquitination specifically in thymic epithelial cells. Conventional CD4⁺ T cell populations in thymus, blood, and spleen of MHCIIKR^KI/KI and March8 ^-/- mice were largely unaltered. In MLRs, March8 ^-/-, but not MHCIIKR^KI/KI, CD4⁺ T cells had reduced reactivity to both self- and allogeneic MHC II. Thymic T_reg were significantly reduced in MHCIIKR^KI/KI mice, but not March8 ^-/- mice, whereas splenic T_reg were unaffected. Neither scenario provoked autoimmunity, with no evidence of immunohistopathology and normal levels of autoantibody. In summary, MHC II ubiquitination in specific APC types does not have a major impact on the conventional CD4⁺ T cell compartment but is important for T_reg development.

The Membrane-Associated MARCH E3 Ligase Family: Emerging Roles in Immune Regulation

The membrane-associated RING-CH-type finger (MARCH) proteins of E3 ubiquitin ligases have emerged as critical regulators of immune responses. MARCH proteins target immune receptors, viral proteins as well as components in innate immune response for polyubiquitination and degradations via distinct routes. This review summarizes the current progress about MARCH proteins and their regulation on immune responses.

Membrane-associated RING-CH (MARCH) 1 and 2 are MARCH family members that inhibit HIV-1 infection

Membrane-associated RING-CH 8 (MARCH8) is one of 11 members of the MARCH family of RING finger E3 ubiquitin ligases and down-regulates several membrane proteins (e.g. major histocompatibility complex II [MHC-II], CD86, and transferrin receptor). We recently reported that MARCH8 also targets HIV-1 envelope glycoproteins and acts as an antiviral factor. However, it remains unclear whether other family members might have antiviral functions similar to those of MARCH8. Here we show that MARCH1 and MARCH2 are MARCH family members that reduce virion incorporation of envelope glycoproteins. Infectivity assays revealed that MARCH1 and MARCH2 dose-dependently suppress viral infection. Treatment with type I interferon enhanced endogenous expression levels of MARCH1 and MARCH2 in monocyte-derived macrophages. Expression of these proteins in virus-producing cells decreased the efficiency of viral entry and down-regulated HIV-1 envelope glycoproteins from the cell surface, resulting in reduced incorporation of envelope glycoproteins into virions, as observed in MARCH8 expression. With the demonstration that MARCH1 and MARCH2 are antiviral MARCH family members as presented here, these two proteins join a growing list of host factors that inhibit HIV-1 infection.

A serine in the first transmembrane domain of the human E3 ubiquitin ligase MARCH9 is critical for down-regulation of its protein substrates

The membrane-associated RING-CH (MARCH) family of membrane-bound E3 ubiquitin ligases regulates the levels of cell-surface membrane proteins, many of which are involved in immune responses. Although their role in ubiquitin-dependent endocytosis and degradation of cell-surface proteins is extensively documented, the features of MARCH proteins and their substrates that drive the molecular recognition events leading to ubiquitin transfer remain poorly defined. In this study, we sought to determine the features of human MARCH9 that are required for regulating the surface levels of its substrate proteins. Consistent with previous studies of other MARCH proteins, we found that susceptibility to MARCH9 activity is encoded in the transmembrane (TM) domains of its substrates. Accordingly, substitutions at specific residues and motifs within MARCH9's TM domains resulted in varying degrees of functional impairment. Most notably, a single serine-to-alanine substitution in the first of its two TM domains rendered MARCH9 completely unable to alter the surface levels of two different substrates: the major histocompatibility class I molecule HLA-A2 and the T-cell co-receptor CD4. Solution NMR analysis of a MARCH9 fragment encompassing the two TM domains and extracellular connecting loop revealed that the residues contributing most to MARCH9 activity are located in the α-helical portions of TM1 and TM2 that are closest to the extracellular face of the lipid bilayer. This observation defines a key region required for substrate regulation. In summary, our biochemical and structural findings demonstrate that specific sequences in the α-helical MARCH9 TM domains make crucial contributions to its ability to down-regulate its protein substrates.

MARCH1-mediated ubiquitination of MHC II impacts the MHC I antigen presentation pathway

Major histocompatibility complex class II (MHC II) expression and turn-over are regulated via its ubiquitination by the membrane associated RING-CH 1 (MARCH1) E3 ligase. Unexpectedly, we show that MHC II ubiquitination also impacts MHC I. Lack of MARCH1 in B cells and dendritic cells (DCs) resulted in a significant reduction in surface MHC I expression. This decrease was not directly caused by changes in MARCH1 ubiquitination of MHC I but indirectly by altered MHC II trafficking in the absence of its ubiquitination. Deletion of MHC II in March1^-/- cells restored normal MHC I surface expression and replacement of wild type MHC II by a variant that could not be ubiquitinated caused a reduction in MHC I expression. Furthermore, these cells displayed inefficient presentation of peptide and protein antigen via MHC I to CD8⁺ T cells. In summary, we describe an unexpected intersection between MHC I and MHC II such that the surface expression of both molecules are indirectly and directly regulated by MARCH1 ubiquitination, respectively.

RING-Domain E3 Ligase-Mediated Host-Virus Interactions: Orchestrating Immune Responses by the Host and Antagonizing Immune Defense by Viruses

The RING-domain E3 ligases (RING E3s), a group of E3 ligases containing one or two RING finger domains, are involved in various cellular processes such as cell proliferation, immune regulation, apoptosis, among others. In the host, a substantial number of the RING E3s have been implicated to inhibit viral replication through regulating immune responses, including activation and inhibition of retinoic acid-inducible gene I-like receptors, toll-like receptors, and DNA receptor signaling pathways, modulation of cell-surface expression of major histocompatibility complex, and co-stimulatory molecules. During the course of evolution and adaptation, viruses encode RING E3s to antagonize host immune defense, such as the infected cell protein 0 of herpes simplex virus type 1, the non-structural protein 1 of rotavirus, and the K3 and K5 of Kaposi’s sarcoma-associated herpesvirus. In addition, recent studies suggest that viruses can hijack the host RING E3s to facilitate viral replication. Based on emerging and interesting discoveries, the RING E3s present novel links among the host and viruses. Herein, we focus on the latest research progresses in the RING E3s-mediated host–virus interactions and discuss the outlooks of the RING E3s for future research.

Increased expression of MARCH8, an E3 ubiquitin ligase, is associated with growth of esophageal tumor

Background

Herein, for the first time, we report aberrant expression of membrane-associated RING-CH8 (MARCH8) in human esophageal squamous cell carcinoma. MARCH8 is a member of the recently discovered MARCH family of really interesting new genes (RING) E3 ligases. Though initial studies primarily focused on its immunomodulatory role, the newly discovered targets of this E3 ligase point towards its possible role in other biological processes such as embryogenesis and inhibition of apoptosis. However, its relevance in cancers is yet to be elucidated.

Methods

We carried out quantitative real time PCR and immunohistochemistry to examine the levels of MARCH8 mRNA and protein in esophageal squamous cell carcinoma tissues. The role of MARCH8 in esophageal cancer cells was evaluated by cell proliferation, clonogenic and migration/invasion assays and flow cytometry with MARCH8 gene knockdown.

Results

Significantly increased expression of MARCH8 mRNA was found in esophageal squamous cell carcinoma as compared to distant matched non-malignant tissues (p = 0.024, AUC = 0.654). Immunohistochemical analysis revealed overexpression of MARCH8 protein in 86% of esophageal squamous cell carcinoma tissues (p < 0.001, AUC = 0.908). Interestingly, intense nuclear staining of MARCH8 protein was detected in cancer cells in addition to its cytoplasmic expression. Knockdown of MARCH8 resulted in decreased proliferation, migration, invasion and clonogenic potential of esophageal cancer cells. In addition to this, silencing of MARCH8 induced apoptosis in esophageal cancer cells which was measured by cell cycle distribution assay which showed increase in sub G0 and G2/M populations (cell death) and decrease in S-phase population. To further check the type of apoptosis induced by MARCH8 silencing, annexin assay was performed which showed significant increase in the number of cells in early apoptotic phase.

Conclusions

Overall, increased expression of MARCH8 gene in preneoplastic and neoplastic esophageal tissues and its knockdown effect on cancer cell properties demonstrated herein points towards the potential role of this protein in esophageal tumorigenesis.

Hijacking of the Ubiquitin/Proteasome Pathway by the HIV Auxiliary Proteins

The ubiquitin-proteasome system (UPS) ensures regulation of the protein pool in the cell by ubiquitination of proteins followed by their degradation by the proteasome. It plays a central role in the cell under normal physiological conditions as well as during viral infections. On the one hand, the UPS can be used by the cell to degrade viral proteins, thereby restricting the viral infection. On the other hand, it can also be subverted by the virus to its own advantage, notably to induce degradation of cellular restriction factors. This makes the UPS a central player in viral restriction and counter-restriction. In this respect, the human immunodeficiency viruses (HIV-1 and 2) represent excellent examples. Indeed, many steps of the HIV life cycle are restricted by cellular proteins, some of which are themselves components of the UPS. However, HIV itself hijacks the UPS to mediate defense against several cellular restriction factors. For example, the HIV auxiliary proteins Vif, Vpx and Vpu counteract specific restriction factors by the recruitment of cellular UPS components. In this review, we describe the interplay between HIV and the UPS to illustrate its role in the restriction of viral infections and its hijacking by viral proteins for counter-restriction.

Molecular cloning of the MARCH family in grass carp (Ctenopharyngodon idellus) and their response to grass carp reovirus challenge

Grass carp (Ctenopharyngodon idellus) is an economical aquaculture species in China, and the Grass Carp Reovirus (GCRV) that causes hemorrhagic disease seriously affects the grass carp cultivation industry. Substantial evidence indicates that there is an association between the membrane-associated RING-CH family of E3 ligase (MARCH) family and immune defense in mammals, while functional studies on non-mammalian MARCH proteins are limited. In order to know the characteristics of the MARCH genes in C. idellus, eight MARCH genes (MARCH1, 2, 5, 6, 7, 8, 9 and 11) were cloned and the open reading frames (ORF) were identified in grass carp. All MARCH proteins in grass carp contained an RING-CH domain, which is characteristic of the MARCH protein. The phylogenetic analysis revealed that different MARCH proteins gathered into their separate clusters. All eight members of the MARCH gene family were detected in all tissues sampled, but the relative expression level differed. In addition, the mRNA expression of all the MARCHs was regulated at different levels in the immune organs after a GCRV challenge, and they responded robustly in both the intestine and liver. The mRNA expression of MARCH8, MHC II, TfR, IL1RAP, EGR1, and DUSP1 in the intestine after GCRV infection was analyzed, and the results showed that MARCH8 could negatively regulate TfR, IL1RAP, EGR1, and DUSP1, which signaled via the MAPK or NF-κB-activation pathways that play vital roles in immunity. Our findings identified a novel gene family in C. idellus and provided novel evidence that MARCH genes are inducible and involved in the immune response. Moreover, MARCH8 might function to negatively regulate immune receptors in C. idellus. Therefore, the MARCH might play a vital role in regulating the immune response of C. idellus.

Identification of an antiviral host transmembrane protein MARCH8

Membrane-associated RING-CH 8 (MARCH8) is one of 11 members of the recently discovered MARCH family of RING-finger E3 ubiquitin ligases. MARCH8 downregulates several host transmembrane proteins; however, its physiological roles remain unknown. Here we identify MARCH8 as a novel antiviral factor. The overexpression of MARCH8 in virus producing cells did not affect levels of lentivirus production, but markedly reduced viral infectivity. MARCH8 blocked the incorporation of HIV-1 envelope glycoprotein into virions by downregulating it from the cell surface, probably through their interaction, resulting in reduced viral entry efficiency. The inhibitory effect of MARCH8 on vesicular stomatitis virus G-glycoprotein was even more remarkable, suggesting a broad-spectrum inhibition of enveloped viruses by MARCH8. Importantly, the endogenous expression of MARCH8 was high in monocyte-derived macrophages and dendritic cells, and MARCH8 depletion in macrophages significantly increased the infectivity of virions produced from these cells. Our findings thus indicate that MARCH8, which is highly expressed in terminally differentiated myeloid cells, is a potent antiviral host transmembrane protein that reduces virion incorporation of viral envelope glycoproteins.

Characterization of E3 ligases involved in lysosomal sorting of the HIV-1 restriction factor BST2

The cellular protein BST2 (also known as tetherin) acts as a major intrinsic antiviral protein that prevents the release of enveloped viruses by trapping nascent viral particles at the surface of infected cells. Viruses have evolved specific strategies to displace BST2 from viral budding sites in order to promote virus egress. In HIV-1, the accessory protein Vpu counters BST2 antiviral activity and promotes sorting of BST2 for lysosomal degradation. Vpu increases polyubiquitylation of BST2, a post-translation modification required for Vpu-induced BST2 downregulation, through recruitment of the E3 ligase complex SCF adaptors β-TrCP1 and β-TrCP2 (two isoforms encoded by BTRC and FBXW11, respectively). Herein, we further investigate the role of the ubiquitylation machinery in the lysosomal sorting of BST2. Using a small siRNA screen, we highlighted two additional regulators of BST2 constitutive ubiquitylation and sorting to the lysosomes: the E3 ubiquitin ligases NEDD4 and MARCH8. Interestingly, Vpu does not hijack the cellular machinery that is constitutively involved in BST2 ubiquitylation to sort BST2 for degradation in the lysosomes but instead promotes the recognition of BST2 by β-TrCP proteins. Altogether, our results provide further understanding of the mechanisms underlying BST2 turnover in cells.

Highlighted Article: We identify two E3 ubiquitin ligases, NEDD4 and MARCH8, as regulators of BST2 (tetherin) – a protein that restricts viral release; we thus provide further understanding of the mechanisms underlying BST2 turnover in cells.

Ubiquitination and the Regulation of Membrane Proteins

Newly synthesized transmembrane proteins undergo a series of steps to ensure that only the required amount of correctly folded protein is localized to the membrane. The regulation of protein quality and its abundance at the membrane are often controlled by ubiquitination, a multistep enzymatic process that results in the attachment of ubiquitin, or chains of ubiquitin to the target protein. Protein ubiquitination acts as a signal for sorting, trafficking, and the removal of membrane proteins via endocytosis, a process through which multiple ubiquitin ligases are known to specifically regulate the functions of a number of ion channels, transporters, and signaling receptors. Endocytic removal of these proteins through ubiquitin-dependent endocytosis provides a way to rapidly downregulate the physiological outcomes, and defects in such controls are directly linked to human pathologies. Recent evidence suggests that ubiquitination is also involved in the shedding of membranes and associated proteins as extracellular vesicles, thereby not only controlling the cell surface levels of some membrane proteins, but also their potential transport to neighboring cells. In this review, we summarize the mechanisms and functions of ubiquitination of membrane proteins and provide specific examples of ubiquitin-dependent regulation of membrane proteins.

HLA class II antigen-processing pathway in tumors: Molecular defects and clinical relevance

The human leukocyte antigen (HLA) class II antigen-processing machinery (APM) presents to cognate CD4⁺ T-cells antigenic peptides mainly generated from exogeneous proteins in the endocytic compartment. These CD4⁺ T cells exert helper function, but may also act as effector cells, thereby recognizing HLA class II antigen-expressing tumor cells. Thus, HLA class II antigen expression by tumor cells influences the tumor antigen (TA)-specific immune responses and, depending on the cancer type, the clinical course of the disease. Many types of human cancers express HLA class II antigens, although with marked differences in their frequency. Some types of cancer lack HLA class II antigen expression, which could be due to structural defects or deregulation affecting different components of the complex HLA class II APM and/or from lack of cytokine(s) in the tumor microenvironment. In this review, we have summarized the information about HLA class II antigen distribution in normal tissues, the structural organization of the HLA class II APM, their expression and regulation in malignant cells, the defects, which have been identified in malignant cells, and their functional and clinical relevance.

Major Histocompatibility Complex Class II HLA-DRα Is Downregulated by Kaposi's Sarcoma-Associated Herpesvirus-Encoded Lytic Transactivator RTA and MARCH8

Kaposi's sarcoma-associated herpesvirus (KSHV) maintains two modes of life cycle, the latent and lytic phases. To evade the attack of the cell host's immune system, KSHV switches from the lytic to the latent phase, a phase in which only a few of viral proteins are expressed. The mechanism by which KSHV evades the attack of the immune system and establishes latency has not been fully understood. Major histocompatibility complex class II (MHC-II) molecules are key components of the immune system defense mechanism against viral infections. Here we report that HLA-DRα, a member of the MHC-II molecules, was downregulated by the replication and transcription activator (RTA) protein encoded by KSHV ORF50, an important regulator of the viral life cycle. RTA not only downregulated HLA-DRα at the protein level through direct binding and degradation through the proteasome pathway but also indirectly downregulated the protein level of HLA-DRα by enhancing the expression of MARCH8, a member of the membrane-associated RING-CH (MARCH) proteins. Our findings indicate that KSHV RTA facilitates evasion of the virus from the immune system through manipulation of HLA-DRα.

IMPORTANCE

Kaposi's sarcoma-associated herpesvirus (KSHV) has a causal role in a number of human cancers, and its persistence in infected cells is controlled by the host's immune system. The mechanism by which KSHV evades an attack by the immune system has not been well understood. This work represents studies which identify a novel mechanism by which the virus can facilitate evasion of an immune system. We now show that RTA, the replication and transcription activator encoded by KSHV (ORF50), can function as an E3 ligase to degrade HLA-DRα. It can directly bind and induce degradation of HLA-DRα through the ubiquitin-proteasome degradation pathway. In addition to the direct regulation of HLA-DRα, RTA can also indirectly downregulate the level of HLA-DRα protein by upregulating transcription of MARCH8. Increased MARCH8 results in the downregulation of HLA-DRα. Furthermore, we also demonstrate that expression of HLA-DRα was impaired in KSHV de novo infection.

Ubiquitin ligase MARCH 8 cooperates with CD83 to control surface MHC II expression in thymic epithelium and CD4 T cell selection

Major histocompatibility complex class II (MHC II) expression is tightly regulated, being subjected to cell type-specific mechanisms that closely control its levels at the cell surface. Ubiquitination by the E3 ubiquitin ligase MARCH 1 regulates MHC II expression in dendritic cells and B cells. In this study, we demonstrate that the related ligase MARCH 8 is responsible for regulating surface MHC II in thymic epithelial cells (TECs). March8(-/-) mice have elevated MHC II at the surface of cortical TECs and autoimmune regulator (AIRE)(-) medullary TECs (mTECs), but not AIRE(+) mTECs. Despite this, thymic and splenic CD4(+) T cell numbers and repertoires remained unaltered in March8(-/-) mice. Notably, the ubiquitination of MHC II by MARCH 8 is controlled by CD83. Mice expressing a mutated form of CD83 (Cd83(anu/anu) mice) have impaired CD4(+) T cell selection, but deleting March8 in Cd83(anu/anu) mice restored CD4(+) T cell selection to normal levels. Therefore, orchestrated regulation of MHC II surface expression in TECs by MARCH 8 and CD83 plays a major role in CD4(+) T cell selection. Our results also highlight the specialized use of ubiquitinating machinery in distinct antigen-presenting cell types, with important functional consequences and implications for therapeutic manipulation.

Thymic CD4 T cell selection requires attenuation of March8-mediated MHCII turnover in cortical epithelial cells through CD83

Deficiency of CD83 in thymic epithelial cells (TECs) dramatically impairs thymic CD4 T cell selection. CD83 can exert cell-intrinsic and -extrinsic functions through discrete protein domains, but it remains unclear how CD83's capacity to operate through these alternative functional modules relates to its crucial role in TECs. In this study, using viral reconstitution of gene function in TECs, we found that CD83's transmembrane domain is necessary and sufficient for thymic CD4 T cell selection. Moreover, a ubiquitination-resistant MHCII variant restored CD4 T cell selection in Cd83(-/-) mice. Although during dendritic cell maturation CD83 is known to stabilize MHCII through opposing the ubiquitin ligase March1, regulation of March1 did not account for CD83's TEC-intrinsic role. Instead, we provide evidence that MHCII in cortical TECs (cTECs) is targeted by March8, an E3 ligase of as yet unknown physiological substrate specificity. Ablating March8 in Cd83(-/-) mice restored CD4 T cell development. Our results identify CD83-mediated MHCII stabilization through antagonism of March8 as a novel functional adaptation of cTECs for T cell selection. Furthermore, these findings suggest an intriguing division of labor between March1 and March8 in controlling inducible versus constitutive MHCII expression in hematopoietic antigen-presenting cells versus TECs.

Metabolites of milk intake: a metabolomic approach in UK twins with findings replicated in two European cohorts

Purpose

Milk provides a significant source of calcium, protein, vitamins and other minerals to Western populations throughout life. Due to its widespread use, the metabolic and health impact of milk consumption warrants further investigation and biomarkers would aid epidemiological studies.

Methods

Milk intake assessed by a validated food frequency questionnaire was analyzed against fasting blood metabolomic profiles from two metabolomic platforms in females from the TwinsUK cohort (n = 3559). The top metabolites were then replicated in two independent populations (EGCUT, n = 1109 and KORA, n = 1593), and the results from all cohorts were meta-analyzed.

Results

Four metabolites were significantly associated with milk intake in the TwinsUK cohort after adjustment for multiple testing (P < 8.08 × 10⁻⁵) and covariates (BMI, age, batch effects, family relatedness and dietary covariates) and replicated in the independent cohorts. Among the metabolites identified, the carnitine metabolite trimethyl-N-aminovalerate (β = 0.012, SE = 0.002, P = 2.98 × 10⁻¹²) and the nucleotide uridine (β = 0.004, SE = 0.001, P = 9.86 × 10⁻⁶) were the strongest novel predictive biomarkers from the non-targeted platform. Notably, the association between trimethyl-N-aminovalerate and milk intake was significant in a group of MZ twins discordant for milk intake (β = 0.050, SE = 0.015, P = 7.53 × 10⁻⁴) and validated in the urine of 236 UK twins (β = 0.091, SE = 0.032, P = 0.004). Two metabolites from the targeted platform, hydroxysphingomyelin C14:1 (β = 0.034, SE = 0.005, P = 9.75 × 10⁻¹⁴) and diacylphosphatidylcholine C28:1 (β = 0.034, SE = 0.004, P = 4.53 × 10⁻¹⁶), were also replicated.

Conclusions

We identified and replicated in independent populations four novel biomarkers of milk intake: trimethyl-N-aminovalerate, uridine, hydroxysphingomyelin C14:1 and diacylphosphatidylcholine C28:1. Together, these metabolites have potential to objectively examine and refine milk-disease associations.

Electronic supplementary material

The online version of this article (doi:10.1007/s00394-016-1278-x) contains supplementary material, which is available to authorized users.

Molecular mechanism and cellular function of MHCII ubiquitination

The major histocompatibility complex class II (MHCII) is ubiquitinated via the evolutionarily conserved lysine in the cytoplasmic tail of the β chain in dendritic cells (DCs) and B cells. The ubiquitination is mediated by the membrane-associated RING-CH1 (MARCH1) ubiquitin ligase although it can be also mediated by the homologous ligase MARCH8 in model cell lines. The ubiquitination promotes MHCII endocytosis and lysosomal sorting that results in a reduction in the level of MHCII at cell surface. Functionally, MHCII ubiquitination serves as a means by which DCs suppress MHCII expression and reduce antigen presentation in response to the immune regulatory cytokine interleukin-10 (IL-10) and regulatory T cells. Recently, additional roles of MHCII ubiquitination have emerged. MHCII ubiquitination promoted DC production of inflammatory cytokines in response to the Toll-like receptor ligands. It also potentiated DC ability to activate antigen-specific naive CD4(+) T cells while limiting the amount of antigens presented at cell surface. Similarly, MHCII ubiquitination promoted DC activation of CD4(+) thymocytes supporting regulatory T-cell development independent of its effect of limiting antigen presentation. Thus, ubiquitination appears to confer MHCII a function independent of presenting antigens by a mechanism yet to be identified.

MARCH8 inhibits HIV-1 infection by reducing virion incorporation of envelope glycoproteins

Membrane-associated RING-CH 8 (MARCH8) is one of 11 members of the recently discovered MARCH family of RING (really interesting new gene)-finger E3 ubiquitin ligases. MARCH8 downregulates several host transmembrane proteins, including major histocompatibility complex (MHC)-II, CD86, interleukin (IL)-1 receptor accessory protein, TNF-related apoptosis-inducing ligand (TRAIL) receptor 1 and the transferrin receptor. However, its physiological roles remain largely unknown. Here we identify MARCH8 as a novel antiviral factor. The ectopic expression of MARCH8 in virus-producing cells does not affect levels of lentivirus production, but it does markedly reduce viral infectivity. MARCH8 blocks the incorporation of HIV-1 envelope glycoprotein into virus particles by downregulating it from the cell surface, probably through their interaction, resulting in a substantial reduction in the efficiency of viral entry. The inhibitory effect of MARCH8 on vesicular stomatitis virus G-glycoprotein is even more remarkable, suggesting a broad-spectrum inhibition of enveloped viruses by MARCH8. Notably, the endogenous expression of MARCH8 is high in monocyte-derived macrophages and dendritic cells, and MARCH8 knockdown or knockout in macrophages significantly increases the infectivity of virions produced by these cells. Our findings thus indicate that MARCH8 is highly expressed in terminally differentiated myeloid cells, and that it is a potent antiviral protein that targets viral envelope glycoproteins and reduces their incorporation into virions.

The Missing Link in Epstein-Barr Virus Immune Evasion: the BDLF3 Gene Induces Ubiquitination and Downregulation of Major Histocompatibility Complex Class I (MHC-I) and MHC-II

The ability of Epstein-Barr virus (EBV) to spread and persist in human populations relies on a balance between host immune responses and EBV immune evasion. CD8(+) cells specific for EBV late lytic cycle antigens show poor recognition of target cells compared to immediate early and early antigen-specific CD8(+) cells. This phenomenon is due in part to the early EBV protein BILF1, whose immunosuppressive activity increases with lytic cycle progression. However, published data suggest the existence of a hitherto unidentified immune evasion protein further enhancing protection against late EBV antigen-specific CD8(+) cells. We have now identified the late lytic BDLF3 gene as the missing link accounting for efficient evasion during the late lytic cycle. Interestingly, BDLF3 also contributes to evasion of CD4(+) cell responses to EBV. We report that BDLF3 downregulates expression of surface major histocompatibility complex (MHC) class I and class II molecules in the absence of any effect upon other surface molecules screened, including CD54 (ICAM-1) and CD71 (transferrin receptor). BDLF3 both enhanced internalization of surface MHC molecules and reduced the rate of their appearance at the cell surface. The reduced expression of surface MHC molecules correlated with functional protection against CD8(+) and CD4(+) T cell recognition. The molecular mechanism was identified as BDLF3-induced ubiquitination of MHC molecules and their subsequent downregulation in a proteasome-dependent manner.

IMPORTANCE

Immune evasion is a necessary feature of viruses that establish lifelong persistent infections in the face of strong immune responses. EBV is an important human pathogen whose immune evasion mechanisms are only partly understood. Of the EBV immune evasion mechanisms identified to date, none could explain why CD8(+) T cell responses to late lytic cycle genes are so infrequent and, when present, recognize lytically infected target cells so poorly relative to CD8(+) T cells specific for early lytic cycle antigens. The present work identifies an additional immune evasion protein, BDLF3, that is expressed late in the lytic cycle and impairs CD8(+) T cell recognition by targeting cell surface MHC class I molecules for ubiquitination and proteasome-dependent downregulation. Interestingly, BDLF3 also targets MHC class II molecules to impair CD4(+) T cell recognition. BDLF3 is therefore a rare example of a viral protein that impairs both the MHC class I and class II antigen-presenting pathways.

Viral immune evasion: Lessons in MHC class I antigen presentation

The MHC class I antigen presentation pathway enables cells infected with intracellular pathogens to signal the presence of the invader to the immune system. Cytotoxic T lymphocytes are able to eliminate the infected cells through recognition of pathogen-derived peptides presented by MHC class I molecules at the cell surface. In the course of evolution, many viruses have acquired inhibitors that target essential stages of the MHC class I antigen presentation pathway. Studies on these immune evasion proteins reveal fascinating strategies used by viruses to elude the immune system. Viral immunoevasins also constitute great research tools that facilitate functional studies on the MHC class I antigen presentation pathway, allowing the investigation of less well understood routes, such as TAP-independent antigen presentation and cross-presentation of exogenous proteins. Viral immunoevasins have also helped to unravel more general cellular processes. For instance, basic principles of ER-associated protein degradation via the ubiquitin-proteasome pathway have been resolved using virus-induced degradation of MHC class I as a model. This review highlights how viral immunoevasins have increased our understanding of MHC class I-restricted antigen presentation.

The ins and outs of MHC class II-mediated antigen processing and presentation

Antigenic peptide-loaded MHC class II molecules (peptide-MHC class II) are constitutively expressed on the surface of professional antigen-presenting cells (APCs), including dendritic cells, B cells, macrophages and thymic epithelial cells, and are presented to antigen-specific CD4(+) T cells. The mechanisms of antigen uptake, the nature of the antigen processing compartments and the lifetime of cell surface peptide-MHC class II complexes can vary depending on the type of APC. It is likely that these differences are important for the function of each distinct APC subset in the generation of effective adaptive immune responses. In this Review, we describe our current knowledge of the mechanisms of uptake and processing of antigens, the intracellular formation of peptide-MHC class II complexes, the intracellular trafficking of peptide-MHC class II complexes to the APC plasma membrane and their ultimate degradation.

The Membrane Associated RING-CH Proteins: A Family of E3 Ligases with Diverse Roles through the Cell

Since the discovery that conjugation of ubiquitin to proteins can drive proteolytic degradation, ubiquitination has been shown to perform a diverse range of functions in the cell. It plays an important role in endocytosis, signal transduction, trafficking of vesicles inside the cell, and even DNA repair. The process of ubiquitination-mediated control has turned out to be remarkably complex, involving a diverse array of proteins and many levels of control. This review focuses on a family of structurally related E3 ligases termed the membrane-associated RING-CH (MARCH) ubiquitin ligases, which were originally discovered as structural homologs to the virals E3s, K3, and K5 from Kaposi's sarcoma-associated herpesvirus (KSHV). These proteins contain a catalytic RING-CH finger and are typically membrane-bound, with some having up to 14 putative transmembrane domains. Despite several lines of evidence showing that the MARCH proteins play a complex and essential role in several cellular processes, this family remains understudied.

Dendritic cells utilize the evolutionarily conserved WASH and retromer complexes to promote MHCII recycling and helper T cell priming

Immature dendritic cells (DCs) maintain a highly dynamic pool of recycling MHCII that promotes sampling of environmental antigens for presentation to T helper cells. However, the molecular basis of MHCII recycling and the cellular machinery that orchestrates MHCII trafficking are incompletely understood. Using a mouse model we show that WASH, an actin regulatory protein that facilitates retromer function, is essential for MHCII recycling and efficient priming of T helper cells. We further demonstrate that WASH deficiency results in impaired MHCII surface levels, recycling, and an accumulation of polyubiquitinated MHCII complexes, which are subsequently slated for premature lysosomal degradation. Consequently, conditional deletion of the Wash gene in DCs impairs priming of both conventional and autoimmune T helper cells in vivo and attenuates disease progression in a model of experimental autoimmune encephalitis (EAE). Thus, we identify a novel mechanism in which DCs employ the evolutionarily conserved WASH and retromer complex for MHCII recycling in order to regulate T helper cell priming.

Cell adhesion in zebrafish embryos is modulated by March 8

March8 is a member of a family of transmembrane E3 ubiquitin ligases that have been studied mostly for their role in the immune system. We find that March8 is expressed in the zebrafish egg and early embryo, suggesting a role in development. Both knock-down and overexpression of March8 leads to abnormal development. The phenotype of zebrafish embryos and Xenopus animal explants overexpressing March8 implicates impairment of cell adhesion as a cause of the effect. In zebrafish embryos and in cultured cells, overexpression of March8 leads to a reduction in the surface levels of E-cadherin, a major cell-cell adhesion molecule. Experiments in cell culture further show that E-cadherin can be ubiquitinated by March8. On the basis of these observations we suggest that March8 functions in the embryo to modulate the strength of cell adhesion by regulating the localization of E-cadherin.

MHC class II antigen presentation by dendritic cells regulated through endosomal sorting

For the initiation of adaptive immune responses, dendritic cells present antigenic peptides in association with major histocompatibility complex class II (MHCII) to naïve CD4(+) T lymphocytes. In this review, we discuss how antigen presentation is regulated through intracellular processing and trafficking of MHCII. Newly synthesized MHCII is chaperoned by the invariant chain to endosomes, where peptides from endocytosed pathogens can bind. In nonactivated dendritic cells, peptide-loaded MHCII is ubiquitinated and consequently sorted by the ESCRT machinery to intraluminal vesicles of multivesicular bodies, ultimately leading to lysosomal degradation. Ubiquitination of newly synthesized MHCII is blocked when dendritic cells are activated, now allowing its transfer to the cell surface. This mode of regulation for MHCII is a prime example of how molecular processing and sorting at multivesicular bodies can determine the expression of signaling receptors at the plasma membrane.