Steric shielding of surface epitopes and impaired immune recognition induced by the ebola virus glycoprotein

Immunological insights into the re-emergence of human metapneumovirus

Human metapneumovirus (hMPV) is a seasonal respiratory virus that typically causes mild, flu-like symptoms. In some cases, it can lead to severe respiratory complications, such as pneumonia, bronchitis, and bronchiolitis, often requiring hospitalization. Recently, a surge in hMPV cases has been reported in China and other countries, raising concerns about a potential pandemic scenario reminiscent of COVID-19. This review explores the genomic structure, replication cycle, genetic diversity, and evolutionary trajectory of hMPV. It also discusses host immune responses and the available animal models to study pathogenesis and to screen for potential vaccines and antivirals. Additionally, we examine the shifting seasonal trends in hMPV circulation, evaluate the low pandemic risk posed by existing hMPV clades, and underscore the need for continued vaccine and antiviral development. Finally, we advocate for strengthened global surveillance, especially in low- and middle-income countries, as a critical strategy to mitigate the risks posed by emerging hMPV clades.

Antiviral defense against filovirus infections: targets and evasion mechanisms

Filoviruses include a number of serious human pathogens, infections with which result in the development of hemorrhagic fevers with high case fatality rates. As for other RNA viruses, viral replication generates both protein and RNA species that can serve as danger signals, leading to the activation of antiviral defense pathways. However, in order to be able to efficiently infect humans these viruses have developed mechanisms that allow them to evade diverse host antiviral defense mechanisms. Consequently, in addition to their functions within the viral lifecycle many filovirus proteins have been shown to have accessory functions involved in the regulation of diverse host pathways. These include those of the type-I interferon response, other pathways involved in dsRNA-sensing, as well as the selective inhibition of interferon stimulated gene activities. Further, filoviruses have developed mechanisms to subvert recognition of infected cells and the generation of neutralizing antibodies. This review focuses on bringing together the evidence to date supporting the existence of diverse mechanisms aimed at regulating these pathways as well as providing details of the mechanisms involved.

Mapping Immunological, Host Receptor Binding Determinants, and Cathepsin Cleavage Site of EBOV Glycoprotein Utilizing the Qubevirus Platform

Ebola virus (EBOV) remains a highly infectious human pathogen that causes a severe and lethal disease known as Ebola virus disease (EVD), despite recent progress in vaccine development based on its only surface glycoprotein (GP). In this study, we modeled and inserted four overlapping fragments (F1-4) of the EBOV GP at the C-terminus of the A1 protein of Qubevirus (Qβ) and used the platform to investigate the tropism and immunological functions of the GP by displaying the peptides with 30 overlapping amino acids. The resulting recombinant phages were used to determine their reactivity with GP-specific antibodies and their binding to the recombinant Niemann-Pick C1 (rNPC1) receptor in an immunoassay. In addition, modified, truncated, and C-terminus-tagged fragment F1 named F5 was utilized to map the cathepsin cleavage sites in an enzymatic assay. We demonstrated that a large GP peptide of 200 AA could be fused to A1 and exposed on the Qβ platform in an accessible manner without significantly affecting its viability and infectivity. Fragments F1 (GP1-200), F2 (GP170-370), and F3 (GP350-550) were shown to contain important immune epitopes through binding to anti-GP-specific antibodies. Further, F1 was found to bind rNPC1, thereby suggesting a receptor binding determinant of the GP that was further confirmed in a competitive assay where the recombinant phages bearing the F1 fragment reduced the infectivity of EBOV pseudovirus by 27%. In addition, the viral infectivity was shown to be reduced by 46.39% by a cyclic peptide selected from an RNA Qβ library. Finally, F5 showed the cleavage sites to be AA191-192 and AA194-195 for CatB and L, respectively, which were further validated using a recombinant EBOV glycoprotein. These results provide insights into the antigenicity and tropism characteristic of the glycoprotein, with implications for the development of subunit vaccines or other biologics against Ebola virus disease.

Molecular virulence determinants of human-pathogenic filoviruses

The Filoviridae family encompasses Ebola virus (EBOV) and Marburg virus (MARV), some of the most lethal viruses known to cause sporadic, recurring outbreaks of severe hemorrhagic fever mainly throughout central Africa. However, other lesser-known viruses also belong to the filovirus family as they are closely related, such as Bundibugyo, Reston and Taï Forest virus. These viruses differ in their virulence in humans significantly: while EBOV and MARV show lethality in humans of up to 90 %, Reston virus appears to be avirulent in humans. Here, underlying molecular factors leading to differences in virulence via changes in filovirus entry, replication and immune evasion strategies are summarized and assessed. While the filovirus glycoprotein contributes towards virulence by facilitating entry into a wide variety of tissues, differences in virus-host interactions and replication efficacies lead to measurable variances of progeny virus production. Additionally, immune evasion strategies lead to alterations in replication efficacy thus changing who has the upper hand between the virus and the host. Understanding and unraveling the contributions of these molecular determinants on filovirus virulence provide insights into the processes causing the underlying pathogenesis. It will further help to assess the pathogenicity of newly discovered filoviruses. Finally, these molecular determinants and processes present attractive targets for therapeutic intervention and development of novel antiviral countermeasures.

Rational design of next-generation filovirus vaccines with glycoprotein stabilization, nanoparticle display, and glycan modification

Filoviruses pose a significant threat to human health with frequent outbreaks and high mortality. Although two vector-based vaccines are available for Ebola virus, a broadly protective filovirus vaccine remains elusive. In this study, we evaluate a general strategy for stabilizing glycoprotein (GP) structures of Ebola, Sudan, and Bundibugyo ebolaviruses and Ravn marburgvirus. A 3.2 Å-resolution crystal structure provides atomic details for the redesigned Ebola virus GP, and cryo-electron microscopy reveals how a pan-ebolavirus neutralizing antibody targets a conserved site on the Sudan virus GP (3.13 Å-resolution), in addition to a low-resolution model of antibody-bound Ravn virus GP. A self-assembling protein nanoparticle (SApNP), I3-01v9, is redesigned at the N-terminus to allow the optimal surface display of filovirus GP trimers. Following detailed in vitro characterization, the lymph node dynamics of Sudan virus GP and GP-presenting SApNPs are investigated in a mouse model. Compared with soluble GP trimer, SApNPs show ~112 times longer retention in lymph node follicles, up-to-28 times greater presentation on follicular dendritic cell dendrites, and up-to-3 times stronger germinal center reactions. Functional antibody responses induced by filovirus GP trimers and SApNPs bearing wildtype and modified glycans are assessed in mice. Our study provides a foundation for next-generation filovirus vaccine development.

Role of surface glycans in enveloped RNA virus infections: A structural perspective

Enveloped RNA viruses have been causative agents of major pandemic outbreaks in the recent past. Glycans present on these virus surface proteins are critical for multiple processes during the viral infection cycle. Presence of glycans serves as a key determinant of immunogenicity, but intrinsic heterogeneity, dynamics, and evolutionary shifting of glycans in heavily glycosylated enveloped viruses confounds typical structure-function analysis. Glycosylation sites are also conserved across different viral families, which further emphasizes their functional significance. In this review, we summarize findings regarding structure-function correlation of glycans on enveloped RNA virus proteins.

Afucosylated anti-EBOV antibody MIL77-3 engages sGP to elicit NK cytotoxicity

MIL77-3 is one component of antibody cocktail that is produced in our lab and represents an effective regimen for animals suffering from Zaire Ebolavirus (EBOV) infection. MIL77-3 is engineered to increase its affinity for the FcγRIIIa (CD16a) by deleting the fucose in the framework region. The potential effects of this modification on host immune responses, however, remain largely unknown. Herein, we demonstrated that MIL77-3 recognized secreted glycoproptein (sGP), produced by EBOV, and formed the immunocomplex to potently augment antibody-dependent cytotoxicity of human peripheral blood-derived natural killer cells (pNKs), including CD56dim and CD56bright subpopulations, in contrast to the counterparts (Mab114, rEBOV548, fucosylated MIL77-3). Intriguingly, this effect was not observed when NK92-CD16a cell line was utilized and restored by the addition of beads-coupled or membrane-anchored sGP in combination with MIL77-3. Furthermore, sGP bound to unrecognized receptors on T cells contaminated in pNKs rather than NK92-CD16a cells. Administration of beads-coupled sGP/MIL77-3 complex in mice elicited NK activation. Overall, this work reveals an immune-stimulating function of sGP/MIL77-3 complex by triggering cytotoxic activity of NK cells, highlighting the necessity to evaluate the potential impact of MIL77-3 on host immune reaction in clinical trials.

IMPORTANCE

Zaire Ebolavirus (EBOV) is highly lethal and causes sporadic outbreaks. The passive administration of monoclonal antibodies (mAbs) represents a promising treatment regimen against EBOV. Mounting evidence has shown that the efficacy of a subset of therapeutic mAbs in vivo is intimately associated with its capacity to trigger NK activity, supporting glycomodification of Fc region of anti-EBOV mAbs as a putative strategy to enhance Fc-mediated immune effector function as well as protection in vivo. Our work here uncovers the potential harmful influence of this modification on host immune responses, especially for mAbs with cross-reactivity to secreted glycoproptein (sGP) (e.g., MIL77-3), and highlights it is necessary to evaluate the NK-stimulating activity of a fucosylated mAb engaged with sGP when a new candidate is developed.

A toxicology study of Csf2ra complementation and pulmonary macrophage transplantation therapy of hereditary PAP in mice

Pulmonary macrophage transplantation (PMT) is a gene and cell transplantation approach in development as therapy for hereditary pulmonary alveolar proteinosis (hPAP), a surfactant accumulation disorder caused by mutations in CSF2RA/B (and murine homologs). We conducted a toxicology study of PMT of Csf2ra gene-corrected macrophages (mGM-Rα+Mϕs) or saline-control intervention in Csf2raKO or wild-type (WT) mice including single ascending dose and repeat ascending dose studies evaluating safety, tolerability, pharmacokinetics, and pharmacodynamics. Lentiviral-mediated Csf2ra cDNA transfer restored GM-CSF signaling in mGM-Rα+Mϕs. Following PMT, mGM-Rα+Mϕs engrafted, remained within the lungs, and did not undergo uncontrolled proliferation or result in bronchospasm, pulmonary function abnormalities, pulmonary or systemic inflammation, anti-transgene product antibodies, or pulmonary fibrosis. Aggressive male fighting caused a similarly low rate of serious adverse events in saline- and PMT-treated mice. Transient, minor pulmonary neutrophilia and exacerbation of pre-existing hPAP-related lymphocytosis were observed 14 days after PMT of the safety margin dose but not the target dose (5,000,000 or 500,000 mGM-Rα+Mϕs, respectively) and only in Csf2raKO mice but not in WT mice. PMT reduced lung disease severity in Csf2raKO mice. Results indicate PMT of mGM-Rα+Mϕs was safe, well tolerated, and therapeutically efficacious in Csf2raKO mice, and established a no adverse effect level and 10-fold safety margin.

A replicating RNA vaccine confers protection in a rhesus macaque model of Crimean-Congo hemorrhagic fever

Crimean-Congo hemorrhagic fever (CCHF) is a tick-borne febrile illness with a wide geographic distribution. In recent years the geographic range of Crimean-Congo hemorrhagic fever virus (CCHFV) and its tick vector have increased, placing an increasing number of people at risk of CCHFV infection. Currently, there are no widely available vaccines, and although the World Health Organization recommends ribavirin for treatment, its efficacy is unclear. Here we evaluate a promising replicating RNA vaccine in a rhesus macaque (Macaca mulatta) model of CCHF. This model provides an alternative to the established cynomolgus macaque model and recapitulates mild-to-moderate human disease. Rhesus macaques infected with CCHFV consistently exhibit viremia, detectable viral RNA in a multitude of tissues, and moderate pathology in the liver and spleen. We used this model to evaluate the immunogenicity and protective efficacy of a replicating RNA vaccine. Rhesus macaques vaccinated with RNAs expressing the CCHFV nucleoprotein and glycoprotein precursor developed robust non-neutralizing humoral immunity against the CCHFV nucleoprotein and had significant protection against the CCHFV challenge. Together, our data report a model of CCHF using rhesus macaques and demonstrate that our replicating RNA vaccine is immunogenic and protective in non-human primates after a prime-boost immunization.

A rabies virus-vectored vaccine expressing two copies of the Marburg virus glycoprotein gene induced neutralizing antibodies against Marburg virus in humanized mice

Marburg virus disease (MVD) is a lethal viral haemorrhagic fever caused by Marburg virus (MARV) with a case fatality rate as high as 88%. There is currently no vaccine or antiviral therapy approved for MVD. Due to high variation among MARV isolates, vaccines developed against one strain fail to protect against other strains. Here we report that three recombinant rabies virus (RABV) vector vaccines encoding two copies of GPs covering both MARV lineages induced pseudovirus neutralizing antibodies in BALB/c mice. Furthermore, high-affinity human neutralizing antibodies were isolated from a humanized mouse model. The three vaccines produced a Th1-biased serological response similar to that of human patients. Adequate sequential immunization enhanced the production of neutralizing antibodies. Virtual docking suggested that neutralizing antibodies induced by the Angola strain seemed to be able to hydrogen bond to the receptor-binding site (RBS) in the GP of the Ravn strain through hypervariable regions 2 (CDR2) and CDR3 of the VH region. These findings demonstrate that three inactivated vaccines are promising candidates against different strains of MARV, and a novel fully humanized neutralizing antibody against MARV was isolated.

The Mucin-Like Domain of the Ebola Glycoprotein Does Not Impact Virulence or Pathogenicity in Ferrets

BACKGROUND

Ebola virus (EBOV) is considered among the most dangerous viruses with case fatality rates approaching 90% depending on the outbreak. While several viral proteins (VPs) including VP24, VP35, and the soluble glycoprotein are understood to contribute to virulence, less is known of the contribution of the highly variable mucin-like domain (MLD) of EBOV. Early studies have defined a potential role in immune evasion of the MLD by providing a glycan shield to critical glycoprotein residues tied to viral entry. Nonetheless, little is known as to what direct role the MLD plays in acute EBOV disease (EVD).

METHODS

We generated an infectious EBOV clone that lacks the MLD and assessed its virulence in ferrets compared with wild-type (WT) virus.

RESULTS

No differences in growth kinetics were observed in vitro, nor were there any differences in time to death, viremia, or clinical picture in ferrets infected with recombinant EBOV (rEBOV)-WT or rEBOV-Δmucin.

CONCLUSIONS

The EBOV MLD does not play a critical role in acute pathogenesis of EVD in ferrets.

Structure and Role of O-Linked Glycans in Viral Envelope Proteins

N- and O-glycans are both important constituents of viral envelope glycoproteins. O-linked glycosylation can be initiated by any of 20 different human polypeptide O-acetylgalactosaminyl transferases, resulting in an important functional O-glycan heterogeneity. O-glycans are organized as solitary glycans or in clusters of multiple glycans forming mucin-like domains. They are functional both in the viral life cycle and in viral colonization of their host. Negatively charged O-glycans are crucial for the interactions between glycosaminoglycan-binding viruses and their host. A novel mechanism, based on controlled electrostatic repulsion, explains how such viruses solve the conflict between optimized viral attachment to target cells and efficient egress of progeny virus. Conserved solitary O-glycans appear important for viral uptake in target cells by contributing to viral envelope fusion. Dual roles of viral O-glycans in the host B cell immune response, either epitope blocking or epitope promoting, may be exploitable for vaccine development. Finally, specific virus-induced O-glycans may be involved in viremic spread.

Development of a dual vaccine against East Coast fever and lumpy skin disease

East Coast fever is an acute bovine disease caused by the apicomplexan parasite Theileria parva and is regarded as one of the most important tick-vectored diseases in Africa. The current vaccination procedure has many drawbacks, as it involves the use of live T. parva sporozoites. As a novel vaccination strategy, we have constructed the recombinant lumpy skin disease virus (LSDV) named LSDV-SODis-p67HA-BLV-Gag, encoding a modified form of the T. parva p67 surface antigen (p67HA), as well as the bovine leukemia virus (BLV) gag gene for the formation of virus-like particles (VLPs) to potentially enhance p67 immunogenicity. In place of the native sequence, the chimeric p67HA antigen has the human tissue plasminogen activator signal sequence and the influenza hemagglutinin A2 transmembrane domain and cytoplasmic tail. p67HA was detected on the surface of infected cells, and VLPs comprising BLV Gag and p67HA were produced. We also show that higher multiple bands observed in western blot analysis are due to glycosylation of p67. The two vaccines, pMExT-p67HA (DNA) and LSDV-SODis-p67HA-BLV-Gag, were tested for immunogenicity in mice. p67-binding antibodies were produced by vaccinated animals, with higher titers detected in mice vaccinated with the recombinant LSDV. This candidate dual vaccine warrants further testing in cattle.

An RSV Live-Attenuated Vaccine Candidate Lacking G Protein Mucin Domains Is Attenuated, Immunogenic, and Effective in Preventing RSV in BALB/c Mice

BACKGROUND

Respiratory syncytial virus (RSV) is a leading viral respiratory pathogen in infants. The objective of this study was to generate RSV live-attenuated vaccine (LAV) candidates by removing the G-protein mucin domains to attenuate viral replication while retaining immunogenicity through deshielding of surface epitopes.

METHODS

Two LAV candidates were generated from recombinant RSV A2-line19F by deletion of the G-protein mucin domains (A2-line19F-G155) or deletion of the G-protein mucin and transmembrane domains (A2-line19F-G155S). Vaccine attenuation was measured in BALB/c mouse lungs by fluorescent focus unit (FFU) assays and real-time polymerase chain reaction (RT-PCR). Immunogenicity was determined by measuring serum binding and neutralizing antibodies in mice following prime/boost on days 28 and 59. Efficacy was determined by measuring RSV lung viral loads on day 4 postchallenge.

RESULTS

Both LAVs were undetectable in mouse lungs by FFU assay and elicited similar neutralizing antibody titers compared to A2-line19F on days 28 and 59. Following RSV challenge, vaccinated mice showed no detectable RSV in the lungs by FFU assay and a significant reduction in RSV RNA in the lungs by RT-PCR of 560-fold for A2-line19F-G155 and 604-fold for A2-line19F-G155S compared to RSV-challenged, unvaccinated mice.

CONCLUSIONS

Removal of the G-protein mucin domains produced RSV LAV candidates that were highly attenuated with retained immunogenicity.

RNF185 regulates proteostasis in Ebolavirus infection by crosstalk between the calnexin cycle, ERAD, and reticulophagy

Virus infection affects cellular proteostasis and provides an opportunity to study this cellular process under perturbation. The proteostasis network in the endoplasmic reticulum (ER) is composed of the calnexin cycle, and the two protein degradation pathways ER-associated protein degradation (ERAD) and ER-to-lysosome-associated degradation (ERLAD/ER-phagy/reticulophagy). Here we show that calnexin and calreticulin trigger Zaire Ebolavirus (EBOV) glycoprotein GP_1,2 misfolding. Misfolded EBOV-GP_1,2 is targeted by ERAD machinery, but this results in lysosomal instead of proteasomal degradation. Moreover, the ER Ub ligase RNF185, usually associated with ERAD, polyubiquitinates EBOV-GP_1,2 on lysine 673 via ubiquitin K27-linkage. Polyubiquinated GP_1,2 is subsequently recruited into autophagosomes by the soluble autophagy receptor sequestosome 1 (SQSTM1/p62), in an ATG3- and ATG5-dependent manner. We conclude that EBOV hijacks all three proteostasis mechanisms in the ER to downregulate GP_1,2 via polyubiquitination and show that this increases viral fitness. This study identifies linkages among proteostasis network components previously thought to function independently.

Little is known about how proteostasis is maintained during viral infection. Here, the authors identify unexpected crosstalk between the calnexin cycle, ERAD, and reticulophagy, resulting in suppression of ebolavirus glycoprotein expression.

Function-based classification of hazardous biological sequences: Demonstration of a new paradigm for biohazard assessments

Bioengineering applies analytical and engineering principles to identify functional biological building blocks for biotechnology applications. While these building blocks are leveraged to improve the human condition, the lack of simplistic, machine-readable definition of biohazards at the function level is creating a gap for biosafety practices. More specifically, traditional safety practices focus on the biohazards of known pathogens at the organism-level and may not accurately consider novel biodesigns with engineered functionalities at the genetic component-level. This gap is motivating the need for a paradigm shift from organism-centric procedures to function-centric biohazard identification and classification practices. To address this challenge, we present a novel methodology for classifying biohazards at the individual sequence level, which we then compiled to distinguish the biohazardous property of pathogenicity at the whole genome level. Our methodology is rooted in compilation of hazardous functions, defined as a set of sequences and associated metadata that describe coarse-level functions associated with pathogens (e.g., adherence, immune subversion). We demonstrate that the resulting database can be used to develop hazardous “fingerprints” based on the functional metadata categories. We verified that these hazardous functions are found at higher levels in pathogens compared to non-pathogens, and hierarchical clustering of the fingerprints can distinguish between these two groups. The methodology presented here defines the hazardous functions associated with bioengineering functional building blocks at the sequence level, which provide a foundational framework for classifying biological hazards at the organism level, thus leading to the improvement and standardization of current biosecurity and biosafety practices.

Protein disulfide isomerases (PDIs) negatively regulate ebolavirus structural glycoprotein expression in the endoplasmic reticulum (ER) via the autophagy-lysosomal pathway

Zaire ebolavirus (EBOV) causes a severe hemorrhagic fever in humans and non-human primates with high morbidity and mortality. EBOV infection is dependent on its structural glycoprotein (GP), but high levels of GP expression also trigger cell rounding, detachment, and downregulation of many surface molecules that is thought to contribute to its high pathogenicity. Thus, EBOV has evolved an RNA editing mechanism to reduce its GP expression and increase its fitness. We now report that the GP expression is also suppressed at the protein level in cells by protein disulfide isomerases (PDIs). Although PDIs promote oxidative protein folding by catalyzing correct disulfide formation in the endoplasmic reticulum (ER), PDIA3/ERp57 adversely triggered the GP misfolding by targeting GP cysteine residues and activated the unfolded protein response (UPR). Abnormally folded GP was targeted by ER-associated protein degradation (ERAD) machinery and, unexpectedly, was degraded via the macroautophagy/autophagy-lysosomal pathway, but not the proteasomal pathway. PDIA3 also decreased the GP expression from other ebolavirus species but increased the GP expression from Marburg virus (MARV), which is consistent with the observation that MARV-GP does not cause cell rounding and detachment, and MARV does not regulate its GP expression via RNA editing during infection. Furthermore, five other PDIs also had a similar inhibitory activity to EBOV-GP. Thus, PDIs negatively regulate ebolavirus glycoprotein expression, which balances the viral life cycle by maximizing their infection but minimizing their cellular effect. We suggest that ebolaviruses hijack the host protein folding and ERAD machinery to increase their fitness via reticulophagy during infection.Abbreviations: 3-MA: 3-methyladenine; 4-PBA: 4-phenylbutyrate; ACTB: β-actin; ATF: activating transcription factor; ATG: autophagy-related; BafA1: bafilomycin A1; BDBV: Bundibugyo ebolavirus; CALR: calreticulin; CANX: calnexin; CHX: cycloheximide; CMA: chaperone-mediated autophagy; ConA: concanamycin A; CRISPR: clusters of regularly interspaced short palindromic repeats; Cas9: CRISPR-associated protein 9; dsRNA: double-stranded RNA; EBOV: Zaire ebolavirus; EDEM: ER degradation enhancing alpha-mannosidase like protein; EIF2AK3/PERK: eukaryotic translation initiation factor 2 alpha kinase 3; Env: envelope glycoprotein; ER: endoplasmic reticulum; ERAD: ER-associated protein degradation; ERN1/IRE1: endoplasmic reticulum to nucleus signaling 1; GP: glycoprotein; HA: hemagglutinin; HDAC6: histone deacetylase 6; HMM: high-molecular-mass; HIV-1: human immunodeficiency virus type 1; HSPA5/BiP: heat shock protein family A (Hsp70) member 5; IAV: influenza A virus; IP: immunoprecipitation; KIF: kifenesine; Lac: lactacystin; LAMP: lysosomal associated membrane protein; MAN1B1/ERManI: mannosidase alpha class 1B member 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MARV: Marburg virus; MLD: mucin-like domain; NHK/SERPINA1: alpha1-antitrypsin variant null (Hong Kong); NTZ: nitazoxanide; PDI: protein disulfide isomerase; RAVV: Ravn virus; RESTV: Reston ebolavirus; SARS-CoV: severe acute respiratory syndrome coronavirus; SBOV: Sudan ebolavirus; sGP: soluble GP; SQSTM1/p62: sequestosome 1; ssGP: small soluble GP; TAFV: Taï Forest ebolavirus; TIZ: tizoxanide; TGN: thapsigargin; TLD: TXN (thioredoxin)-like domain; Ub: ubiquitin; UPR: unfolded protein response; VLP: virus-like particle; VSV: vesicular stomatitis virus; WB: Western blotting; WT: wild-type; XBP1: X-box binding protein 1.

Immune escape facilitation by mutations of epitope residues in RdRp of SARS-CoV-2

Mutations drive viral evolution and genome variability that causes viruses to escape host immunity and to develop drug resistance. SARS-CoV-2 has considerably higher mutation rate. SARS-CoV-2 possesses a RNA dependent RNA polymerase (RdRp) which helps to replicate its genome. The mutation P323L in RdRp is associated with the loss of a particular epitope (321-327) from this protein. We consider the effects of mutations in some of the epitope region including the naturally occurring mutation P323L on the structure of the epitope and their interface with paratope using all-atom molecular dynamics (MD) simulation studies. We observe that the mutations cause conformational changes in the epitope region by opening up the region associated with increase in the radius of gyration and intramolecular hydrogen bonds, making the region less accessible. Moreover, we study the conformational stability of the epitope region and epitope:paratope interface under the mutation from the fluctuations in the dihedral angles. We observe that the mutation renders the epitope and the epitope:paratope interface unstable compared to the corresponding wild type ones. Thus, the mutations may help in escaping antibody mediated immunity of the hostCommunicated by Ramaswamy H. Sarma.

Sagacious epitope selection for vaccines, and both antibody-based therapeutics and diagnostics: tips from virology and oncology

The target of an antibody plays a significant role in the success of antibody-based therapeutics and diagnostics, and vaccine development. This importance is focused on the target binding site—epitope, where epitope selection as a part of design thinking beyond traditional antigen selection using whole cell or whole protein immunization can positively impact success. With purified recombinant protein production and peptide synthesis to display limited/selected epitopes, intrinsic factors that can affect the functioning of resulting antibodies can be more easily selected for. Many of these factors stem from the location of the epitope that can impact accessibility of the antibody to the epitope at a cellular or molecular level, direct inhibition of target antigen activity, conservation of function despite escape mutations, and even non-competitive inhibition sites. By incorporating novel computational methods for predicting antigen changes to model-informed drug discovery and development, superior vaccines and antibody-based therapeutics or diagnostics can be easily designed to mitigate failures. With detailed examples, this review highlights the new opportunities, factors and methods of predicting antigenic changes for consideration in sagacious epitope selection.

Antibody bivalency improves antiviral efficacy by inhibiting virion release independently of Fc gamma receptors

Across the animal kingdom, multivalency discriminates antibodies from all other immunoglobulin superfamily members. The evolutionary forces conserving multivalency above other structural hallmarks of antibodies remain, however, incompletely defined. Here, we engineer monovalent either Fc-competent or -deficient antibody formats to investigate mechanisms of protection of neutralizing antibodies (nAbs) and non-neutralizing antibodies (nnAbs) in virus-infected mice. Antibody bivalency enables the tethering of virions to the infected cell surface, inhibits the release of virions in cell culture, and suppresses viral loads in vivo independently of Fc gamma receptor (FcγR) interactions. In return, monovalent antibody formats either do not inhibit virion release and fail to protect in vivo or their protective efficacy is largely FcγR dependent. Protection in mice correlates with virus-release-inhibiting activity of nAb and nnAb rather than with their neutralizing capacity. These observations provide mechanistic insights into the evolutionary conservation of antibody bivalency and help refining correlates of nnAb protection for vaccine development.

New Insights into the Role of the Complement System in Human Viral Diseases

The complement system (CS) is part of the human immune system, consisting of more than 30 proteins that play a vital role in the protection against various pathogens and diseases, including viral diseases. Activated via three pathways, the classical pathway (CP), the lectin pathway (LP), and the alternative pathway (AP), the complement system leads to the formation of a membrane attack complex (MAC) that disrupts the membrane of target cells, leading to cell lysis and death. Due to the increasing number of reports on its role in viral diseases, which may have implications for research on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), this review aims to highlight significant progress in understanding and defining the role of the complement system in four groups of diseases of viral etiology: (1) respiratory diseases; (2) acute liver failure (ALF); (3) disseminated intravascular coagulation (DIC); and (4) vector-borne diseases (VBDs). Some of these diseases already present a serious global health problem, while others are a matter of concern and require the collaboration of relevant national services and scientists with the World Health Organization (WHO) to avoid their spread.

Structural and Functional Aspects of Ebola Virus Proteins

Ebola virus (EBOV), member of genus Ebolavirus, family Filoviridae, have a non-segmented, single-stranded RNA that contains seven genes: (a) nucleoprotein (NP), (b) viral protein 35 (VP35), (c) VP40, (d) glycoprotein (GP), (e) VP30, (f) VP24, and (g) RNA polymerase (L). All genes encode for one protein each except GP, producing three pre-proteins due to the transcriptional editing. These pre-proteins are translated into four products, namely: (a) soluble secreted glycoprotein (sGP), (b) Δ-peptide, (c) full-length transmembrane spike glycoprotein (GP), and (d) soluble small secreted glycoprotein (ssGP). Further, shed GP is released from infected cells due to cleavage of GP by tumor necrosis factor α-converting enzyme (TACE). This review presents a detailed discussion on various functional aspects of all EBOV proteins and their residues. An introduction to ebolaviruses and their life cycle is also provided for clarity of the available analysis. We believe that this review will help understand the roles played by different EBOV proteins in the pathogenesis of the disease. It will help in targeting significant protein residues for therapeutic and multi-protein/peptide vaccine development.

Altering the Immunogenicity of Hemagglutinin Immunogens by Hyperglycosylation and Disulfide Stabilization

Influenza virus alters glycosylation patterns on its surface exposed glycoproteins to evade host adaptive immune responses. The viral hemagglutinin (HA), in particular the H3 subtype, has increased its overall surface glycosylation since its introduction in 1968. We previously showed that modulating predicted N-linked glycosylation sites on H3 A/Hong Kong/1/1968 HA identified a conserved epitope at the HA interface. This epitope is occluded on the native HA trimer but is likely exposed during HA "breathing" on the virion surface. Antibodies directed to this site are protective via an ADCC-mediated mechanism. This glycan engineering strategy made an otherwise subdominant epitope dominant in the murine model. Here, we asked whether cysteine stabilization of the hyperglycosylated HA trimer could reverse this immunodominance by preventing access to the interface epitope and focus responses to the HA receptor binding site (RBS). While analysis of serum responses from immunized mice did not show a redirection to the RBS, cysteine stabilization did result in an overall reduction in immunogenicity of the interface epitope. Thus, glycan engineering and cysteine stabilization are two strategies that can be used together to alter immunodominance patterns to HA. These results add to rational immunogen design approaches used to manipulate immune responses for the development of next-generation influenza vaccines.

Glycan-Lectin Interactions in Cancer and Viral Infections and How to Disrupt Them

Glycan-lectin interactions play an essential role in different cellular processes. One of their main functions is involvement in the immune response to pathogens or inflammation. However, cancer cells and viruses have adapted to avail themselves of these interactions. By displaying specific glycosylation structures, they are able to bind to lectins, thus promoting pathogenesis. While glycan-lectin interactions promote tumor progression, metastasis, and/or chemoresistance in cancer, in viral infections they are important for viral entry, release, and/or immune escape. For several years now, a growing number of investigations have been devoted to clarifying the role of glycan-lectin interactions in cancer and viral infections. Various overviews have already summarized and highlighted their findings. In this review, we consider the interactions of the lectins MGL, DC-SIGN, selectins, and galectins in both cancer and viral infections together. A possible transfer of ways to target and disrupt them might lead to new therapeutic approaches in different pathological backgrounds.

Review: Insights on Current FDA-Approved Monoclonal Antibodies Against Ebola Virus Infection

The unprecedented 2013-2016 West Africa Ebola outbreak accelerated several medical countermeasures (MCMs) against Ebola virus disease (EVD). Several investigational products (IPs) were used throughout the outbreak but were not conclusive for efficacy results. Only the Randomized Controlled Trial (RCT) on ZMapp was promising but inconclusive. More recently, during the second-largest Ebola outbreak in North Kivu and Ituri provinces, Democratic Republic of the Congo (DRC), four IPs, including one small molecule (Remdesivir), two monoclonal antibody (mAb) cocktails (ZMapp and REGN-EB3) and a single mAb (mAb114), were evaluated in an RCT, the Pamoja Tulinde Maisha (PALM) study. Two products (REGN-EB3 and mAb114) demonstrated efficacy as compared to the control arm, ZMapp. There were remarkably few side effects recorded in the trial. The FDA approved both medications in this scientifically sound study, marking a watershed moment in the field of EVD therapy. These products can be produced relatively inexpensively and can be stockpiled. The administration of mAbs in EVD patients appears to be safe and effective, while several critical knowledge gaps remain; the impact of early administration of Ebola-specific mAbs on developing a robust immune response for future Ebola virus exposure is unknown. The viral mutation escape, leading to resistance, presents a potential limitation for single mAb therapy; further improvements need to be explored. Understanding the contribution of Fc-mediated antibody functions such as antibody-dependent cellular cytotoxicity (ADCC) of those approved mAbs is still critical. The potential merit of combination therapy and post-exposure prophylaxis (PEP) need to be demonstrated. Furthermore, the PALM trial has accounted for 30% of mortality despite the administration of specific treatments. The putative role of EBOV soluble Glycoprotein (sGP) as a decoy to the immune system, the virus persistence, and relapses might be investigated for treatment failure. The development of pan-filovirus or pan-species mAbs remains essential for protection. The interaction between FDA-approved mAbs and vaccines remains unclear and needs to be investigated. In this review, we summarize the efficacy and safety results of the PALM study and review current research questions for the further development of mAbs in pre-exposure or emergency post-exposure use.

Cell-Free Glycoengineering of the Recombinant SARS-CoV-2 Spike Glycoprotein

The baculovirus-insect cell expression system is readily utilized to produce viral glycoproteins for research as well as for subunit vaccines and vaccine candidates, for instance against SARS-CoV-2 infections. However, the glycoforms of recombinant proteins derived from this expression system are inherently different from mammalian cell-derived glycoforms with mainly complex-type N-glycans attached, and the impact of these differences in protein glycosylation on the immunogenicity is severely under investigated. This applies also to the SARS-CoV-2 spike glycoprotein, which is the antigen target of all licensed vaccines and vaccine candidates including virus like particles and subunit vaccines that are variants of the spike protein. Here, we expressed the transmembrane-deleted human β-1,2 N-acetlyglucosamintransferases I and II (MGAT1ΔTM and MGAT2ΔTM) and the β-1,4-galactosyltransferase (GalTΔTM) in E. coli to in-vitro remodel the N-glycans of a recombinant SARS-CoV-2 spike glycoprotein derived from insect cells. In a cell-free sequential one-pot reaction, fucosylated and afucosylated paucimannose-type N-glycans were converted to complex-type galactosylated N-glycans. In the future, this in-vitro glycoengineering approach can be used to efficiently generate a wide range of N-glycans on antigens considered as vaccine candidates for animal trials and preclinical testing to better characterize the impact of N-glycosylation on immunity and to improve the efficacy of protein subunit vaccines.

The murine cytomegalovirus immunoevasin gp40/m152 inhibits NKG2D receptor RAE-1γ by intracellular retention and cell surface masking

NKG2D (also known as KLRK1) is a crucial natural killer (NK) cell-activating receptor, and the murine cytomegalovirus (MCMV) employs multiple immunoevasins to avoid NKG2D-mediated activation. One of the MCMV immunoevasins, gp40 (m152), downregulates the cell surface NKG2D ligand RAE-1γ (also known as Raet1c) thus limiting NK cell activation. This study establishes the molecular mechanism by which gp40 retains RAE-1γ in the secretory pathway. Using flow cytometry and pulse-chase analysis, we demonstrate that gp40 retains RAE-1γ in the early secretory pathway, and that this effect depends on the binding of gp40 to a host protein, TMED10, a member of the p24 protein family. We also show that the TMED10-based retention mechanism can be saturated, and that gp40 has a backup mechanism as it masks RAE-1γ on the cell surface, blocking the interaction with the NKG2D receptor and thus NK cell activation.

Hepatitis C virus modulates signal peptide peptidase to alter host protein processing

The mechanism by which hepatitis C virus (HCV) evades immune surveillance and causes chronic infection is unclear. We demonstrate here that HCV core protein interferes with the maturation of major histocompatibility complex (MHC) class I catalyzed by signal peptide peptidase (SPP) and induces degradation via HMG-CoA reductase degradation 1 homolog. In addition, we found that the core protein transmembrane domain is homologous to the human cytomegalovirus US2 protein, whose transmembrane region also targets SPP to impair MHC class I molecule expression in a similar manner. Therefore, our data suggest that SPP represents a potential target for the impairment of MHC class I molecules by DNA and RNA viruses.

Immunoevasins are viral proteins that prevent antigen presentation on major histocompatibility complex (MHC) class I, thus evading host immune recognition. Hepatitis C virus (HCV) evades immune surveillance to induce chronic infection; however, how HCV-infected hepatocytes affect immune cells and evade immune recognition remains unclear. Herein, we demonstrate that HCV core protein functions as an immunoevasin. Its expression interfered with the maturation of MHC class I molecules catalyzed by the signal peptide peptidase (SPP) and induced their degradation via HMG-CoA reductase degradation 1 homolog, thereby impairing antigen presentation to CD8⁺ T cells. The expression of MHC class I in the livers of HCV core transgenic mice and chronic hepatitis C patients was impaired but was restored in patients achieving sustained virological response. Finally, we show that the human cytomegalovirus US2 protein, possessing a transmembrane region structurally similar to the HCV core protein, targets SPP to impair MHC class I molecule expression. Thus, SPP represents a potential target for the impairment of MHC class I molecules by DNA and RNA viruses.

COVID-19: Risk Factors Associated with Infectivity and Severity

COVID‐19 is highly transmissible; however, its severity varies from one individual to another. Variability among different isolates of the virus and among its receptor (ACE2) may contribute to this severity, but comorbidity plays a major role on disease prognosis. Many comorbidities have been reported to be associated with severe COVID‐19 patients. We have collected data from retrospective studies which include clinical and epidemiological features of patients and categorize them into severe/mild, ICU/non‐ICU and survivors/dead patients. In this review, we give an update about SARS‐CoV‐2 structure with emphasis on the possible reasons for the severity of the virus in patients. We also collected information and patients’ data to highlight the relation between COVID‐19 patients and comorbidities.

A cross-neutralizing antibody between HIV-1 and influenza virus

Incessant antigenic evolution enables the persistence and spread of influenza virus in the human population. As the principal target of the immune response, the hemagglutinin (HA) surface antigen on influenza viruses continuously acquires and replaces N-linked glycosylation sites to shield immunogenic protein epitopes using host-derived glycans. Anti-glycan antibodies, such as 2G12, target the HIV-1 envelope protein (Env), which is even more extensively glycosylated and contains under-processed oligomannose-type clusters on its dense glycan shield. Here, we illustrate that 2G12 can also neutralize human seasonal influenza A H3N2 viruses that have evolved to present similar oligomannose-type clusters on their HAs from around 20 years after the 1968 pandemic. Using structural biology and mass spectrometric approaches, we find that two N-glycosylation sites close to the receptor binding site (RBS) on influenza hemagglutinin represent the oligomannose cluster recognized by 2G12. One of these glycan sites is highly conserved in all human H3N2 strains and the other emerged during virus evolution. These two N-glycosylation sites have also become crucial for fitness of recent H3N2 strains. These findings shed light on the evolution of the glycan shield on influenza virus and suggest 2G12-like antibodies can potentially act as broad neutralizers to target human enveloped viruses.

Characterisation of the T-cell response to Ebola virus glycoprotein amongst survivors of the 2013-16 West Africa epidemic

Zaire ebolavirus (EBOV) is a highly pathogenic filovirus which can result in Ebola virus disease (EVD); a serious medical condition that presents as flu like symptoms but then often leads to more serious or fatal outcomes. The 2013-16 West Africa epidemic saw an unparalleled number of cases. Here we show characterisation and identification of T cell epitopes in surviving patients from Guinea to the EBOV glycoprotein. We perform interferon gamma (IFNγ) ELISpot using a glycoprotein peptide library to identify T cell epitopes and determine the CD4+ or CD8+ T cell component response. Additionally, we generate data on the T cell phenotype and measure polyfunctional cytokine secretion by these antigen specific cells. We show candidate peptides able to elicit a T cell response in EBOV survivors and provide inferred human leukocyte antigen (HLA) allele restriction. This data informs on the long-term T cell response to Ebola virus disease and highlights potentially important immunodominant peptides.

Genetic engineering of T cells for immunotherapy

Genetically engineered T cell immunotherapies have provided remarkable clinical success to treat B cell acute lymphoblastic leukaemia by harnessing a patient's own T cells to kill cancer, and these approaches have the potential to provide therapeutic benefit for numerous other cancers, infectious diseases and autoimmunity. By introduction of either a transgenic T cell receptor or a chimeric antigen receptor, T cells can be programmed to target cancer cells. However, initial studies have made it clear that the field will need to implement more complex levels of genetic regulation of engineered T cells to ensure both safety and efficacy. Here, we review the principles by which our knowledge of genetics and genome engineering will drive the next generation of adoptive T cell therapies.

Designs and Characterization of Subunit Ebola GP Vaccine Candidates: Implications for Immunogenicity

The humoral responses of Ebola virus (EBOV) survivors mainly target the surface glycoprotein GP, and anti-GP neutralizing antibodies have been associated with protection against EBOV infection. In order to elicit protective neutralizing antibodies through vaccination a native-like conformation of the antigen is required. We therefore engineered and expressed in CHO cells several GP variants from EBOV (species Zaire ebolavirus, Mayinga variant), including a soluble GP ΔTM, a mucin-like domain-deleted GP ΔTM-ΔMUC, as well as two GP ΔTM-ΔMUC variants with C-terminal trimerization motifs in order to favor their native trimeric conformation. Inclusion of the trimerization motifs resulted in proteins mimicking GP metastable trimer and showing increased stability. The mucin-like domain appeared not to be critical for the retention of the native conformation of the GP protein, and its removal unmasked several neutralizing epitopes, especially in the trimers. The soluble GP variants inhibited mAbs neutralizing activity in a pseudotype transduction assay, further confirming the proteins' structural integrity. Interestingly, the trimeric GPs, a native-like GP complex, showed stronger affinity for antibodies raised by natural infection in EBOV disease survivors rather than for antibodies raised in volunteers that received the ChAd3-EBOZ vaccine. These results support our hypothesis that neutralizing antibodies are preferentially induced when using a native-like conformation of the GP antigen. The soluble trimeric recombinant GP proteins we developed represent a novel and promising strategy to develop prophylactic vaccines against EBOV and other filoviruses.

Structure of human endo-α-1,2-mannosidase (MANEA), an antiviral host-glycosylation target

Mammalian protein N-linked glycosylation is critical for glycoprotein folding, quality control, trafficking, recognition, and function. N-linked glycans are synthesized from Glc₃Man₉GlcNAc₂ precursors that are trimmed and modified in the endoplasmic reticulum (ER) and Golgi apparatus by glycoside hydrolases and glycosyltransferases. Endo-α-1,2-mannosidase (MANEA) is the sole endo-acting glycoside hydrolase involved in N-glycan trimming and is located within the Golgi, where it allows ER-escaped glycoproteins to bypass the classical N-glycosylation trimming pathway involving ER glucosidases I and II. There is considerable interest in the use of small molecules that disrupt N-linked glycosylation as therapeutic agents for diseases such as cancer and viral infection. Here we report the structure of the catalytic domain of human MANEA and complexes with substrate-derived inhibitors, which provide insight into dynamic loop movements that occur on substrate binding. We reveal structural features of the human enzyme that explain its substrate preference and the mechanistic basis for catalysis. These structures have inspired the development of new inhibitors that disrupt host protein N-glycan processing of viral glycans and reduce the infectivity of bovine viral diarrhea and dengue viruses in cellular models. These results may contribute to efforts aimed at developing broad-spectrum antiviral agents and help provide a more in-depth understanding of the biology of mammalian glycosylation.

Non-synonymous mutations of SARS-CoV-2 leads epitope loss and segregates its variants

The non-synonymous mutations of SARS-CoV-2 isolated from across the world have been identified during the last few months. The surface glycoprotein spike of SARS-CoV-2 forms the most important hotspot for amino acid alterations followed by the ORF1a/ORF1ab poly-proteins. It is evident that the D614G mutation in spike glycoprotein and P4715L in RdRp is the important determinant of SARS-CoV-2 evolution since its emergence. P4715L in RdRp, G251V in ORF3a and S1498F of Nsp3 is associated with the epitope loss that may influence pathogenesis caused by antibody escape variants. The phylogenomics distinguished the ancestral viral samples from China and most part of Asia, isolated since the initial outbreak and the later evolved variants isolated from Europe and Americas. The evolved variants have been found to predominant globally with the loss of epitopes from its proteins. These have implications for SARS-CoV-2 transmission, pathogenesis and immune interventions.

Immunological Perspective for Ebola Virus Infection and Various Treatment Measures Taken to Fight the Disease

Ebolaviruses, discovered in 1976, belongs to the Filoviridae family, which also includes Marburg and Lloviu viruses. They are negative-stranded RNA viruses with six known species identified to date. Ebola virus (EBOV) is a member of Zaire ebolavirus species and can cause the Ebola virus disease (EVD), an emerging zoonotic disease that results in homeostatic imbalance and multi-organ failure. There are three EBOV outbreaks documented in the last six years resulting in significant morbidity (> 32,000 cases) and mortality (> 13,500 deaths). The potential factors contributing to the high infectivity of this virus include multiple entry mechanisms, susceptibility of the host cells, employment of multiple immune evasion mechanisms and rapid person-to-person transmission. EBOV infection leads to cytokine storm, disseminated intravascular coagulation, host T cell apoptosis as well as cell mediated and humoral immune response. In this review, a concise recap of cell types targeted by EBOV and EVD symptoms followed by detailed run-through of host innate and adaptive immune responses, virus-driven regulation and their combined effects contributing to the disease pathogenesis has been presented. At last, the vaccine and drug development initiatives as well as challenges related to the management of infection have been discussed.

Case Fatality Ratio Estimates for the 2013-2016 West African Ebola Epidemic: Application of Boosted Regression Trees for Imputation

BACKGROUND

The 2013-2016 West African Ebola epidemic has been the largest to date with >11 000 deaths in the affected countries. The data collected have provided more insight into the case fatality ratio (CFR) and how it varies with age and other characteristics. However, the accuracy and precision of the naive CFR remain limited because 44% of survival outcomes were unreported.

METHODS

Using a boosted regression tree model, we imputed survival outcomes (ie, survival or death) when unreported, corrected for model imperfection to estimate the CFR without imputation, with imputation, and adjusted with imputation. The method allowed us to further identify and explore relevant clinical and demographic predictors of the CFR.

RESULTS

The out-of-sample performance (95% confidence interval [CI]) of our model was good: sensitivity, 69.7% (52.5-75.6%); specificity, 69.8% (54.1-75.6%); percentage correctly classified, 69.9% (53.7-75.5%); and area under the receiver operating characteristic curve, 76.0% (56.8-82.1%). The adjusted CFR estimates (95% CI) for the 2013-2016 West African epidemic were 82.8% (45.6-85.6%) overall and 89.1% (40.8-91.6%), 65.6% (61.3-69.6%), and 79.2% (45.4-84.1%) for Sierra Leone, Guinea, and Liberia, respectively. We found that district, hospitalisation status, age, case classification, and quarter (date of case reporting aggregated at three-month intervals) explained 93.6% of the variance in the naive CFR.

CONCLUSIONS

The adjusted CFR estimates improved the naive CFR estimates obtained without imputation and were more representative. Used in conjunction with other resources, adjusted estimates will inform public health contingency planning for future Ebola epidemics, and help better allocate resources and evaluate the effectiveness of future inventions.

Mucin-Like Domain of Ebola Virus Glycoprotein Enhances Selective Oncolytic Actions against Brain Tumors

Given that the Ebola virus (EBOV) infects a wide array of organs and cells yet displays a relative lack of neurotropism, we asked whether a chimeric vesicular stomatitis virus (VSV) expressing the EBOV glycoprotein (GP) might selectively target brain tumors. The mucin-like domain (MLD) of the EBOV GP may enhance virus immune system evasion. Here, we compared chimeric VSVs in which EBOV GP replaces the VSV glycoprotein, thereby reducing the neurotoxicity associated with wild-type VSV. A chimeric VSV expressing the full-length EBOV GP (VSV-EBOV) containing the MLD was substantially more effective and safer than a parallel construct with an EBOV GP lacking the MLD (VSV-EBOVΔMLD). One-step growth, reverse transcription-quantitative PCR, and Western blotting assessments showed that VSV-EBOVΔMLD produced substantially more progeny faster than VSV-EBOV. Using immunodeficient SCID mice, we focused on targeting human brain tumors with these VSV-EBOVs. Similar to the findings of our previous study in which we used an attenuated VSV-EBOV with no MLD that expressed green fluorescent protein (GFP) (VSV-EBOVΔMLD-GFP), VSV-EBOVΔMLD without GFP targeted glioma but yielded only a modest extension of survival. In contrast, VSV-EBOV containing the MLD showed substantially better targeting and elimination of brain tumors after intravenous delivery and increased the survival of brain tumor-bearing mice. Despite the apparent destruction of most tumor cells by VSV-EBOVΔMLD, the virus remained active within the SCID mouse brain and showed widespread infection of normal brain cells. In contrast, VSV-EBOV eliminated the tumors and showed relatively little infection of normal brain cells. Parallel experiments with direct intracranial virus infection generated similar results. Neither VSV-EBOV nor VSV-EBOVΔMLD showed substantive infection of the brains of normal immunocompetent mice.IMPORTANCE The Ebola virus glycoprotein contains a mucin-like domain which may play a role in immune evasion. Chimeric vesicular stomatitis viruses with the EBOV glycoprotein substituted for the VSV glycoprotein show greater safety and efficacy in targeting brain tumors in immunodeficient mice when the MLD was expressed within the EBOV glycoprotein than when EBOV lacked the mucin-like domain.

N-Glycans Mediate the Ebola Virus-GP1 Shielding of Ligands to Immune Receptors and Immune Evasion

The Ebola Virus (EBOV) glycoprotein (GP) sterically shields cell-membrane ligands to immune receptors such as human leukocyte antigen class-1 (HLA-I) and MHC class I polypeptide-related sequence A (MICA), thus mediating immunity evasion. It was suggested that the abundant N-glycosylation of the EBOV-GP is involved in this steric shielding. We aimed to characterize (i) the GP N-glycosylation sites contributing to the shielding, and (ii) the effect of mutating these sites on immune subversion by the EBOV-GP. The two highly glycosylated domains of GP are the mucin-like domain (MLD) and the glycan cap domain (GCD) with three and six N-glycosylation sites, respectively. We mutated the N-glycosylation sites either in MLD or in GCD or in both domains. We showed that the glycosylation sites in both the MLD and GCD domains contribute to the steric shielding. This was shown for the steric shielding of either HLA-I or MICA. We then employed the fluorescence resonance energy transfer (FRET) method to measure the effect of N-glycosylation site removal on the distance in the cell membrane between the EBOV-GP and HLA-I (HLA.A^*0201 allele). We recorded high FRET values for the interaction of CFP-fused HLA.A^*0201 and YFP-fused EBOV-GP, demonstrating the very close distance (<10 nm) between these two proteins on the cell membrane of GP-expressing cells. The co-localization of HLA-I and Ebola GP was unaffected by the disruption of steric shielding, as the removal of N-glycosylation sites on Ebola GP revealed similar FRET values with HLA-I. However, these mutations directed to N-glycosylation sites had restored immune cell function otherwise impaired due to steric shielding over immune cell ligands by WT Ebola GP. Overall, we showed that the GP-mediated steric shielding aimed to impair immune function is facilitated by the N-glycans protruding from its MLD and GCD domains, but these N-glycans are not controlling the close distance between GP and its shielded proteins.

Efficient Expression and Processing of Ebola Virus Glycoprotein Induces Morphological Changes in BmN Cells but Cannot Rescue Deficiency of Bombyx Mori Nucleopolyhedrovirus GP64

Ebola virus (EBOV) disease outbreaks have resulted in many fatalities, yet no licensed vaccines are available to prevent infection. Recombinant glycoprotein (GP) production may contribute to finding a cure for Ebola virus disease, which is the key candidate protein for vaccine preparation. To explore GP_1,2 expression in BmN cells, EBOV-GP_1,2 with its native signal peptide or the GP64 signal peptide was cloned and transferred into a normal or gp64 null Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid via transposition. The infectivity of the recombinant bacmids was investigated after transfection, expression and localization of EBOV-GP were investigated, and cell morphological changes were analyzed by TEM. The GP64 signal peptide, but not the GP_1,2 native signal peptide, caused GP_1,2 localization to the cell membrane, and the differentially localized GP_1,2 proteins were cleaved into GP₁ and GP₂ fragments in BmN cells. GP_1,2 expression resulted in dramatic morphological changes in BmN cells in the early stage of infection. However, GP_1,2 expression did not rescue GP64 deficiency in BmNPV infection. This study provides a better understanding of GP expression and processing in BmN cells, which may lay a foundation for EBOV-GP expression using the BmNPV baculovirus expression system.

Efficacy of Ebola Glycoprotein-Specific Equine Polyclonal Antibody Product Against Lethal Ebola Virus Infection in Guinea Pigs

Background

Filoviruses including Ebola, Sudan, and other species are emerging zoonotic pathogens representing a significant public health concern with high outbreak potential, and they remain a potential bioterrorism-related threat. We have developed a despeciated equine Ebola polyclonal antibody (E-EIG) postexposure treatment against Ebola virus (EBOV) and evaluated its efficacy in the guinea pig model of EBOV infection.

Methods

Guinea pigs were infected with guinea pig-adapted EBOV (Mayinga strain) and treated with various dose levels of E-EIG (20-100 mg/kg) twice daily for 6 days starting at 24 h postinfection. The E-EIG was also assessed for neutralization activity against related filoviruses including EBOV strains Mayinga, Kikwit, and Makona and the Bundibugyo and Taï Forest ebolavirus species.

Results

Treatment with E-EIG conferred 83% to 100% protection in guinea pigs. The results demonstrated a comparable neutralization activity (range, 1:512-1:896) of E-EIG against all tested strains, suggesting the potential for cross-protection with the polyclonal antibody therapeutic.

Conclusions

This study showed that equine-derived polyclonal antibodies are efficacious against lethal EBOV disease in a relevant animal model. Furthermore, the studies support the utility of the equine antibody platform for the rapid production of a therapeutic product in the event of an outbreak by a filovirus or other zoonotic pathogen.

Exploitation of glycosylation in enveloped virus pathobiology

Glycosylation is a ubiquitous post-translational modification responsible for a multitude of crucial biological roles. As obligate parasites, viruses exploit host-cell machinery to glycosylate their own proteins during replication. Viral envelope proteins from a variety of human pathogens including HIV-1, influenza virus, Lassa virus, SARS, Zika virus, dengue virus, and Ebola virus have evolved to be extensively glycosylated. These host-cell derived glycans facilitate diverse structural and functional roles during the viral life-cycle, ranging from immune evasion by glycan shielding to enhancement of immune cell infection. In this review, we highlight the imperative and auxiliary roles glycans play, and how specific oligosaccharide structures facilitate these functions during viral pathogenesis. We discuss the growing efforts to exploit viral glycobiology in the development of anti-viral vaccines and therapies.

Dendritic Cells/Macrophages-Targeting Feature of Ebola Glycoprotein and its Potential as Immunological Facilitator for Antiviral Vaccine Approach

In the prevention of epidemic and pandemic viral infection, the use of the antiviral vaccine has been the most successful biotechnological and biomedical approach. In recent times, vaccine development studies have focused on recruiting and targeting immunogens to dendritic cells (DCs) and macrophages to induce innate and adaptive immune responses. Interestingly, Ebola virus (EBOV) glycoprotein (GP) has a strong binding affinity with DCs and macrophages. Shreds of evidence have also shown that the interaction between EBOV GP with DCs and macrophages leads to massive recruitment of DCs and macrophages capable of regulating innate and adaptive immune responses. Therefore, studies for the development of vaccine can utilize the affinity between EBOV GP and DCs/macrophages as a novel immunological approach to induce both innate and acquired immune responses. In this review, we will discuss the unique features of EBOV GP to target the DC, and its potential to elicit strong immune responses while targeting DCs/macrophages. This review hopes to suggest and stimulate thoughts of developing a stronger and effective DC-targeting vaccine for diverse virus infection using EBOV GP.

Inhibition of Ebola Virus by a Molecularly Engineered Banana Lectin

Ebolaviruses cause an often rapidly fatal syndrome known as Ebola virus disease (EVD), with average case fatality rates of ~50%. There is no licensed vaccine or treatment for EVD, underscoring the urgent need to develop new anti-ebolavirus agents, especially in the face of an ongoing outbreak in the Democratic Republic of the Congo and the largest ever outbreak in Western Africa in 2013-2016. Lectins have been investigated as potential antiviral agents as they bind glycans present on viral surface glycoproteins, but clinical use of them has been slowed by concerns regarding their mitogenicity, i.e. ability to cause immune cell proliferation. We previously engineered a banana lectin (BanLec), a carbohydrate-binding protein, such that it retained antiviral activity but lost mitogenicity by mutating a single amino acid, yielding H84T BanLec (H84T). H84T shows activity against viruses containing high-mannose N-glycans, including influenza A and B, HIV-1 and -2, and hepatitis C virus. Since ebolavirus surface glycoproteins also contain many high-mannose N-glycans, we assessed whether H84T could inhibit ebolavirus replication. H84T inhibited Ebola virus (EBOV) replication in cell cultures. In cells, H84T inhibited both virus-like particle (VLP) entry and transcription/replication of the EBOV mini-genome at high micromolar concentrations, while inhibiting infection by transcription- and replication-competent VLPs, which measures the full viral life cycle, in the low micromolar range. H84T did not inhibit assembly, budding, or release of VLPs. These findings suggest that H84T may exert its anti-ebolavirus effect(s) by blocking both entry and transcription/replication. In a mouse model, H84T partially (maximally, ~50-80%) protected mice from an otherwise lethal mouse-adapted EBOV infection. Interestingly, a single dose of H84T pre-exposure to EBOV protected ~80% of mice. Thus, H84T shows promise as a new anti-ebolavirus agent with potential to be used in combination with vaccination or other agents in a prophylactic or therapeutic regimen.

Glycosylation of viral surface proteins probed by mass spectrometry

Glycosylation is a common and biologically significant post-translational modification that is found on numerous virus surface proteins (VSPs). Many of these glycans affect virulence through modulating virus receptor binding, masking antigenic sites, or by stimulating the host immune response. Mass spectrometry (MS) has arisen as a pivotal technique for the characterization of VSP glycosylation. This review will cover how MS-based analyses, such as released glycan profiles, glycan site localization, site-occupancy, and site-specific heterogeneity, are being utilized to map VSP glycosylation. Furthermore, this review will provide information on how MS glycoprofiling data are being used in conjunction with molecular and structural experiments to provide a better understanding of the role of specific glycans in VSP function.

Longitudinal Analysis of the Human B Cell Response to Ebola Virus Infection

Ebola virus (EBOV) remains a public health threat. We performed a longitudinal study of B cell responses to EBOV in four survivors of the 2014 West African outbreak. Infection induced lasting EBOV-specific immunoglobulin G (IgG) antibodies, but their subclass composition changed over time, with IgG1 persisting, IgG3 rapidly declining, and IgG4 appearing late. Striking changes occurred in the immunoglobulin repertoire, with massive recruitment of naive B cells that subsequently underwent hypermutation. We characterized a large panel of EBOV glycoprotein-specific monoclonal antibodies (mAbs). Only a small subset of mAbs that bound glycoprotein by ELISA recognized cell-surface glycoprotein. However, this subset contained all neutralizing mAbs. Several mAbs protected against EBOV disease in animals, including one mAb that targeted an epitope under evolutionary selection during the 2014 outbreak. Convergent antibody evolution was seen across multiple donors, particularly among VH3-13 neutralizing antibodies specific for the GP1 core. Our study provides a benchmark for assessing EBOV vaccine-induced immunity.

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Ebola virus infection causes massive recruitment of naive B cells

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Virus-specific antibodies continue to class-switch and mutate for months after acute infection

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Protective antibodies can be neutralizing or non-neutralizing and can appear early

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Convergent, protective antibody rearrangements are seen in multiple donors

Site-specific glycosylation of Ebola virus glycoprotein by human polypeptide GalNAc-transferase 1 induces cell adhesion defects

The surface glycoprotein (GP) of Ebola virus causes many of the virus's pathogenic effects, including a dramatic loss of endothelial cell adhesion associated with widespread hemorrhaging during infection. Although the GP-mediated deadhesion depends on its extracellular mucin-like domain, it is unknown whether any, or all, of this domain's densely clustered O-glycosylation sites are required. It is also unknown whether any of the 20 distinct polypeptide GalNAc-transferases (ppGalNAc-Ts) that initiate mucin-type O-glycosylation in human cells are functionally required. Here, using HEK293 cell lines lacking specific glycosylation enzymes, we demonstrate that GP requires extended O-glycans to exert its deadhesion effect. We also identified ppGalNAc-T1 as largely required for the GP-mediated adhesion defects. Despite its profound effect on GP function, the absence of ppGalNAc-T1 only modestly reduced the O-glycan mass of GP, indicating that even small changes in the bulky glycodomain can cause loss of GP function. Indeed, protein-mapping studies identified a small segment of the mucin-like domain critical for function and revealed that mutation of five glycan acceptor sites within this segment are sufficient to abrogate GP function. Together, these results argue against a mechanism of Ebola GP-induced cell detachment that depends solely on ectodomain bulkiness and identify a single host-derived glycosylation enzyme, ppGalNAc-T1, as a potential target for therapeutic intervention against Ebola virus disease.

Removal of the N-Glycosylation Sequon at Position N116 Located in p27 of the Respiratory Syncytial Virus Fusion Protein Elicits Enhanced Antibody Responses after DNA Immunization

Prevention of severe lower respiratory tract infections in infants caused by the human respiratory syncytial virus (hRSV) remains a major public health priority. Currently, the major focus of vaccine development relies on the RSV fusion (F) protein since it is the main target protein for neutralizing antibodies induced by natural infection. The protein conserves 5 N-glycosylation sites, two of which are located in the F2 subunit (N27 and N70), one in the F1 subunit (N500) and two in the p27 peptide (N116 and N126). To study the influence of the loss of one or more N-glycosylation sites on RSV F immunogenicity, BALB/c mice were immunized with plasmids encoding RSV F glycomutants. In comparison with F WT DNA immunized mice, higher neutralizing titres were observed following immunization with F N116Q. Moreover, RSV A2-K-line19F challenge of mice that had been immunized with mutant F N116Q DNA was associated with lower RSV RNA levels compared with those in challenged WT F DNA immunized animals. Since p27 is assumed to be post-translationally released after cleavage and thus not present on the mature RSV F protein, it remains to be elucidated how deletion of this glycan can contribute to enhanced antibody responses and protection upon challenge. These findings provide new insights to improve the immunogenicity of RSV F in potential vaccine candidates.

Viral glycoproteomes: technologies for characterization and outlook for vaccine design

It has long been known that surface proteins of most enveloped viruses are covered with glycans. It has furthermore been demonstrated that glycosylation is essential for propagation and immune evasion for many viruses. The recent development of high-resolution mass spectrometry techniques has enabled identification not only of the precise structures but also the positions of such post-translational modifications on viruses, revealing substantial differences in extent of glycosylation and glycan maturation for different classes of viruses. In-depth characterization of glycosylation and other post-translational modifications of viral envelope glycoproteins is essential for rational design of vaccines and antivirals. In this Review, we provide an overview of techniques used to address viral glycosylation and summarize information on glycosylation of enveloped viruses representing ongoing public health challenges. Furthermore, we discuss how knowledge on glycosylation can be translated to means to prevent and combat viral infections.

The Ebola-Glycoprotein Modulates the Function of Natural Killer Cells

The Ebola virus (EBOV) uses evasion mechanisms that directly interfere with host T-cell antiviral responses. By steric shielding of human leukocyte antigen class-1, the Ebola glycoprotein (GP) blocks interaction with T-cell receptors (TCRs), thus rendering T cells unable to attack virus-infected cells. It is likely that this mechanism could promote increased natural killer (NK) cell activity against GP-expressing cells by preventing the engagement of NK inhibitory receptors; however, we found that primary human NK cells were less reactive to GP-expressing HEK293T cells. This was manifested as reduced cytokine secretion, a reduction in NK degranulation, and decreased lysis of GP-expressing target cells. We also demonstrated reduced recognition of GP-expressing cells by recombinant NKG2D and NKp30 receptors. In accordance, we showed a reduced monoclonal antibody-based staining of NKG2D and NKp30 ligands on GP-expressing target cells. Trypsin digestion of the membrane-associated GP led to a recovery of the recognition of membrane-associated NKG2D and NKp30 ligands. We further showed that membrane-associated GP did not shield recognition by KIR2DL receptors; in accordance, GP expression by target cells significantly perturbed signal transduction through activating, but not through inhibitory, receptors. Our results suggest a novel evasion mechanism employed by the EBOV to specifically avoid the NK cell immune response.