HM1.24 is internalized from lipid rafts by clathrin-mediated endocytosis through interaction with alpha-adaptin

Antagonism of BST2/Tetherin, a new restriction factor of respiratory syncytial virus, requires the viral NS1 protein

Human respiratory syncytial virus (RSV) is an enveloped RNA virus and the leading viral agent responsible for severe pediatric respiratory infections worldwide. Identification of cellular factors able to restrict viral infection is one of the key strategies used to design new drugs against infection. Here, we report for the first time that the cellular protein BST2/Tetherin (a widely known host antiviral molecule) behaves as a restriction factor of RSV infection. We showed that BST2 silencing resulted in a significant rise in viral production during multi-cycle infection, suggesting an inhibitory role during the late steps of RSV's multiplication cycle. Conversely, BST2 overexpression resulted in the decrease of the viral production. Furthermore, BST2 was found associated with envelope proteins and co-localized with viral filaments, suggesting that BST2 tethers RSV particles. Interestingly, RSV naturally downregulates cell surface and global BST2 expression, possibly through a mechanism dependent on ubiquitin. RSV's ability to enhance BST2 degradation was also validated in a model of differentiated cells infected by RSV. Additionally, we found that a virus deleted of NS1 is unable to downregulate BST2 and is significantly more susceptible to BST2 restriction compared to the wild type virus. Moreover, NS1 and BST2 interact in a co- immunoprecipitation experiment. Overall, our data support a model in which BST2 is a restriction factor against RSV infection and that the virus counteracts this effect by limiting the cellular factor's expression through a mechanism involving the viral protein NS1.

Interactions between HIV proteins and host restriction factors: implications for potential therapeutic intervention in HIV infection

Different host proteins target different HIV proteins and antagonize their functions, depending on the stage of the HIV life cycle and the stage of infection. Concurrently, HIV proteins also target and antagonize various different host proteins to facilitate HIV replication within host cells. The preceding quite specific area of knowledge in HIV pathogenesis, however, remains insufficiently understood. We therefore propose, in this review article, to examine and discuss the HIV proteins that counteract those host restriction proteins which results directly in increased infectivity of HIV. We elaborate on HIV proteins that antagonize host cellular proteins to promote HIV replication, and thus HIV infection. We examine the functions and mechanisms via which Nef, Vif, Vpu, Env, Vpr, and Vpx counteract host proteins such as Ser5, PSGL-1, IFITMS, A3G, tetherin, GBP5, SAMHD1, STING, HUSH, REAF, and TET2 to increase HIV infectivity. Nef antagonizes three host proteins, viz., Ser5, PSGL1, and IFITIMs, while Vpx also antagonizes three host restriction factors, viz., SAMHD1, STING, and HUSH complex; therefore, these proteins may be potential candidates for therapeutic intervention in HIV infection. Tetherin is targeted by Vpu and Env, PSGL1 is targeted by Nef and Vpu, while Ser5 is targeted by Nef and Env proteins. Finally, conclusive remarks and future perspectives are also presented.

Bone marrow stromal cell antigen 2: Tumor biology, signaling pathway and therapeutic targeting (Review)

Bone marrow stromal cell antigen 2 (BST2) is a type II transmembrane protein that serves critical roles in antiretroviral defense in the innate immune response. In addition, it has been suggested that BST2 is highly expressed in various types of human cancer and high BST2 expression is related to different clinicopathological parameters in cancer. The molecular mechanism underlying BST2 as a potential tumor biomarker in human solid tumors has been reported on; however, to the best of our knowledge, there has been no review published on the molecular mechanism of BST2 in human solid tumors. The present review focuses on human BST2 expression, structure and functions; the molecular mechanisms of BST2 in breast cancer, hepatocellular carcinoma, gastrointestinal tumor and other solid tumors; the therapeutic potential of BST2; and the possibility of BST2 as a potential marker. BST2 is involved in cell membrane integrity and lipid raft formation, which can activate epidermal growth factor receptor signaling pathways, providing a potential mechanistic link between BST2 and tumorigenesis. Notably, BST2 may be considered a universal tumor biomarker and a potential therapeutical target.

Multi-functional BST2/tetherin against HIV-1, other viruses and LINE-1

Bone marrow stromal cell antigen 2 (BST2), also known as CD317, HM1.24, or tetherin, is a type II transmembrane glycoprotein. Its expression is induced by IFN-I, and it initiates host immune responses by directly trapping enveloped HIV-1 particles onto the cell surface. This antagonistic mechanism toward the virus is attributable to the unique structure of BST2. In addition to its antiviral activity, BST2 restricts retrotransposon LINE-1 through a distinct mechanism. As counteractive measures, different viruses use a variety of proteins to neutralize the function or even stability of BST2. Interestingly, BST2 seems to have both a positive and a negative influence on immunomodulation and virus propagation. Here, we review the relationship between the structural and functional bases of BST2 in anti-HIV-1 and suppressing retrotransposon LINE-1 activation and focus on its dual features in immunomodulation and regulating virus propagation.

Murine BST2/tetherin promotes measles virus infection of neurons

BST2/tetherin is a transmembrane protein with antiviral activity; it is synthesized following exposure to interferons, and restricts the release of budding virus particles by tethering them to the host cell membrane. We previously showed that BST2 is induced in primary neurons following measles virus (MV) infection or type I interferon; however, BST2 was dispensable for protection against challenge with neuron-restricted MV. Here, we define the contribution of BST-2 in neuronal MV infection. Surprisingly, and in contrast to its antiviral role in non-neuronal cells, murine BST2 promotes MV infection in brains of permissive mice and in primary neuron cultures. Moreover, BST2 expression was predominantly observed in the non-synaptic fraction of purified neurons. These studies highlight a cell-type dependent role of a well-characterized antiviral protein in enhancing neuronal infection.

Role of Viral Protein U (Vpu) in HIV-1 Infection and Pathogenesis

Human immunodeficiency virus (HIV)-1 and HIV-2 originated from cross-species transmission of simian immunodeficiency viruses (SIVs). Most of these transfers resulted in limited spread of these viruses to humans. However, one transmission event involving SIVcpz from chimpanzees gave rise to group M HIV-1, with M being the principal strain of HIV-1 responsible for the AIDS pandemic. Vpu is an HIV-1 accessory protein generated from Env/Vpu encoded bicistronic mRNA and localized in cytosolic and membrane regions of cells capable of being infected by HIV-1 and that regulate HIV-1 infection and transmission by downregulating BST-2, CD4 proteins levels, and immune evasion. This review will focus of critical aspects of Vpu including its zoonosis, the adaptive hurdles to cross-species transmission, and future perspectives and broad implications of Vpu in HIV-1 infection and dissemination.

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.

LAPTM4α is targeted from the Golgi to late endosomes/lysosomes in a manner dependent on the E3 ubiquitin ligase Nedd4-1 and ESCRT proteins

Lysosome-associated protein transmembrane 4α (LAPTM4α) is a four transmembrane-spanning protein primarily localized in endosomes and lysosomes and has several putative lysosomal targeting signals at its C-terminal cytoplasmic domain, including tyrosine-based motifs (YxxΦ) and PY motifs (L/PxxY). LAPTM4α has been previously shown to be ubiquitinated by the E3 ubiquitin ligase Nedd4-1 through binding to its PY motifs and sorted to lysosomes, however, the molecular mechanisms underlying the localization of LAPTM4α to endosomes/lysosomes have not yet been fully elucidated. In the present study, we show that LAPTM4α binds Nedd4-1 in a manner dependent on PY motifs, while the PY motifs and Nedd4-1 are not necessarily required for LAPTM4α ubiquitination. The binding of LAPTM4α with Nedd4-1, however, is necessary for an effective sorting of LAPTM4α from the Golgi to late endosomes/lysosomes. An unexpected finding is that LAPTM4α is localized in the lumen, but not in the limiting membrane, of late endosomes, and degraded in lysosomes over time. Interestingly, we further found that siRNA knockdown of endosomal sorting complexes required for transport (ESCRT) components that mediate sorting of ubiquitinated membrane proteins into intralumenal vesicles (ILVs) of endosomes selectively blocks the transport of LAPTM4α to endosomes. Collectively, these results suggest that trafficking of LAPTM4α from the Golgi to endosomes is promoted by the interaction with Nedd4-1, which further requires ESCRT components. Furthermore, our findings highlight a novel function for ESCRT proteins in mediating protein and/or vesicle trafficking from the Golgi to endosomes/lysosomes.

CD317 mediates immunocytolysis resistance by RICH2/cytoskeleton-dependent membrane protection

Immune evasion is a common hallmark of cancers. Immunotherapies that aim at restoring or increasing the immune response against cancers have revolutionized outcomes for patients, but the mechanisms of resistance remain poorly defined. Here, we report that CD317, a surface molecule with a unique topology that is double anchored into the membrane, protects tumor cells from immunocytolysis. CD317 knockdown in tumor cells renders more severe death in response to NK or chimeric antigen receptor-modified NK cells challenge. Such effects of CD317 silencing might be the results of increasing sensitivity of tumor cells to immune killing rather than strengthening immune response, since neither effector-target cell contact nor the activation of effector cells was affected, and the enhanced cytolysis was also not counteracted by the addition of recombinant CD317 proteins. Mechanistically, CD317 might endow tumor cells with more flexibility to modulate cytoskeleton through its association with RICH2, thereby protects membrane integrity against perforin and consequently promotes survival in response to immunocytolysis. These results reveal a new mechanism of immunocytolysis resistance and suggest CD317 as an attractive target which can be exploited for improving the efficacy of cancer immunotherapies.

HM1.24/BST-2 is constitutively poly-ubiquitinated at the N-terminal amino acid in the cytoplasmic domain

HM1.24 (also known as BST-2, CD317, and Tetherin) is a type II single-pass transmembrane glycoprotein, which traverses membranes using an N-terminal transmembrane helix and is anchored in membrane lipid rafts via a C-terminal glycosylphosphatidylinositol (GPI). HM1.24 plays a role in diverse cellular functions, including cell signaling, immune modulation, and malignancy. In addition, it also functions as an interferon-induced cellular antiviral restriction factor that inhibits the replication and release of diverse enveloped viruses, and which is counteracted by Vpu, an HIV-1 accessory protein. Vpu induces down-regulation and ubiquitin conjugation to the cytoplasmic domain of HM1.24. However, evidence for ubiquitination site(s) of HM1.24 remains controversial. We demonstrated that HM1.24 is constitutively poly-ubiquitinated at the N-terminal cytoplasmic domain, and that the mutation of all potential ubiquitination sites, including serine, threonine, cysteine, and lysine in the cytoplasmic domain of HM1.24, does not affect the ubiquitination of HM1.24. We further demonstrated that although a GPI anchor is necessary and sufficient for HM1.24 antiviral activities and virion-trapping, the deleted mutant of GPI does not influence the ubiquitination of HM1.24. These results suggest that the lipid raft localization of HM1.24 is not a prerequisite for the ubiquitination. Collectively, our findings demonstrate that the ubiquitination of HM1.24 occurs at the N-terminal amino acid in the cytoplasmic domain and indicate that the constitutive ubiquitination machinery of HM1.24 may differ from the Vpu-induced machinery.

Antibody development and property analysis of RhBST2 for accurate cell biological and viral research

BST2 is a single-pass type II transmembrane (TM) protein, which has a cytoplasmic domain, a transmembrane domain, and an extracellular domain, each domain is important for biologic function of BST2. BST2 is a host restriction factor that can effectively inhibit retrovirus release. Rhesus monkeys are considered as relevant natural animal models for studying AIDS in humans. In order to recognize rhesus BST2 (RhBST2) protein and detect its function accurately, we prepared a polyclonal antibody (pAb) especially for RhBST2. Meanwhile, we constructed RhBST2 proteins with the addition of an HA-tag at the N-terminus (RhBST2-NHA) or inside of the ectodomain (RhBST2-IHA) to compare the recognition ability of rabbit anti-RhBST2 pAb and anti-HA mAb. The results showed that the anti-HA mAb and rabbit anti-RhBST2 pAb had the same ability to identify RhBST2. RhBST2 demonstrated antiviral activity and the ability to activate NF-κB. Moreover, the N-glycosylation states, cell surface level and intracellular localization of RhBST2 were detected. However, HA tags relatively changed part of the biological function of RhBST2. These results show that the RhBST2 polyclonal antibody is more suitable for analyzing the properties and functions of RhBST2, and the natural domain of RhBST2 is very important for its function.

Lymphocyte-specific protein 1 (LSP1) regulates bone marrow stromal cell antigen 2 (BST-2)-mediated intracellular trafficking of HIV-1 in dendritic cells

Human immunodeficiency virus type 1 (HIV-1) subverts intracellular trafficking pathways to avoid its degradation and elimination, thereby enhancing its survival and spread. The molecular mechanisms involved in intracellular transport of HIV-1 are not yet fully defined. We demonstrate that the actin-binding protein lymphocyte-specific protein 1 (LSP1) interacts with the interferon-inducible protein bone marrow stromal antigen 2 (BST-2) in dendritic cells (DCs) to facilitate both endocytosis of surface-bound HIV-1 and the formation of early endosomes. Analysis of the molecular interaction between LSP1 and BST-2 reveals that the N terminus of LSP1 interacts with BST-2. Overall, we identify a novel mechanism of intracellular trafficking of HIV-1 in DCs centering on the LSP1/BST-2 complex. We also show that the HIV-1 accessory protein Vpu subverts this pathway by inducing proteasomal degradation of LSP1, augmenting cell-cell transmission of HIV-1.

Rapamycin Re-Directs Lysosome Network, Stimulates ER-Remodeling, Involving Membrane CD317 and Affecting Exocytosis, in Campylobacter Jejuni-Lysate-Infected U937 Cells

The Gram-negative Campylobacter jejuni is a major cause of foodborne gastroenteritis in humans worldwide. The cytotoxic effects of Campylobacter have been mainly ascribed to the actions of the cytolethal distending toxin (CDT): it is mandatory to put in evidence risk factors for sequela development, such as reactive arthritis (ReA) and Guillain–Barré syndrome (GBS). Several researches are directed to managing symptom severity and the possible onset of sequelae. We found for the first time that rapamycin (RM) is able to largely inhibit the action of C. jejuni lysate CDT in U937 cells, and to partially avoid the activation of specific sub-lethal effects. In fact, we observed that the ability of this drug to redirect lysosomal compartment, stimulate ER-remodeling (highlighted by ER–lysosome and ER–mitochondria contacts), protect mitochondria network, and downregulate CD317/tetherin, is an important component of membrane microdomains. In particular, lysosomes are involved in the process of the reduction of intoxication, until the final step of lysosome exocytosis. Our results indicate that rapamycin confers protection against C. jejuni bacterial lysate insults to myeloid cells.

Characterization of Zika Virus Endocytic Pathways in Human Glioblastoma Cells

Zika virus (ZIKV) infections can cause microcephaly and neurological disorders. However, the early infection events of ZIKV in neural cells remain to be characterized. Here, by using a combination of pharmacological and molecular approaches and the human glioblastoma cell T98G as a model, we first observed that ZIKV infection was inhibited by chloroquine and NH₄Cl, indicating a requirement of low intracellular pH. We further showed that dynamin is required as the ZIKV entry was affected by the specific inhibitor dynasore, small interfering RNA (siRNA) knockdown of dynamin, or by expressing the dominant-negative K44A mutant. Moreover, the ZIKV entry was significantly inhibited by chlorpromazine, pitstop2, or siRNA knockdown of clathrin heavy chain, indicating an involvement of clathrin-mediated endocytosis. In addition, genistein treatment, siRNA knockdown of caveolin-1, or overexpression of a dominant-negative caveolin mutant impacted the ZIKV entry, with ZIKV particles being observed to colocalize with caveolin-1, implying that caveola endocytosis can also be involved. Furthermore, we found that the endocytosis of ZIKV is dependent on membrane cholesterol, microtubules, and actin cytoskeleton. Importantly, ZIKV infection was inhibited by silencing of Rab5 and Rab7, while confocal microscopy showed that ZIKV particles localized in Rab5- and Rab7-postive endosomes. These results indicated that, after internalization, ZIKV likely moves to Rab5-positive early endosome and Rab7-positive late endosomes before delivering its RNA into the cytoplasm. Taken together, our study, for the first time, described the early infection events of ZIKV in human glioblastoma cell T98G.

The N-glycosylation of Equine Tetherin Affects Antiviral Activity by Regulating Its Subcellular Localization

Tetherin is an interferon-inducible type II transmembrane glycoprotein which inhibits the release of viruses, including retroviruses, through a “physical tethering” model. However, the role that the glycosylation of tetherin plays in its antiviral activity remains controversial. In this study, we found that mutation of N-glycosylation sites resulted in an attenuation of the antiviral activity of equine tetherin (eqTHN), as well as a reduction in the expression of eqTHN at the plasma membrane (PM). In addition, eqTHN N-glycosylation mutants colocalize obviously with ER, CD63, LAMP1 and endosomes, while WT eqTHN do not. Furthermore, we also found that N-glycosylation impacts the transport of eqTHN in the cell not by affecting the endocytosis, but rather by influencing the anterograde trafficking of the protein. These results suggest that the N-glycosylation of eqTHN is important for the antiviral activity of the protein through regulating its normal subcellular localization. This finding will enhance our understanding of the function of this important restriction factor.

Beyond Tethering the Viral Particles: Immunomodulatory Functions of Tetherin (BST-2)

Host response to viral infection is a highly regulated process involving engagement of various host factors, cytokines, chemokines, and stimulatory signals that pave the way for an antiviral immune response. The response is manifested in terms of viral sequestration, phagocytosis, and inhibition of genome replication, and, finally, if required, lymphocyte-mediated clearance of virally infected cells. During this process, cross-talk between viral and host factors can shape disease outcomes and immunopathology. Bone marrow stromal antigen 2 (BST-2), also know as tetherin, is induced by type I interferon produced in response to viral infections, as well as in certain cancers. BST-2 has been shown to be a host restriction factor of virus multiplication through its ability to physically tether budding virions and restrict viral spread. However, BST-2 has other roles in the host antiviral response. This review focuses on the diverse functions of BST-2 and its downstream signaling pathways in regulating host immune responses.

Mechanism of Tetherin Inhibition of Alphavirus Release

Tetherin is an interferon-inducible, antiviral host factor that broadly restricts enveloped virus release by tethering budded viral particles to the plasma membrane. In response, many viruses have evolved tetherin antagonists. The human tetherin gene can express two isoforms, long and short, due to alternative translation initiation sites in the N-terminal cytoplasmic tail. The long isoform (L-tetherin) contains 12 extra amino acids in its N terminus, including a dual tyrosine motif (YDYCRV) that is an internalization signal for clathrin-mediated endocytosis and a determinant of NF-κB activation. Tetherin restricts alphaviruses, which are highly organized enveloped RNA viruses that bud from the plasma membrane. L-tetherin is more efficient than S-tetherin in inhibiting alphavirus release in 293 cells. Here, we demonstrated that alphaviruses do not encode an antagonist for either of the tetherin isoforms. Instead, the isoform specificity reflected a requirement for tetherin endocytosis. The YXY motif in L-tetherin was necessary for alphavirus restriction in 293 cells but was not required for rhabdovirus restriction. L-tetherin's inhibition of alphavirus release correlated with its internalization but did not involve NF-κB activation. In contrast, in U-2 OS cells, the YXY motif and the L-tetherin N-terminal domain were not required for either robust tetherin internalization or alphavirus inhibition. Tetherin forms that were negative for restriction accumulated at the surface of infected cells, while the levels of tetherin forms that restrict were decreased. Together, our results suggest that tetherin-mediated virus internalization plays an important role in the restriction of alphavirus release and that cell-type-specific cofactors may promote tetherin endocytosis.IMPORTANCE The mechanisms of tetherin's antiviral activities and viral tetherin antagonism have been studied in detail for a number of different viruses. Although viral countermeasures against tetherin can differ significantly, overall, tetherin's antiviral activity correlates with physical tethering of virus particles to prevent their release. While tetherin can mediate virus endocytic uptake and clearance, this has not been observed to be required for restriction. Here we show that efficient tetherin inhibition of alphavirus release requires efficient tetherin endocytosis. Our data suggest that this endocytic uptake can be mediated by tetherin itself or by a tetherin cofactor that promotes uptake of an endocytosis-deficient variant of tetherin.

CRISPR-mediated activation of endogenous BST-2/tetherin expression inhibits wild-type HIV-1 production

The CRISPR technology not only can knock out target genes by using the RNA-guided Cas9 nuclease but also can activate their expression when a nuclease-deficient Cas9 (dCas9) is employed. Using the latter function, we here show the effect of the CRISPR-mediated pinpoint activation of endogenous expression of BST-2 (also known as tetherin), a virus restriction factor with a broad antiviral spectrum. Single-guide RNA (sgRNA) sequences targeting the BST-2 promoter were selected by promoter assays. Potential sgRNAs and dCas9 fused to the VP64 transactivation domain, along with an accessory transcriptional activator complex, were introduced into cells by lentiviral transduction. Increased expression of BST-2 mRNA in transduced cells was confirmed by real-time RT-PCR. Cells in which BST-2 expression was highly enhanced showed the effective inhibition of HIV-1 production and replication even in the presence of the viral antagonist Vpu against BST-2. These findings confirm that the physiological stoichiometry between host restriction factors and viral antagonists may determine the outcome of the battle with viruses.

Limiting Respiratory Viral Infection by Targeting Antiviral and Immunological Functions of BST-2/Tetherin: Knowledge and Gaps

Recent findings regarding the cellular biology and immunology of BST-2 (also known as tetherin) indicate that its function could be exploited as a universal replication inhibitor of enveloped respiratory viruses (e.g., influenza, respiratory syncytial virus, etc.). BST-2 inhibits viral replication by preventing virus budding from the plasma membrane and by inducing an antiviral state in cells adjacent to infection via unique inflammatory signaling mechanisms. This review presents the first comprehensive summary of what is currently known about BST-2 anti-viral function against respiratory viruses, how these viruses construct countermeasures to antagonize BST-2, and how BST-2 function might be targeted to develop therapies to treat respiratory virus infections. The authors address the current gaps in knowledge, including the need for mechanistic understanding of BST-2 antagonism by respiratory viruses, that should be bridged to achieve that goal.

Vaccination with a DNA vaccine encoding CD317-targeting HBs antigen elicits enhanced immunity in mice

BACKGROUND

Conventional hepatitis B virus (HBV) vaccines fail to induce protective antibody titers in 5-10% of immune-competent vaccines. Therefore, safe and effective HBV vaccines are still clinically needed.

METHODS

In this study, we developed a plasmid DNA vaccine encoding CD317 single-chain fragment variable (α317scFv) linked with the hepatitis B surface antigen (HBsAg) and detected the humoral and cellular immune responses elicited by this vaccine in BALB/c mice.

RESULTS

Vaccination with this fusion DNA vaccine in BALB/c mice induced more robust antiviral T cell and antibody immunity against HBsAg. Compared with mice vaccinated with control vaccine encoding HBsAg, the level of serum-circulating anti-HBsAg antibody (HBsAb) was nearly double in fusion DNA-vaccinated mice. More interesting, splenic lymphocytes isolated from fusion DNA-vaccinated mice showed more potent proliferation and IFN-γ production after being re-stimulated with recombinant HBsAg in vitro. And not only that, the cytotoxicity of fusion DNA vaccine-sensitized splenocytes was ∼3-fold higher than that of controls.

CONCLUSION

Taken together, our results reveal that the fusion DNA vaccine can induce more effective immunological protection against HBV, and is a promising candidate for preventing HBV infection.

Strain-Specific Antagonism of the Human H1N1 Influenza A Virus against Equine Tetherin

Tetherin/BST-2/CD317 is an interferon-induced host restriction factor that can block the budding of enveloped viruses by tethering them to the cell surface. Many viruses use certain proteins to counteract restriction by tetherin from their natural hosts, but not from other species. The influenza A virus (FLUAV) has a wide range of subtypes with different host tropisms. Human tetherin (huTHN) has been reported to restrict only specific FLUAV strains and the viral hemagglutinin (HA) and neuraminidase (NA) genes determine the sensitivity to huTHN. Whether tetherins from other hosts can block human FLUAV is still unknown. Here, we evaluate the impact of equine tetherin (eqTHN) and huTHN on the replication of A/Sichuan/1/2009 (H1N1) and A/equine/Xinjiang/1/2007 (H3N8) strains. Our results show that eqTHN had higher restriction activity towards both viruses, and its shorter cytoplasmic tail contributed to that activity. We further demonstrated that HA and NA of A/Hamburg/4/2009 (H1N1) could counteract eqTHN. Notably, our results indicate that four amino acids, 13T and 49L of HA and 32T and 80V of NA, were involved in blocking the restriction activity of eqTHN. These findings reveal interspecies restriction by eqTHN towards FLUAV, and the role of the HA and NA proteins in overcoming this restriction.

High-Mannose But Not Complex-Type Glycosylation of Tetherin Is Required for Restriction of HIV-1 Release

Tetherin is an interferon-inducible antiviral protein that inhibits the release of a broad spectrum of enveloped viruses by retaining virions at the surface of infected cells. While the role of specific tetherin domains in antiviral activity is clearly established, the role of glycosylation in tetherin function is not clear. In this study, we carried out a detailed investigation of this question by using tetherin variants in which one or both sites of N-linked glycosylation were mutated (N65A, N92A, and N65,92A), and chemical inhibitors that prevent glycosylation at specific stages of oligosaccharide were added or modified. The single N-linked glycosylation mutants, N65A and N92A, efficiently inhibited the release of Vpu-defective human immunodeficiency virus type 1 (HIV-1). In contrast, the non-glycosylated double mutant, N65,92A, lost its ability to block HIV-1 release. The inability of the N65,92A mutant to inhibit HIV-1 release is associated with a lack of cell-surface expression. A role for glycosylation in cell-surface tetherin expression is supported by tunicamycin treatment, which inhibits the first step of N-linked glycosylation and impairs both cell-surface expression and antiviral activity. Inhibition of complex-type glycosylation with kifunensine, an inhibitor of the oligosaccharide processing enzyme mannosidase 1, had no effect on either the cell-surface expression or antiviral activity of tetherin. These results demonstrate that high-mannose modification of a single asparagine residue is necessary and sufficient, while complex-type glycosylation is dispensable, for cell-surface tetherin expression and antiviral activity.

Viperin Targets Flavivirus Virulence by Inducing Assembly of Noninfectious Capsid Particles

Efficient antiviral immunity requires interference with virus replication at multiple layers targeting diverse steps in the viral life cycle. We describe here a novel flavivirus inhibition mechanism that results in interferon-mediated obstruction of tick-borne encephalitis virus particle assembly and involves release of malfunctioning membrane-associated capsid (C) particles. This mechanism is controlled by the activity of the interferon-induced protein viperin, a broad-spectrum antiviral interferon-stimulated gene. Through analysis of the viperin-interactome, we identified the Golgi brefeldin A-resistant guanine nucleotide exchange factor 1 (GBF1) as the cellular protein targeted by viperin. Viperin-induced antiviral activity, as well as C-particle release, was stimulated by GBF1 inhibition and knockdown and reduced by elevated levels of GBF1. Our results suggest that viperin targets flavivirus virulence by inducing the secretion of unproductive noninfectious virus particles via a GBF1-dependent mechanism. This as-yet-undescribed antiviral mechanism allows potential therapeutic intervention.

IMPORTANCE The interferon response can target viral infection on almost every level; however, very little is known about the interference of flavivirus assembly. We show here that interferon, through the action of viperin, can disturb the assembly of tick-borne encephalitis virus. The viperin protein is highly induced after viral infection and exhibit broad-spectrum antiviral activity. However, the mechanism of action is still elusive and appears to vary between the different viruses, indicating that cellular targets utilized by several viruses might be involved. In this study, we show that viperin induces capsid particle release by interacting and inhibiting the function of the cellular protein Golgi brefeldin A-resistant guanine nucleotide exchange factor 1 (GBF1). GBF1 is a key protein in the cellular secretory pathway and is essential in the life cycle of many viruses, also targeted by viperin, implicating GBF1 as a novel putative drug target.

Structural determinant of BST-2-mediated regulation of breast cancer cell motility: a role for cytoplasmic tail tyrosine residues

There is now irrefutable evidence that overexpression of the innate immunity protein―BST-2, in breast cancer cells is implicated in tumor growth and progression. The cellular mechanisms that control BST-2-mediated effect in tumor progression involve enhancement of cancer cell motility―migration/invasion. However, the distinct structural elements of BST-2 that mediate breast cancer cell motility remain unknown. Here, we used various motility assays and different variants of BST-2 to examine the cellular and structural mechanisms controlling BST-2-mediated cell motility. We show that BST-2 silencing in various cancer cell lines inhibits cell motility. Restoration of BST-2 expression using construct expressing wild type BST-2 rescues cell motility. Mutational analysis identifies the cytoplasmic tail of BST-2 as a novel regulator of cancer cell motility, because cell motility was significantly abrogated by substitution of the BST-2 cytoplasmic tail tyrosine residues to alanine residues. Furthermore, in a spheroid invasion model, BST-2-expressing tumor spheroids are highly invasive inside 3D Matrigel matrices. In this model, the spreading distance of BST-2-expressing spheroids was significantly higher than that of BST-2-suppressed spheroids. Collectively, our data reveal that i) BST-2-expressing breast cancer cells in spheroids are more motile than their BST-2-supressed counterparts; ii) BST-2 cytoplasmic tail regulates non-proteolytic (migration) and proteolytic (invasion) mechanisms of breast cancer cell motility; and iii) replacement of the tyrosine residues at positions 6 and 8 in the cytoplasmic tail of BST-2 with alanine residues inhibits cell motility.

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.

Efficient Vpu-Mediated Tetherin Antagonism by an HIV-1 Group O Strain

Simian immunodeficiency viruses (SIVs) use their Nef proteins to counteract the restriction factor tetherin. However, a deletion in human tetherin prevents antagonism by the Nef proteins of SIVcpz and SIVgor, which represent the ape precursors of human immunodeficiency virus type 1 (HIV-1). To promote virus release from infected cells, pandemic HIV-1 group M strains evolved Vpu as a tetherin antagonist, while the Nef protein of less widespread HIV-1 group O strains acquired the ability to target a region adjacent to this deletion. In this study, we identified an unusual HIV-1 group O strain (RBF206) that evolved Vpu as an effective antagonist of human tetherin. While both RBF206 Vpu and Nef exert anti-tetherin activity in transient-transfection assays, mainly Vpu promotes RBF206 release in infected CD4+ T cells. Although mutations distinct from the adaptive changes observed in group M Vpus (M-Vpus) were critical for the acquisition of its anti-tetherin activity, RBF206 O-Vpu potently suppresses NF-κB activation and reduces CD4 cell surface expression. Interestingly, RBF206 Vpu counteracts tetherin in a largely species-independent manner, degrading both the long and short isoforms of human tetherin. Downmodulation of CD4, but not counteraction of tetherin, by RBF206 Vpu was dependent on the cellular ubiquitin ligase machinery. Our data present the first example of an HIV-1 group O Vpu that efficiently antagonizes human tetherin and suggest that counteraction by O-Nefs may be suboptimal.IMPORTANCE Previous studies showed that HIV-1 groups M and O evolved two alternative strategies to counteract the human ortholog of the restriction factor tetherin. While HIV-1 group M switched from Nef to Vpu due to a deletion in the cytoplasmic domain of human tetherin, HIV-1 group O, which lacks Vpu-mediated anti-tetherin activity, acquired a Nef protein that is able to target a region adjacent to the deletion. Here we report an unusual exception, identifying a strain of HIV-1 group O (RBF206) whose Vpu protein evolved an effective antagonism of human tetherin. Interestingly, the adaptive changes in RBF206 Vpu are distinct from those found in M-Vpus and mediate efficient counteraction of both the long and short isoforms of this restriction factor. Our results further illustrate the enormous flexibility of HIV-1 in counteracting human defense mechanisms.

Tetherin/BST-2: Restriction Factor or Immunomodulator?

BACKGROUND

Cell-mediated immune (CMI) responses are critical for the control of HIV-1 infection and their importance was highlighted by the existence of viral proteins, particularly Vpu and Nef, that antagonize these responses. Pandemic HIV-1 Vpu counteracts Tetherin/BST-2, a host factor that could prevent the release of HIV-1 virions by tethering virions on the cell surface, but a link between Tetherin and HIV-1 CMI responses has not yet been demonstrated in vivo. In vitro, the virological and immunological impact of Tetherin-mediated accumulation of virions ranged from enhanced or diminished cell-to-cell spread to enhanced recognition by virus-specific antibodies for natural killer cellmediated lysis. However, Tetherin-restricted virions could be internalized through an endocytosis motif in the Tetherin cytoplasmic tail.

METHODS

Given the uncertainties on which in vitro results manifest in vivo and the dearth of knowledge on how Tetherin influences retroviral immunity, in vivo retrovirus infections in mice encoding wild-type, null and endocytosis-defective Tetherin were performed. Here, we review and highlight the results from these in vivo studies.

RESULTS

Current data suggests that endocytosis-defective Tetherin functions as a potent innate restriction factor. By contrast, endocytosis-competent Tetherin, the form found in most mammals including humans and the form counteracted by HIV-1 Vpu, was linked to stronger CMI responses in mice.

CONCLUSION

We propose that the main role of endocytosis-competent Tetherin is not to directly restrict retroviral replication, but to promote a more effective CMI response against retroviruses.

BST-2: at the crossroads of viral pathogenesis and oncogenesis

BST-2 is a moonlight protein with several protective and deleterious functions. Regulation of virus restriction and tumor aggressiveness are the most studied aspects of BST-2 function and thus, the main focus of this perspective. Virus inhibition roles of BST-2 have therapeutic potential that, if properly harnessed, could result in near broad spectrum antiviral. However, the involvement of BST-2 in cancer calls for additional studies on BST-2 biology and re-evaluation of the overall role of BST-2 in host protection, as it appears that BST-2 has pleiotropic effects in the host. Here, we analyze the antiviral and protumor roles of BST-2. We also discuss potential therapeutic options for BST-2 against viral infection and cancer.

Filamin A Is Involved in HIV-1 Vpu-mediated Evasion of Host Restriction by Modulating Tetherin Expression

Tetherin, also known as bone marrow stromal antigen 2 (BST-2), inhibits the release of a wide range of enveloped viruses, including human immunodeficiency virus, type 1 (HIV-1) by directly tethering nascent virions to the surface of infected cells. The HIV-1 accessary protein Vpu counteracts tetherin restriction via sequestration, down-regulation, and/or displacement mechanisms to remove tetherin from sites of virus budding. However, the exact mechanism of Vpu-mediated antagonism of tetherin restriction remains to be fully understood. Here we report a novel role for the actin cross-linking regulator filamin A (FLNa) in Vpu anti-tetherin activities. We demonstrate that FLNa associates with tetherin and that FLNa modulates tetherin turnover. FLNa deficiency was found to enhance cell surface and steady-state levels of tetherin expression. In contrast, we observed that overexpression of FLNa reduced tetherin expression levels both on the plasma membrane and in intracellular compartments. Although FLNb shows high amino acid sequence similarity with FLNa, we reveal that only FLNa, but not FLNb, plays an essential role in tetherin turnover. We further showed that FLNa deficiency inhibited Vpu-mediated enhancement of virus release through interfering with the activity of Vpu to down-regulate cellular tetherin. Taken together, our studies suggest that Vpu hijacks the FLNa function in the modulation of tetherin to neutralize the antiviral factor tetherin. These findings may provide novel strategies for the treatment of HIV-1 infection.

The role of BST-2/Tetherin in host protection and disease manifestation

Host cells respond to viral infections by activating immune response genes that are not only involved in inflammation, but may also predispose cells to cancerous transformation. One such gene is BST‐2, a type II transmembrane protein with a unique topology that endows it tethering and signaling potential. Through this ability to tether and signal, BST‐2 regulates host response to viral infection either by inhibiting release of nascent viral particles or in some models inhibiting viral dissemination. However, despite its antiviral functions, BST‐2 is involved in disease manifestation, a function linked to the ability of BST‐2 to promote cell‐to‐cell interaction. Therefore, modulating BST‐2 expression and/or activity has the potential to influence course of disease.

Increased BST2 expression during simian immunodeficiency virus infection is not a determinant of disease progression in rhesus monkeys

Background

Bone marrow stromal cell antigen 2 (BST2), also known as tetherin, HM1.24 or CD317 represents a type 2 integral membrane protein, which has been described to restrict the production of some enveloped viruses by inhibiting the virus release from the cell surface. This innate antiviral mechanism is counteracted by the HIV-1 viral factor Vpu, targeting BST2 for cellular degradation. Since antiviral BST2 activity has been mainly confirmed by in vitro data, we investigated its role in vivo on the disease progression using the SIV/macaque model for AIDS. We determined BST2 expression in PBMC and leukocyte subsets of uninfected and SIV-infected rhesus macaques by real-time PCR and flow cytometry and correlated it with disease progression and viral load.

Results

Compared to pre-infection levels, we found increased BST2 expression in PBMC, purified CD4⁺ lymphocytes and CD14⁺ monocytes of SIV-infected animals, which correlated with viral load. Highest BST2 levels were found in progressors and lowest levels comparable to uninfected macaques were observed in long-term non-progressors (LTNPs). During acute viremia, BST2 mRNA increased in parallel with MX1, a prototype interferon-stimulated gene. This association was maintained during the whole disease course.

Conclusion

The detected relationship between BST2 expression and viral load as well as with MX1 indicate a common regulation by the interferon response and suggest rather limited influence of BST2 in vivo on the disease outcome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0219-8) contains supplementary material, which is available to authorized users.

Requirements within the Ebola Viral Glycoprotein for Tetherin Antagonism

Tetherin is an interferon-induced, intrinsic cellular response factor that blocks release of numerous viruses, including Ebola virus, from infected cells. As with many viruses targeted by host factors, Ebola virus employs a tetherin antagonist, the viral glycoprotein (EboGP), to counteract restriction and promote virus release. Unlike other tetherin antagonists such as HIV-1 Vpu or KSHV K5, the features within EboGP needed to overcome tetherin are not well characterized. Here, we describe sequences within the EboGP ectodomain and membrane spanning domain (msd) as necessary to relieve tetherin restriction of viral particle budding. Fusing the EboGP msd to a normally secreted form of the glycoprotein effectively promotes Ebola virus particle release. Cellular protein or lipid anchors could not substitute for the EboGP msd. The requirement for the EboGP msd was not specific for filovirus budding, as similar results were seen with HIV particles. Furthermore trafficking of chimeric proteins to budding sites did not correlate with an ability to counter tetherin. Additionally, we find that a glycoprotein construct, which mimics the cathepsin-activated species by proteolytic removal of the EboGP glycan cap and mucin domains, is unable to counteract tetherin. Combining these results suggests an important role for the EboGP glycan cap and msd in tetherin antagonism.

Viral Restriction Activity of Feline BST2 Is Independent of Its N-Glycosylation and Induction of NF-κB Activation

BST2 (CD317, tetherin, HM1.24) is an interferon-inducible transmembrane protein which can directly inhibit the release of enveloped virus particles from infected cells, and its anti-viral activity is reported to be related to the specific topological arrangement of its four structural domains. The N-terminal cytoplasmic tail of feline BST2 (fBST2) is characterized by a shorter N-terminal region compared to those of other known homologs. In this study, we investigated the functional impact of modifying the cytoplasmic tail region of fBST2 and its molecular mechanism. The fBST2 protein with the addition of a peptide at the N-terminus retained anti-release activity against human immunodeficiency virus type-1 and pseudovirus based on feline immunodeficiency virus at a weaker level compared with the wild-type fBST2. However, the fBST2 protein with addition of a peptide internally in the ectodomain proximal to the GPI anchor still retained its anti-viral activity well. Notably, the N-glycosylation state and the cell surface level of the N-terminally modified variants were unlike those of the wild-type protein, while no difference was observed in their intracellular localizations. However, in contrast to human BST2, the wild-type fBST2 did not show the ability to activate NF-κB. Consistent with previous reports, our findings showed that adding a peptide in the cytoplasmic tail region of fBST2 may influence its anti-viral activity. The shorter N-terminal cytoplasmic region of fBST2 compared with human BST2 did not apparently affect its anti-viral activity, which is independent of its N-glycosylation and ability to activate NF-κB.

Bst2/Tetherin Is Induced in Neurons by Type I Interferon and Viral Infection but Is Dispensable for Protection against Neurotropic Viral Challenge

In permissive mouse central nervous system (CNS) neurons, measles virus (MV) spreads in the absence of hallmark viral budding or neuronal death, with transmission occurring efficiently and exclusively via the synapse. MV infection also initiates a robust type I interferon (IFN) response, resulting in the synthesis of a large number of genes, including bone marrow stromal antigen 2 (Bst2)/tetherin/CD317. Bst2 restricts the release of some enveloped viruses, but to date, its role in viral infection of neurons has not been assessed. Consequently, we investigated how Bst2 was induced and what role it played in MV neuronal infection. The magnitude of induction of neuronal Bst2 RNA and protein following IFN exposure and viral infection was notably higher than in similarly treated mouse embryo fibroblasts (MEFs). Bst2 synthesis was both IFN and Stat1 dependent. Although Bst2 prevented MV release from nonneuronal cells, its deletion had no effect on viral pathogenesis in MV-challenged mice. Our findings underscore how cell-type-specific differences impact viral infection and pathogenesis.

IMPORTANCE

Viral infections of the central nervous system can lead to debilitating disease and death. Moreover, it is becoming increasingly clear that nonrenewable cells, including most central nervous system neurons, combat neurotropic viral infections in fundamentally different ways than other rapidly dividing and renewable cell populations. Here we identify type I interferon signaling as a key inducer of a known antiviral protein (Bst2) in neurons. Unexpectedly, the gene is dispensable for clearance of neurotropic viral infection despite its well-defined contribution to limiting the spread of enveloped viruses in proliferating cells. A deeper appreciation of the importance of cell type heterogeneity in antiviral immunity will aid in the identification of unique therapeutic targets for life-threatening viral infections.

Intracellular Dynamics and Fate of a Humanized Anti-Interleukin-6 Receptor Monoclonal Antibody, Tocilizumab

Tocilizumab (TCZ), a humanized anti-interleukin-6 (IL-6) receptor (IL-6R) monoclonal antibody, abrogates signal transducer protein gp130-mediated IL-6 signaling by competitively inhibiting the binding of IL-6 to the receptor, and shows clinical efficacy in autoimmune and inflammatory diseases. Despite accumulating evidence for therapeutic efficacy, the behavior and fate of TCZ at the cellular level remain largely unknown. To address this, we evaluated the endocytosis and intracellular trafficking of IL-6R in HeLa cells. The results of our study provide evidence that IL-6R is constitutively internalized from the cell surface by ligand or TCZ binding and the expression of gp130 in an independent manner and is targeted via endosomes without being significantly directed to the recycling pathway to, and degraded in, lysosomes. Furthermore, the cytoplasmic tail of IL-6R is required for constitutive endocytosis of the receptor, which is mediated by the clathrin and AP-2 complex. We further demonstrate that FcRn, whose function is to regulate the serum persistence of IgG, is confined primarily to early/recycling endosomes and rapidly transits between these compartments and late endosomes/lysosomes without being degraded. Importantly, the expression of FcRn induces the segregation of TCZ from IL-6R, resulting in extensive colocalization of TCZ and FcRn in IL-6R-depleted endosomal compartments. Collectively, our results suggest that FcRn can accelerate the retrieval of the internalized TCZ, not only from endosomes but also from lysosomes. Our findings provide new insight into the mechanism by which the antibody internalized into cells is rescued from lysosomal degradation and into how its serum levels are maintained.

BST2/tetherin inhibition of alphavirus exit

Alphaviruses such as chikungunya virus (CHIKV) and Semliki Forest virus (SFV) are small enveloped RNA viruses that bud from the plasma membrane. Tetherin/BST2 is an interferon-induced host membrane protein that inhibits the release of many enveloped viruses via direct tethering of budded particles to the cell surface. Alphaviruses have highly organized structures and exclude host membrane proteins from the site of budding, suggesting that their release might be insensitive to tetherin inhibition. Here, we demonstrated that exogenously-expressed tetherin efficiently inhibited the release of SFV and CHIKV particles from host cells without affecting virus entry and infection. Alphavirus release was also inhibited by the endogenous levels of tetherin in HeLa cells. While rubella virus (RuV) and dengue virus (DENV) have structural similarities to alphaviruses, tetherin inhibited the release of RuV but not DENV. We found that two recently identified tetherin isoforms differing in length at the N-terminus exhibited distinct capabilities in restricting alphavirus release. SFV exit was efficiently inhibited by the long isoform but not the short isoform of tetherin, while both isoforms inhibited vesicular stomatitis virus exit. Thus, in spite of the organized structure of the virus particle, tetherin specifically blocks alphavirus release and shows an interesting isoform requirement.

Positioning of cysteine residues within the N-terminal portion of the BST-2/tetherin ectodomain is important for functional dimerization of BST-2

BST-2/tetherin is a cellular host factor capable of restricting the release of a variety of enveloped viruses, including HIV-1. Structurally, BST-2 consists of an N-terminal cytoplasmic domain, a transmembrane domain, an ectodomain, and a C-terminal membrane anchor. The BST-2 ectodomain encodes three cysteine residues in its N-terminal half, each of which can contribute to the formation of cysteine-linked dimers. We previously reported that any one of the three cysteine residues is sufficient to produce functional BST-2 dimers. Here we investigated the importance of cysteine positioning on the ectodomain for functional dimerization of BST-2. Starting with a cysteine-free monomeric form of BST-2, individual cysteine residues were reintroduced at various locations throughout the ectodomain. The resulting BST-2 variants were tested for expression, dimerization, surface presentation, and inhibition of HIV-1 virus release. We found significant flexibility in the positioning of cysteine residues, although the propensity to form cysteine-linked dimers generally decreased with increasing distance from the N terminus. Interestingly, all BST-2 variants, including the one lacking all three ectodomain cysteines, retained the ability to form non-covalent dimers, and all of the BST-2 variants were efficiently expressed at the cell surface. Importantly, not all BST-2 variants capable of forming cysteine-linked dimers were functional, suggesting that cysteine-linked dimerization of BST-2 is necessary but not sufficient for inhibiting virus release. Our results expose new structural constraints governing the functional dimerization of BST-2, a property essential to its role as a restriction factor tethering viruses to the host cell.

Epitope tags beside the N-terminal cytoplasmic tail of human BST-2 alter its intracellular trafficking and HIV-1 restriction

BST-2 blocks the particle release of various enveloped viruses including HIV-1, and this antiviral activity is dependent on the topological arrangement of its four structural domains. Several functions of the cytoplasmic tail (CT) of BST-2 have been previously discussed, but the exact role of this domain remains to be clearly defined. In this study, we investigated the impact of truncation and commonly-used tags addition into the CT region of human BST-2 on its intracellular trafficking and signaling as well as its anti-HIV-1 function. The CT-truncated BST-2 exhibited potent inhibition on Vpu-defective HIV-1 and even wild-type HIV-1. However, the N-terminal HA-tagged CT-truncated BST-2 retained little antiviral activity and dramatically differed from its original protein in the cell surface level and intracellular localization. Further, we showed that the replacement of the CT domain with a hydrophobic tag altered BST-2 function possibly by preventing its normal vesicular trafficking. Notably, we demonstrated that a positive charged motif “KRXK” in the conjunctive region between the cytotail and the transmembrane domain which is conserved in primate BST-2 is important for the protein trafficking and the antiviral function. These results suggest that although the CT of BST-2 is not essential for its antiviral activity, the composition of residues in this region may play important roles in its normal trafficking which subsequently affected its function. These observations provide additional implications for the structure-function model of BST-2.

Targeted therapy for HM1.24 (CD317) on multiple myeloma cells

Multiple myeloma (MM) still remains an incurable disease, at least because of the existence of cell-adhesion mediated drug-resistant MM cells and/or continuous recruitment of presumed MM cancer stem cell-like cells (CSCs). As a new alternative treatment modality, immunological approaches using monoclonal antibodies (mAbs) and/or cytotoxic T lymphocytes (CTLs) are now attracting much attention as a novel strategy attacking MM cells. We have identified that HM1.24 [also known as bone marrow stromal cell antigen 2 (BST2) or CD317] is overexpressed on not only mature MM cells but also MM CSCs. We then have developed a humanized mAb to HM1.24 and defucosylated version of the mAb to adapt to clinical practice. Moreover, we have successfully induced HM1.24-specific CTLs against MM cells. The combination of these innovative therapeutic modalities may likely exert an anti-MM activity by evading the drug resistance mechanism and eliminating presumed CSCs in MM.

Tetherin can restrict cell-free and cell-cell transmission of HIV from primary macrophages to T cells

Bst-2/Tetherin inhibits the release of HIV by tethering newly formed virus particles to the plasma membrane of infected cells. Although the mechanisms of Tetherin-mediated restriction are increasingly well understood, the biological relevance of this restriction in the natural target cells of HIV is unclear. Moreover, whether Tetherin exerts any restriction on the direct cell-cell spread of HIV across intercellular contacts remains controversial. Here we analyse the restriction endogenous Tetherin imposes on HIV transmission from primary human macrophages, one of the main targets of HIV in vivo. We find that the mRNA and protein levels of Tetherin in macrophages are comparable to those in T cells from the same donors, and are highly upregulated by type I interferons. Improved immunocytochemistry protocols enable us to demonstrate that Tetherin localises to the cell surface, the trans-Golgi network, and the macrophage HIV assembly compartments. Tetherin retains budded virions in the assembly compartments, thereby impeding the release and cell-free spread of HIV, but it is not required for the maintenance of these compartments per se. Notably, using a novel assay to quantify cell-cell spread, we show that Tetherin promotes the transfer of virus clusters from macrophages to T cells and thereby restricts the direct transmission of a dual-tropic HIV-1. Kinetic analyses provide support for the notion that this direct macrophage-T cell spread is mediated, at least in part, by so-called virological synapses. Finally, we demonstrate that the viral Vpu protein efficiently downregulates the cell surface and overall levels of Tetherin, and thereby abrogates this HIV restriction in macrophages. Together, our study shows that Tetherin, one of the most potent HIV restriction factors identified to date, can inhibit virus spread from primary macrophages, regardless of the mode of transmission.

The novel immunotoxin HM1.24-ETA' induces apoptosis in multiple myeloma cells

Despite new treatment modalities, the clinical outcome in a substantial number of patients with multiple myeloma (MM) has yet to be improved. Antibody-based targeted therapies for myeloma patients could make use of the HM1.24 antigen (CD317), a surface molecule overexpressed on malignant plasma cells and efficiently internalized. Here, a novel immunotoxin, HM1.24-ETA', is described. HM1.24-ETA' was generated by genetic fusion of a CD317-specific single-chain Fv (scFv) antibody and a truncated variant of Pseudomonas aeruginosa exotoxin A (ETA'). HM1.24-ETA' inhibited growth of interleukin 6 (IL-6)-dependent and -independent myeloma cell lines. Half-maximal growth inhibition was observed at concentrations as low as 0.3 nM. Target cell killing occurred via induction of apoptosis and was unaffected in co-culture experiments with bone marrow stromal cells. HM1.24-ETA' efficiently triggered apoptosis of freshly isolated/cryopreserved cells of patients with plasma cell leukemia and MM and was active in a preclinical severe combined immunodeficiency (SCID) mouse xenograft model. Importantly, HM1.24-ETA' was not cytotoxic against CD317-positive cells from healthy tissue (monocytes, human umbilical vein endothelial cells). These results indicate that CD317 may represent a promising target structure for specific and efficient immunotoxin therapy for patients with plasma cell tumors.

Mechanisms underlying HIV-1 Vpu-mediated viral egress

Viruses such as lentiviruses that are responsible for long lasting infections have to evade several levels of cellular immune mechanisms to persist and efficiently disseminate in the host. Over the past decades, much evidence has emerged regarding the major role of accessory proteins of primate lentiviruses, human immunodeficiency virus and simian immunodeficiency virus, in viral evasion from the host immune defense. This short review will provide an overview of the mechanism whereby the accessory protein Vpu contributes to this escape. Vpu is a multifunctional protein that was shown to contribute to viral egress by down-regulating several mediators of the immune system such as CD4, CD1d, NTB-A and the restriction factor BST2. The mechanisms underlying its activity are not fully characterized but rely on its ability to interfere with the host machinery regulating protein turnover and vesicular trafficking. This review will focus on our current understanding of the mechanisms whereby Vpu down-regulates CD4 and BST2 expression levels to favor viral egress.

Structural basis of HIV-1 Vpu-mediated BST2 antagonism via hijacking of the clathrin adaptor protein complex 1

BST2/tetherin, an antiviral restriction factor, inhibits the release of enveloped viruses from the cell surface. Human immunodeficiency virus-1 (HIV-1) antagonizes BST2 through viral protein u (Vpu), which downregulates BST2 from the cell surface. We report the crystal structure of a protein complex containing Vpu and BST2 cytoplasmic domains and the core of the clathrin adaptor protein complex 1 (AP1). This, together with our biochemical and functional validations, reveals how Vpu hijacks the AP1-dependent membrane trafficking pathways to mistraffick BST2. Vpu mimics a canonical acidic dileucine-sorting motif to bind AP1 in the cytosol, while simultaneously interacting with BST2 in the membrane. These interactions enable Vpu to build on an intrinsic interaction between BST2 and AP1, presumably causing the observed retention of BST2 in juxtanuclear endosomes and stimulating its degradation in lysosomes. The ability of Vpu to hijack AP-dependent trafficking pathways suggests a potential common theme for Vpu-mediated downregulation of host proteins.

DOI:http://dx.doi.org/10.7554/eLife.02362.001

HIV is a retrovirus that attacks the immune system, making the body increasingly susceptible to opportunistic infections and disease and eventually leading to AIDS. While antiretroviral drugs have allowed people with AIDS to live longer, there is no cure or vaccine for HIV.

Two types of HIV exist, with HIV-1 being much more common and pathogenic than HIV-2. Like other ‘complex’ retroviruses, the HIV-1 genome contains genes that encode various proteins that allow the virus to disrupt the immune response of the host it is attacking. Viral protein u is a protein encoded by HIV-1 (but not HIV-2) that counteracts an antiviral protein called BST2 in the host. BST2, which is part of the host's innate immune response, prevents newly formed viruses from leaving the surface of infected cells. By counteracting BST2, viral protein u allows the virus to spread in the host more efficiently.

Like many proteins, newly produced BST2 is packaged inside structures called vesicles in a part of the cell called the trans-Golgi network, and then sent to its destination. Complexes formed by various proteins make sure that the vesicles take their cargo to their correct destinations within the cell. Two adaptor protein complexes—known as AP1 and AP2—are thought to be involved the transport of BST2. However, it is not known how viral protein u stops BST2 from reaching the cell surface, or how it decreases the amount of BST2 in the cell as a whole. Jia et al. show how viral protein u and BST2 jointly interact with AP1. This interaction leads to the mistrafficking and degradation of BST2 and the counteraction of its antiviral activity.

DOI:http://dx.doi.org/10.7554/eLife.02362.002

Counteraction of the multifunctional restriction factor tetherin

The interferon-inducible restriction factor tetherin (also known as CD317, BST-2 or HM1.24) has emerged as a key component of the antiviral immune response. Initially, tetherin was shown to restrict replication of various enveloped viruses by inhibiting the release of budding virions from infected cells. More recently, it has become clear that tetherin also acts as a pattern recognition receptor inducing NF-κB-dependent proinflammatory gene expression in virus infected cells. Whereas the ability to restrict virion release is highly conserved among mammalian tetherin orthologs and thus probably an ancient function of this protein, innate sensing seems to be an evolutionarily recent activity. The potent and broad antiviral activity of tetherin is reflected by the fact that many viruses evolved means to counteract this restriction factor. A continuous arms race with viruses has apparently driven the evolution of different isoforms of tetherin with different functional properties. Interestingly, tetherin has also been implicated in cellular processes that are unrelated to immunity, such as the organization of the apical actin network and membrane microdomains or stabilization of the Golgi apparatus. In this review, I summarize our current knowledge of the different functions of tetherin and describe the molecular strategies that viruses have evolved to antagonize or evade this multifunctional host restriction factor.

HIV-1 Vpu antagonism of tetherin inhibits antibody-dependent cellular cytotoxic responses by natural killer cells

The type I interferon-inducible factor tetherin retains virus particles on the surfaces of cells infected with vpu-deficient human immunodeficiency virus type 1 (HIV-1). While this mechanism inhibits cell-free viral spread, the immunological implications of tethered virus have not been investigated. We found that surface tetherin expression increased the antibody opsonization of vpu-deficient HIV-infected cells. The absence of Vpu also stimulated NK cell-activating FcγRIIIa signaling and enhanced NK cell degranulation and NK cell-mediated antibody-dependent cellular cytotoxicity (ADCC). The deletion of vpu in HIV-1-infected primary CD4(+) T cells enhanced the levels of antibody binding and Fc receptor signaling mediated by HIV-positive-patient-derived antibodies. The magnitudes of antibody binding and Fc signaling were both highly correlated to the levels of tetherin on the surfaces of infected primary CD4 T cells. The affinity of antibody binding to FcγRIIIa was also found to be critical in mediating efficient Fc activation. These studies implicate Vpu antagonism of tetherin as an ADCC evasion mechanism that prevents antibody-mediated clearance of virally infected cells.

IMPORTANCE

The ability of the HIV-1 accessory factor to antagonize tetherin has been considered to primarily function by limiting the spread of virus by preventing the release of cell-free virus. This study supports the hypothesis that a major function of Vpu is to decrease the recognition of infected cells by anti-HIV antibodies at the cell surface, thereby reducing recognition by antibody-dependent clearance by natural killer cells.

BST-2/tetherin: viral tether, viral sensor or both?

In the fields of virology and innate immunity, BST-2/tetherin is well known for its ability to block the egress of enveloped viruses from infected cells. This appears to be accomplished by 'tethering' virions to the cell surface, thereby limiting virion release. In the past year, several groups have discovered that BST-2/tetherin can activate NF-κB, a transcriptional activator that leads to the rapid expression of both proinflammatory cytokines and proteins involved in cell survival. While this new BST-2 function has been interpreted as a possible viral-sensing mechanism, there may also be broader implications for HIV gene regulation. This article reviews the evidence for BST-2-dependent NF-κB activation, and explores the significance of these exciting new results.

Feline immunodeficiency virus envelope glycoproteins antagonize tetherin through a distinctive mechanism that requires virion incorporation

BST2/tetherin inhibits the release of enveloped viruses from cells. Primate lentiviruses have evolved specific antagonists (Vpu, Nef, and Env). Here we characterized tetherin proteins of species representing both branches of the order Carnivora. Comparison of tiger and cat (Feliformia) to dog and ferret (Caniformia) genes demonstrated that the tiger and cat share a start codon mutation that truncated most of the tetherin cytoplasmic tail early in the Feliformia lineage (19 of 27 amino acids, including the dual tyrosine motif). Alpha interferon (IFN-α) induced tetherin and blocked feline immunodeficiency virus (FIV) replication in lymphoid and nonlymphoid feline cells. Budding of bald FIV and HIV particles was blocked by carnivore tetherins. However, infectious FIV particles were resistant, and spreading FIV replication was uninhibited. Antagonism mapped to the envelope glycoprotein (Env), which rescued FIV from carnivore tetherin restriction when expressed in trans but, in contrast to known antagonists, did not rescue noncognate particles. Also unlike the primate lentiviral antagonists, but similar to the Ebola virus glycoprotein, FIV Env did not reduce intracellular or cell surface tetherin levels. Furthermore, FIV-enveloped FIV particles actually required tetherin for optimal release from cells. The results show that FIV Envs mediate a distinctive tetherin evasion. Well adapted to a phylogenetically ancient tetherin tail truncation in the Felidae, it requires functional virion incorporation of Env, and it shields the budding particle without downregulating plasma membrane tetherin. Moreover, FIV has evolved dependence on this protein: particles containing FIV Env need tetherin for optimal release from the cell, while Env(-) particles do not.

IMPORTANCE

HIV-1 antagonizes the restriction factor tetherin with the accessory protein Vpu, while HIV-2 and the filovirus Ebola use their envelope (Env) glycoproteins for this purpose. It turns out that the FIV tetherin antagonist is also its Env protein, but the mechanism is distinctive. Unlike other tetherin antagonists, FIV Env cannot act in trans to rescue vpu-deficient HIV-1. It must be incorporated specifically into FIV virions to be active. Also unlike other retroviral antagonists, but similar to Ebola virus Env, it does not act by downregulating or degrading tetherin. FIV Env might exclude tetherin locally or direct assembly to tetherin-negative membrane domains. Other distinctive features are apparent, including evidence that this virus evolved an equilibrium in which tetherin is both restriction factor and cofactor, as FIV requires tetherin for optimal particle release.