SARS-CoV-2 growth, furin-cleavage-site adaptation and neutralization using serum from acutely infected hospitalized COVID-19 patients

A measles-vectored vaccine candidate expressing prefusion-stabilized SARS-CoV-2 spike protein brought to phase I/II clinical trials: protection of African green monkeys from COVID-19 disease

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged at the end of 2019 and is responsible for the largest human pandemic in 100 years. Thirty-four vaccines are currently approved for use worldwide, and approximately 67% of the world population has received a complete primary series of one, yet countries are dealing with new waves of infections, variant viruses continue to emerge, and breakthrough infections are frequent secondary to waning immunity. Here, we evaluate a measles virus (MV)-vectored vaccine expressing a stabilized prefusion SARS-CoV-2 spike (S) protein (MV-ATU3-S2PΔF2A; V591) with demonstrated immunogenicity in mouse models (see companion article [J. Brunet, Z. Choucha, M. Gransagne, H. Tabbal, M.-W. Ku et al., J Virol 98:e01693-23, 2024, https://doi.org/10.1128/jvi.01693-23]) in an established African green monkey model of disease. Animals were vaccinated with V591 or the control vaccine (an equivalent MV-vectored vaccine with an irrelevant antigen) intramuscularly using a prime/boost schedule, followed by challenge with an early pandemic isolate of SARS-CoV-2 at 56 days post-vaccination. Pre-challenge, only V591-vaccinated animals developed S-specific antibodies that had virus-neutralizing activity as well as S-specific T cells. Following the challenge, V591-vaccinated animals had lower infectious virus and viral (v) RNA loads in mucosal secretions and stopped shedding virus in these secretions earlier. vRNA loads were lower in these animals in respiratory and gastrointestinal tract tissues at necropsy. This correlated with a lower disease burden in the lungs as quantified by PET/CT at early and late time points post-challenge and by pathological analysis at necropsy.IMPORTANCESevere acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for the largest human pandemic in 100 years. Even though vaccines are currently available, countries are dealing with new waves of infections, variant viruses continue to emerge, breakthrough infections are frequent, and vaccine hesitancy persists. This study uses a safe and effective measles vaccine as a platform for vaccination against SARS-CoV-2. The candidate vaccine was used to vaccinate African green monkeys (AGMs). All vaccinated AGMs developed robust antigen-specific immune responses. After challenge, these AGMs produced less virus in mucosal secretions, for a shorter period, and had a reduced disease burden in the lungs compared to control animals. At necropsy, lower levels of viral RNA were detected in tissue samples from vaccinated animals, and the lungs of these animals lacked the histologic hallmarks of SARS-CoV-2 disease observed exclusively in the control AGMs.

Machine learning-driven SERS fingerprinting of disintegrated viral components for rapid detection of SARS-CoV-2 in environmental dust

Surveillance of airborne viruses in crowded indoor spaces is crucial for managing outbreaks, as highlighted by the SARS-CoV-2 pandemic. However, the rapid and on-site detection of fast-mutating viruses, such as SARS-CoV-2, in complex environmental backgrounds remains challenging. Our study introduces a machine learning (ML)-driven surface-enhanced Raman spectroscopy (SERS) approach for detecting viruses within environmental dust matrices. By decomposing intact virions into individual structural components via a Raman-background-free lysis protocol and concentrating them into nanogap SERS hotspots, we significantly enhance the SERS signal intensity and fingerprint information density from viral structural components. Utilizing Principal Component Analysis (PCA), we establish a robust connection between the SERS data of these structural components and their biological sequences, laying a solid foundation for virus detection through SERS. Furthermore, we demonstrate reliable quantitative detection of SARS-CoV-2 using identified SARS-CoV-2 peaks at concentrations down to 102 pfu/ml through Gaussian Process Regression (GPR) and a digital SERS methodology. Finally, applying a Principal Component Analysis-Linear Discriminant Analysis (PCA-LDA) algorithm, we identify SARS-CoV-2, influenza A virus, and Zika virus within an environmental dust background with over 86% accuracy. Therefore, our ML-driven SERS approach holds promise for rapid environmental virus monitoring to manage future outbreaks.

Relevance of the Entry by Fusion at the Cytoplasmic Membrane vs. Fusion After Endocytosis in the HIV and SARS-Cov-2 Infections

HIV-1 and SARS-Cov-2 fuse at the cell surface or at endosomal compartments for entry into target cells; entry at the cell surface associates to productive infection, whereas endocytosis of low pH-independent viruses may lead to virus inactivation, slow replication, or alternatively, to productive infection. Endocytosis and fusion at the cell surface are conditioned by cell type-specific restriction factors and the presence of enzymes required for activation of the viral fusogen. Whereas fusion with the plasma membrane is considered the main pathway to productive infection of low pH-independent entry viruses, endocytosis is also productive and may be the main route of the highly efficient cell-to-cell dissemination of viruses. Alternative receptors, membrane cofactors, and the presence of enzymes processing the fusion protein at the cell membrane, determine the balance between fusion and endocytosis in specific target cells. Characterization of the mode of entry in particular cell culture conditions is desirable to better assess the effect of neutralizing and blocking agents and their mechanism of action. Whatever the pathway of virus internalization, production of the viral proteins into the cells can lead to the expression of the viral fusion protein on the cell surface; if this protein is able to induce membrane fusion at physiological pH, it promotes the fusion of the infected cell with surrounding uninfected cells, leading to the formation of syncytia or heterokaryons. Importantly, particular membrane proteins and lipids act as cofactors to support fusion. Virus-induced cell-cell fusion leads to efficient virus replication into fused cells, cell death, inflammation, and severe disease.

Nanoisland SERS-Substrates for Specific Detection and Quantification of Influenza A Virus

Surface-enhanced Raman spectroscopy (SERS)-based aptasensors for virus determination have attracted a lot of interest recently. This approach provides both specificity due to an aptamer component and a low limit of detection due to signal enhancement by a SERS substrate. The most successful SERS-based aptasensors have a limit of detection (LoD) of 10-100 viral particles per mL (VP/mL) that is advantageous compared to polymerase chain reactions. These characteristics of the sensors require the use of complex substrates. Previously, we described silver nanoisland SERS-substrate with a reproducible and uniform surface, demonstrating high potency for industrial production and a suboptimal LoD of 4 × 105 VP/mL of influenza A virus. Here we describe a study of the sensor morphology, revealing an unexpected mechanism of signal enhancement through the distortion of the nanoisland layer. A novel modification of the aptasensor was proposed with chromium-enhanced adhesion of silver nanoparticles to the surface as well as elimination of the buffer-dependent distortion-triggering steps. As a result, the LoD of the Influenza A virus was decreased to 190 VP/mL, placing the nanoisland SERS-based aptasensors in the rank of the most powerful sensors for viral detection.

Generation of quality-controlled SARS-CoV-2 variant stocks

One of the main challenges in the fight against coronavirus disease 2019 (COVID-19) stems from the ongoing evolution of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) into multiple variants. To address this hurdle, research groups around the world have independently developed protocols to isolate these variants from clinical samples. These isolates are then used in translational and basic research-for example, in vaccine development, drug screening or characterizing SARS-CoV-2 biology and pathogenesis. However, over the course of the COVID-19 pandemic, we have learned that the introduction of artefacts during both in vitro isolation and subsequent propagation to virus stocks can lessen the validity and reproducibility of data. We propose a rigorous pipeline for the generation of high-quality SARS-CoV-2 variant clonal isolates that minimizes the acquisition of mutations and introduces stringent controls to detect them. Overall, the process includes eight stages: (i) cell maintenance, (ii) isolation of SARS-CoV-2 from clinical specimens, (iii) determination of infectious virus titers by plaque assay, (iv) clonal isolation by plaque purification, (v) whole-virus-genome deep-sequencing, (vi and vii) amplification of selected virus clones to master and working stocks and (viii) sucrose purification. This comprehensive protocol will enable researchers to generate reliable SARS-CoV-2 variant inoculates for in vitro and in vivo experimentation and will facilitate comparisons and collaborative work. Quality-controlled working stocks for most applications can be generated from acquired biorepository virus within 1 month. An additional 5-8 d are required when virus is isolated from clinical swab material, and another 6-7 d is needed for sucrose-purifying the stocks.

Transchromosomic bovine-derived anti-SARS-CoV-2 polyclonal human antibodies protects hACE2 transgenic hamsters against multiple variants

Pandemic SARS-CoV-2 has undergone rapid evolution resulting in the emergence of many variants with mutations in the spike protein, some of which appear to evade antibody neutralization, transmit more efficiently, and/or exhibit altered virulence. This raises significant concerns regarding the efficacy of anti-S monoclonal antibody-based therapeutics which have failed against variant SARS-CoV-2 viruses. To address this concern, SAB-185, a human anti-SARS-CoV-2 polyclonal antibody was generated in the DiversitAb platform. SAB-185 exhibited equivalent, robust in vitro neutralization for Munich, Alpha, Beta, Gamma, and Δ144-146 variants and, although diminished, retained PRNT50 and PRNT80 neutralization endpoints for Delta and Omicron variants. Human ACE2 transgenic Syrian hamsters, which exhibit lethal SARS-CoV-2 disease, were protected from mortality after challenge with the Munich, Alpha, Beta, Delta, and Δ144-146 variants and clinical signs after non-lethal Omicron BA.1 infection. This suggests that SAB-185 may be an effective immunotherapy even in the presence of ongoing viral mutation.

Isolation of ACE2-dependent and -independent sarbecoviruses from Chinese horseshoe bats

While the spike proteins from severe acute respiratory syndrome coronaviruses-1 and 2 (SARS-CoV and SARS-CoV-2) bind to host angiotensin-converting enzyme 2 (ACE2) to infect cells, the majority of bat sarbecoviruses cannot use ACE2 from any species. Despite their discovery almost 20 years ago, ACE2-independent sarbecoviruses have never been isolated from field samples, leading to the assumption these viruses pose little risk to humans. We have previously shown how spike proteins from a small group of ACE2-independent bat sarbecoviruses may possess the ability to infect human cells in the presence of exogenous trypsin. Here, we adapted our earlier findings into a virus isolation protocol and recovered two new ACE2-dependent viruses, RsYN2012 and RsYN2016A, as well as an ACE2-independent virus, RsHuB2019A. Although our stocks of RsHuB2019A rapidly acquired a tissue-culture adaption that rendered the spike protein resistant to trypsin, trypsin was still required for viral entry, suggesting limitations on the exogenous entry factors that support bat sarbecoviruses. Electron microscopy revealed that ACE2-independent sarbecoviruses have a prominent spike corona and share similar morphology to other coronaviruses. Our findings demonstrate a broader zoonotic threat posed by sarbecoviruses and shed light on the intricacies of coronavirus isolation and propagation in vitro. IMPORTANCE Several coronaviruses have been transmitted from animals to people, and 20 years of virus discovery studies have uncovered thousands of new coronavirus sequences in nature. Most of the animal-derived sarbecoviruses have never been isolated in culture due to cell incompatibilities and a poor understanding of the in vitro requirements for their propagation. Here, we built on our growing body of work characterizing viral entry mechanisms of bat sarbecoviruses in human cells and have developed a virus isolation protocol that allows for the exploration of these understudied viruses. Our protocol is robust and practical, leading to successful isolation of more sarbecoviruses than previous approaches and from field samples that had been collected over a 10-year longitudinal study.

Targeting spike glycans to inhibit SARS-CoV2 viral entry

SARS-CoV-2 spike harbors glycans which function as ligands for lectins. Therefore, it should be possible to exploit lectins to target SARS-CoV-2 and inhibit cellular entry by binding glycans on the spike protein. Burkholderia oklahomensis agglutinin (BOA) is an antiviral lectin that interacts with viral glycoproteins via N-linked high mannose glycans. Here, we show that BOA binds to the spike protein and is a potent inhibitor of SARS-CoV-2 viral entry at nanomolar concentrations. Using a variety of biophysical approaches, we demonstrate that the interaction is avidity driven and that BOA cross-links the spike protein into soluble aggregates. Furthermore, using virus neutralization assays, we demonstrate that BOA effectively inhibits all tested variants of concern as well as SARS-CoV 2003, establishing that multivalent glycan-targeting molecules have the potential to act as pan-coronavirus inhibitors.

Intra-host variation in the spike S1/S2 region of a feline coronavirus type-1 in a cat with persistent infection

Feline coronavirus type 1 (FCoV-1) is widely known for causing feline infectious peritonitis (FIP), a systemic infection that is often fatal, with the virus known as the FIPV biotype. However, subclinical disease also occurs, in which cats may not show signs and intermittently shed the virus, including in feces, possibly for long periods of time. This virus is known as the FECV biotype. Progression of FECV to FIPV has been linked to several genomic changes, however a specific region of the viral spike protein at the interface of the spike S1 and S2 domains has been especially implicated. In this study, we followed a cat (#576) for six years from 2017, at which time FCoV-1 was detected in feces and conjunctival swabs, until 2022, when the animal was euthanized based on a diagnosis of alimentary small cell lymphoma. Over this time period, the cat was clinically diagnosed with inflammatory bowel disease and chronic rhinitis, and cardiac problems were also suspected. Using hybridization capture targeting the spike (S) gene of FCoV followed by next-generation sequencing, we screened 27 clinical samples. We detected FCoV-1 in 4 samples taken in 2017 (intestine and nasal tissue, feces, and conjunctiva), and 3 samples taken in 2022 (feces, and intestinal and heart tissue), but not in fecal samples taken in 2019 and 2020. Next, we focused on the S1/S2 region within S, which contains the furin cleavage site (FCS), a key regulator of viral transmission and pathogenesis. We show that the FCoV-1 variants obtained from feces in 2017 and 2022 were identical, while the ones from conjunctiva (2017), heart (2022), and intestine (2017 and 2022) were distinct. Sequence comparison of all the variants obtained showed that most of the non-synonymous changes in the S1/S2 region occur within the FCS. In the heart, we found two variants that differed by a single nucleotide, resulting in distinct FCS motifs that differ in one amino acid. It is predicted that one of these FCS motifs will down-regulate spike cleavability. The variant from the conjunctiva (2017) had a 6-nucleotide in-frame insertion that resulted in a longer and more exposed S1/S2 loop, which is predicted to be more accessible to the furin protease. Our studies indicate that FCoV-1 can independently persist in the gastrointestinal tract and heart of a cat over a long period of time without evidence of typical FIP signs, with intermittent viral shedding from the gastrointestinal and respiratory tracts.

A non-transmissible live attenuated SARS-CoV-2 vaccine

Live attenuated vaccines (LAVs) administered via the mucosal route may offer better control of the COVID-19 pandemic than non-replicating vaccines injected intramuscularly. Conceptionally, LAVs have several advantages, including presentation of the entire antigenic repertoire of the virus, and the induction of strong mucosal immunity. Thus, immunity induced by LAV could offer superior protection against future surges of COVID-19 cases caused by emerging SARS-CoV-2 variants. However, LAVs carry the risk of unintentional transmission. To address this issue, we investigated whether transmission of a SARS-CoV-2 LAV candidate can be blocked by removing the furin cleavage site (FCS) from the spike protein. The level of protection and immunity induced by the attenuated virus with the intact FCS was virtually identical to the one induced by the attenuated virus lacking the FCS. Most importantly, removal of the FCS completely abolished horizontal transmission of vaccine virus between cohoused hamsters. Furthermore, the vaccine was safe in immunosuppressed animals and showed no tendency to recombine in vitro or in vivo with a SARS-CoV-2 field strain. These results indicate that removal of the FCS from SARS-CoV-2 LAV is a promising strategy to increase vaccine safety and prevent vaccine transmission without compromising vaccine efficacy.

Viral infectivity in paediatric SARS-CoV-2 clinical samples does not vary by age

At the start of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, there was much uncertainty about the role of children in infection and transmission dynamics. Through the course of the pandemic, it became clear that children were susceptible to SARS-CoV-2 infection, although they were experiencing a notable lack of severe disease outcomes as compared to the adult population. This trend held true with the emergence of new SARS-CoV-2 variants, even in paediatric populations that were ineligible to be vaccinated. The difference in disease outcomes has prompted questions about the virological features of SARS-CoV-2 infection in this population. In order to determine if there was any difference in the infectivity of the virus produced by children with coronavirus disease 2019 (COVID-19), we compared viral RNA levels (clinical RT-qPCR C T) and infectious virus titres from 144 SARS-CoV-2-positive clinical samples collected from children aged 0 to 18 years old. We found that age had no impact on the infectiousness of SARS-CoV-2 within our cohort, with children of all ages able to produce high levels of infectious virus.

Multiplex Lithographic SERS Aptasensor for Detection of Several Respiratory Viruses in One Pot

Rapid and reliable techniques for virus identification are required in light of recurring epidemics and pandemics throughout the world. Several techniques have been distributed for testing the flow of patients. Polymerase chain reaction with reverse transcription is a reliable and sensitive, though not rapid, tool. The antibody-based strip is a rapid, though not reliable, and sensitive tool. A set of alternative tools is being developed to meet all the needs of the customer. Surface-enhanced Raman spectroscopy (SERS) provides the possibility of single molecule detection taking several minutes. Here, a multiplex lithographic SERS aptasensor was developed aiming at the detection of several respiratory viruses in one pot within 17 min. The four labeled aptamers were anchored onto the metal surface of four SERS zones; the caught viruses affect the SERS signals of the labels, providing changes in the analytical signals. The sensor was able to decode mixes of SARS-CoV-2 (severe acute respiratory syndrome coronavirus two), influenza A virus, respiratory syncytial virus, and adenovirus within a single experiment through a one-stage recognition process.

In SARS-CoV-2 delta variants, Spike-P681R and D950N promote membrane fusion, Spike-P681R enhances spike cleavage, but neither substitution affects pathogenicity in hamsters

BACKGROUND

The SARS-CoV-2 delta (B.1.617.2 lineage) variant was first identified at the end of 2020 and possessed two unique amino acid substitutions in its spike protein: S-P681R, at the S1/S2 cleavage site, and S-D950N, in the HR1 of the S2 subunit. However, the roles of these substitutions in virus phenotypes have not been fully characterized.

METHODS

We used reverse genetics to generate Wuhan-D614G viruses with these substitutions and delta viruses lacking these substitutions and explored how these changes affected their viral characteristics in vitro and in vivo.

FINDINGS

S-P681R enhanced spike cleavage and membrane fusion, whereas S-D950N slightly promoted membrane fusion. Although S-681R reduced the virus replicative ability especially in VeroE6 cells, neither substitution affected virus replication in Calu-3 cells and hamsters. The pathogenicity of all recombinant viruses tested in hamsters was slightly but not significantly affected.

INTERPRETATION

Our observations suggest that the S-P681R and S-D950N substitutions alone do not increase virus pathogenicity, despite of their enhancement of spike cleavage or fusogenicity.

FUNDING

A full list of funding bodies that contributed to this study can be found under Acknowledgments.

Cryo-EM reveals binding of linoleic acid to SARS-CoV-2 spike glycoprotein, suggesting an antiviral treatment strategy

The COVID-19 pandemic and concomitant lockdowns presented a global health challenge and triggered unprecedented research efforts to elucidate the molecular mechanisms and pathogenicity of SARS-CoV-2. The spike glycoprotein decorating the surface of SARS-CoV-2 virions is a prime target for vaccine development, antibody therapy and serology as it binds the host cell receptor and is central for viral cell entry. The electron cryo-microscopy structure of the spike protein revealed a hydrophobic pocket in the receptor-binding domain that is occupied by an essential fatty acid, linoleic acid (LA). The LA-bound spike protein adopts a non-infectious locked conformation which is more stable than the infectious form and shields important immunogenic epitopes. Here, the impact of LA binding on viral infectivity and replication, and the evolutionary conservation of the pocket in other highly pathogenic coronaviruses, including SARS-CoV-2 variants of concern (VOCs), are reviewed. The importance of LA metabolic products, the eicosanoids, in regulating the human immune response and inflammation is highlighted. Lipid and fatty-acid binding to a hydrophobic pocket in proteins on the virion surface appears to be a broader strategy employed by viruses, including picornaviruses and Zika virus. Ligand binding stabilizes their protein structure and assembly, and downregulates infectivity. In the case of rhinoviruses, this has been exploited to develop small-molecule antiviral drugs that bind to the hydrophobic pocket. The results suggest a COVID-19 antiviral treatment based on the LA-binding pocket.

Aptamer-coated track-etched membranes with a nanostructured silver layer for single virus detection in biological fluids

Aptasensors based on surface-enhanced Raman spectroscopy (SERS) are of high interest due to the superior specificity and low limit of detection. It is possible to produce stable and cheap SERS-active substrates and portable equipment meeting the requirements of point-of-care devices. Here we combine the membrane filtration and SERS-active substrate in the one pot. This approach allows efficient adsorption of the viruses from the solution onto aptamer-covered silver nanoparticles. Specific determination of the viruses was provided by the aptamer to influenza A virus labeled with the Raman-active label. The SERS-signal from the label was decreased with a descending concentration of the target virus. Even several virus particles in the sample provided an increase in SERS-spectra intensity, requiring only a few minutes for the interaction between the aptamer and the virus. The limit of detection of the aptasensor was as low as 10 viral particles per mL (VP/mL) of influenza A virus or 2 VP/mL per probe. This value overcomes the limit of detection of PCR techniques (∼10³ VP/mL). The proposed biosensor is very convenient for point-of-care applications.

The D614G mutation redirects SARS-CoV-2 spike to lysosomes and suppresses deleterious traits of the furin cleavage site insertion mutation

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) egress occurs by lysosomal exocytosis. We show that the Spike D614G mutation enhances Spike trafficking to lysosomes, drives Spike-mediated reprogramming of lysosomes, and reduces cell surface Spike expression by ~3-fold. D614G is not a human-specific adaptation. Rather, it is an adaptation to the earlier furin cleavage site insertion (FCSI) mutation that occurred at the genesis of SARS-CoV-2. While advantageous to the virus, furin cleavage of spike has deleterious effects on spike structure and function, inhibiting its trafficking to lysosomes and impairing its infectivity by the transmembrane serine protease 2(TMPRSS2)-independent, endolysosomal pathway. D614G restores spike trafficking to lysosomes and enhances the earliest events in SARS-CoV-2 infectivity, while spike mutations that restore SARS-CoV-2's TMPRSS2-independent infectivity restore spike's trafficking to lysosomes. Together, these and other results show that D614G is an intragenic suppressor of deleterious traits linked to the FCSI and lend additional support to the endolysosomal model of SARS-CoV-2 egress and entry.

Targeting Spike Glycans to Inhibit SARS-CoV2 Viral Entry

SARS-CoV-2 Spike harbors glycans which function as ligands for lectins. Therefore, it should be possible to exploit lectins to target SARS-CoV-2 and inhibit cellular entry by binding glycans on the Spike protein. Burkholderia oklahomensis agglutinin (BOA) is an antiviral lectin that interacts with viral glycoproteins via N-linked high mannose glycans. Here, we show that BOA binds to the Spike protein and is a potent inhibitor of SARS-CoV-2 viral entry at nanomolar concentrations. Using a variety of biophysical tools, we demonstrate that the interaction is avidity driven and that BOA crosslinks the Spike protein into soluble aggregates. Furthermore, using virus neutralization assays, we demonstrate that BOA effectively inhibits all tested variants of concern as well as SARS-CoV 2003, establishing that glycan-targeting molecules have the potential to be pan-coronavirus inhibitors.

Immune durability and protection against SARS-CoV-2 re-infection in Syrian hamsters

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has caused a pandemic. As immunity to endemic human coronaviruses (i.e. NL63 or OC43) wanes leading to re-infection, it was unknown if SARS-CoV-2 immunity would also decline permitting repeat infections. Recent case reports confirm previously infected individuals can become re-infected; however, re-infection may be due to heterogeneity in the initial infection or the host immune response, or may be the result of infection with a variant strain that escapes pre-existing immunity. To control these variables, we utilized the Syrian hamster model to evaluate the duration of immunity and susceptibility to re-infection with SARS-CoV-2. Hamsters were given a primary mock or SARS-CoV-2 infection (culture media or 105 TCID50 USA/WA1/2020 isolate, respectively). Mock and SARS-CoV-2 infected hamsters were then given a secondary SARS-CoV-2 infection at 1, 2, 4, or 6 months post-primary infection (n = 14/time point/group). After the primary SARS-CoV-2 infection, hamsters developed anti-spike protein IgG, IgA, and neutralizing antibodies, and these antibodies were maintained for at least 6 months. Upon secondary SARS-CoV-2 challenge, previously SARS-CoV-2 infected animals were protected from weight loss, while all previously mock-infected animals became infected and lost weight. Importantly, despite having high titres of antibodies, one SARS-CoV-2 infected animal re-challenged at 4 months had a breakthrough infection with replicating virus in the upper and lower respiratory tract. These studies demonstrate immunity to SARS-CoV-2 is maintained for 6 months; however, protection may be incomplete and, even in the presence of high antibody titres, previously infected hosts may become re-infected.

Single domain antibodies derived from ancient animals as broadly neutralizing agents for SARS-CoV-2 and other coronaviruses

With severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as an emergent human virus since December 2019, the world population is susceptible to coronavirus disease 2019 (COVID-19). SARS-CoV-2 has higher transmissibility than the previous coronaviruses, associated by the ribonucleic acid (RNA) virus nature with high mutation rate, caused SARS-CoV-2 variants to arise while circulating worldwide. Neutralizing antibodies are identified as immediate and direct-acting therapeutic against COVID-19. Single-domain antibodies (sdAbs), as small biomolecules with non-complex structure and intrinsic stability, can acquire antigen-binding capabilities comparable to conventional antibodies, which serve as an attractive neutralizing solution. SARS-CoV-2 spike protein attaches to human angiotensin-converting enzyme 2 (ACE2) receptor on lung epithelial cells to initiate viral infection, serves as potential therapeutic target. sdAbs have shown broad neutralization towards SARS-CoV-2 with various mutations, effectively stop and prevent infection while efficiently block mutational escape. In addition, sdAbs can be developed into multivalent antibodies or inhaled biotherapeutics against COVID-19.

The origins of COVID-19 pandemic: A brief overview

The novel coronavirus disease (COVID-19) outbreak that emerged at the end of 2019 has now swept the world for more than 2 years, causing immeasurable damage to the lives and economies of the world. It has drawn so much attention to discovering how the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) originated and entered the human body. The current argument revolves around two contradictory theories: a scenario of laboratory spillover events and human contact with zoonotic diseases. Here, we reviewed the transmission, pathogenesis, possible hosts, as well as the genome and protein structure of SARS-CoV-2, which play key roles in the COVID-19 pandemic. We believe the coronavirus was originally transmitted to human by animals rather than by a laboratory leak. However, there still needs more investigations to determine the source of the pandemic. Understanding how COVID-19 emerged is vital to developing global strategies for mitigating future outbreaks.

A Deletion Encompassing the Furin Cleavage Site in the Spike Encoding Gene Does Not Alter SARS-CoV-2 Replication in Lung Tissues of Mink and Neutralization by Convalescent Human Serum Samples

SARS-CoV-2 has been shown to lose the furin polybasic cleavage site (FCS) following adaptation on cell culture. Deletion occurring in this region, which may include also the FCS flanking regions, seem not to affect virus replication in vitro; however, a chimeric SARS-CoV-2 virus without the sole FCS motif has been associated with lower virulence in mice and lower neutralization values. Moreover, SARS-CoV-2 virus lacking the FCS was shed to lower titers from experimentally infected ferrets and was not transmitted to cohoused sentinel animals, unlike wild-type virus. In this study, we investigated the replication kinetics and cellular tropism of a SARS-CoV-2 isolate carrying a 10-amino acid deletion in the spike protein spanning the FCS in lung ex vivo organ cultures of mink. Furthermore, we tested the neutralization capabilities of human convalescent SARS-CoV-2 positive serum samples against this virus. We showed that this deletion did not significantly hamper neither ex vivo replication nor neutralization activity by convalescent serum samples. This study highlights the importance of the preliminary phenotypic characterization of emerging viruses in ex vivo models and demonstrates that mink lung tissues are permissive to the replication of a mutant form of SARS-CoV-2 showing a deletion spanning the FCS. Notably, we also highlight the need for sequencing viral stocks before any infection study as large deletions may occur leading to the misinterpretation of results.

Improved Neutralisation of the SARS-CoV-2 Omicron Variant following a Booster Dose of Pfizer-BioNTech (BNT162b2) COVID-19 Vaccine

In late November 2021, the World Health Organization declared the SARS-CoV-2 lineage B.1.1.529 the fifth variant of concern, Omicron. This variant has acquired over 30 mutations in the spike protein (with 15 in the receptor-binding domain), raising concerns that Omicron could evade naturally acquired and vaccine-derived immunity. We utilized an authentic virus, multicycle neutralisation assay to demonstrate that sera collected one, three, and six months post-two doses of Pfizer-BioNTech BNT162b2 had a limited ability to neutralise SARS-CoV-2. However, four weeks after a third dose, neutralising antibody titres were boosted. Despite this increase, neutralising antibody titres were reduced fourfold for Omicron compared to lineage A.2.2 SARS-CoV-2.

Acquisition of Furin Cleavage Site and Further SARS-CoV-2 Evolution Change the Mechanisms of Viral Entry, Infection Spread, and Cell Signaling

Circulation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the human population leads to further viral evolution. The new variants that arise during this evolution are more infectious. Our data suggest that newer variants have shifted from utilizing both cathepsin/endosome- and TMPRSS2-mediated entry mechanisms to rely on a TMPRSS2-dependent entry pathway. Accordingly, only the early lineages of SARS-CoV-2 are capable of infecting and forming syncytia in Vero/ACE2 cells which lack TMPRSS2 expression. The presence of an intact multibasic furin cleavage site (FCS) in the S protein was a key requirement for cell-to-cell fusion. Deletion of FCS makes SARS-CoV-2 more infectious in vitro but renders it incapable of syncytium formation. Cell-to-cell fusion likely represents an alternative means of virus spread and is resistant to the presence of high levels of neutralizing monoclonal antibodies (MAbs) and immune sera in the media. In this study, we also noted that cells infected with SARS-CoV-2 with an intact FCS or alphavirus replicon expressing S protein (VEErep/S) released high levels of free S1 subunit. The released S1 is capable of activating the TLR4 receptor and inducing a pro-inflammatory response. Thus, S1 activation of TLR4 may be an important contributor to SARS-CoV-2-induced COVID-19 disease and needs to be considered in the design of COVID mRNA vaccines. Lastly, a VEErep/S-replicon was shown to produce large amounts of infectious, syncytium-forming pseudoviruses and thus could represent alternative experimental system for screening inhibitors of virus entry and syncytium formation. IMPORTANCE The results of this study demonstrate that the late lineages of SARS-CoV-2 evolved to more efficient use of the TMPRSS2-mediated entry pathway and gradually lost an ability to employ the cathepsins/endosome-mediated entry. The acquisition of a furin cleavage site (FCS) by SARS-CoV-2-specific S protein made the virus a potent producer of syncytia. Their formation is also determined by expression of ACE2 and TMPRSS2 and is resistant to neutralizing human MAbs and immune sera. Syncytium formation appears to be an alternative means of infection spread following the development of an adaptive immune response. Cells infected with SARS-CoV-2 with an intact FCS secrete high levels of the S1 subunit. The released S1 demonstrates an ability to activate the TLR4 receptor and induce pro-inflammatory cytokines, which represent a hallmark of SARS-CoV-2 pathogenesis. Alphavirus replicons encoding SARS-CoV-2 S protein cause spreading, syncytium-forming infection, and they can be applied as an experimental tool for studying the mechanism of syncytium formation.

Modeling within-host and aerosol dynamics of SARS-CoV-2: The relationship with infectiousness

The relationship between transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the amount of virus present in the proximity of a susceptible host is not understood. Here, we developed a within-host and aerosol mathematical model and used it to determine the relationship between viral kinetics in the upper respiratory track, viral kinetics in the aerosols, and new transmissions in golden hamsters challenged with SARS-CoV-2. We determined that infectious virus shedding early in infection correlates with transmission events, shedding of infectious virus diminishes late in the infection, and high viral RNA levels late in the infection are a poor indicator of transmission. We further showed that viral infectiousness increases in a density dependent manner with viral RNA and that their relative ratio is time-dependent. Such information is useful for designing interventions.

Quantifying the relationship between SARS-CoV-2 dynamics in upper respiratory tract and in aerosols is key to understanding SARS-CoV-2 transmission and evaluating intervention strategies. Of particular interest is the link between the viral RNA measured by PCR and a subject’s infectiousness. Here, we developed a mechanistic model of viral transmission in golden hamsters and used data in upper respiratory tract and aerosols to evaluate key within-host and environment based viral parameters. The significance of our research is in identifying the timing and duration of viral shedding, how long it stays infectious, and the link between infectious virus and total viral RNA. Such knowledge enhances our understanding of the SARS-CoV-2 transmission window.

Preclinical immunogenicity and efficacy of a candidate COVID-19 vaccine based on a vesicular stomatitis virus-SARS-CoV-2 chimera

Background

To investigate a vaccine technology with potential to protect against coronavirus disease 2019 (COVID-19) and reduce transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with a single vaccine dose, we developed a SARS-CoV-2 candidate vaccine using the live vesicular stomatitis virus (VSV) chimeric virus approach previously used to develop a licensed Ebola virus vaccine.

Methods

We generated a replication-competent chimeric VSV-SARS-CoV-2 vaccine candidate by replacing the VSV glycoprotein (G) gene with coding sequence for the SARS-CoV-2 Spike glycoprotein (S). Immunogenicity of the lead vaccine candidate (VSV∆G-SARS-CoV-2) was evaluated in cotton rats and golden Syrian hamsters, and protection from SARS-CoV-2 infection also was assessed in hamsters.

Findings

VSV∆G-SARS-CoV-2 delivered with a single intramuscular (IM) injection was immunogenic in cotton rats and hamsters and protected hamsters from weight loss following SARS-CoV-2 challenge. When mucosal vaccination was evaluated, cotton rats did not respond to the vaccine, whereas mucosal administration of VSV∆G-SARS-CoV-2 was found to be more immunogenic than IM injection in hamsters and induced immunity that significantly reduced SARS-CoV-2 challenge virus loads in both lung and nasal tissues.

Interpretation

VSV∆G-SARS-CoV-2 delivered by IM injection or mucosal administration was immunogenic in golden Syrian hamsters, and both vaccination methods effectively protected the lung from SARS-CoV-2 infection. Hamsters vaccinated by mucosal application of VSV∆G-SARS-CoV-2 also developed immunity that controlled SARS-CoV-2 replication in nasal tissue.

Funding

The study was funded by Merck Sharp & Dohme, Corp., a subsidiary of Merck & Co., Inc., Rahway, NJ, USA, and The International AIDS Vaccine Initiative, Inc. (IAVI), New York, USA. Parts of this research was supported by the Biomedical Advanced Research and Development Authority (BARDA) and the Defense Threat Reduction Agency (DTRA) of the US Department of Defense.

A Combination of Membrane Filtration and Raman-Active DNA Ligand Greatly Enhances Sensitivity of SERS-Based Aptasensors for Influenza A Virus

Biosensors combining the ultrahigh sensitivity of surface-enhanced Raman scattering (SERS) and the specificity of nucleic acid aptamers have recently drawn attention in the detection of respiratory viruses. The most sensitive SERS-based aptasensors allow determining as low as 104 virus particles per mL that is 100-fold lower than any antibody-based lateral flow tests but 10-100-times higher than a routine polymerase chain reaction with reversed transcription (RT-PCR). Sensitivity of RT-PCR has not been achieved in SERS-based aptasensors despite the usage of sophisticated SERS-active substrates. Here, we proposed a novel design of a SERS-based aptasensor with the limit of detection of just 103 particles per ml of the influenza A virus that approaches closely to RT-PCR sensitivity. The sensor utilizes silver nanoparticles with the simplest preparation instead of sophisticated SERS-active surfaces. The analytical signal is provided by a unique Raman-active dye that competes with the virus for the binding to the G-quadruplex core of the aptamer. The aptasensor functions even with aliquots of the biological fluids due to separation of the off-target molecules by pre-filtration through a polymeric membrane. The aptasensor detects influenza viruses in the range of 1·103-5·1010 virus particles per ml.

SARS-CoV-2 Spike Furin Cleavage Site and S2' Basic Residues Modulate the Entry Process in a Host Cell-Dependent Manner

SARS-CoV-2 spike (S) envelope glycoprotein constitutes the main determinant of virus entry and the target of host immune response, thus being of great interest for antiviral research. It is constituted of S1 and S2 subunits, which are involved in ACE2 receptor binding and fusion between the viral envelope and host cell membrane, respectively. Induction of the fusion process requires S cleavage at the S1-S2 junction and the S2′ site located upstream of the fusion peptide. Interestingly, the SARS-CoV-2 spike harbors a 4-residue insertion at the S1-S2 junction that is absent in its closest relatives and constitutes a polybasic motif recognized by furin-like proteases. In addition, the S2′ site is characterized by the presence of conserved basic residues. Here, we sought to determine the importance of the furin cleavage site (FCS) and the S2′ basic residues for S-mediated entry functions. We determined the impact of mutations introduced at these sites on S processing, fusogenic activity, and its ability to mediate entry in different cellular backgrounds. Strikingly, mutation phenotypes were highly dependent on the host cell background. We confirmed that although the FCS was not absolutely required for virus entry, it contributed to extending the fusogenic potential of S. Cleavage site mutations, as well as inhibition of furin protease activity, affected the cell surface expression of S in a host cell-dependent manner. Finally, inhibition of furin activity differentially affected SARS-CoV-2 virus infectivity in the tested host cells, thereby confirming the host cell-dependent effect of spike processing for the viral life cycle.

IMPORTANCE SARS-CoV-2 is responsible for the current global pandemic that has resulted in several million deaths. As the key determinant of virus entry into host cells and the main target of host immune response, the spike glycoprotein constitutes an attractive target for therapeutics development. Entry functions of spike rely on its processing at two sites by host cell proteases. While SARS-CoV-2 spike differs from its closest relatives by the insertion of a basic furin cleavage motif at the first site, it harbors conserved basic residues at the second cleavage site. Characterization of the importance of the basic sequences present at the two cleavage sites revealed that they were influencing spike processing, intracellular localization, induction of fusion, and entry in a host cell-dependent manner. Thus, our results revealed a high heterogeneity in spike sequence requirement for entry functions in the different host cells, in agreement with the high adaptability of the SARS-CoV-2 virus.

Development of a Rapid Live SARS-CoV-2 Neutralization Assay Based on a qPCR Readout

Measuring SARS-CoV-2 neutralizing antibodies after vaccination or natural infection remains a priority in the ongoing COVID-19 pandemic to determine immunity, especially against newly emerging variants. The gold standard for assessing antibody-mediated immunity against SARS-CoV-2 are cell-based live virus neutralization assays. These assays usually take several days, thereby limiting test capacities and the availability of rapid results. In this study, therefore, we developed a faster live virus assay, which detects neutralizing antibodies through the early measurement of antibody-mediated intracellular virus reduction by SARS-CoV-2 qRT-PCR. In our assay, Vero E6 cells are infected with virus isolates preincubated with patient sera and controls. After 24 h, the intracellular viral load is determined by qRT-PCR using a standard curve to calculate percent neutralization. Utilizing COVID-19 convalescent-phase sera, we show that our novel assay generates results with high sensitivity and specificity as we detected antiviral activity for all tested convalescent-phase sera, but no antiviral activity in prepandemic sera. The assay showed a strong correlation with a conventional virus neutralization assay (r_S = 0.8910), a receptor-binding domain ELISA (r_S = 0.8485), and a surrogate neutralization assay (r_S = 0.8373), proving that quantifying intracellular viral RNA can be used to measure seroneutralization. Our assay can be adapted easily to new variants, as demonstrated by our cross-neutralization experiments. This characteristic is key for rapidly determining immunity against newly emerging variants. Taken together, the novel assay presented here reduces turnaround time significantly while making use of a highly standardized and sensitive SARS-CoV-2 qRT-PCR method as a readout.

Assessing the Reliability of SARS-CoV-2 Neutralization Studies That Use Post-Vaccination Sera

Assessing COVID-19 vaccine effectiveness against emerging SARS-CoV-2 variants is crucial for determining future vaccination strategies and other public health strategies. When clinical effectiveness data are unavailable, a common method of assessing vaccine performance is to utilize neutralization assays using post-vaccination sera. Neutralization studies are typically performed across a wide array of settings, populations and vaccination strategies, and using different methodologies. For any comparison and meta-analysis to be meaningful, the design and methodology of the studies used must at minimum address aspects that confer a certain degree of reliability and comparability. We identified and characterized three important categories in which studies differ (cohort details, assay details and data reporting details) and that can affect the overall reliability and/or usefulness of neutralization assay results. We define reliability as a measure of methodological accuracy, proper study setting concerning subjects, samples and viruses, and reporting quality. Each category comprises a set of several relevant key parameters. To each parameter, we assigned a possible impact (ranging from low to high) on overall study reliability depending on its potential to influence the results. We then developed a reliability assessment tool that assesses the aggregate reliability of a study across all parameters. The reliability assessment tool provides explicit selection criteria for inclusion of comparable studies in meta-analyses of neutralization activity of SARS-CoV-2 variants in post-vaccination sera and can also both guide the design of future neutralization studies and serve as a checklist for including important details on key parameters in publications.

The Reassessed Potential of SARS-CoV-2 Attenuation for COVID-19 Vaccine Development—A Systematic Review

Live-attenuated SARS-CoV-2 vaccines received relatively little attention during the COVID-19 pandemic. Despite this, several methods of obtaining attenuated coronaviruses are known. In this systematic review, the strategies of coronavirus attenuation, which may potentially be applied to SARS-CoV-2, were identified. PubMed, Scopus, Web of Science and Embase databases were searched to identify relevant articles describing attenuating mutations tested in vivo. In case of coronaviruses other than SARS-CoV-2, sequence alignment was used to exclude attenuating mutations that cannot be applied to SARS-CoV-2. Potential immunogenicity, safety and efficacy of the attenuated SARS-CoV-2 vaccine were discussed based on animal studies data. A total of 27 attenuation strategies, used to create 101 different coronaviruses, have been described in 56 eligible articles. The disruption of the furin cleavage site in the SARS-CoV-2 spike protein was identified as the most promising strategy. The replacement of core sequences of transcriptional regulatory signals, which prevents recombination with wild-type viruses, also appears particularly advantageous. Other important attenuating mutations encompassed mostly the prevention of evasion of innate immunity. Sufficiently attenuated coronaviruses typically caused no meaningful disease in susceptible animals and protected them from challenges with virulent virus. This indicates that attenuated COVID-19 vaccines may be considered as a potential strategy to fight the threat posed by SARS-CoV-2.

SARS-CoV-2 pathogenesis

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a devastating pandemic. Although most people infected with SARS-CoV-2 develop a mild to moderate disease with virus replication restricted mainly to the upper airways, some progress to having a life-threatening pneumonia. In this Review, we explore recent clinical and experimental advances regarding SARS-CoV-2 pathophysiology and discuss potential mechanisms behind SARS-CoV-2-associated acute respiratory distress syndrome (ARDS), specifically focusing on new insights obtained using novel technologies such as single-cell omics, organoid infection models and CRISPR screens. We describe how SARS-CoV-2 may infect the lower respiratory tract and cause alveolar damage as a result of dysfunctional immune responses. We discuss how this may lead to the induction of a 'leaky state' of both the epithelium and the endothelium, promoting inflammation and coagulation, while an influx of immune cells leads to overexuberant inflammatory responses and immunopathology. Finally, we highlight how these findings may aid the development of new therapeutic interventions against COVID-19.

Human immunoglobulin from transchromosomic bovines hyperimmunized with SARS-CoV-2 spike antigen efficiently neutralizes viral variants

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants with amino-acid substitutions and deletions in spike protein (S) can reduce the effectiveness of monoclonal antibodies (mAbs) and may compromise immunity induced by vaccines. We report a polyclonal, fully human, anti-SARS-CoV-2 immunoglobulin produced in transchromosomic bovines (Tc-hIgG-SARS-CoV-2) hyperimmunized with two doses of plasmid DNA encoding the SARS-CoV-2 Wuhan strain S gene, followed by repeated immunization with S protein purified from insect cells. The resulting Tc-hIgG-SARS-CoV-2, termed SAB-185, efficiently neutralizes SARS-CoV-2, and vesicular stomatitis virus (VSV) SARS-CoV-2 chimeras in vitro. Neutralization potency was retained for S variants including S477N, E484K, and N501Y, substitutions present in recent variants of concern. In contrast to the ease of selection of escape variants with mAbs and convalescent human plasma, we were unable to isolate VSV-SARS-CoV-2 mutants resistant to Tc-hIgG-SARS-CoV-2 neutralization. This fully human immunoglobulin that potently inhibits SARS-CoV-2 infection may provide an effective therapeutic to combat COVID-19.

Replication kinetics and infectivity of SARS-CoV-2 variants of concern in common cell culture models

BACKGROUND

During the ongoing Covid-19 pandemic caused by the emerging virus SARS-CoV-2, research in the field of coronaviruses has expanded tremendously. The genome of SARS-CoV-2 has rapidly acquired numerous mutations, giving rise to several Variants of Concern (VOCs) with altered epidemiological, immunological, and pathogenic properties.

METHODS

As cell culture models are important tools to study viruses, we investigated replication kinetics and infectivity of SARS-CoV-2 in the African Green Monkey-derived Vero E6 kidney cell line and the two human cell lines Caco-2, a colon epithelial carcinoma cell line, and the airway epithelial carcinoma cell line Calu-3. We assessed viral RNA copy numbers and infectivity of viral particles in cell culture supernatants at different time points ranging from 2 to 96 h post-infection.

RESULTS

We here describe a systematic comparison of growth kinetics of the five SARS-CoV-2 VOCs Alpha/B.1.1.7, Beta/B.1.351, Gamma/P.1, Delta/B.1.617.2, and Omicron/B.1.1.529 and a non-VOC/B.1.1 strain on three different cell lines to provide profound information on the differential behaviour of VOCs in different cell lines for researchers worldwide. We show distinct differences in viral replication kinetics of the SARS-CoV-2 non-VOC and five VOCs on the three cell culture models Vero E6, Caco-2, and Calu-3.

CONCLUSION

This is the first systematic comparison of all SARS-CoV-2 VOCs on three different cell culture models. This data provides support for researchers worldwide in their experimental design for work on SARS-CoV-2. It is recommended to perform virus isolation and propagation on Vero E6 while infection studies or drug screening and antibody-based assays should rather be conducted on the human cell lines Caco-2 and Calu-3.

Detection of the ORF1 Gene Is an Indicator of the Possible Isolation of Severe Acute Respiratory Syndrome Coronavirus 2

In the ongoing coronavirus diseases 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), real-time RT-PCR based diagnostic assays have been used for the detection of infection, but the positive signal of real-time RT-PCR does not necessarily indicate the infectivity of the patient. Due to the unique replication system of the coronavirus, primer/probe sets targeted nucleocapsid (N) and spike (S) protein detect the abundantly synthesized subgenomic RNAs as well as the virus genome, possibly making the assay unsuitable for estimation of the infectivity of the specimen, although it has an advantage for the diagnostic tests. In this study, the primer/probe set targeting the open reading frame 1a (ORF1a) gene was developed to specifically detect viral genomic RNA. Then the relation between the ORF1a signal and infectivity of the clinical specimens was validated by virus isolation using VeroE6 cells, which constitutively express transmembrane protease, serine 2, (VeroE6/TMPRSS2). The analytical sensitivity of developed ORF1a set was similar to that of previously developed N and S sets. Nevertheless, in the assay of the clinical specimen, detection rate of the ORF1a gene was lower than that of the N and S genes. These data indicated that clinical specimens contain a significant amount of subgenomic RNAs. However, as expected, the isolation-succeeded specimen always showed an RT-PCR-positive signal for the ORF1a gene, suggesting ORF1a detection in combination with N and S sets could be a more rational indicator for the possible infectivity of the clinical specimens.

Neutralization of SARS-CoV-2 Variants by rVSV-ΔG-Spike-Elicited Human Sera

The emergence of rapidly spreading variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) poses a major challenge to the ability of vaccines and therapeutic antibodies to provide immunity. These variants contain mutations of specific amino acids that might impede vaccine efficacy. BriLife® (rVSV-ΔG-spike) is a newly developed SARS-CoV-2 vaccine candidate currently in phase II clinical trials. It is based on a replication-competent vesicular stomatitis virus (VSV) platform. The rVSV-ΔG-spike contains several spontaneously acquired spike mutations that correspond to SARS-CoV-2 variants' mutations. We show that human sera from BriLife® vaccinees preserve comparable neutralization titers towards alpha, gamma, and delta variants and show less than a three-fold reduction in the neutralization capacity of beta and omicron compared to the original virus. Taken together, we show that human sera from BriLife® vaccinees overall maintain a neutralizing antibody response against all tested variants. We suggest that BriLife®-acquired mutations may prove advantageous against future SARS-CoV-2 VOCs.

Virus-like Particles: Measures and Biological Functions

Virus-like particles resemble infectious virus particles in size, shape, and molecular composition; however, they fail to productively infect host cells. Historically, the presence of virus-like particles has been inferred from total particle counts by microscopy, and infectious particle counts or plaque-forming-units (PFUs) by plaque assay; the resulting ratio of particles-to-PFUs is often greater than one, easily 10 or 100, indicating that most particles are non-infectious. Despite their inability to hijack cells for their reproduction, virus-like particles and the defective genomes they carry can exhibit a broad range of behaviors: interference with normal virus growth during co-infections, cell killing, and activation or inhibition of innate immune signaling. In addition, some virus-like particles become productive as their multiplicities of infection increase, a sign of cooperation between particles. Here, we review established and emerging methods to count virus-like particles and characterize their biological functions. We take a critical look at evidence for defective interfering virus genomes in natural and clinical isolates, and we review their potential as antiviral therapeutics. In short, we highlight an urgent need to better understand how virus-like genomes and particles interact with intact functional viruses during co-infection of their hosts, and their impacts on the transmission, severity, and persistence of virus-associated diseases.

Comparison of SARS-CoV-2 Evolution in Paediatric Primary Airway Epithelial Cell Cultures Compared with Vero-Derived Cell Lines

SARS-CoV-2 can efficiently infect both children and adults, albeit with morbidity and mortality positively associated with increasing host age and presence of co-morbidities. SARS-CoV-2 continues to adapt to the human population, resulting in several variants of concern (VOC) with novel properties, such as Alpha and Delta. However, factors driving SARS-CoV-2 fitness and evolution in paediatric cohorts remain poorly explored. Here, we provide evidence that both viral and host factors co-operate to shape SARS-CoV-2 genotypic and phenotypic change in primary airway cell cultures derived from children. Through viral whole-genome sequencing, we explored changes in genetic diversity over time of two pre-VOC clinical isolates of SARS-CoV-2 during passage in paediatric well-differentiated primary nasal epithelial cell (WD-PNEC) cultures and in parallel, in unmodified Vero-derived cell lines. We identified a consistent, rich genetic diversity arising in vitro, variants of which could rapidly rise to near fixation within two passages. Within isolates, SARS-CoV-2 evolution was dependent on host cells, with paediatric WD-PNECs showing a reduced diversity compared to Vero (E6) cells. However, mutations were not shared between strains. Furthermore, comparison of both Vero-grown isolates on WD-PNECs disclosed marked growth attenuation mapping to the loss of the polybasic cleavage site (PBCS) in Spike, while the strain with mutations in Nsp12 (T293I), Spike (P812R) and a truncation of Orf7a remained viable in WD-PNECs. Altogether, our work demonstrates that pre-VOC SARS-CoV-2 efficiently infects paediatric respiratory epithelial cells, and its evolution is restrained compared to Vero (E6) cells, similar to the case of adult cells. We highlight the significant genetic plasticity of SARS-CoV-2 while uncovering an influential role for collaboration between viral and host cell factors in shaping viral evolution and ultimately fitness in human respiratory epithelium.

Measuring infectious SARS-CoV-2 in clinical samples reveals a higher viral titer:RNA ratio for Delta and Epsilon vs. Alpha variants

Novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants pose a challenge to controlling the COVID-19 pandemic. Previous studies indicate that clinical samples collected from individuals infected with the Delta variant may contain higher levels of RNA than previous variants, but the relationship between levels of viral RNA and infectious virus for individual variants is unknown. We measured infectious viral titer (using a microfocus-forming assay) and total and subgenomic viral RNA levels (using RT-PCR) in a set of 162 clinical samples containing SARS-CoV-2 Alpha, Delta, and Epsilon variants that were collected in identical swab kits from outpatient test sites and processed soon after collection. We observed a high degree of variation in the relationship between viral titers and RNA levels. Despite this, the overall infectivity differed among the three variants. Both Delta and Epsilon had significantly higher infectivity than Alpha, as measured by the number of infectious units per quantity of viral E gene RNA (5.9- and 3.0-fold increase; P < 0.0001, P = 0.014, respectively) or subgenomic E RNA (14.3- and 6.9-fold increase; P < 0.0001, P = 0.004, respectively). In addition to higher viral RNA levels reported for the Delta variant, the infectivity (amount of replication competent virus per viral genome copy) may be increased compared to Alpha. Measuring the relationship between live virus and viral RNA is an important step in assessing the infectivity of novel SARS-CoV-2 variants. An increase in the infectivity for Delta may further explain increased spread, suggesting a need for increased measures to prevent viral transmission.

Glycosaminoglycan binding by arboviruses: a cautionary tale

Arboviruses are medically important arthropod-borne viruses that cause a range of diseases in humans from febrile illness to arthritis, encephalitis and hemorrhagic fever. Given their transmission cycles, these viruses face the challenge of replicating in evolutionarily divergent organisms that can include ticks, flies, mosquitoes, birds, rodents, reptiles and primates. Furthermore, their cell attachment receptor utilization may be affected by the opposing needs for generating high and sustained serum viremia in vertebrates such that virus particles are efficiently collected during a hematophagous arthropod blood meal but they must also bind sufficiently to cellular structures on divergent organisms such that productive infection can be initiated and viremia generated. Sulfated polysaccharides of the glycosaminoglycan (GAG) groups, primarily heparan sulfate (HS), have been identified as cell attachment moieties for many arboviruses. Original identification of GAG binding as a phenotype of arboviruses appeared to involve this attribute arising solely as a consequence of adaptation of virus isolates to growth in cell culture. However, more recently, naturally circulating strains of at least one arbovirus, eastern equine encephalitis, have been shown to bind HS efficiently and the GAG binding phenotype continues to be associated with arbovirus infection in published studies. If GAGs are attachment receptors for many naturally circulating arboviruses, this could lead to development of broad-spectrum antiviral therapies through blocking of the virus-GAG interaction. This review summarizes the available data for GAG/HS binding as a phenotype of naturally circulating arbovirus strains emphasizing the importance of avoiding tissue culture amplification and artifactual phenotypes during their isolation.

Structural insights in cell-type specific evolution of intra-host diversity by SARS-CoV-2

As the global burden of SARS-CoV-2 infections escalates, so does the evolution of viral variants with increased transmissibility and pathology. In addition to this entrenched diversity, RNA viruses can also display genetic diversity within single infected hosts with co-existing viral variants evolving differently in distinct cell types. The BriSΔ variant, originally identified as a viral subpopulation from SARS-CoV-2 isolate hCoV-19/England/02/2020, comprises in the spike an eight amino-acid deletion encompassing a furin recognition motif and S1/S2 cleavage site. We elucidate the structure, function and molecular dynamics of this spike providing mechanistic insight into how the deletion correlates to viral cell tropism, ACE2 receptor binding and infectivity of this SARS-CoV-2 variant. Our results reveal long-range allosteric communication between functional domains that differ in the wild-type and the deletion variant and support a view of SARS-CoV-2 probing multiple evolutionary trajectories in distinct cell types within the same infected host.

Intra-Host SARS-CoV-2 Evolution in the Gut of Mucosally-Infected Chlorocebus aethiops (African Green Monkeys)

In recent months, several SARS-CoV-2 variants have emerged that enhance transmissibility and escape host humoral immunity. Hence, the tracking of viral evolutionary trajectories is clearly of great importance. Little is known about SARS-CoV-2 evolution in nonhuman primate models used to test vaccines and therapies and to model human disease. Viral RNA was sequenced from rectal swabs from Chlorocebus aethiops (African green monkeys) after experimental respiratory SARS-CoV-2 infection. Two distinct patterns of viral evolution were identified that were shared between all collected samples. First, mutations in the furin cleavage site that were initially present in the virus as a consequence of VeroE6 cell culture adaptation were not detected in viral RNA recovered in rectal swabs, confirming the necessity of this motif for viral infection in vivo. Three amino acid changes were also identified; ORF 1a S2103F, and spike D215G and H655Y, which were detected in rectal swabs from all sampled animals. These findings are demonstrative of intra-host SARS-CoV-2 evolution and may identify a host-adapted variant of SARS-CoV-2 that would be useful in future primate models involving SARS-CoV-2 infection.

Proteolytic activation of SARS-CoV-2 spike protein

Spike (S) protein cleavage is a crucial step in coronavirus infection. In this review, this process is discussed, with particular focus on the novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Compared with influenza virus and paramyxovirus membrane fusion proteins, the cleavage activation mechanism of coronavirus S protein is much more complex. The S protein has two cleavage sites (S1/S2 and S2'), and the cleavage motif for furin protease at the S1/S2 site that results from a unique four-amino acid insertion is one of the distinguishing features of SARS-CoV-2. The viral particle incorporates the S protein, which has already undergone S1/S2 cleavage by furin, and then undergoes further cleavage at the S2' site, mediated by the type II transmembrane serine protease transmembrane protease serine 2 (TMPRSS2), after binding to the receptor angiotensin-converting enzyme 2 (ACE2) to facilitate membrane fusion at the plasma membrane. In addition, SARS-CoV-2 can enter the cell by endocytosis and be proteolytically activated by cathepsin L, although this is not a major mode of SARS-CoV-2 infection. SARS-CoV-2 variants with enhanced infectivity have been emerging throughout the ongoing pandemic, and there is a close relationship between enhanced infectivity and changes in S protein cleavability. All four variants of concern carry the D614G mutation, which indirectly enhances S1/S2 cleavability by furin. The P681R mutation of the delta variant directly increases S1/S2 cleavability, enhancing membrane fusion and SARS-CoV-2 virulence. Changes in S protein cleavability can significantly impact viral infectivity, tissue tropism, and virulence. Understanding these mechanisms is critical to counteracting the coronavirus pandemic.

Mechanisms of SARS-CoV-2 entry into cells

The unprecedented public health and economic impact of the COVID-19 pandemic caused by infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been met with an equally unprecedented scientific response. Much of this response has focused, appropriately, on the mechanisms of SARS-CoV-2 entry into host cells, and in particular the binding of the spike (S) protein to its receptor, angiotensin-converting enzyme 2 (ACE2), and subsequent membrane fusion. This Review provides the structural and cellular foundations for understanding the multistep SARS-CoV-2 entry process, including S protein synthesis, S protein structure, conformational transitions necessary for association of the S protein with ACE2, engagement of the receptor-binding domain of the S protein with ACE2, proteolytic activation of the S protein, endocytosis and membrane fusion. We define the roles of furin-like proteases, transmembrane protease, serine 2 (TMPRSS2) and cathepsin L in these processes, and delineate the features of ACE2 orthologues in reservoir animal species and S protein adaptations that facilitate efficient human transmission. We also examine the utility of vaccines, antibodies and other potential therapeutics targeting SARS-CoV-2 entry mechanisms. Finally, we present key outstanding questions associated with this critical process.

Reduced antibody activity against SARS-CoV-2 B.1.617.2 delta virus in serum of mRNA-vaccinated individuals receiving tumor necrosis factor-α inhibitors

BACKGROUND

Although vaccines effectively prevent coronavirus disease 2019 (COVID-19) in healthy individuals, they appear to be less immunogenic in individuals with chronic inflammatory disease (CID) or receiving chronic immunosuppression therapy.

METHODS

Here we assessed a cohort of 77 individuals with CID treated as monotherapy with chronic immunosuppressive drugs for antibody responses in serum against historical and variant severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viruses after immunization with the BNT162b2 mRNA vaccine.

FINDINGS

Longitudinal analysis showed the greatest reductions in neutralizing antibodies and Fc effector function capacity in individuals treated with tumor necrosis factor alpha (TNF-α) inhibitors (TNFi), and this pattern appeared to be worse against the B.1.617.2 delta virus. Within 5 months of vaccination, serum neutralizing titers of all TNFi-treated individuals tested fell below the presumed threshold correlate for antibody-mediated protection. However, TNFi-treated individuals receiving a third mRNA vaccine dose boosted their serum neutralizing antibody titers by more than 16-fold.

CONCLUSIONS

Vaccine boosting or administration of long-acting prophylaxis (e.g., monoclonal antibodies) will likely be required to prevent SARS-CoV-2 infection in this susceptible population.

FUNDING

This study was supported by grants and contracts from the NIH (R01 AI157155, R01AI151178, and HHSN75N93019C00074; NIAID Centers of Excellence for Influenza Research and Response (CEIRR) contracts HHSN272201400008C and 75N93021C00014; and Collaborative Influenza Vaccine Innovation Centers [CIVIC] contract 75N93019C00051).

Site-Specific Glycosylation Patterns of the SARS-CoV-2 Spike Protein Derived From Recombinant Protein and Viral WA1 and D614G Strains

The SARS-CoV-2 spike protein is heavily glycosylated, having 22 predicted N-glycosylation sites per monomer. It is also O-glycosylated, although the number of O-glycosites is less defined. Recent studies show that spike protein glycans play critical roles in viral entry and infection. The spike monomer has two subdomains, S1 and S2, and a receptor-binding domain (RBD) within the S1 domain. In this study, we have characterized the site-specific glycosylation patterns of the HEK293 recombinant spike RBD and S1 domains as well as the intact spike derived from the whole virus produced in Vero cells. The Vero cell-derived spike from the WA1 strain and a D614G variant was analyzed. All spike proteins, S1, and RBDs were analyzed using hydrophilic interaction chromatography (HILIC) and LC-MS/MS on an Orbitrap Eclipse Tribrid mass spectrometer. N-glycans identified in HEK293-derived S1 were structurally diverse. Those found in the HEK293-derived RBD were highly similar to those in HEK293 S1 where N-glycosites were shared. Comparison of the whole cell-derived WA1 and D614G spike proteins revealed that N-glycosites local to the mutation site appeared to be more readily detected, hinting that these sites are more exposed to glycosylation machinery. Moreover, recombinant HEK293-derived S1 was occupied almost completely with complex glycan, while both WA1 and D614G derived from the Vero E6 cell whole virus were predominantly high-mannose glycans. This stands in stark contrast to glycosylation patterns seen in both CHO- and HEK cell-derived recombinant S1, S2, and the whole spike previously reported. Concerning O-glycosylation, our analyses revealed that HEK293 recombinant proteins possessed a range of O-glycosites with compositions consistent with Core type 1 and 2 glycans. The O-glycosites shared between the S1 and RBD constructs, sites T323 and T523, were occupied by a similar range of Core 1 and 2 type O-glycans. Overall, this study reveals that the sample nature and cell substrate used for production of these proteins can have a dramatic impact on the glycosylation profile. SARS-CoV-2 spike glycans are associated with host ACE2 receptor interaction efficiency. Therefore, understanding such differences will serve to better understand these host–pathogen interactions and inform the choice of cell substrates to suite downstream investigations.

Propagation of SARS-CoV-2 in Calu-3 Cells to Eliminate Mutations in the Furin Cleavage Site of Spike

SARS-CoV-2 pathogenesis, vaccine, and therapeutic studies rely on the use of animals challenged with highly pathogenic virus stocks produced in cell cultures. Ideally, these virus stocks should be genetically and functionally similar to the original clinical isolate, retaining wild-type properties to be reliably used in animal model studies. It is well-established that SARS-CoV-2 isolates serially passaged on Vero cell lines accumulate mutations and deletions in the furin cleavage site; however, these can be eliminated when passaged on Calu-3 lung epithelial cell lines, as presented in this study. As numerous stocks of SARS-CoV-2 variants of concern are being grown in cell cultures with the intent for use in animal models, it is essential that propagation methods generate virus stocks that are pathogenic in vivo. Here, we found that the propagation of a B.1.351 SARS-CoV-2 stock on Calu-3 cells eliminated viruses that previously accumulated mutations in the furin cleavage site. Notably, there were alternative variants that accumulated at the same nucleotide positions in virus populations grown on Calu-3 cells at multiple independent facilities. When a Calu-3-derived B.1.351 virus stock was used to infect hamsters, the virus remained pathogenic and the Calu-3-specific variants persisted in the population. These results suggest that Calu-3-derived virus stocks are pathogenic but care should still be taken to evaluate virus stocks for newly arising mutations during propagation.

Hydroxychloroquine and azithromycin used alone or combined are not effective against SARS-CoV-2 ex vivo and in a hamster model

Drug repositioning has been used extensively since the beginning of the COVID-19 pandemic in an attempt to identify antiviral molecules for use in human therapeutics. Hydroxychloroquine and azithromycin have shown inhibitory activity against SARS-CoV-2 replication in different cell lines. Based on such in vitro data and despite the weakness of preclinical assessment, many clinical trials were set up using these molecules. In the present study, we show that hydroxychloroquine and azithromycin alone or combined does not block SARS-CoV-2 replication in human bronchial airway epithelia. When tested in a Syrian hamster model, hydroxychloroquine and azithromycin administrated alone or combined displayed no significant effect on viral replication, clinical course of the disease and lung impairments, even at high doses. Hydroxychloroquine quantification in lung tissues confirmed strong exposure to the drug, above in vitro inhibitory concentrations. Overall, this study does not support the use of hydroxychloroquine and azithromycin as antiviral drugs for the treatment of SARS-CoV-2 infections.

Evaluation of Residual Infectivity after SARS-CoV-2 Aerosol Transmission in a Controlled Laboratory Setting

Severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) is mainly transmitted through respiratory droplets, aerosols, or direct contact with fomites from an infected subject. It has been reported that SARS-CoV-2 is stable and viable in aerosol up to 16 h in controlled laboratory conditions. However, the aerosolization conditions varied a lot between the studies. In this work, an experimental laboratory model of SARS-CoV-2 aerosolization was established, employing an impinger nebulizer, a cylindrical chamber for aerosol travel, and a SKC biosampler for the collection of particles. The efficiency of the system was assessed based on the molecular determination of the viral load in the nebulizer after the aerosolization and in the aerosol collected at the end of the travel. Moreover, the residual infectivity was tested in vitro on the Vero E6 cell line, through the observation of the cytopathic effect (CPE), and the quantification of the viral load in the supernatants at 7 days post inoculation (dpi). A high RNA viral load was found in the SKC biosampler after aerosolization, indicating that it was possible to transport a high virus titer through the 30-cm chamber with all the dilutions (initial 105, 104, 103 plaque forming unit-PFU/mL). At the 7 dpi, an increment of the RNA viral load was determined for the dilutions 105 and 104 PFU/mL tested, while only the initial 105 PFU/mL resulted in visible CPE. Our findings allowed us to achieve the resilience of SARS-CoV-2 in aerosol form, at a concentration comparable to those reported for clinical samples. This mode of transmission should be considered for the mitigation and preventive measures to counteract SARS-CoV-2 spreading.

Natural and Recombinant SARS-CoV-2 Isolates Rapidly Evolve In Vitro to Higher Infectivity through More Efficient Binding to Heparan Sulfate and Reduced S1/S2 Cleavage

One of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virulence factors is the ability to interact with high affinity to the ACE2 receptor, which mediates viral entry into cells. The results of our study demonstrate that within a few passages in cell culture, both the natural isolate of SARS-CoV-2 and the recombinant cDNA-derived variant acquire an additional ability to bind to heparan sulfate (HS). This promotes a primary attachment of viral particles to cells before their further interactions with the ACE2. Interaction with HS is acquired through multiple mechanisms. These include (i) accumulation of point mutations in the N-terminal domain (NTD) of the S protein, which increases the positive charge of the surface of this domain, (ii) insertions into the NTD of heterologous peptides containing positively charged amino acids, and (iii) mutation of the first amino acid downstream of the furin cleavage site. This last mutation affects S protein processing, transforms the unprocessed furin cleavage site into the heparin-binding peptide, and makes viruses less capable of syncytium formation. These viral adaptations result in higher affinity of viral particles to heparin, dramatic increase in plaque sizes, more efficient viral spread, higher infectious titers, and 2 orders of magnitude higher infectivity. The detected adaptations also suggest an active role of NTD in virus attachment and entry. As in the case of other RNA-positive (RNA⁺) viruses, evolution to HS binding may result in virus attenuation in vivo.

IMPORTANCE The spike protein of SARS-CoV-2 is a major determinant of viral pathogenesis. It mediates binding to the ACE2 receptor and, later, fusion of viral envelope and cellular membranes. The results of our study demonstrate that SARS-CoV-2 rapidly evolves during propagation in cultured cells. Its spike protein acquires mutations in the NTD and in the P1′ position of the furin cleavage site (FCS). The amino acid substitutions or insertions of short peptides in NTD are closely located on the protein surface and increase its positive charge. They strongly increase affinity of the virus to heparan sulfate, make it dramatically more infectious for the cultured cells, and decrease the genome equivalent to PFU (GE/PFU) ratio by orders of magnitude. The S686G mutation also transforms the FCS into the heparin-binding peptide. Thus, the evolved SARS-CoV-2 variants efficiently use glycosaminoglycans on the cell surface for primary attachment before the high-affinity interaction of the spikes with the ACE2 receptor.

Genetic and structural basis for SARS-CoV-2 variant neutralization by a two-antibody cocktail

Understanding the molecular basis for immune recognition of SARS-CoV-2 spike glycoprotein antigenic sites will inform the development of improved therapeutics. We determined the structures of two human monoclonal antibodies-AZD8895 and AZD1061-which form the basis of the investigational antibody cocktail AZD7442, in complex with the receptor-binding domain (RBD) of SARS-CoV-2 to define the genetic and structural basis of neutralization. AZD8895 forms an 'aromatic cage' at the heavy/light chain interface using germ line-encoded residues in complementarity-determining regions (CDRs) 2 and 3 of the heavy chain and CDRs 1 and 3 of the light chain. These structural features explain why highly similar antibodies (public clonotypes) have been isolated from multiple individuals. AZD1061 has an unusually long LCDR1; the HCDR3 makes interactions with the opposite face of the RBD from that of AZD8895. Using deep mutational scanning and neutralization escape selection experiments, we comprehensively mapped the crucial binding residues of both antibodies and identified positions of concern with regards to virus escape from antibody-mediated neutralization. Both AZD8895 and AZD1061 have strong neutralizing activity against SARS-CoV-2 and variants of concern with antigenic substitutions in the RBD. We conclude that germ line-encoded antibody features enable recognition of the SARS-CoV-2 spike RBD and demonstrate the utility of the cocktail AZD7442 in neutralizing emerging variant viruses.