DC-SIGN and DC-SIGNR bind ebola glycoproteins and enhance infection of macrophages and endothelial cells

Thermodynamics-Guided Design Reveals a Cooperative Hydrogen Bond in DC-SIGN-targeted Glycomimetics

Due to the shallow and hydrophilic binding sites of carbohydrate-binding proteins, the design of glycomimetics is often complicated by high desolvation costs as well as competition with solvent. Therefore, a careful optimization of interaction vectors and ligand properties is required in the design and optimization of glycomimetics. Here, we employ thermodynamics-guided design to optimize mannose-based glycomimetics targeting the human C-type lectin receptor dendritic cell-specific intercellular adhesion molecule 3 grabbing nonintegrin (DC-SIGN), a pathogenic host factor in viral infections. By exploring ligand rigidification and hydrogen bond engineering, a monovalent glycomimetic with an unprecedented affinity for DC-SIGN in the low μM range was discovered. A matched molecular pair analysis based on microcalorimetric data revealed a stereospecific hydrogen bond interaction with Glu358/Ser360 as the origin of this cooperative and enthalpically dominated interaction. This detailed insight into the binding mechanism paves the way for an improvement of monovalent glycomimetics targeting DC-SIGN.

Myeloid C-type lectin receptors in innate immune recognition

C-type lectin receptors (CLRs) expressed by myeloid cells constitute a versatile family of receptors that play a key role in innate immune recognition. Myeloid CLRs exhibit a remarkable ability to recognize an extensive array of ligands, from carbohydrates and beyond, and encompass pattern-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), and markers of altered self. These receptors, classified into distinct subgroups, play pivotal roles in immune recognition and modulation of immune responses. Their intricate signaling pathways orchestrate a spectrum of cellular responses, influencing processes such as phagocytosis, cytokine production, and antigen presentation. Beyond their contributions to host defense in viral, bacterial, fungal, and parasitic infections, myeloid CLRs have been implicated in non-infectious diseases such as cancer, allergies, and autoimmunity. A nuanced understanding of myeloid CLR interactions with endogenous and microbial triggers is starting to uncover the context-dependent nature of their roles in innate immunity, with implications for therapeutic intervention.

Common pathways targeted by viral hemorrhagic fever viruses to infect the placenta and increase the risk of stillbirth

Viral hemorrhagic fevers (VHF) are endemic to Africa, South America and Asia and contribute to significant maternal and fetal morbidity and mortality. Viruses causing VHFs are typically zoonotic, spreading to humans through livestock, wildlife, or mosquito vectors. Some of the most lethal VHF viruses also impart a high-risk of stillbirth including ebolaviruses, Marburg virus (MARV), Lassa virus (LASV), and Rift Valley Fever Virus (RVFV). Large outbreaks and epidemics are common, though the impact on the mother, fetus and placenta is understudied from a public health, clinical and basic science perspective. Notably, these viruses utilize ubiquitous cellular surface entry receptors critical for normal placental function to enable viral invasion into multiple key cell types of the placenta and set the stage for maternal-fetal transmission and stillbirth. We employ insights from molecular virology and viral immunology to discuss how trophoblast expression of viral entry receptors for VHF viruses may increase the risk for viral transmission to the fetus and stillbirth. As the frequency of VHF outbreaks is expected to increase with worsening climate change, understanding the pathogenesis of VHF-related diseases in the placenta is paramount to predicting the impact of emerging viruses on the placenta and perinatal outcomes.

Role of Siglecs in viral infections: A double-edged sword interaction

Sialic-acid-binding immunoglobulin-like lectins are cell surface immune receptors known as Siglecs that play a paramount role as modulators of immunity. In recent years, research has underscored how the underlaying biology of this family of receptors influences the outcome of viral infections. While Siglecs are needed to promote effective antiviral immune responses, they can also pave the way to viral dissemination within tissues. Here, we review how recent preclinical findings focusing on the interplay between Siglecs and viruses may translate into promising broad-spectrum therapeutic interventions or key biomarkers to monitor the course of viral infections.

Pseudotyped Viruses for Marburgvirus and Ebolavirus

Marburg virus (MARV) and Ebola virus (EBOV) of the Filoviridae family are the most lethal viruses in terms of mortality rate. However, the development of antiviral treatment is hampered by the requirement for biosafety level-4 (BSL-4) containment. The establishment of BSL-2 pseudotyped viruses can provide important tools for the study of filoviruses. This chapter summarizes general information on the filoviruses and then focuses on the construction of replication-deficient pseudotyped MARV and EBOV (e.g., lentivirus system and vesicular stomatitis virus system). It also details the potential applications of the pseudotyped viruses, including neutralization antibody detection, the study of infection mechanisms, the evaluation of antibody-dependent enhancement, virus entry inhibitor screening, and glycoprotein mutation analysis.

Filoviruses: Innate Immunity, Inflammatory Cell Death, and Cytokines

Filoviruses are a group of single-stranded negative sense RNA viruses. The most well-known filoviruses that affect humans are ebolaviruses and marburgviruses. During infection, they can cause life-threatening symptoms such as inflammation, tissue damage, and hemorrhagic fever, with case fatality rates as high as 90%. The innate immune system is the first line of defense against pathogenic insults such as filoviruses. Pattern recognition receptors (PRRs), including toll-like receptors, retinoic acid-inducible gene-I-like receptors, C-type lectin receptors, AIM2-like receptors, and NOD-like receptors, detect pathogens and activate downstream signaling to induce the production of proinflammatory cytokines and interferons, alert the surrounding cells to the threat, and clear infected and damaged cells through innate immune cell death. However, filoviruses can modulate the host inflammatory response and innate immune cell death, causing an aberrant immune reaction. Here, we discuss how the innate immune system senses invading filoviruses and how these deadly pathogens interfere with the immune response. Furthermore, we highlight the experimental difficulties of studying filoviruses as well as the current state of filovirus-targeting therapeutics.

Macrophage infection, activation, and histopathological findings in ebolavirus infection

Macrophages contribute to Ebola virus disease through their susceptibility to direct infection, their multi-faceted response to ebolaviruses, and their association with pathological findings in tissues throughout the body. Viral attachment and entry factors, as well as the more recently described influence of cell polarization, shape macrophage susceptibility to direct infection. Moreover, the study of Toll-like receptor 4 and the RIG-I-like receptor pathway in the macrophage response to ebolaviruses highlight important immune signaling pathways contributing to the breadth of macrophage responses. Lastly, the deep histopathological catalogue of macrophage involvement across numerous tissues during infection has been enriched by descriptions of tissues involved in sequelae following acute infection, including: the eye, joints, and the nervous system. Building upon this knowledge base, future opportunities include characterization of macrophage phenotypes beneficial or deleterious to survival, delineation of the specific roles macrophages play in pathological lesion development in affected tissues, and the creation of macrophage-specific therapeutics enhancing the beneficial activities and reducing the deleterious contributions of macrophages to the outcome of Ebola virus disease.

Glycan shield of the ebolavirus envelope glycoprotein GP

The envelope glycoprotein GP of the ebolaviruses is essential for host cell entry and the primary target of the host antibody response. GP is heavily glycosylated with up to 17 N-linked sites, numerous O-linked glycans in its disordered mucin-like domain (MLD), and three predicted C-linked mannosylation sites. Glycosylation is important for host cell attachment, GP stability and fusion activity, and shielding from neutralization by serum antibodies. Here, we use glycoproteomics to profile the site-specific glycosylation patterns of ebolavirus GP. We detect up to 16 unique O-linked glycosylation sites in the MLD, and two O-linked sites in the receptor-binding GP1 subunit. Multiple O-linked glycans are observed within N-linked glycosylation sequons, suggesting crosstalk between the two types of modifications. We confirmed C-mannosylation of W288 in full-length trimeric GP. We find complex glycosylation at the majority of N-linked sites, while the conserved sites N257 and especially N563 are enriched in unprocessed glycans, suggesting a role in host-cell attachment via DC-SIGN/L-SIGN. Our findings illustrate how N-, O-, and C-linked glycans together build the heterogeneous glycan shield of GP, guiding future immunological studies and functional interpretation of ebolavirus GP-antibody interactions.

Site-specific N-, O-, and C-linked glycans are characterized in the ebolavirus envelope glycoprotein GP using mass spectrometry-based glycoproteomics.

Adenovirus receptors on antigen-presenting cells of the skin

Skin, the largest human organ, is part of the first line of physical and immunological defense against many pathogens. Understanding how skin antigen-presenting cells (APCs) respond to viruses or virus-based vaccines is crucial to develop antiviral pharmaceutics, and efficient and safe vaccines. Here, we discuss the way resident and recruited skin APCs engage adenoviruses and the impact on innate immune responses. This article is protected by copyright. All rights reserved.

Tinker, tailor, soldier, cell: the role of C-type lectins in the defense and promotion of disease

C-type lectins (CTLs) represent a large family of soluble and membrane-bound proteins which bind calcium dependently via carbohydrate recognition domains (CRDs) to glycan residues presented on the surface of a variety of pathogens. The deconvolution of a cell's glycan code by CTLs underpins several important physiological processes in mammals such as pathogen neutralization and opsonization, leukocyte trafficking, and the inflammatory response. However, as our knowledge of CTLs has developed it has become apparent that the role of this innate immune family of proteins can be double-edged, where some pathogens have developed approaches to subvert and exploit CTL interactions to promote infection and sustain the pathological state. Equally, CTL interactions with host glycoproteins can contribute to inflammatory diseases such as arthritis and cancer whereby, in certain contexts, they exacerbate inflammation and drive malignant progression. This review discusses the 'dual agent' roles of some of the major mammalian CTLs in both resolving and promoting infection, inflammation and inflammatory disease and highlights opportunities and emerging approaches for their therapeutic modulation.

Comprehensive Profiling Analysis of CD209 in Malignancies Reveals the Therapeutic Implication for Tumor Patients Infected With SARS-CoV-2

Coronavirus disease 2019 (COVID-19), which is known to be caused by the virus severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), is characterized by pneumonia, cytokine storms, and lymphopenia. Patients with malignant tumors may be particularly vulnerable to SARS-CoV-2 infection and possibly more susceptible to severe complications due to immunosuppression. Recent studies have found that CD209 (DC-SIGN) might be a potential binding receptor for SARS-CoV-2 in addition to the well-known receptor ACE2. However, pan-cancer studies of CD209 remain unclear. In this study, we first comprehensively investigated the expression profiles of CD209 in malignancies in both pan-carcinomas and healthy tissues based on bioinformatic techniques. The CD209 expression declined dramatically in various cancer types infected by SARS-CoV-2. Remarkably, CD209 was linked with diverse immune checkpoint genes and infiltrating immune cells. These findings indicate that the elevation of CD209 among specific cancer patients may delineate a mechanism accounting for a higher vulnerability to infection by SARS-CoV-2, as well as giving rise to cytokine storms. Taken together, CD209 plays critical roles in both immunology and metabolism in various cancer types. Pharmacological inhibition of CD209 antigen (D-mannose), together with other anti-SARS-CoV-2 strategies, might provide beneficial therapeutic effects in specific cancer patients.

Platelets in Viral Infections - Brave Soldiers or Trojan Horses

Viral infections are often associated with platelet activation and haemostatic complications. In line, low platelet counts represent a hallmark for poor prognosis in many infectious diseases. The underlying cause of platelet dysfunction in viral infections is multifaceted and complex. While some viruses directly interact with platelets and/or megakaryocytes to modulate their function, also immune and inflammatory responses directly and indirectly favour platelet activation. Platelet activation results in increased platelet consumption and degradation, which contributes to thrombocytopenia in these patients. The role of platelets is often bi-phasic. Initial platelet hyper-activation is followed by a state of platelet exhaustion and/or hypo-responsiveness, which together with low platelet counts promotes bleeding events. Thereby infectious diseases not only increase the thrombotic but also the bleeding risk or both, which represents a most dreaded clinical complication. Treatment options in these patients are limited and new therapeutic strategies are urgently needed to prevent adverse outcome. This review summarizes the current literature on platelet-virus interactions and their impact on viral pathologies and discusses potential intervention strategies. As pandemics and concomitant haemostatic dysregulations will remain a recurrent threat, understanding the role of platelets in viral infections represents a timely and pivotal challenge.

CD164 is a host factor for lymphocytic choriomeningitis virus entry

Lymphocytic choriomeningitis virus (LCMV) is the prototypic arenavirus and has been utilized for decades as a model to understand the host immune response against viral infection. LCMV infection can lead to fatal meningitis in immunocompromised people and can lead to congenital birth defects and spontaneous abortion if acquired during pregnancy. Using a genetic screen, we uncover host factors involved in LCMV entry that were previously unknown and are candidate therapeutic targets to combat LCMV infection. This study expands our understanding of the entry pathway of LCMV, revealing that its glycoprotein switches from utilizing the known receptor α-DG and heparan sulfate at the plasma membrane to binding the lysosomal mucin CD164 at pH levels found in endolysosomal compartments, facilitating membrane fusion.

Lymphocytic choriomeningitis virus (LCMV) is a rodent-borne zoonotic arenavirus that causes congenital abnormalities and can be fatal for transplant recipients. Using a genome-wide loss-of-function screen, we identify host factors required for LCMV entry into cells. We identify the lysosomal mucin CD164, glycosylation factors, the heparan sulfate biosynthesis machinery, and the known receptor alpha-dystroglycan (α-DG). Biochemical analysis revealed that the LCMV glycoprotein binds CD164 at acidic pH and requires a sialylated glycan at residue N104. We demonstrate that LCMV entry proceeds by the virus switching binding from heparan sulfate or α-DG at the plasma membrane to CD164 prior to membrane fusion, thus identifying additional potential targets for therapeutic intervention.

TET2 is Required for Type I IFN-mediated Inhibition of Bat-Origin Swine Acute Diarrhea Syndrome Coronavirus

Swine acute diarrhea syndrome coronavirus (SADS-CoV) is a newly discovered bat-origin coronavirus with fatal pathogenicity for neonatal piglets. There is no vaccine to prevent SADS-CoV infection or clinically approved drugs targeting SADS-CoV. Therefore, unraveling cellular factors that regulate SADS-CoV for cell entry is critical to understanding the viral transmission mechanism and provides a potential therapeutic target for SADS-CoV cure. Here, we showed that type I interferon (IFN-I) pretreatment potently blocks SADS-CoV entry into cells using lentiviral pseudo-virions as targets whose entry is driven by the SADS-CoV Spike glycoprotein. IFN-I-mediated inhibition of SADS-CoV entry and replication was dramatically impaired in the absence of TET2. These results suggest TET2 is found to serve as a checkpoint of IFN-I-meditated inhibition on the cell entry of SADS-CoV, and our discovery might constitute a novel treatment option to combat against SADS-CoV. This article is protected by copyright. All rights reserved.

Stepwise Enzymatic-Dependent Mechanism of Ebola Virus Binding to Cell Surface Receptors Monitored by AFM

Ebola virus (EBOV) is responsible for several outbreaks of hemorrhagic fever with high mortality, raising great public concern. Several cell surface receptors have been identified to mediate EBOV binding and internalization, including phosphatidylserine (PS) receptors (TIM-1) and C-type lectin receptors (DC-SIGNR). However, the role of TIM-1 during early cell surface binding remains elusive and in particular whether TIM-1 acts as a specific receptor for EBOV. Here, we used force-distance curve-based atomic force microscopy (FD-based AFM) to quantify the binding between TIM-1/DC-SIGNR and EBOV glycoprotein (GP) and observed that both receptors specifically bind to GP with high-affinity. Since TIM-1 can also directly interact with PS at the single-molecule level, we also confirmed that TIM-1 acts as dual-function receptors of EBOV. These results highlight the direct involvement of multiple high-affinity receptors in the first steps of binding to cell surfaces, thus offering new perspectives for the development of anti-EBOV therapeutic molecules.

Characterization of Covid-19 infected pregnant women sera using laboratory indexes, vibrational spectroscopy, and machine learning classifications

Herein, we show differences in blood serum of asymptomatic and symptomatic pregnant women infected with COVID-19 and correlate them with laboratory indexes, ATR FTIR and multivariate machine learning methods. We collected the sera of COVID-19 diagnosed pregnant women, in the second trimester (n = 12), third-trimester (n = 7), and second-trimester with severe symptoms (n = 7) compared to the healthy pregnant (n = 11) women, which makes a total of 37 participants. To assign the accuracy of FTIR spectra regions where peak shifts occurred, the Random Forest algorithm, traditional C5.0 single decision tree algorithm and deep neural network approach were used. We verified the correspondence between the FTIR results and the laboratory indexes such as: the count of peripheral blood cells, biochemical parameters, and coagulation indicators of pregnant women. CH₂ scissoring, amide II, amide I vibrations could be used to differentiate the groups. The accuracy calculated by machine learning methods was higher than 90%. We also developed a method based on the dynamics of the absorbance spectra allowing to determine the differences between the spectra of healthy and COVID-19 patients. Laboratory indexes of biochemical parameters associated with COVID-19 validate changes in the total amount of proteins, albumin and lipase.

Immune Response to Viruses

Viruses are essentially, obligate intracellular parasites. They require a host to replicate their genetic material, spread to other cells, and eventually to other hosts. For humans, most viral infections are not considered lethal, regardless if at the cellular level, the virus can obliterate individual cells. Constant genomic mutations, (which can alter the antigenic content of viruses such as influenza or coronaviruses), zoonosis or immunosuppression/immunocompromisation, is when viruses achieve higher host mortality. Frequent examples of the severe consequenses of viral infection can be seen in children and the elderly. In most instances, the immune system will take a multifaceted approach in defending the host against viruses. Depending on the virus, the individual, and the point of entry, the immune system will initiate a robust response which involves multiple components. In this chapter, we expand on the total immune system, breaking it down to the two principal types: Innate and Adaptive Immunity, their different roles in viral recognition and clearance. Finally, how different viruses activate and evade different arms of the immune system.

Medicinal chemistry of the myeloid C-type lectin receptors Mincle, Langerin, and DC-SIGN

In their role as pattern-recognition receptors on cells of the innate immune system, myeloid C-type lectin receptors (CLRs) assume important biological functions related to immunity, homeostasis, and cancer. As such, this family of receptors represents an appealing target for therapeutic interventions for modulating the outcome of many pathological processes, in particular related to infectious diseases. This review summarizes the current state of research into glycomimetic or drug-like small molecule ligands for the CLRs Mincle, Langerin, and DC-SIGN, which have potential therapeutic applications in vaccine research and anti-infective therapy.

C-type lectin receptor DCIR contributes to hippocampal injury in acute neurotropic virus infection

Neurotropic viruses target the brain and contribute to neurologic diseases. C-type lectin receptors (CLRs) are pattern recognition receptors that recognize carbohydrate structures on endogenous molecules and pathogens. The myeloid CLR dendritic cell immunoreceptor (DCIR) is expressed by antigen presenting cells and mediates inhibitory intracellular signalling. To investigate the effect of DCIR on neurotropic virus infection, mice were infected experimentally with Theiler’s murine encephalomyelitis virus (TMEV). Brain tissue of TMEV-infected C57BL/6 mice and DCIR^−/− mice were analysed by histology, immunohistochemistry and RT-qPCR, and spleen tissue by flow cytometry. To determine the impact of DCIR deficiency on T cell responses upon TMEV infection in vitro, antigen presentation assays were utilised. Genetic DCIR ablation in C57BL/6 mice was associated with an ameliorated hippocampal integrity together with reduced cerebral cytokine responses and reduced TMEV loads in the brain. Additionally, absence of DCIR favoured increased peripheral cytotoxic CD8⁺ T cell responses following TMEV infection. Co-culture experiments revealed that DCIR deficiency enhances the activation of antigen-specific CD8⁺ T cells by virus-exposed dendritic cells (DCs), indicated by increased release of interleukin-2 and interferon-γ. Results suggest that DCIR deficiency has a supportive influence on antiviral immune mechanisms, facilitating virus control in the brain and ameliorates neuropathology during acute neurotropic virus infection.

Characterization of the therapeutic effect of antibodies targeting the Ebola glycoprotein using a novel BSL2-compliant rVSVΔG-EBOV-GP infection model

Ebola virus (EBOV) infections cause haemorrhagic fever, multi-organ failure and death, and survivors can experience neurological sequelae. Licensing of monoclonal antibodies targeting EBOV glycoprotein (EBOV-GP) improved its prognosis, however, this treatment is primarily effective during early stages of disease and its effectiveness in reducing neurological sequela remains unknown. Currently, the need for BSL4 containment hinders research and therapeutic development; development of an accessible BSL-2 in vivo mouse model would facilitate preclinical studies to screen and select therapeutics. Previously, we have shown that a subcutaneous inoculation with replicating EBOV-GP pseudotyped vesicular stomatitis virus (rVSVΔG-EBOV-GP or VSV-EBOV) in neonatal mice causes transient viremia and infection of the mid and posterior brain resulting in overt neurological symptoms and death. Here, we demonstrate that the model can be used to test therapeutics that target the EBOV-GP, by using an anti-EBOV-GP therapeutic (SAB-139) previously shown to block EBOV infection in mice and primates. We show that SAB-139 treatment decreases the severity of neurological symptoms and improves survival when administered before (1 day prior to infection) or up to 3 dpi, by which time animals have high virus titres in their brains. Improved survival was associated with reduced viral titres, microglia loss, cellular infiltration/activation, and inflammatory responses in the brain. Interestingly, SAB-139 treatment significantly reduced the severe VSV-EBOV-induced long-term neurological sequalae although convalescent mice showed modest evidence of abnormal fear responses. Together, these data suggest that the neonatal VSV-EBOV infection system can be used to facilitate assessment of therapeutics targeting EBOV-GP in the preclinical setting.

Production of Recombinant Replication-defective Lentiviruses Bearing the SARS-CoV or SARS-CoV-2 Attachment Spike Glycoprotein and Their Application in Receptor Tropism and Neutralisation Assays

For enveloped viruses, such as SARS-CoV-2, transmission relies on the binding of viral glycoproteins to cellular receptors. Conventionally, this process is recapitulated in the lab by infection of cells with isolated live virus. However, such studies can be restricted due to the availability of high quantities of replication-competent virus, biosafety precautions and associated trained staff. Here, we present a protocol based on pseudotyping to produce recombinant replication-defective lentiviruses bearing the SARS-CoV or SARS-CoV-2 attachment Spike glycoprotein, allowing the investigation of viral entry in a lower-containment facility. Pseudoparticles are produced by cells transiently transfected with plasmids encoding retroviral RNA packaging signals and Gag-Pol proteins, for the reconstitution of lentiviral particles, and a plasmid coding for the viral attachment protein of interest. This approach allows the investigation of different aspects of viral entry, such as the identification of receptor tropism, the prediction of virus host range, and zoonotic transmission potential, as well as the characterisation of antibodies (sera or monoclonal antibodies) and pharmacological inhibitors that can block entry. Graphic abstract: SARS-CoV and SARS-CoV-2 pseudoparticle generation and applications.

Glycan Epitopes and Potential Glycoside Antagonists of DC-SIGN Involved in COVID-19: In Silico Study

Glycosylation is an important post-translational modification that affects a wide variety of physiological functions. DC-SIGN (Dendritic Cell-Specific Intercellular adhesion molecule-3-Grabbing Non-integrin) is a protein expressed in antigen-presenting cells that recognizes a variety of glycan epitopes. Until now, the binding of DC-SIGN to SARS-CoV-2 Spike glycoprotein has been reported in various articles and is regarded to be a factor in systemic infection and cytokine storm. The mechanism of DC-SIGN recognition offers an alternative method for discovering new medication for COVID-19 treatment. Here, we discovered three potential pockets that hold different glycan epitopes by performing molecular dynamics simulations of previously reported oligosaccharides. The "EPN" motif, "NDD" motif, and Glu354 form the most critical pocket, which is known as the Core site. We proposed that the type of glycan epitopes, rather than the precise amino acid sequence, determines the recognition. Furthermore, we deduced that oligosaccharides could occupy an additional site, which adds to their higher affinity than monosaccharides. Based on our findings and previously described glycoforms on the SARS-CoV-2 Spike, we predicted the potential glycan epitopes for DC-SIGN. It suggested that glycan epitopes could be recognized at multiple sites, not just Asn234, Asn149 and Asn343. Subsequently, we found that Saikosaponin A and Liquiritin, two plant glycosides, were promising DC-SIGN antagonists in silico.

Sweet Drugs for Bad Bugs: A Glycomimetic Strategy against the DC-SIGN-Mediated Dissemination of SARS-CoV-2

The C-type lectin receptor DC-SIGN is a pattern recognition receptor expressed on macrophages and dendritic cells. It has been identified as a promiscuous entry receptor for many pathogens, including epidemic and pandemic viruses such as SARS-CoV-2, Ebola virus, and HIV-1. In the context of the recent SARS-CoV-2 pandemic, DC-SIGN-mediated virus dissemination and stimulation of innate immune responses has been implicated as a potential factor in the development of severe COVID-19. Inhibition of virus binding to DC-SIGN, thus, represents an attractive host-directed strategy to attenuate overshooting innate immune responses and prevent the progression of the disease. In this study, we report on the discovery of a new class of potent glycomimetic DC-SIGN antagonists from a focused library of triazole-based mannose analogues. Structure-based optimization of an initial screening hit yielded a glycomimetic ligand with a more than 100-fold improved binding affinity compared to methyl α-d-mannopyranoside. Analysis of binding thermodynamics revealed an enthalpy-driven improvement of binding affinity that was enabled by hydrophobic interactions with a loop region adjacent to the binding site and displacement of a conserved water molecule. The identified ligand was employed for the synthesis of multivalent glycopolymers that were able to inhibit SARS-CoV-2 spike glycoprotein binding to DC-SIGN-expressing cells, as well as DC-SIGN-mediated trans-infection of ACE2⁺ cells by SARS-CoV-2 spike protein-expressing viruses, in nanomolar concentrations. The identified glycomimetic ligands reported here open promising perspectives for the development of highly potent and fully selective DC-SIGN-targeted therapeutics for a broad spectrum of viral infections.

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.

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.

The Methanolic Extract of Perilla frutescens Robustly Restricts Ebola Virus Glycoprotein-Mediated Entry

Ebola virus (EBOV), one of the most infectious human viruses and a leading cause of viral hemorrhagic fever, imposes a potential public health threat with several recent outbreaks. Despite the difficulties associated with working with this pathogen in biosafety level-4 containment, a protective vaccine and antiviral therapeutic were recently approved. However, the high mortality rate of EBOV infection underscores the necessity to continuously identify novel antiviral strategies to help expand the scope of prophylaxis/therapeutic management against future outbreaks. This includes identifying antiviral agents that target EBOV entry, which could improve the management of EBOV infection. Herein, using EBOV glycoprotein (GP)-pseudotyped particles, we screened a panel of natural medicinal extracts, and identified the methanolic extract of Perilla frutescens (PFME) as a robust inhibitor of EBOV entry. We show that PFME dose-dependently impeded EBOV GP-mediated infection at non-cytotoxic concentrations, and exerted the most significant antiviral activity when both the extract and the pseudoparticles are concurrently present on the host cells. Specifically, we demonstrate that PFME could block viral attachment and neutralize the cell-free viral particles. Our results, therefore, identified PFME as a potent inhibitor of EBOV entry, which merits further evaluation for development as a therapeutic strategy against EBOV infection.

Activating Natural Killer Cell Receptors, Selectins, and Inhibitory Siglecs Recognize Ebolavirus Glycoprotein

Expression of the extensively glycosylated Ebolavirus glycoprotein (EBOV-GP) induces physical alterations of surface molecules and plays a crucial role in viral pathogenicity. Here we investigate the interactions of EBOV-GP with host surface molecules using purified EBOV-GP, EBOV-GP-transfected cell lines, and EBOV-GP-pseudotyped lentiviral particles. Subsequently, we wanted to examine which receptors are involved in this recognition by binding studies to cells transfected with the EBOV-GP as well as to recombinant soluble EBOV-GP. As the viral components can also bind to inhibitory receptors of immune cells (e.g., Siglecs, TIM-1), they can even suppress the activity of immune effector cells. Our data show that natural killer (NK) cell receptors NKp44 and NKp46, selectins (CD62E/P/L), the host factors DC-SIGNR/DC-SIGN, and inhibitory Siglecs function as receptors for EBOV-GP. Our results show also moderate to strong avidity of homing receptors (P-, L-, and E-selectin) and DC-SIGNR/DC-SIGN to purified EBOV-GP, to cells transfected with EBOV-GP, as well as to the envelope of a pseudotyped lentiviral vector carrying the EBOV-GP. The concomitant activation and inhibition of the immune system exemplifies the evolutionary antagonism between the immune system and pathogens. Altogether these interactions with activating and inhibitory receptors result in a reduced NK cell-mediated lysis of EBOV-GP-expressing cells. Modulation of these interactions may provide new strategies for treating infections caused by this virus.

Maturation signatures of conventional dendritic cell subtypes in COVID-19 suggest direct viral sensing

Growing evidence suggests that conventional dendritic cells (cDCs) undergo aberrant maturation in COVID‐19, which negatively affects T‐cell activation. The presence of effector T cells in patients with mild disease and dysfunctional T cells in severely ill patients suggests that adequate T‐cell responses limit disease severity. Understanding how cDCs cope with SARS‐CoV‐2 can help elucidate how protective immune responses are generated. Here, we report that cDC2 subtypes exhibit similar infection‐induced gene signatures, with the upregulation of IFN‐stimulated genes and IL‐6 signaling pathways. Furthermore, comparison of cDCs between patients with severe and mild disease showed severely ill patients to exhibit profound downregulation of genes encoding molecules involved in antigen presentation, such as MHCII, TAP, and costimulatory proteins, whereas we observed the opposite for proinflammatory molecules, such as complement and coagulation factors. Thus, as disease severity increases, cDC2s exhibit enhanced inflammatory properties and lose antigen presentation capacity. Moreover, DC3s showed upregulation of anti‐apoptotic genes and accumulated during infection. Direct exposure of cDC2s to the virus in vitro recapitulated the activation profile observed in vivo. Our findings suggest that SARS‐CoV‐2 interacts directly with cDC2s and implements an efficient immune escape mechanism that correlates with disease severity by downregulating crucial molecules required for T‐cell activation.

Impact of Protein Glycosylation on the Design of Viral Vaccines

Glycans play crucial roles in various biological processes such as cell proliferation, cell-cell interactions, and immune responses. Since viruses co-opt cellular biosynthetic pathways, viral glycosylation mainly depends on the host cell glycosylation machinery. Consequently, several viruses exploit the cellular glycosylation pathway to their advantage. It was shown that viral glycosylation is strongly dependent on the host system selected for virus propagation and/or protein expression. Therefore, the use of different expression systems results in various glycoforms of viral glycoproteins that may differ in functional properties. These differences clearly illustrate that the choice of the expression system can be important, as the resulting glycosylation may influence immunological properties. In this review, we will first detail protein N- and O-glycosylation pathways and the resulting glycosylation patterns; we will then discuss different aspects of viral glycosylation in pathogenesis and in vaccine development; and finally, we will elaborate on how to harness viral glycosylation in order to optimize the design of viral vaccines. To this end, we will highlight specific examples to demonstrate how glycoengineering approaches and exploitation of different expression systems could pave the way towards better self-adjuvanted glycan-based viral vaccines.

CD209L/L-SIGN and CD209/DC-SIGN Act as Receptors for SARS-CoV-2

As the COVID-19 pandemic continues to spread, investigating the processes underlying the interactions between SARS-CoV-2 and its hosts is of high importance. Here, we report the identification of CD209L/L-SIGN and the related protein CD209/DC-SIGN as receptors capable of mediating SARS-CoV-2 entry into human cells. Immunofluorescence staining of human tissues revealed prominent expression of CD209L in the lung and kidney epithelia and endothelia. Multiple biochemical assays using a purified recombinant SARS-CoV-2 spike receptor-binding domain (S-RBD) or S1 encompassing both N termal domain and RBD and ectopically expressed CD209L and CD209 revealed that CD209L and CD209 interact with S-RBD. CD209L contains two N-glycosylation sequons, at sites N92 and N361, but we determined that only site N92 is occupied. Removal of the N-glycosylation at this site enhances the binding of S-RBD with CD209L. CD209L also interacts with ACE2, suggesting a role for heterodimerization of CD209L and ACE2 in SARS-CoV-2 entry and infection in cell types where both are present. Furthermore, we demonstrate that human endothelial cells are permissive to SARS-CoV-2 infection, and interference with CD209L activity by a knockdown strategy or with soluble CD209L inhibits virus entry. Our observations demonstrate that CD209L and CD209 serve as alternative receptors for SARS-CoV-2 in disease-relevant cell types, including the vascular system. This property is particularly important in tissues where ACE2 has low expression or is absent and may have implications for antiviral drug development.

Poly-l-lysine Glycoconjugates Inhibit DC-SIGN-mediated Attachment of Pandemic Viruses

The C-type lectin receptor DC-SIGN mediates interactions with envelope glycoproteins of many viruses such as SARS-CoV-2, ebola, and HIV and contributes to virus internalization and dissemination. In the context of the recent SARS-CoV-2 pandemic, involvement of DC-SIGN has been linked to severe cases of COVID-19. Inhibition of the interaction between DC-SIGN and viral glycoproteins has the potential to generate broad spectrum antiviral agents. Here, we demonstrate that mannose-functionalized poly-l-lysine glycoconjugates efficiently inhibit the attachment of viral glycoproteins to DC-SIGN-presenting cells with picomolar affinity. Treatment of these cells leads to prolonged receptor internalization and inhibition of virus binding for up to 6 h. Furthermore, the polymers are fully bio-compatible and readily cleared by target cells. The thermodynamic analysis of the multivalent interactions reveals enhanced enthalpy-driven affinities and promising perspectives for the future development of multivalent therapeutics.

CD209L/L-SIGN and CD209/DC-SIGN act as receptors for SARS-CoV-2

As the COVID-19 pandemic continues to spread, investigating the processes underlying the interactions between SARS-CoV-2 and its hosts is of high importance. Here, we report the identification of CD209L/L-SIGN and the related protein CD209/DC-SIGN as receptors capable of mediating SARS-CoV-2 entry into human cells. Immunofluorescence staining of human tissues revealed prominent expression of CD209L in the lung and kidney epithelium and endothelium. Multiple biochemical assays using a purified recombinant SARS-CoV-2 spike receptor binding domain (S-RBD) or S1 encompassing both NTB and RBD and ectopically expressed CD209L and CD209 revealed that CD209L and CD209 interact with S-RBD. CD209L contains two N-glycosylation sequons, at sites N92 and N361, but we determined that only site N92 is occupied. Removal of the N-glycosylation at this site enhances the binding of S-RBD with CD209L. CD209L also interacts with ACE2, suggesting a role for heterodimerization of CD209L and ACE2 in SARS-CoV-2 entry and infection in cell types where both are present. Furthermore, we demonstrate that human endothelial cells are permissive to SARS-CoV-2 infection and interference with CD209L activity by knockdown strategy or with soluble CD209L inhibits virus entry. Our observations demonstrate that CD209L and CD209 serve as alternative receptors for SARS-CoV-2 in disease-relevant cell types, including the vascular system. This property is particularly important in tissues where ACE2 has low expression or is absent, and may have implications for antiviral drug development.

Developments in Treatment Methodologies Using Dendrimers for Infectious Diseases

Dendrimers comprise a specific group of macromolecules, which combine structural properties of both single molecules and long expanded polymers. The three-dimensional form of dendrimers and the extensive possibilities for use of additional substrates for their construction creates a multivalent potential and a wide possibility for medical, diagnostic and environmental purposes. Depending on their composition and structure, dendrimers have been of interest in many fields of science, ranging from chemistry, biotechnology to biochemical applications. These compounds have found wide application from the production of catalysts for their use as antibacterial, antifungal and antiviral agents. Of particular interest are peptide dendrimers as a medium for transport of therapeutic substances: synthetic vaccines against parasites, bacteria and viruses, contrast agents used in MRI, antibodies and genetic material. This review focuses on the description of the current classes of dendrimers, the methodology for their synthesis and briefly drawbacks of their properties and their use as potential therapies against infectious diseases.

Ceramide and Related Molecules in Viral Infections

Ceramide is a lipid messenger at the heart of sphingolipid metabolism. In concert with its metabolizing enzymes, particularly sphingomyelinases, it has key roles in regulating the physical properties of biological membranes, including the formation of membrane microdomains. Thus, ceramide and its related molecules have been attributed significant roles in nearly all steps of the viral life cycle: they may serve directly as receptors or co-receptors for viral entry, form microdomains that cluster entry receptors and/or enable them to adopt the required conformation or regulate their cell surface expression. Sphingolipids can regulate all forms of viral uptake, often through sphingomyelinase activation, and mediate endosomal escape and intracellular trafficking. Ceramide can be key for the formation of viral replication sites. Sphingomyelinases often mediate the release of new virions from infected cells. Moreover, sphingolipids can contribute to viral-induced apoptosis and morbidity in viral diseases, as well as virus immune evasion. Alpha-galactosylceramide, in particular, also plays a significant role in immune modulation in response to viral infections. This review will discuss the roles of ceramide and its related molecules in the different steps of the viral life cycle. We will also discuss how novel strategies could exploit these for therapeutic benefit.

Engineered Human Monoclonal scFv to Receptor Binding Domain of Ebolavirus

(1) Background: Ebolavirus (EBOV) poses as a significant threat for human health by frequently causing epidemics of the highly contagious Ebola virus disease (EVD). EBOV glycoprotein (GP), as a sole surface glycoprotein, needs to be cleaved in endosomes to fully expose a receptor-binding domain (RBD) containing a receptor-binding site (RBS) for receptor binding and genome entry into cytoplasm for replication. RBDs are highly conserved among EBOV species, so they are an attractive target for broadly effective anti-EBOV drug development. (2) Methods: Phage display technology was used as a tool to isolate human single-chain antibodies (HuscFv) that bind to recombinant RBDs from a human scFv (HuscFv) phage display library. The RBD-bound HuscFvs were fused with cell-penetrating peptide (CPP), and cell-penetrating antibodies (transbodies) were made, produced from the phage-infected E. coli clones and characterized. (3) Results: Among the HuscFvs obtained from phage-infected E. coli clones, HuscFvs of three clones, HuscFv4, HuscFv11, and HuscFv14, the non-cell-penetrable or cell-penetrable HuscFv4 effectively neutralized cellular entry of EBOV-like particles (VLPs). While all HuscFvs were found to bind cleaved GP (GPcl), their presumptive binding sites were markedly different, as determined by molecular docking. (4) Conclusions: The HuscFv4 could be a promising therapeutic agent against EBOV infection.

The SARS-CoV-2 and other human coronavirus spike proteins are fine-tuned towards temperature and proteases of the human airways

The high transmissibility of SARS-CoV-2 is related to abundant replication in the upper airways, which is not observed for the other highly pathogenic coronaviruses SARS-CoV and MERS-CoV. We here reveal features of the coronavirus spike (S) protein, which optimize the virus towards the human respiratory tract. First, the S proteins exhibit an intrinsic temperature preference, corresponding with the temperature of the upper or lower airways. Pseudoviruses bearing the SARS-CoV-2 spike (SARS-2-S) were more infectious when produced at 33°C instead of 37°C, a property shared with the S protein of HCoV-229E, a common cold coronavirus. In contrast, the S proteins of SARS-CoV and MERS-CoV favored 37°C, in accordance with virus preference for the lower airways. Next, SARS-2-S-driven entry was efficiently activated by not only TMPRSS2, but also the TMPRSS13 protease, thus broadening the cell tropism of SARS-CoV-2. Both proteases proved relevant in the context of authentic virus replication. TMPRSS13 appeared an effective spike activator for the virulent coronaviruses but not the low pathogenic HCoV-229E virus. Activation of SARS-2-S by these surface proteases requires processing of the S1/S2 cleavage loop, in which both the furin recognition motif and extended loop length proved critical. Conversely, entry of loop deletion mutants is significantly increased in cathepsin-rich cells. Finally, we demonstrate that the D614G mutation increases SARS-CoV-2 stability, particularly at 37°C, and, enhances its use of the cathepsin L pathway. This indicates a link between S protein stability and usage of this alternative route for virus entry. Since these spike properties may promote virus spread, they potentially explain why the spike-G614 variant has replaced the early D614 variant to become globally predominant. Collectively, our findings reveal adaptive mechanisms whereby the coronavirus spike protein is adjusted to match the temperature and protease conditions of the airways, to enhance virus transmission and pathology.

The devastating COVID-19 pandemic is caused by SARS-CoV-2, a novel virus that despite recent zoonotic introduction is already very well adapted to its human host. Its rapid spread is related to abundant replication in the upper airways, which is not observed for other highly pathogenic human coronaviruses. To understand the role of the viral spike protein in this airway adaptation, we constructed pseudoviruses of SARS-CoV-2 and other coronaviruses that cause severe pneumonia or, on the contrary, a mild common cold. The key findings were verified with authentic virus. We reveal features of the spike proteins, which optimize the coronavirus towards specific parts of the respiratory tract. Namely, we show that the spike proteins exhibit intrinsic temperature preference to precisely match the upper (~33°C) or lower (37°C) airways. We recognized which proteases of human airways activate the spike for virus entry, in particular one protease that may mediate coronavirus virulence. Finally, a link was perceived between spike stability and entry via endosomal proteases. We propose that these mechanisms of spike fine-tuning may have contributed to a global shift in SARS-CoV-2 epidemiology, from the early spike-D614 to the currently predominating G614 variant.

Structure-Based Design of Prefusion-Stabilized Filovirus Glycoprotein Trimers

Ebola virus causes severe hemorrhagic fever, often leading to death in humans. The trimeric fusion glycoprotein (GP) is the sole target for neutralizing antibodies and is the major focus of vaccine development. Soluble GP ectodomains are unstable and mostly monomeric when not fused to a heterologous trimerization domain. Here, we report structure-based designs of Ebola and Marburg GP trimers based on a stabilizing mutation in the hinge loop in refolding region 1 and substitution of a partially buried charge at the interface of the GP1 and GP2 subunits. The combined substitutions (T577P and K588F) substantially increased trimer expression for Ebola GP proteins. We determined the crystal structure of stabilized GP from the Makona Zaire ebolavirus strain without a trimerization domain or complexed ligand. The structure reveals that the stabilized GP adopts the same trimeric prefusion conformation, provides insight into triggering of GP conformational changes, and should inform future filovirus vaccine development.

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Filovirus GP expression increases by stabilizing mutations in hinge loop and base helix

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Charged lysine in base helix and GP1 N terminus are trapped in metastable conformation

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Crystal structure of stabilized Makona Δmucin GP confirms successful stabilization

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These findings may be useful for understanding fusion mechanisms and vaccine design

The sphingosine kinase 1 activator, K6PC-5, attenuates Ebola virus infection

Ebola virus (EBOV) is responsible for outbreaks with case fatality rates of up to 90% and for an epidemic in West Africa with more than ten thousand deaths. EBOV glycoprotein (EBOV-GP) is the only viral surface protein and is responsible for viral entry into cells. Here, by employing pseudotyped EBOV-GP viral particles, we uncover a critical role for sphingolipids in inhibiting viral entry. Sphingosine kinase 1 (SphK1) catalyzes the phosphorylation of sphingosine to sphingosine 1-phosphate (S1P). The administration of the SphK1 activator, K6PC-5, or S1P, or the overexpression of SphK1 consistently exhibited striking inhibitory effects in EBOV-GP-driven entry in diverse cell lines. Finally, K6PC-5 markedly reduced the EBOV titer in infected cells and the de novo production of viral proteins. These data present K6PC-5 as an efficient tool to inhibit EBOV infection in endothelial cells and suggest further studies to evaluate its systemic effects.

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K6PC-5, a sphingosine kinase 1 activator, inhibits Ebola virus infection

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Sphingosine 1-phosphate, the product of SphK1, attenuates the viral entry

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Inhibiton/activation of S1P receptors has no influence on Ebola virus entry

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These data support the endogen effect of S1P in Ebola virus infection

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.

Immunotherapeutic strategies to target vulnerabilities in the Ebolavirus glycoprotein

The 2014 Ebola virus disease (EVD) outbreak in West Africa and the subsequent outbreaks of 2018-2020 in Equator and North Kivu provinces of the Democratic Republic of the Congo illustrate the public health challenges of emerging and reemerging viruses. EVD has a high case fatality rate with a rapidly progressing syndrome of fever, rash, vomiting, diarrhea, and bleeding diathesis. Recently, two monoclonal-antibody-based therapies received United States Food and Drug Administration (FDA) approval, and there are several other passive immunotherapies that hold promise as therapeutics against other species of Ebolavirus. Here, we review concepts needed to understand mechanisms of action, present an expanded schema to define additional sites of vulnerability on the viral glycoprotein, and review current antibody-based therapeutics. The concepts described are used to gain insights into the key characteristics that represent functional targets for immunotherapies against Zaire Ebolavirus and other emerging viruses within the Ebolavirus genus.

Endothelial Cells in Emerging Viral Infections

There are several reasons to consider the role of endothelial cells in COVID-19 and other emerging viral infections. First, severe cases of COVID-19 show a common breakdown of central vascular functions. Second, SARS-CoV-2 replicates in endothelial cells. Third, prior deterioration of vascular function exacerbates disease, as the most common comorbidities of COVID-19 (obesity, hypertension, and diabetes) are all associated with endothelial dysfunction. Importantly, SARS-CoV-2's ability to infect endothelium is shared by many emerging viruses, including henipaviruses, hantavirus, and highly pathogenic avian influenza virus, all specifically targeting endothelial cells. The ability to infect endothelium appears to support generalised dissemination of infection and facilitate the access to certain tissues. The disturbed vascular function observed in severe COVID-19 is also a prominent feature of many other life-threatening viral diseases, underscoring the need to understand how viruses modulate endothelial function. We here review the role of vascular endothelial cells in emerging viral infections, starting with a summary of endothelial cells as key mediators and regulators of vascular and immune responses in health and infection. Next, we discuss endotheliotropism as a possible virulence factor and detail features that regulate viruses' ability to attach to and enter endothelial cells. We move on to review how endothelial cells detect invading viruses and respond to infection, with particular focus on pathways that may influence vascular function and the host immune system. Finally, we discuss how endothelial cell function can be dysregulated in viral disease, either by viral components or as bystander victims of overshooting or detrimental inflammatory and immune responses. Many aspects of how viruses interact with the endothelium remain poorly understood. Considering the diversity of such mechanisms among different emerging viruses allows us to highlight common features that may be of general validity and point out important challenges.

Innate Immune Sensing of Viruses and Its Consequences for the Central Nervous System

Viral infections remain a global public health concern and cause a severe societal and economic burden. At the organismal level, the innate immune system is essential for the detection of viruses and constitutes the first line of defense. Viral components are sensed by host pattern recognition receptors (PRRs). PRRs can be further classified based on their localization into Toll-like receptors (TLRs), C-type lectin receptors (CLR), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), NOD-like receptors (NLRs) and cytosolic DNA sensors (CDS). TLR and RLR signaling results in production of type I interferons (IFNα and -β) and pro-inflammatory cytokines in a cell-specific manner, whereas NLR signaling leads to the production of interleukin-1 family proteins. On the other hand, CLRs are capable of sensing glycans present in viral pathogens, which can induce phagocytic, endocytic, antimicrobial, and pro- inflammatory responses. Peripheral immune sensing of viruses and the ensuing cytokine response can significantly affect the central nervous system (CNS). But viruses can also directly enter the CNS via a multitude of routes, such as the nasal epithelium, along nerve fibers connecting to the periphery and as cargo of infiltrating infected cells passing through the blood brain barrier, triggering innate immune sensing and cytokine responses directly in the CNS. Here, we review mechanisms of viral immune sensing and currently recognized consequences for the CNS of innate immune responses to viruses.

Identification of Important N-Linked Glycosylation Sites in the Hemagglutinin Protein and Their Functional Impact on DC-SIGN Mediated Avian Influenza H5N1 Infection

DC-SIGN, a C-type lectin mainly expressed in dendritic cells (DCs), has been reported to mediate several viral infections. We previously reported that DC-SIGN mediated H5N1 influenza A virus (AIVs) infection, however, the important DC-SIGN interaction with N-glycosylation sites remain unknown. This study aims to identify the optimal DC-SIGN interacting N-glycosylation sites in HA proteins of H5N1-AIVs. Results from NetNGlyc program analyzed the H5 hemagglutinin sequences of isolates during 2004–2020, revealing that seven and two conserved N-glycosylation sites were detected in HA1 and HA2 domain, respectively. A lentivirus pseudotyped A/Vietnam/1203/04 H5N1 envelope (H5N1-PVs) was generated which displayed an abundance of HA5 proteins on the virions via immuno-electron microscope observation. Further, H5N1-PVs or reverse-genetics (H5N1-RG) strains carrying a serial N-glycosylated mutation was generated by site-directed mutagenesis assay. Human recombinant DC-SIGN (rDC-SIGN) coated ELISA showed that H5N1-PVs bound to DC-SIGN, however, mutation on the N27Q, N39Q, and N181Q significantly reduced this binding (p < 0.05). Infectivity and capture assay demonstrated that N27Q and N39Q mutations significantly ameliorated DC-SIGN mediated H5N1 infection. Furthermore, combined mutations (N27Q&N39Q) significantly waned the interaction on either H5N1-PVs or -RG infection in cis and in trans (p < 0.01). This study concludes that N27 and N39 are two essential N-glycosylation contributing to DC-SIGN mediating H5N1 infection.

C-type Lectin CD209L/L-SIGN and CD209/DC-SIGN: Cell Adhesion Molecules Turned to Pathogen Recognition Receptors

C-type lectin CD209/DC-SIGN and CD209L/L-SIGN proteins are distinct cell adhesion and pathogen recognition receptors that mediate cellular interactions and recognize a wide range of pathogens, including viruses such as SARS, SARS-CoV-2, bacteria, fungi and parasites. Pathogens exploit CD209 family proteins to promote infection and evade the immune recognition system. CD209L and CD209 are widely expressed in SARS-CoV-2 target organs and can contribute to infection and pathogenesis. CD209 family receptors are highly susceptible to alternative splicing and genomic polymorphism, which may influence virus tropism and transmission in vivo. The carbohydrate recognition domain (CRD) and the neck/repeat region represent the key features of CD209 family proteins that are also central to facilitating cellular ligand interactions and pathogen recognition. While the neck/repeat region is involved in oligomeric dimerization, the CRD recognizes the mannose-containing structures present on specific glycoproteins such as those found on the SARS-CoV-2 spike protein. Considering the role of CD209L and related proteins in diverse pathogen recognition, this review article discusses the recent advances in the cellular and biochemical characterization of CD209 and CD209L and their roles in viral uptake, which has important implications in understanding the host-pathogen interaction, the viral pathobiology and driving vaccine development of SARS-CoV-2.

Virus-Mediated Cell-Cell Fusion

Cell-cell fusion between eukaryotic cells is a general process involved in many physiological and pathological conditions, including infections by bacteria, parasites, and viruses. As obligate intracellular pathogens, viruses use intracellular machineries and pathways for efficient replication in their host target cells. Interestingly, certain viruses, and, more especially, enveloped viruses belonging to different viral families and including human pathogens, can mediate cell-cell fusion between infected cells and neighboring non-infected cells. Depending of the cellular environment and tissue organization, this virus-mediated cell-cell fusion leads to the merge of membrane and cytoplasm contents and formation of multinucleated cells, also called syncytia, that can express high amount of viral antigens in tissues and organs of infected hosts. This ability of some viruses to trigger cell-cell fusion between infected cells as virus-donor cells and surrounding non-infected target cells is mainly related to virus-encoded fusion proteins, known as viral fusogens displaying high fusogenic properties, and expressed at the cell surface of the virus-donor cells. Virus-induced cell-cell fusion is then mediated by interactions of these viral fusion proteins with surface molecules or receptors involved in virus entry and expressed on neighboring non-infected cells. Thus, the goal of this review is to give an overview of the different animal virus families, with a more special focus on human pathogens, that can trigger cell-cell fusion.

Robust and persistent SARS-CoV-2 infection in the human intestinal brush border expressing cells

Studies on patients with the coronavirus disease-2019 (COVID-19) have implicated that the gastrointestinal (GI) tract is a major site of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. We established a human GI tract cell line model highly permissive to SARS-CoV-2. These cells, C2BBe1 intestinal cells with a brush border having high levels of transmembrane serine protease 2 (TMPRSS2), showed robust viral propagation, and could be persistently infected with SARS-CoV-2, supporting the clinical observations of persistent GI infection in COVID-19 patients. Ectopic expression of viral receptors revealed that the levels of angiotensin-converting enzyme 2 (ACE2) expression confer permissiveness to SARS-CoV-2 infection, and TMPRSS2 greatly facilitates ACE2-mediated SARS-CoV-2 dissemination. Interestingly, ACE2 but not TMPRSS2 expression was significantly promoted by enterocytic differentiation, suggesting that the state of enterocytic differentiation may serve as a determining factor for viral propagation. Thus, our study sheds light on the pathogenesis of SARS-CoV-2 in the GI tract.

HER2-mediated enhancement of Ebola virus entry

Multiple cell surface molecules including TAM receptors (TYRO3, AXL, and MERTK), a family of tyrosine kinase receptors, can serve as attachment receptors for Ebola virus (EBOV) entry into cells. The interaction of these receptors with EBOV particles is believed to trigger the initial internalization events that lead to macropinocytosis. However, the details of how these interactions lead to EBOV internalization have yet to be elucidated. Here, we screened receptor tyrosine kinase (RTK) inhibitors for anti-EBOV activity by using our previously established biologically contained Ebola virus that lacks the VP30 gene (EBOVΔVP30) and identified several RTKs, including human epidermal growth factor receptor 2 (HER2), as potential targets of anti-EBOV inhibitors and as novel host factors that have a role in EBOV infection. Of these identified RTKs, it was only HER2 whose knockdown by siRNAs impaired EBOVΔVP30-induced AKT1 phosphorylation, an event that is required for AKT1 activation and subsequent macropinocytosis. Stable expression of HER2 resulted in constitutive activation of AKT1, resulting in the enhancement of EBOVΔVP30 growth, EBOV GP-mediated entry, and macropinocytosis. Moreover, we found that HER2 interacts with the TAM receptors, and in particular forms a complex with TYRO3 and EBOVΔVP30 particles on the cell surface. Interestingly, HER2 was required for EBOVΔVP30-induced TYRO3 and AKT1 activation, but the other TAM receptors (TYRO3 and MERTK) were not essential for EBOVΔVP30-induced HER2 and AKT1 activation. Our findings demonstrate that HER2 plays an important role in EBOV entry and provide novel insights for the development of therapeutics against the virus.

The Interaction of Fluorinated Glycomimetics with DC-SIGN: Multiple Binding Modes Disentangled by the Combination of NMR Methods and MD Simulations

Fluorinated glycomimetics are frequently employed to study and eventually modulate protein–glycan interactions. However, complex glycans and their glycomimetics may display multiple binding epitopes that enormously complicate the access to a complete picture of the protein–ligand complexes. We herein present a new methodology based on the synergic combination of experimental ¹⁹F-based saturation transfer difference (STD) NMR data with computational protocols, applied to analyze the interaction between DC-SIGN, a key lectin involved in inflammation and infection events with the trifluorinated glycomimetic of the trimannoside core, ubiquitous in human glycoproteins. A novel 2D-STD-TOCSYreF NMR experiment was employed to obtain the experimental STD NMR intensities, while the Complete Relaxation Matrix Analysis (CORCEMA-ST) was used to predict that expected for an ensemble of geometries extracted from extensive MD simulations. Then, an in-house built computer program was devised to find the ensemble of structures that provide the best fit between the theoretical and the observed STD data. Remarkably, the experimental STD profiles obtained for the ligand/DC-SIGN complex could not be satisfactorily explained by a single binding mode, but rather with a combination of different modes coexisting in solution. Therefore, the method provides a precise view of those ligand–receptor complexes present in solution.