Extracellular vesicles from Zika virus-infected cells display viral E protein that binds ZIKV-neutralizing antibodies to prevent infection enhancement.

Mosquito-borne flaviviruses including Zika virus (ZIKV) represent a public health problem in some parts of the world. Although ZIKV infection is predominantly asymptomatic or associated with mild symptoms, it can lead to neurological complications. ZIKV infection can also cause antibody-dependent enhancement (ADE) of infection with similar viruses, warranting further studies of virion assembly and the function of envelope (E) protein-specific antibodies. Although extracellular vesicles (EVs) from flavivirus-infected cells have been reported to transmit infection, this interpretation is challenged by difficulties in separating EVs from flavivirions due to their similar biochemical composition and biophysical properties. In the present study, a rigorous EV-virion separation method combining sequential ultracentrifugation and affinity capture was developed to study EVs from ZIKV-infected cells. We find that these EVs do not transmit infection, but EVs display abundant E proteins which have an antigenic landscape similar to that of virions carrying E. ZIKV E-coated EVs attenuate antibody-dependent enhancement mediated by ZIKV E-specific and DENV-cross-reactive antibodies in both cell culture and mouse models. We thus report an alternative route for Flavivirus E protein secretion. These results suggest that modulation of E protein release via virions and EVs may present a new approach to regulating flavivirus-host interactions.

A Spike-destructing human antibody effectively neutralizes Omicron-included SARS-CoV-2 variants with therapeutic efficacy.

Neutralizing antibodies (nAbs) are important assets to fight COVID-19, but most existing nAbs lose the activities against Omicron subvariants. Here, we report a human monoclonal antibody (Ab08) isolated from a convalescent patient infected with the prototype strain (Wuhan-Hu-1). Ab08 binds to the receptor-binding domain (RBD) with pico-molar affinity (230 pM), effectively neutralizes SARS-CoV-2 and variants of concern (VOCs) including Alpha, Beta, Gamma, Mu, Omicron BA.1 and BA.2, and to a lesser extent for Delta and Omicron BA.4/BA.5 which bear the L452R mutation. Of medical importance, Ab08 shows therapeutic efficacy in SARS-CoV-2-infected hACE2 mice. X-ray crystallography of the Ab08-RBD complex reveals an antibody footprint largely in the ß-strand core and away from the ACE2-binding motif. Negative staining electron-microscopy suggests a neutralizing mechanism through which Ab08 destructs the Spike trimer. Together, our work identifies a nAb with therapeutic potential for COVID-19.

Antibody-mediated spike activation promotes cell-cell transmission of SARS-CoV-2.

The COVID pandemic fueled by emerging SARS-CoV-2 new variants of concern remains a major global health concern, and the constantly emerging mutations present challenges to current therapeutics. The spike glycoprotein is not only essential for the initial viral entry, but is also responsible for the transmission of SARS-CoV-2 components via syncytia formation. Spike-mediated cell-cell transmission is strongly resistant to extracellular therapeutic and convalescent antibodies via an unknown mechanism. Here, we describe the antibody-mediated spike activation and syncytia formation on cells displaying the viral spike. We found that soluble antibodies against receptor binding motif (RBM) are capable of inducing the proteolytic processing of spike at both the S1/S2 and S2' cleavage sites, hence triggering ACE2-independent cell-cell fusion. Mechanistically, antibody-induced cell-cell fusion requires the shedding of S1 and exposure of the fusion peptide at the cell surface. By inhibiting S1/S2 proteolysis, we demonstrated that cell-cell fusion mediated by spike can be re-sensitized towards antibody neutralization in vitro. Lastly, we showed that cytopathic effect mediated by authentic SARS-CoV-2 infection remain unaffected by the addition of extracellular neutralization antibodies. Hence, these results unveil a novel mode of antibody evasion and provide insights for antibody selection and drug design strategies targeting the SARS-CoV-2 infected cells.

SARS-CoV-2 spike engagement of ACE2 primes S2' site cleavage and fusion initiation.

The COVID-19 pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has resulted in tremendous loss worldwide. Although viral spike (S) protein binding of angiotensin-converting enzyme 2 (ACE2) has been established, the functional consequences of the initial receptor binding and the stepwise fusion process are not clear. By utilizing a cell-cell fusion system, in complement with a pseudoviral infection model, we found that the spike engagement of ACE2 primed the generation of S2' fragments in target cells, a key proteolytic event coupled with spike-mediated membrane fusion. Mutagenesis of an S2' cleavage site at the arginine (R) 815, but not an S2 cleavage site at arginine 685, was sufficient to prevent subsequent syncytia formation and infection in a variety of cell lines and primary cells isolated from human ACE2 knock-in mice. The requirement for S2' cleavage at the R815 site was also broadly shared by other SARS-CoV-2 spike variants, such as the Alpha, Beta, and Delta variants of concern. Thus, our study highlights an essential role for host receptor engagement and the key residue of spike for proteolytic activation, and uncovers a targetable mechanism for host cell infection by SARS-CoV-2.

A high-affinity RBD-targeting nanobody improves fusion partner's potency against SARS-CoV-2.

A key step to the SARS-CoV-2 infection is the attachment of its Spike receptor-binding domain (S RBD) to the host receptor ACE2. Considerable research has been devoted to the development of neutralizing antibodies, including llama-derived single-chain nanobodies, to target the receptor-binding motif (RBM) and to block ACE2-RBD binding. Simple and effective strategies to increase potency are desirable for such studies when antibodies are only modestly effective. Here, we identify and characterize a high-affinity synthetic nanobody (sybody, SR31) as a fusion partner to improve the potency of RBM-antibodies. Crystallographic studies reveal that SR31 binds to RBD at a conserved and 'greasy' site distal to RBM. Although SR31 distorts RBD at the interface, it does not perturb the RBM conformation, hence displaying no neutralizing activities itself. However, fusing SR31 to two modestly neutralizing sybodies dramatically increases their affinity for RBD and neutralization activity against SARS-CoV-2 pseudovirus. Our work presents a tool protein and an efficient strategy to improve nanobody potency.

Correction: Histone demethylase LSD1 restricts influenza A virus infection by erasing IFITM3-K88 monomethylation.

[This corrects the article DOI: 10.1371/journal.ppat.1006773.].

The SARS-CoV-2 envelope and membrane proteins modulate maturation and retention of the spike protein, allowing assembly of virus-like particles.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a ß-coronavirus, is the causative agent of the COVID-19 pandemic. Like for other coronaviruses, its particles are composed of four structural proteins: spike (S), envelope (E), membrane (M), and nucleoprotein (N) proteins. The involvement of each of these proteins and their interactions are critical for assembly and production of ß-coronavirus particles. Here, we sought to characterize the interplay of SARS-CoV-2 structural proteins during the viral assembly process. By combining biochemical and imaging assays in infected versus transfected cells, we show that E and M regulate intracellular trafficking of S as well as its intracellular processing. Indeed, the imaging data reveal that S is relocalized at endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) or Golgi compartments upon coexpression of E or M, as observed in SARS-CoV-2-infected cells, which prevents syncytia formation. We show that a C-terminal retrieval motif in the cytoplasmic tail of S is required for its M-mediated retention in the ERGIC, whereas E induces S retention by modulating the cell secretory pathway. We also highlight that E and M induce a specific maturation of N-glycosylation of S, independently of the regulation of its localization, with a profile that is observed both in infected cells and in purified viral particles. Finally, we show that E, M, and N are required for optimal production of virus-like-particles. Altogether, these results highlight how E and M proteins may influence the properties of S proteins and promote the assembly of SARS-CoV-2 viral particles.

A protein coevolution method uncovers critical features of the Hepatitis C Virus fusion mechanism.

Amino-acid coevolution can be referred to mutational compensatory patterns preserving the function of a protein. Viral envelope glycoproteins, which mediate entry of enveloped viruses into their host cells, are shaped by coevolution signals that confer to viruses the plasticity to evade neutralizing antibodies without altering viral entry mechanisms. The functions and structures of the two envelope glycoproteins of the Hepatitis C Virus (HCV), E1 and E2, are poorly described. Especially, how these two proteins mediate the HCV fusion process between the viral and the cell membrane remains elusive. Here, as a proof of concept, we aimed to take advantage of an original coevolution method recently developed to shed light on the HCV fusion mechanism. When first applied to the well-characterized Dengue Virus (DENV) envelope glycoproteins, coevolution analysis was able to predict important structural features and rearrangements of these viral protein complexes. When applied to HCV E1E2, computational coevolution analysis predicted that E1 and E2 refold interdependently during fusion through rearrangements of the E2 Back Layer (BL). Consistently, a soluble BL-derived polypeptide inhibited HCV infection of hepatoma cell lines, primary human hepatocytes and humanized liver mice. We showed that this polypeptide specifically inhibited HCV fusogenic rearrangements, hence supporting the critical role of this domain during HCV fusion. By combining coevolution analysis and in vitro assays, we also uncovered functionally-significant coevolving signals between E1 and E2 BL/Stem regions that govern HCV fusion, demonstrating the accuracy of our coevolution predictions. Altogether, our work shed light on important structural features of the HCV fusion mechanism and contributes to advance our functional understanding of this process. This study also provides an important proof of concept that coevolution can be employed to explore viral protein mediated-processes, and can guide the development of innovative translational strategies against challenging human-tropic viruses.

Correction: Histone demethylase LSD1 restricts influenza A virus infection by erasing IFITM3-K88 monomethylation.

[This corrects the article DOI: 10.1371/journal.ppat.1006773.].

Histone demethylase LSD1 restricts influenza A virus infection by erasing IFITM3-K88 monomethylation.

The histone demethylase LSD1 has been known as a key transcriptional coactivator for DNA viruses such as herpes virus. Inhibition of LSD1 was found to block viral genome transcription and lytic replication of DNA viruses. However, RNA virus genomes do not rely on chromatin structure and histone association, and the role of demethylase activity of LSD1 in RNA virus infections is not anticipated. Here, we identify that, contrary to its role in enhancing DNA virus replication, LSD1 limits RNA virus replication by demethylating and activating IFITM3 which is a host restriction factor for many RNA viruses. We have found that LSD1 is recruited to demethylate IFITM3 at position K88 under IFNα treatment. However, infection by either Vesicular Stomatitis Virus (VSV) or Influenza A Virus (IAV) triggers methylation of IFITM3 by promoting its disassociation from LSD1. Accordingly, inhibition of the enzymatic activity of LSD1 by Trans-2-phenylcyclopropylamine hydrochloride (TCP) increases IFITM3 monomethylation which leads to more severe disease outcomes in IAV-infected mice. In summary, our findings highlight the opposite role of LSD1 in fighting RNA viruses comparing to DNA viruses infection. Our data suggest that the demethylation of IFITM3 by LSD1 is beneficial for the host to fight against RNA virus infection.

Specialization of Hepatitis C Virus Envelope Glycoproteins for B Lymphocytes in Chronically Infected Patients.

UNLABELLED: Hepatitis C virus (HCV) productively infects hepatocytes. Virion surface glycoproteins E1 and E2 play a major role in this restricted cell tropism by mediating virus entry into particular cell types. However, several pieces of evidence have suggested the ability of patient-derived HCV particles to infect peripheral blood mononuclear cells. The viral determinants and mechanisms mediating such events remain poorly understood. Here, we aimed at isolating viral determinants of HCV entry into B lymphocytes. For this purpose, we constructed a library of full E1E2 sequences isolated from serum and B lymphocytes of four chronically infected patients. We observed a strong phylogenetic compartmentalization of E1E2 sequences isolated from B lymphocytes in one patient, indicating that E1E2 glycoproteins can represent important mediators of the strong segregation of two specialized populations in some patients. Most of the E1E2 envelope glycoproteins were functional and allowed transduction of hepatocyte cell lines using HCV-derived pseudoparticles. Strikingly, introduction of envelope glycoproteins isolated from B lymphocytes into the HCV JFH-1 replicating virus switched the entry tropism of this nonlymphotropic virus from hepatotropism to lymphotropism. Significant detection of viral RNA and viral proteins within B cells was restricted to infections with JFH-1 harboring E1E2 from lymphocytes and depended on an endocytic, pH-dependent entry pathway. Here, we achieved for the first time the isolation of HCV viral proteins carrying entry-related lymphotropism determinants. The identification of genetic determinants within E1E2 represents a first step for a better understanding of the complex relationship between HCV infection, viral persistence, and extrahepatic disorders. IMPORTANCE: Hepatitis C virus (HCV) mainly replicates within the liver. However, it has been shown that patient-derived HCV particles can slightly infect lymphocytes in vitro and in vivo, highlighting the existence of lymphotropism determinants within HCV viral proteins. We isolated HCV envelope glycoproteins from patient B lymphocytes that conferred to a nonlymphotropic HCV the ability to enter B cells, thus providing a platform for characterization of HCV entry into lymphocytes. This unusual tropism was accompanied by a loss of entry function into hepatocytes, suggesting that HCV lymphotropic variants likely constitute a distinct but parallel source for viral persistence and immune escape within chronically infected patients. Moreover, the level of genetic divergence of B-cell-derived envelopes correlated with their degree of lymphotropism, underlining a long-term specialization of some viral populations for B-lymphocytes. Consequently, the clearance of both hepatotropic and nonhepatotropic HCV populations may be important for effective treatment of chronically infected patients.

Co-Infection of Mosquitoes with Chikungunya and Dengue Viruses Reveals Modulation of the Replication of Both Viruses in Midguts and Salivary Glands of Aedes aegypti Mosquitoes.

Arthropod-borne virus (arbovirus) infections cause several emerging and resurgent infectious diseases in humans and animals. Chikungunya-affected areas often overlap with dengue-endemic areas. Concurrent dengue virus (DENV) and chikungunya virus (CHIKV) infections have been detected in travelers returning from regions of endemicity. CHIKV and DENV co-infected Aedes albopictus have also been collected in the vicinity of co-infected human cases, emphasizing the need to study co-infections in mosquitoes. We thus aimed to study the pathogen-pathogen interaction involved in these co-infections in DENV/CHIKV co-infected Aedes aegypti mosquitoes. In mono-infections, we detected CHIKV antigens as early as 4 days post-virus exposure in both the midgut (MG) and salivary gland (SG), whereas we detected DENV serotype 2 (DENV-2) antigens from day 5 post-virus exposure in MG and day 10 post-virus exposure in SG. Identical infection rates were observed for singly and co-infected mosquitoes, and facilitation of the replication of both viruses at various times post-viral exposure. We observed a higher replication for DENV-2 in SG of co-infected mosquitoes. We showed that mixed CHIKV and DENV infection facilitated viral replication in Ae. aegypti. The outcome of these mixed infections must be further studied to increase our understanding of pathogen-pathogen interactions in host cells.

Functional and Biochemical Characterization of Hepatitis C Virus (HCV) Particles Produced in a Humanized Liver Mouse Model.

Lipoprotein components are crucial factors for hepatitis C virus (HCV) assembly and entry. As hepatoma cells producing cell culture-derived HCV (HCVcc) particles are impaired in some aspects of lipoprotein metabolism, it is of upmost interest to biochemically and functionally characterize the in vivo produced viral particles, particularly regarding how lipoprotein components modulate HCV entry by lipid transfer receptors such as scavenger receptor BI (SR-BI). Sera from HCVcc-infected liver humanized FRG mice were separated by density gradients. Viral subpopulations, termed HCVfrg particles, were characterized for their physical properties, apolipoprotein association, and infectivity. We demonstrate that, in contrast to the widely spread distribution of apolipoproteins across the different HCVcc subpopulations, the most infectious HCVfrg particles are highly enriched in apoE, suggesting that such apolipoprotein enrichment plays a role for entry of in vivo derived infectious particles likely via usage of apolipoprotein receptors. Consistent with this salient feature, we further reveal previously undefined functionalities of SR-BI in promoting entry of in vivo produced HCV. First, unlike HCVcc, SR-BI is a particularly limiting factor for entry of HCVfrg subpopulations of very low density. Second, HCVfrg entry involves SR-BI lipid transfer activity but not its capacity to bind to the viral glycoprotein E2. In conclusion, we demonstrate that composition and biophysical properties of the different subpopulations of in vivo produced HCVfrg particles modulate their levels of infectivity and receptor usage, hereby featuring divergences with in vitro produced HCVcc particles and highlighting the powerfulness of this in vivo model for the functional study of the interplay between HCV and liver components.

The sheep tetherin paralog oBST2B blocks envelope glycoprotein incorporation into nascent retroviral virions.

UNLABELLED: Bone marrow stromal cell antigen 2 (BST2) is a cellular restriction factor with a broad antiviral activity. In sheep, the BST2 gene is duplicated into two paralogs termed oBST2A and oBST2B. oBST2A impedes viral exit of the Jaagsiekte sheep retroviruses (JSRV), most probably by retaining virions at the cell membrane, similar to the "tethering" mechanism exerted by human BST2. In this study, we provide evidence that unlike oBST2A, oBST2B is limited to the Golgi apparatus and disrupts JSRV envelope (Env) trafficking by sequestering it. In turn, oBST2B leads to a reduction in Env incorporation into viral particles, which ultimately results in the release of virions that are less infectious. Furthermore, the activity of oBST2B does not seem to be restricted to retroviruses, as it also acts on vesicular stomatitis virus glycoproteins. Therefore, we suggest that oBST2B exerts antiviral activity using a mechanism distinct from the classical tethering restriction observed for oBST2A. IMPORTANCE: BST2 is a powerful cellular restriction factor against a wide range of enveloped viruses. Sheep possess two paralogs of the BST2 gene called oBST2A and oBST2B. JSRV, the causative agent of a transmissible lung cancer of sheep, is known to be restricted by oBST2A. In this study, we show that unlike oBST2A, oBST2B impairs the normal cellular trafficking of JSRV envelope glycoproteins by sequestering them within the Golgi apparatus. We also show that oBST2B decreases the incorporation of envelope glycoprotein into JSRV viral particles, which in turn reduces virion infectivity. In conclusion, oBST2B exerts a novel antiviral activity that is distinct from those of BST2 proteins of other species.

Determinants Involved in Hepatitis C Virus and GB Virus B Primate Host Restriction.

UNLABELLED: Hepatitis C virus (HCV) only infects humans and chimpanzees, while GB virus B (GBV-B), another hepatotropic hepacivirus, infects small New World primates (tamarins and marmosets). In an effort to develop an immunocompetent small primate model for HCV infection to study HCV pathogenesis and vaccine approaches, we investigated the HCV life cycle step(s) that may be restricted in small primate hepatocytes. First, we found that replication-competent, genome-length chimeric HCV RNAs encoding GBV-B structural proteins in place of equivalent HCV sequences designed to allow entry into simian hepatocytes failed to induce viremia in tamarins following intrahepatic inoculation, nor did they lead to progeny virus in permissive, transfected human Huh7.5 hepatoma cells upon serial passage. This likely reflected the disruption of interactions between distantly related structural and nonstructural proteins that are essential for virion production, whereas such cross talk could be restored in similarly designed HCV intergenotypic recombinants via adaptive mutations in NS3 protease or helicase domains. Next, HCV entry into small primate hepatocytes was examined directly using HCV-pseudotyped retroviral particles (HCV-pp). HCV-pp efficiently infected tamarin hepatic cell lines and primary marmoset hepatocyte cultures through the use of the simian CD81 ortholog as a coreceptor, indicating that HCV entry is not restricted in small New World primate hepatocytes. Furthermore, we observed genomic replication and modest virus secretion following infection of primary marmoset hepatocyte cultures with a highly cell culture-adapted HCV strain. Thus, HCV can successfully complete its life cycle in primary simian hepatocytes, suggesting the possibility of adapting some HCV strains to small primate hosts. IMPORTANCE: Hepatitis C virus (HCV) is an important human pathogen that infects over 150 million individuals worldwide and leads to chronic liver disease. The lack of a small animal model for this infection impedes the development of a preventive vaccine and pathogenesis studies. In seeking to establish a small primate model for HCV, we first attempted to generate recombinants between HCV and GB virus B (GBV-B), a hepacivirus that infects small New World primates (tamarins and marmosets). This approach revealed that the genetic distance between these hepaciviruses likely prevented virus morphogenesis. We next showed that HCV pseudoparticles were able to infect tamarin or marmoset hepatocytes efficiently, demonstrating that there was no restriction in HCV entry into these simian cells. Furthermore, we found that a highly cell culture-adapted HCV strain was able to achieve a complete viral cycle in primary marmoset hepatocyte cultures, providing a promising basis for further HCV adaptation to small primate hosts.

New Insights into the Understanding of Hepatitis C Virus Entry and Cell-to-Cell Transmission by Using the Ionophore Monensin A.

UNLABELLED: In our study, we characterized the effect of monensin, an ionophore that is known to raise the intracellular pH, on the hepatitis C virus (HCV) life cycle. We showed that monensin inhibits HCV entry in a pangenotypic and dose-dependent manner. Monensin induces an alkalization of intracellular organelles, leading to an inhibition of the fusion step between viral and cellular membranes. Interestingly, we demonstrated that HCV cell-to-cell transmission is dependent on the vesicular pH. Using the selective pressure of monensin, we selected a monensin-resistant virus which has evolved to use a new entry route that is partially pH and clathrin independent. Characterization of this mutant led to the identification of two mutations in envelope proteins, the Y297H mutation in E1 and the I399T mutation in hypervariable region 1 (HVR1) of E2, which confer resistance to monensin and thus allow HCV to use a pH-independent entry route. Interestingly, the I399T mutation introduces an N-glycosylation site within HVR1 and increases the density of virions and their sensitivity to neutralization with anti-apolipoprotein E (anti-ApoE) antibodies, suggesting that this mutation likely induces conformational changes in HVR1 that in turn modulate the association with ApoE. Strikingly, the I399T mutation dramatically reduces HCV cell-to-cell spread. In summary, we identified a mutation in HVR1 that overcomes the vesicular pH dependence, modifies the biophysical properties of particles, and drastically reduces cell-to-cell transmission, indicating that the regulation by HVR1 of particle association with ApoE might control the pH dependence of cell-free and cell-to-cell transmission. Thus, HVR1 and ApoE are critical regulators of HCV propagation. IMPORTANCE: Although several cell surface proteins have been identified as entry factors for hepatitis C virus (HCV), the precise mechanisms regulating its transmission to hepatic cells are still unclear. In our study, we used monensin A, an ionophore that is known to raise the intracellular pH, and demonstrated that cell-free and cell-to-cell transmission pathways are both pH-dependent processes. We generated monensin-resistant viruses that displayed different entry routes and biophysical properties. Thanks to these mutants, we highlighted the importance of hypervariable region 1 (HVR1) of the E2 envelope protein for the association of particles with apolipoprotein E, which in turn might control the pH dependency of cell-free and cell-to-cell transmission.

Baboon envelope pseudotyped LVs outperform VSV-G-LVs for gene transfer into early-cytokine-stimulated and resting HSCs.

Hematopoietic stem cell (HSC)-based gene therapy holds promise for the cure of many diseases. The field is now moving toward the use of lentiviral vectors (LVs) as evidenced by 4 successful clinical trials. These trials used vesicular-stomatitis-virus-G protein (VSV-G)-LVs at high doses combined with strong cytokine-cocktail stimulation to obtain therapeutically relevant transduction levels; however, they might compromise the HSC character. Summarizing all these disadvantages, alternatives to VSV-G-LVs are urgently needed. We generated here high-titer LVs pseudotyped with a baboon retroviral envelope glycoprotein (BaEV-LVs), resistant to human complement. Under mild cytokine prestimulation to preserve the HSC characteristics, a single BaEV-LV application at a low dose, resulted in up to 90% of hCD34(+) cell transduction. Even more striking was that these new BaEV-LVs allowed, at low doses, efficient transduction of up to 30% of quiescent hCD34(+) cells, whereas high-dose VSV-G-LVs were insufficient. Importantly, reconstitution of NOD/Lt-SCID/γc(-/-) (NSG) mice with BaEV-LV-transduced hCD34(+) cells maintained these high transduction levels in all myeloid and lymphoid lineages, including early progenitors. This transduction pattern was confirmed or even increased in secondary NSG recipient mice. This suggests that BaEV-LVs efficiently transduce true HSCs and could improve HSC-based gene therapy, for which high-level HSC correction is needed for life-long cure.

A Lentiviral Vector Allowing Physiologically Regulated Membrane-anchored and Secreted Antibody Expression Depending on B-cell Maturation Status.

The development of lentiviral vectors (LVs) for expression of a specific antibody can be achieved through the transduction of mature B-cells. This approach would provide a versatile tool for active immunotherapy strategies for infectious diseases or cancer, as well as for protein engineering. Here, we created a lentiviral expression system mimicking the natural production of these two distinct immunoglobulin isoforms. We designed a LV (FAM2-LV) expressing an anti-HCV-E2 surface glycoprotein antibody (AR3A) as a membrane-anchored Ig form or a soluble Ig form, depending on the B-cell maturation status. FAM2-LV induced high-level and functional membrane expression of the transgenic antibody in a nonsecretory B-cell line. In contrast, a plasma cell (PC) line transduced with FAM2-LV preferentially produced the secreted transgenic antibody. Similar results were obtained with primary B-cells transduced ex vivo. Most importantly, FAM2-LV transduced primary B-cells efficiently differentiated into PCs, which secreted the neutralizing anti-HCV E2 antibody upon adoptive transfer into immunodeficient NSG (NOD/SCIDγc(-/-)) recipient mice. Altogether, these results demonstrate that the conditional FAM2-LV allows preferential expression of the membrane-anchored form of an antiviral neutralizing antibody in B-cells and permits secretion of a soluble antibody following B-cell maturation into PCs in vivo.

Critical interaction between E1 and E2 glycoproteins determines binding and fusion properties of hepatitis C virus during cell entry.

UNLABELLED: Hepatitis C virus (HCV) envelope glycoproteins E1 and E2 are important mediators for productive cell entry. However, knowledge about their structure, intra- or intermolecular dialogs, and conformational changes is scarce, limiting the design of therapeutic strategies targeting E1E2. Here we sought to investigate how certain domains of E1 and E2 have coevolved to optimize their interactions to promote efficient HCV entry. For this purpose we generated chimeric E1E2 heterodimers derived from two HCV 1a strains to identify and characterize crosstalk between their domains. We found an E1E2 combination that drastically impaired the infectivity of cell culture-derived HCV particles, whereas the reciprocal E1E2 combination led to increased infectivity. Using HCV pseudoparticle assays, we confirmed the opposing entry phenotypes of these heterodimers. By mutagenesis analysis, we identified a particular crosstalk between three amino acids of E1 and the domain III of E2. Its modulation leads to either a full restoration of the functionality of the suboptimal heterodimer or a destabilization of the functional heterodimer. Interestingly, we found that this crosstalk modulates E1E2 binding to HCV entry receptors SR-BI and CD81. In addition, we found for the first time that E1E2 complexes can interact with the first extracellular loop of Claudin-1, whereas soluble E2 did not. These results highlight the critical role of E1 in the modulation of HCV binding to receptors. Finally, we demonstrated that this crosstalk is involved in membrane fusion. CONCLUSIONS: These results reveal a multifunctional and crucial interaction between E1 and E2 for HCV entry into cells. Our study highlights the role of E1 as a modulator of HCV binding to receptors and membrane fusion, underlining its potential as an antiviral target.

The antimalarial ferroquine is an inhibitor of hepatitis C virus.

UNLABELLED: Hepatitis C virus (HCV) is a major cause of chronic liver disease. Despite recent success in improving anti-HCV therapy, additional progress is still needed to develop cheaper and interferon (IFN)-free treatments. Here, we report that ferroquine (FQ), an antimalarial ferrocenic analog of chloroquine, is a novel inhibitor of HCV. FQ potently inhibited HCV infection of hepatoma cell lines by affecting an early step of the viral life cycle. The antiviral activity of FQ on HCV entry was confirmed with pseudoparticles expressing HCV envelope glycoproteins E1 and E2 from six different genotypes. In addition to its effect on HCV entry, FQ also inhibited HCV RNA replication, albeit at a higher concentration. We also showed that FQ has no effect on viral assembly and virion secretion. Using a binding assay at 4°C, we showed that FQ does not prevent attachment of the virus to the cell surface. Furthermore, virus internalization was not affected by FQ, whereas the fusion process was impaired in the presence of FQ as shown in a cell-cell fusion assay. Finally, virus with resistance to FQ was selected by sequential passage in the presence of the drug, and resistance was shown to be conferred by a single mutation in E1 glycoprotein (S327A). By inhibiting cell-free virus transmission using a neutralizing antibody, we also showed that FQ inhibits HCV cell-to-cell spread between neighboring cells. Combinations of FQ with IFN, or an inhibitor of HCV NS3/4A protease, also resulted in additive to synergistic activity. CONCLUSION: FQ is a novel, interesting anti-HCV molecule that could be used in combination with other direct-acting antivirals.