The Envelope Proteins of the Bunyavirales

The Bunyavirales Order encompasses nine families of enveloped viruses containing a single-stranded negative-sense RNA genome divided into three segments. The small (S) and large (L) segments encode proteins participating in genome replication in the infected cell cytoplasm. The middle (M) segment encodes the viral glycoproteins Gn and Gc, which are derived from a precursor polyprotein by host cell proteases. Entry studies are available only for a few viruses in the Order, and in each case they were shown to enter cells via receptor-mediated endocytosis. The acidic endosomal pH triggers the fusion of the viral envelope with the membrane of an endosome. Structural studies on two members of this Order, the phleboviruses and the hantaviruses, have shown that the membrane fusion protein Gc displays a class II fusion protein fold and is homologous to its counterparts in flaviviruses and alphaviruses, which are positive-sense, single-stranded RNA viruses. We analyze here recent data on the structure and function of the structure of the phlebovirus Gc and hantavirus Gn and Gc glycoproteins, and extrapolate common features identified in the amino acid sequences to understand also the structure and function of their counterparts in other families of the Bunyavirales Order. Our analysis also identified clear structural homology between the hantavirus Gn and alphavirus E2 glycoproteins, which make a heterodimer with the corresponding fusion proteins Gc and E1, respectively, revealing that not only the fusion protein has been conserved across viral families.

Mechanisms of Filovirus Entry

Filovirus entry into cells is complex, perhaps as complex as any viral entry mechanism identified to date. However, over the past 10 years, the important events required for filoviruses to enter into the endosomal compartment and fuse with vesicular membranes have been elucidated (Fig. 1). Here, we highlight the important steps that are required for productive entry of filoviruses into mammalian cells.

Two Distinctive Phenotypes of AcMNPV Display Different Immune Abilities and Intracellular Destiny

The budded phenotype (BV) of the baculovirus AcMNPV has been demonstrated to have strong immunostimulatory properties that are relevant for the development of vaccines and antiviral therapies. Although the occluded phenotype (ODV) shares the main structural proteins and its genome with BV, it has been poorly studied in mammals. In this study, we assessed the capacity of ODV to induce immune responses in mice. In contrast to BVs, ODVs failed to promote the secretion of IFN-gamma, IL-6 and Il-12 and to induce antiviral activity against VSV in the short term. Furthermore, ODVs were unable to induce cellular immunity against a coadministered antigen 7 days after inoculation. By analyzing the interaction of ODVs with BMDCs, we observed that although ODVs entered the cells reaching late and acidic endosomes, they did not induce their maturation. Finally, we also analyzed if BVs and ODVs followed different routes in the cell during the infection. BVs, but not ODVs, colocalized with the protein ovalbumin in compartments with the presence of proteases. The results suggest that structural differences could be responsible for their different destinies in the dendritic cell and this could lead to a different impact on the immune response.

Constitutively Expressed IFITM3 Protein in Human Endothelial Cells Poses an Early Infection Block to Human Influenza Viruses

A role for pulmonary endothelial cells in the orchestration of cytokine production and leukocyte recruitment during influenza virus infection, leading to severe lung damage, has been recently identified. As the mechanistic pathway for this ability is not fully known, we extended previous studies on influenza virus tropism in cultured human pulmonary endothelial cells. We found that a subset of avian influenza viruses, including potentially pandemic H5N1, H7N9, and H9N2 viruses, could infect human pulmonary endothelial cells (HULEC) with high efficiency compared to human H1N1 or H3N2 viruses. In HULEC, human influenza viruses were capable of binding to host cellular receptors, becoming internalized and initiating hemifusion but failing to uncoat the viral nucleocapsid and to replicate in host nuclei. Unlike numerous cell types, including epithelial cells, we found that pulmonary endothelial cells constitutively express a high level of the restriction protein IFITM3 in endosomal compartments. IFITM3 knockdown by small interfering RNA (siRNA) could partially rescue H1N1 virus infection in HULEC, suggesting IFITM3 proteins were involved in blocking human influenza virus infection in endothelial cells. In contrast, selected avian influenza viruses were able to escape IFITM3 restriction in endothelial cells, possibly by fusing in early endosomes at higher pH or by other, unknown mechanisms. Collectively, our study demonstrates that the human pulmonary endothelium possesses intrinsic immunity to human influenza viruses, in part due to the constitutive expression of IFITM3 proteins. Notably, certain avian influenza viruses have evolved to escape this restriction, possibly contributing to virus-induced pneumonia and severe lung disease in humans.

IMPORTANCE

Avian influenza viruses, including H5N1 and H7N9, have been associated with severe respiratory disease and fatal outcomes in humans. Although acute respiratory distress syndrome (ARDS) and progressive pulmonary endothelial damage are known to be present during severe human infections, the role of pulmonary endothelial cells in the pathogenesis of avian influenza virus infections is largely unknown. By comparing human seasonal influenza strains to avian influenza viruses, we provide greater insight into the interaction of influenza virus with human pulmonary endothelial cells. We show that human influenza virus infection is blocked during the early stages of virus entry, which is likely due to the relatively high expression of the host antiviral factors IFITMs (interferon-induced transmembrane proteins) located in membrane-bound compartments inside cells. Overall, this study provides a mechanism by which human endothelial cells limit replication of human influenza virus strains, whereas avian influenza viruses overcome these restriction factors in this cell type.

Hepatitis C Virus Is Released via a Noncanonical Secretory Route

We analyzed hepatitis C virus (HCV) morphogenesis using viral genomes encoding a mCherry-tagged E1 glycoprotein. HCV-E1-mCherry polyprotein expression, intracellular localization, and replication kinetics were comparable to those of untagged HCV, and E1-mCherry-tagged viral particles were assembled and released into cell culture supernatants. Expression and localization of structural E1 and nonstructural NS5A followed a temporospatial pattern with a succinct decrease in the number of replication complexes and the appearance of E1-mCherry punctae. Interaction of the structural proteins E1, Core, and E2 increased at E1-mCherry punctae in a time-dependent manner, indicating that E1-mCherry punctae represent assembled or assembling virions. E1-mCherry did not colocalize with Golgi markers. Furthermore, the bulk of viral glycoproteins within released particles revealed an EndoH-sensitive glycosylation pattern, indicating an absence of viral glycoprotein processing by the Golgi apparatus. In contrast, HCV-E1-mCherry trafficked with Rab9-positive compartments and inhibition of endosomes specifically suppressed HCV release. Our data suggest that assembled HCV particles are released via a noncanonical secretory route involving the endosomal compartment.

IMPORTANCE

The goal of this study was to shed light on the poorly understood trafficking and release routes of hepatitis C virus (HCV). For this, we generated novel HCV genomes which resulted in the production of fluorescently labeled viral particles. We used live-cell microscopy and other imaging techniques to follow up on the temporal dynamics of virus particle formation and trafficking in HCV-expressing liver cells. While viral particles and viral structural protein were found in endosomal compartments, no overlap of Golgi structures could be observed. Furthermore, biochemical and inhibitor-based experiments support a HCV release route which is distinguishable from canonical Golgi-mediated secretion. Since viruses hijack cellular pathways to generate viral progeny, our results point toward the possible existence of a not-yet-described cellular secretion route.

[A novel assay for analysis of adenovirusmediated endosome lysis of T lymphocytes]

OBJECTIVE

To analyze adenovirus-mediated endosome lysis of T cells, we developed a novel approach based on pHrodo dextran (pH-sensitive fluorescent dye).

METHODS

After incubating Jurkat cells (T cell leukemia) with serotype 5 adenovirus (Ad5) and pHrodo dextran, we determined the optimal incubation time and concentration of pHrodo dextran. To assess viral lysis of the endosome, we monitored the ratio changes of mean fluorescence intensity in different time points by laser scanning confocal microscopy.

RESULTS

After incubating Jurkat cells with Ad5 and 80 μg/mL pHrodo dextran for 10 minutes, we observed the fluorescence intensity was significantly reduced at 30 minutes compared with that of endosomes at 0 minute. However, we found the mean fluorescence intensity was only slightly reduced by inhibiting V-ATPase with the bafilomycin A1 treatment.

CONCLUSION

The method based on pH-sensitive dye can be used to analyze the adenovirus-mediated endosome lysis of T cells.

A "Trojan horse" bispecific-antibody strategy for broad protection against ebolaviruses

There is an urgent need for monoclonal antibody (mAb) therapies that broadly protect against Ebola virus and other filoviruses. The conserved, essential interaction between the filovirus glycoprotein, GP, and its entry receptor Niemann-Pick C1 (NPC1) provides an attractive target for such mAbs but is shielded by multiple mechanisms, including physical sequestration in late endosomes. Here, we describe a bispecific-antibody strategy to target this interaction, in which mAbs specific for NPC1 or the GP receptor-binding site are coupled to a mAb against a conserved, surface-exposed GP epitope. Bispecific antibodies, but not parent mAbs, neutralized all known ebolaviruses by coopting viral particles themselves for endosomal delivery and conferred postexposure protection against multiple ebolaviruses in mice. Such "Trojan horse" bispecific antibodies have potential as broad antifilovirus immunotherapeutics.

Herpesvirus Entry into Host Cells Mediated by Endosomal Low pH

Herpesviral pathogenesis stems from infection of multiple cell types including the site of latency and cells that support lytic replication. Herpesviruses utilize distinct cellular pathways, including low pH endocytic pathways, to enter different pathophysiologically relevant target cells. This review details the impact of the mildly acidic milieu of endosomes on the entry of herpesviruses, with particular emphasis on herpes simplex virus 1 (HSV-1). Epithelial cells, the portal of primary HSV-1 infection, support entry via low pH endocytosis mechanisms. Mildly acidic pH triggers reversible conformational changes in the HSV-1 class III fusion protein glycoprotein B (gB). In vitro treatment of herpes simplex virions with a similar pH range inactivates infectivity, likely by prematurely activating the viral entry machinery in the absence of a target membrane. How a given herpesvirus mediates both low pH and pH-independent entry events is a key unresolved question.

Alphavirus Restriction by IFITM Proteins

Interferon inducible transmembrane proteins (IFITMs) are broad-spectrum antiviral factors. In cell culture the entry of many enveloped viruses, including orthomyxo-, flavi-, and filoviruses, is inhibited by IFITMs, though the mechanism(s) involved remain unclear and may vary between viruses. We demonstrate that Sindbis and Semliki Forest virus (SFV), which both use endocytosis and acid-induced membrane fusion in early endosomes to infect cells, are restricted by the early endosomal IFITM3. The late endosomal IFITM2 is less restrictive and the plasma membrane IFITM1 does not inhibit normal infection by either virus. IFITM3 inhibits release of the SFV capsid into the cytosol, without inhibiting binding, internalization, trafficking to endosomes or low pH-induced conformational changes in the envelope glycoprotein. Infection by SFV fusion at the cell surface was inhibited by IFITM1, but was equally inhibited by IFITM3. Furthermore, an IFITM3 mutant (Y20A) that is localized to the plasma membrane inhibited infection by cell surface fusion more potently than IFITM1. Together, these results indicate that IFITMs, in particular IFITM3, can restrict alphavirus infection by inhibiting viral fusion with cellular membranes. That IFITM3 can restrict SFV infection by fusion at the cell surface equivalently to IFITM1 suggests that IFITM3 has greater antiviral potency against SFV.

African Swine Fever Virus Undergoes Outer Envelope Disruption, Capsid Disassembly and Inner Envelope Fusion before Core Release from Multivesicular Endosomes

African swine fever virus (ASFV) is a nucleocytoplasmic large DNA virus (NCLDV) that causes a highly lethal disease in domestic pigs. As other NCLDVs, the extracellular form of ASFV possesses a multilayered structure consisting of a genome-containing nucleoid successively wrapped by a thick protein core shell, an inner lipid membrane, an icosahedral protein capsid and an outer lipid envelope. This structural complexity suggests an intricate mechanism of internalization in order to deliver the virus genome into the cytoplasm. By using flow cytometry in combination with pharmacological entry inhibitors, as well as fluorescence and electron microscopy approaches, we have dissected the entry and uncoating pathway used by ASFV to infect the macrophage, its natural host cell. We found that purified extracellular ASFV is internalized by both constitutive macropinocytosis and clathrin-mediated endocytosis. Once inside the cell, ASFV particles move from early endosomes or macropinosomes to late, multivesicular endosomes where they become uncoated. Virus uncoating requires acidic pH and involves the disruption of the outer membrane as well as of the protein capsid. As a consequence, the inner viral membrane becomes exposed and fuses with the limiting endosomal membrane to release the viral core into the cytosol. Interestingly, virus fusion is dependent on virus protein pE248R, a transmembrane polypeptide of the inner envelope that shares sequence similarity with some members of the poxviral entry/fusion complex. Collective evidence supports an entry model for ASFV that might also explain the uncoating of other multienveloped icosahedral NCLDVs.

Virus entry is a crucial initial event for productive infection, being therefore a potential target for antiviral strategies. African swine fever virus (ASFV) is the causative agent of a frequently fatal swine disease for which there is no vaccine. ASFV belongs to the superfamily of nucleocytoplasmic large DNA viruses (NCLDV), which are among the most complex viruses known. ASFV genome locates at a core structure that is wrapped by two lipid membranes separated by an icosahedral protein capsid. Here we have dissected the internalization process of ASFV into host macrophages. Our results indicate that ASFV uses two alternative endocytic mechanisms, clathrin-mediated endocytosis and macropinocytosis, an ongoing process in macrophages. Once internalized, ASFV particles move to multivesicular endosomes, where they undergo a disassembly process leading to the loss of the two outermost layers. This exposes the inner viral envelope, which fuses to the limiting endosome membrane to deliver the viral core into the cytosol. ASFV penetration depends on acidic pH and on the inner envelope viral protein pE248R. Our findings point to an internalization model that could also explain the uncoating of other icosahedral enveloped NCLDVs. Also, they provide new cellular and viral targets for the development of antiviral strategies against ASFV.

Recycling Endosomes and Viral Infection

Many viruses exploit specific arms of the endomembrane system. The unique composition of each arm prompts the development of remarkably specific interactions between viruses and sub-organelles. This review focuses on the viral–host interactions occurring on the endocytic recycling compartment (ERC), and mediated by its regulatory Ras-related in brain (Rab) GTPase Rab11. This protein regulates trafficking from the ERC and the trans-Golgi network to the plasma membrane. Such transport comprises intricate networks of proteins/lipids operating sequentially from the membrane of origin up to the cell surface. Rab11 is also emerging as a critical factor in an increasing number of infections by major animal viruses, including pathogens that provoke human disease. Understanding the interplay between the ERC and viruses is a milestone in human health. Rab11 has been associated with several steps of the viral lifecycles by unclear processes that use sophisticated diversified host machinery. For this reason, we first explore the state-of-the-art on processes regulating membrane composition and trafficking. Subsequently, this review outlines viral interactions with the ERC, highlighting current knowledge on viral-host binding partners. Finally, using examples from the few mechanistic studies available we emphasize how ERC functions are adjusted during infection to remodel cytoskeleton dynamics, innate immunity and membrane composition.

A novel peptide with potent and broad-spectrum antiviral activities against multiple respiratory viruses

A safe, potent and broad-spectrum antiviral is urgently needed to combat emerging respiratory viruses. In light of the broad antiviral activity of β-defensins, we tested the antiviral activity of 11 peptides derived from mouse β-defensin-4 and found that a short peptide, P9, exhibited potent and broad-spectrum antiviral effects against multiple respiratory viruses in vitro and in vivo, including influenza A virus H1N1, H3N2, H5N1, H7N7, H7N9, SARS-CoV and MERS-CoV. The antiviral activity of P9 was attributed to its high-affinity binding to viral glycoproteins, as well as the abundance of basic amino acids in its composition. After binding viral particles through viral surface glycoproteins, P9 entered into cells together with the viruses via endocytosis and prevented endosomal acidification, which blocked membrane fusion and subsequent viral RNA release. This study has paved the avenue for developing new prophylactic and therapeutic agents with broad-spectrum antiviral activities.

Visualization of Content Release from Cell Surface-Attached Single HIV-1 Particles Carrying an Extra-Viral Fluorescent pH-Sensor

HIV-1 fusion leading to productive entry has long been thought to occur at the plasma membrane. However, our previous single virus imaging data imply that, after Env engagement of CD4 and coreceptors at the cell surface, the virus enters into and fuses with intracellular compartments. We were unable to reliably detect viral fusion at the plasma membrane. Here, we implement a novel virus labeling strategy that biases towards detection of virus fusion that occurs in a pH-neutral environment-at the plasma membrane or, possibly, in early pH-neutral vesicles. Virus particles are co-labeled with an intra-viral content marker, which is released upon fusion, and an extra-viral pH sensor consisting of ecliptic pHluorin fused to the transmembrane domain of ICAM-1. This sensor fully quenches upon virus trafficking to a mildly acidic compartment, thus precluding subsequent detection of viral content release. As an interesting secondary observation, the incorporation of the pH-sensor revealed that HIV-1 particles occasionally shuttle between neutral and acidic compartments in target cells expressing CD4, suggesting a small fraction of viral particles is recycled to the plasma membrane and re-internalized. By imaging viruses bound to living cells, we found that HIV-1 content release in neutral-pH environment was a rare event (~0.4% particles). Surprisingly, viral content release was not significantly reduced by fusion inhibitors, implying that content release was due to spontaneous formation of viral membrane defects occurring at the cell surface. We did not measure a significant occurrence of HIV-1 fusion at neutral pH above this defect-mediated background loss of content, suggesting that the pH sensor may destabilize the membrane of the HIV-1 pseudovirus and, thus, preclude reliable detection of single virus fusion events at neutral pH.

Infectious Entry Pathway of Enterovirus B Species

Enterovirus B species (EV-B) are responsible for a vast number of mild and serious acute infections. They are also suspected of remaining in the body, where they cause persistent infections contributing to chronic diseases such as type I diabetes. Recent studies of the infectious entry pathway of these viruses revealed remarkable similarities, including non-clathrin entry of large endosomes originating from the plasma membrane invaginations. Many cellular factors regulating the efficient entry have recently been associated with macropinocytic uptake, such as Rac1, serine/threonine p21-activated kinase (Pak1), actin, Na/H exchanger, phospholipace C (PLC) and protein kinase Cα (PKCα). Another characteristic feature is the entry of these viruses to neutral endosomes, independence of endosomal acidification and low association with acidic lysosomes. The biogenesis of neutral multivesicular bodies is crucial for their infection, at least for echovirus 1 (E1) and coxsackievirus A9 (CVA9). These pathways are triggered by the virus binding to their receptors on the plasma membrane, and they are not efficiently recycled like other cellular pathways used by circulating receptors. Therefore, the best “markers” of these pathways may be the viruses and often their receptors. A deeper understanding of this pathway and associated endosomes is crucial in elucidating the mechanisms of enterovirus uncoating and genome release from the endosomes to start efficient replication.

Follicular Dendritic Cells Retain Infectious HIV in Cycling Endosomes

Despite the success of antiretroviral therapy (ART), it does not cure Human Immunodeficiency Virus (HIV) and discontinuation results in viral rebound. Follicular dendritic cells (FDC) are in direct contact with CD4+ T cells and they retain intact antigen for prolonged periods. We found that human FDC isolated from patients on ART retain infectious HIV within a non-degradative cycling compartment and transmit infectious virus to uninfected CD4 T cells in vitro. Importantly, treatment of the HIV+ FDC with a soluble complement receptor 2 purges the FDC of HIV virions and prevents viral transmission in vitro. Our results provide an explanation for how FDC can retain infectious HIV for extended periods and suggest a therapeutic strategy to achieve cure in HIV-infected humans.

Human immunodeficiency virus (HIV) can lead to acquired immunodeficiency syndrome, or AIDS. Before the introduction of anti retroviral therapy (ART) in the mid-1990s, people with HIV could progress to AIDS in just a few years. Today patients with HIV have a close to normal life expectancy. Worldwide, there are about 2 million new cases of HIV per year. Currently about 35 million people are living with HIV of which around 13 million receive ART. Still an estimated 1.5 million people die from the consequences of HIV each year. Despite the success of ART, it does not cure HIV and discontinuation results in viral rebound. Follicular dendritic cells (FDC), located central to the B cell follicle, are also in direct contact with T cells. FDCs retain intact antigen for prolonged periods. We found that human FDCs isolated from patients on ART retain infectious HIV and can transmit virus to uninfected T cells in vitro. Treatment of the HIV+ FDC with a soluble complement receptor 2 purges the FDC of HIV virions and prevents viral transmission to T cells in vitro. Our results can explain how FDCs retain infectious HIV and suggest a therapeutic strategy to come closer to a cure.

Human beta-defensins 2 and -3 cointernalize with human immunodeficiency virus via heparan sulfate proteoglycans and reduce infectivity of intracellular virions in tonsil epithelial cells

We previously showed that expression of the anti-HIV innate proteins human beta-defensin 2 (hBD2) and hBD3 in adult oral epithelial cells reduces HIV transepithelial transmission by inactivation of virus. However, fetal/infant oral epithelia lack beta-defensin expression, leading to transmission of HIV. The mechanisms of hBD2- and hBD3-mediated HIV inactivation in adult oral epithelial cells are poorly understood. Here we found that heparan sulfate proteoglycans (HSPGs) on the apical surfaces of epithelial cells facilitate simultaneous binding of hBDs and HIV gp120 to the cell surface. HSPG-facilitated binding of hBDs and HIV gp120 to the cell surface did not affect viral attachment. HBD2 or -3 cointernalized with virions in endosomes, formed oligomers, and reduced infectivity of HIV. The anti-HIV effect of combining hBD2 and hBD3 was substantially higher than that of the individual peptides. These findings advance our understanding of the mechanisms of anti-HIV resistance in adult oral epithelium.

Emerging intracellular receptors for hemorrhagic fever viruses

Ebola virus and Lassa virus belong to different virus families that can cause viral hemorrhagic fever, a life-threatening disease in humans with limited treatment options. To infect a target cell, Ebola and Lassa viruses engage receptors at the cell surface and are subsequently shuttled into the endosomal compartment. Upon arrival in late endosomes/lysosomes, the viruses trigger membrane fusion to release their genome into the cytoplasm. Although contact sites at the cell surface were recognized for Ebola virus and Lassa virus, it was postulated that Ebola virus requires a critical receptor inside the cell. Recent screens for host factors identified such internal receptors for both viruses: Niemann-Pick disease type C1 protein (NPC1) for Ebola virus and lysosome-associated membrane protein 1 (LAMP1) for Lassa virus. A cellular trigger is needed to permit binding of the viral envelope protein to these intracellular receptors. This 'receptor switch' represents a previously unnoticed step in virus entry with implications for host-pathogen interactions and viral tropism.

Rab27a controls HIV-1 assembly by regulating plasma membrane levels of phosphatidylinositol 4,5-bisphosphate

During the late stages of the HIV-1 replication cycle, the viral polyprotein Pr55(Gag) is recruited to the plasma membrane (PM), where it binds phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and directs HIV-1 assembly. We show that Rab27a controls the trafficking of late endosomes carrying phosphatidylinositol 4-kinase type 2 α (PI4KIIα) toward the PM of CD4(+) T cells. Hence, Rab27a promotes high levels of PM phosphatidylinositol 4-phosphate and the localized production of PI(4,5)P2, therefore controlling Pr55(Gag) membrane association. Rab27a also controls PI(4,5)P2 levels at the virus-containing compartments of macrophages. By screening Rab27a effectors, we identified that Slp2a, Slp3, and Slac2b are required for the association of Pr55(Gag) with the PM and that Slp2a cooperates with Rab27a in the recruitment of PI4KIIα to the PM. We conclude that by directing the trafficking of PI4KIIα-positive endosomes toward the PM, Rab27a controls PI(4,5)P2 production and, consequently, HIV-1 replication.

Coronavirus cell entry occurs through the endo-/lysosomal pathway in a proteolysis-dependent manner

Enveloped viruses need to fuse with a host cell membrane in order to deliver their genome into the host cell. While some viruses fuse with the plasma membrane, many viruses are endocytosed prior to fusion. Specific cues in the endosomal microenvironment induce conformational changes in the viral fusion proteins leading to viral and host membrane fusion. In the present study we investigated the entry of coronaviruses (CoVs). Using siRNA gene silencing, we found that proteins known to be important for late endosomal maturation and endosome-lysosome fusion profoundly promote infection of cells with mouse hepatitis coronavirus (MHV). Using recombinant MHVs expressing reporter genes as well as a novel, replication-independent fusion assay we confirmed the importance of clathrin-mediated endocytosis and demonstrated that trafficking of MHV to lysosomes is required for fusion and productive entry to occur. Nevertheless, MHV was shown to be less sensitive to perturbation of endosomal pH than vesicular stomatitis virus and influenza A virus, which fuse in early and late endosomes, respectively. Our results indicate that entry of MHV depends on proteolytic processing of its fusion protein S by lysosomal proteases. Fusion of MHV was severely inhibited by a pan-lysosomal protease inhibitor, while trafficking of MHV to lysosomes and processing by lysosomal proteases was no longer required when a furin cleavage site was introduced in the S protein immediately upstream of the fusion peptide. Also entry of feline CoV was shown to depend on trafficking to lysosomes and processing by lysosomal proteases. In contrast, MERS-CoV, which contains a minimal furin cleavage site just upstream of the fusion peptide, was negatively affected by inhibition of furin, but not of lysosomal proteases. We conclude that a proteolytic cleavage site in the CoV S protein directly upstream of the fusion peptide is an essential determinant of the intracellular site of fusion.

Enveloped viruses need to fuse with a host cell membrane in order to deliver their genome into the host cell. In the present study we investigated the entry of coronaviruses (CoVs). CoVs are important pathogens of animals and man with high zoonotic potential as demonstrated by the emergence of SARS- and MERS-CoVs. Previous studies resulted in apparently conflicting results with respect to CoV cell entry, particularly regarding the fusion-activating requirements of the CoV S protein. By combining cell-biological, infection, and fusion assays we demonstrated that murine hepatitis virus (MHV), a prototypic member of the CoV family, enters cells via clathrin-mediated endocytosis. Moreover, although MHV does not depend on a low pH for fusion, the virus was shown to rely on trafficking to lysosomes for proteolytic cleavage of its spike (S) protein and membrane fusion to occur. Based on these results we predicted and subsequently demonstrated that MERS- and feline CoV require cleavage by different proteases and escape the endo/lysosomal system from different compartments. In conclusion, we elucidated the MHV entry pathway in detail and demonstrate that a proteolytic cleavage site in the S protein of different CoVs is an essential determinant of the intracellular site of fusion.

Role of Autophagy in Innate Viral Recognition

Plasmacytoid dendritic cells (pDCs) detect viruses in the acidified endosomes via Toll-like receptors (TLRs) upon endocytosis of virions. Yet, pDC responses to certain single-stranded RNA viruses occur only following live viral infection. In our recent study, we presented evidence that the recognition of such viruses by TLR7 requires autophagy. We speculate that the requirement for autophagy in viral recognition reflects the necessity for transportation of cytosolic viral replication intermediates into the lysosome where TLR7 is activated. In addition, autophagy was found to be required for pDCs to produce type I interferon (IFN) in response to both ssRNA and dsDNA viruses. These results indicated that autophagy plays a key role in mediating virus detection and IFNalpha secretion in pDCs, and suggest that cytosolic replication intermediates of ssRNA viruses serve as pathogen signatures recognized by TLR7.

Late endosomal trafficking of alternative serotype adenovirus vaccine vectors augments antiviral innate immunity

Adenovirus (Ad) vaccine vectors have found widespread use as vaccine platforms against multiple infections and cancers, and multiple serotypes have been shown to differ significantly in their biological properties and immune phenotypes. Our laboratory and others have previously described differential innate immune stimulation elicited by various Ad serotypes. Here, we show that Ad serotype 5 (Ad5) traffics rapidly to the nucleus following infection, whereas Ad35 and Ad26 accumulate in late endosomes 2 to 8 h postinfection. Innate immune cytokine elicitation by all Ad serotypes was abrogated by blockade of endosomal acidification, cathepsin B, and caspase 1, suggesting that virus interactions with acid-dependent sensors, such as Toll-like receptor- and cathepsin-dependent inflammasome activation in late endosomes, may trigger innate immunity. These data suggest a mechanism by which Ad vectors from various serotypes differentially trigger innate antiviral pathways via distinct intracellular trafficking to late endosomes.

IMPORTANCE

Adenoviruses (Ad) are widely used for vaccination and gene therapy applications. Importantly, Ad vectors have been shown to differ significantly in their innate immune profiles both in vivo and in vitro. The molecular mechanism that underlies these observed differences has important implications for the development of improved vaccines. In this study, we propose a mechanism in which the degree of late endosomal trafficking of Ad vectors results in differential stimulation of late endosomal pattern recognition receptors.

Genome-wide small interfering RNA screens reveal VAMP3 as a novel host factor required for Uukuniemi virus late penetration

The Bunyaviridae constitute a large family of enveloped animal viruses, many of which are important emerging pathogens. How bunyaviruses enter and infect mammalian cells remains largely uncharacterized. We used two genome-wide silencing screens with distinct small interfering RNA (siRNA) libraries to investigate host proteins required during infection of human cells by the bunyavirus Uukuniemi virus (UUKV), a late-penetrating virus. Sequence analysis of the libraries revealed that many siRNAs in the screens inhibited infection by silencing not only the intended targets but additional genes in a microRNA (miRNA)-like manner. That the 7-nucleotide seed regions in the siRNAs can cause a perturbation in infection was confirmed by using synthetic miRNAs (miRs). One of the miRs tested, miR-142-3p, was shown to interfere with the intracellular trafficking of incoming viruses by regulating the v-SNARE VAMP3, a strong hit shared by both siRNA screens. Inactivation of VAMP3 by the tetanus toxin led to a block in infection. Using fluorescence-based techniques in fixed and live cells, we found that the viruses enter VAMP3(+) endosomal vesicles 5 min after internalization and that colocalization was maximal 15 min thereafter. At this time, LAMP1 was associated with the VAMP3(+) virus-containing endosomes. In cells depleted of VAMP3, viruses were mainly trapped in LAMP1-negative compartments. Together, our results indicated that UUKV relies on VAMP3 for penetration, providing an indication of added complexity in the trafficking of viruses through the endocytic network.

IMPORTANCE

Bunyaviruses represent a growing threat to humans and livestock globally. Unfortunately, relatively little is known about these emerging pathogens. We report here the first human genome-wide siRNA screens for a bunyavirus. The screens resulted in the identification of 562 host cell factors with a potential role in cell entry and virus replication. To demonstrate the robustness of our approach, we confirmed and analyzed the role of the v-SNARE VAMP3 in Uukuniemi virus entry and infection. The information gained lays the basis for future research into the cell biology of bunyavirus infection and new antiviral strategies. In addition, by shedding light on serious caveats in large-scale siRNA screening, our experimental and bioinformatics procedures will be valuable in the comprehensive analysis of past and future high-content screening data.

Superior in vitro stimulation of human CD8+ T-cells by whole virus versus split virus influenza vaccines

Pandemic and seasonal influenza viruses cause considerable morbidity and mortality in the general human population. Protection from severe disease may result from vaccines that activate antigen-presenting DC for effective stimulation of influenza-specific memory T cells. Special attention is paid to vaccine-induced CD8+ T-cell responses, because they are mainly directed against conserved internal influenza proteins thereby presumably mediating cross-protection against circulating seasonal as well as emerging pandemic virus strains. Our study showed that influenza whole virus vaccines of major seasonal A and B strains activated DC more efficiently than those of pandemic swine-origin H1N1 and pandemic-like avian H5N1 strains. In contrast, influenza split virus vaccines had a low ability to activate DC, regardless which strain was investigated. We also observed that whole virus vaccines stimulated virus-specific CD8+ memory T cells much stronger compared to split virus counterparts, whereas both vaccine formats activated CD4+ Th cell responses similarly. Moreover, our data showed that whole virus vaccine material is delivered into the cytosolic pathway of DC for effective activation of virus-specific CD8+ T cells. We conclude that vaccines against seasonal and pandemic (-like) influenza strains that aim to stimulate cross-reacting CD8+ T cells should include whole virus rather than split virus formulations.

Coat as a dagger: the use of capsid proteins to perforate membranes during non-enveloped DNA viruses trafficking

To get access to the replication site, small non-enveloped DNA viruses have to cross the cell membrane using a limited number of capsid proteins, which also protect the viral genome in the extracellular environment. Most of DNA viruses have to reach the nucleus to replicate. The capsid proteins involved in transmembrane penetration are exposed or released during endosomal trafficking of the virus. Subsequently, the conserved domains of capsid proteins interact with cellular membranes and ensure their efficient permeabilization. This review summarizes our current knowledge concerning the role of capsid proteins of small non-enveloped DNA viruses in intracellular membrane perturbation in the early stages of infection.