Short- and long-term outcomes of pulmonary exposure to a sublethal dose of ricin in mice.

Ricin, an extremely potent toxin produced from the seeds of castor plant, Ricinus communis, is ribosome-inactivating protein that blocks cell-protein synthesis. It is considered a biological threat due to worldwide availability of castor beans, massive quantities as a by-product of castor oil production, high stability and ease of production. The consequence of exposure to lethal dose of ricin was extensively described in various animal models. However, it is assumed that in case of aerosolized ricin bioterror attack, the majority of individuals would be exposed to sublethal doses rather than to lethal ones. Therefore, the purpose of current study was to assess short- and long-term effects on physiological parameters and function following sublethal pulmonary exposure. We show that in the short-term, sublethal exposure of mice to ricin resulted in acute lung injury, including interstitial pneumonia, cytokine storm, neutrophil influx, edema and cellular death. This damage was manifested in reduced lung performance and physiological function. Interestingly, although in the long-term, mice recovered from acute lung damage and restored pulmonary and physiological functionality, the reparative process was associated with lasting fibrotic lesions. Therefore, restriction of short-term acute phase of the disease and management of long-term pulmonary fibrosis by medical countermeasures is expected to facilitate the quality of life of exposed survivors.

Induction of Superior Systemic and Mucosal Protective Immunity to SARS-CoV-2 by Nasal Administration of a VSV-ΔG-Spike Vaccine.

The emergence of rapidly spreading variants of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) poses a major challenge to vaccines' protective efficacy. Intramuscular (IM) vaccine administration induces short-lived immunity but does not prevent infection and transmission. New vaccination strategies are needed to extend the longevity of vaccine protection, induce mucosal and systemic immunity and prevent viral transmission. The intranasal (IN) administration of the VSV-ΔG-spike vaccine candidate directly to mucosal surfaces yielded superior mucosal and systemic immunity at lower vaccine doses. Compared to IM vaccination in the K18-hACE2 model, IN vaccination preferentially induced mucosal IgA and T-cells, reduced the viral load at the site of infection, and ameliorated disease-associated brain gene expression. IN vaccination was protective even one year after administration. As most of the world population has been vaccinated by IM injection, we demonstrate the potential of a heterologous IM + IN vaccination regimen to induce mucosal immunity while maintaining systemic immunity. Furthermore, the IM + IN regimen prevented virus transmission in a golden Syrian hamster co-caging model. Taken together, we show that IN vaccination with VSV-ΔG-spike, either as a homologous IN + IN regimen or as a boost following IM vaccination, has a favorable potential over IM vaccination in inducing efficient mucosal immunity, long-term protection and preventing virus transmission.

Doxycycline, levofloxacin, and moxifloxacin are superior to ciprofloxacin in treating anthrax meningitis in rabbits and NHP.

Efficient treatment of anthrax-related meningitis in patients poses a significant therapeutic challenge. Previously, we demonstrated in our anthrax meningitis rabbit model that ciprofloxacin treatment is ineffective with most of the treated animals succumbing to the infection. Herein we tested the efficacy of doxycycline in our rabbit model and found it highly effective. Since all of our findings are based on a rabbit model, we test the efficacy of ciprofloxacin or doxycycline in a specific central nervous system (CNS) model developed in non-human primates (NHPs). Similar to rabbits, ciprofloxacin treatment was ineffective, while doxycycline protected the infected rhesus macaques (n = 2) from the lethal CNS Bacillus anthracis infection. To test whether the low efficacy of Ciprofloxacin is an example of low efficacy of all fluoroquinolones or only this substance, we treated rabbits that were inoculated intracisterna magna (ICM) with levofloxacin or moxifloxacin. We found that in contrast to ciprofloxacin, levofloxacin and moxifloxacin were highly efficacious in treating lethal anthrax-related meningitis in rabbits and NHP (levofloxacin). We demonstrated (in naïve rabbits) that this difference probably results from variances in blood-brain-barrier penetration of the different fluoroquinolones. The combined treatment of doxycycline and any one of the tested fluoroquinolones was highly effective in the rabbit CNS infection model. The combined treatment of doxycycline and levofloxacin was effective in an inhalation rabbit model, as good as the doxycycline mono-therapy. These findings imply that while ciprofloxacin is highly effective as a post-exposure prophylactic drug, using this drug to treat symptomatic patients should be reconsidered.

Open science discovery of potent noncovalent SARS-CoV-2 main protease inhibitors.

We report the results of the COVID Moonshot, a fully open-science, crowdsourced, and structure-enabled drug discovery campaign targeting the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) main protease. We discovered a noncovalent, nonpeptidic inhibitor scaffold with lead-like properties that is differentiated from current main protease inhibitors. Our approach leveraged crowdsourcing, machine learning, exascale molecular simulations, and high-throughput structural biology and chemistry. We generated a detailed map of the structural plasticity of the SARS-CoV-2 main protease, extensive structure-activity relationships for multiple chemotypes, and a wealth of biochemical activity data. All compound designs (>18,000 designs), crystallographic data (>490 ligand-bound x-ray structures), assay data (>10,000 measurements), and synthesized molecules (>2400 compounds) for this campaign were shared rapidly and openly, creating a rich, open, and intellectual property-free knowledge base for future anticoronavirus drug discovery.

In Vitro and In Vivo Therapeutic Potential of 6,6'-Dihydroxythiobinupharidine (DTBN) from Nuphar lutea on Cells and K18-hACE2 Mice Infected with SARS-CoV-2.

We have previously published research on the anti-viral properties of an alkaloid mixture extracted from Nuphar lutea, the major components of the partially purified mixture found by NMR analysis. These are mostly dimeric sesquiterpene thioalkaloids called thiobinupharidines and thiobinuphlutidines against the negative strand RNA measles virus (MV). We have previously reported that this extract inhibits the MV as well as its ability to downregulate several MV proteins in persistently MV-infected cells, especially the P (phospho)-protein. Based on our observation that the Nuphar extract is effective in vitro against the MV, and the immediate need that the coronavirus disease 2019 (COVID-19) pandemic created, we tested here the ability of 6,6'-dihydroxythiobinupharidine DTBN, an active small molecule, isolated from the Nuphar lutea extract, on COVID-19. As shown here, DTBN effectively inhibits SARS-CoV-2 production in Vero E6 cells at non-cytotoxic concentrations. The short-term daily administration of DTBN to infected mice delayed the occurrence of severe clinical outcomes, lowered virus levels in the lungs and improved survival with minimal changes in lung histology. The viral load on lungs was significantly reduced in the treated mice. DTBN is a pleiotropic small molecule with multiple targets. Its anti-inflammatory properties affect a variety of pathogens including SARS-CoV-2 as shown here. Its activity appears to target both pathogen specific (as suggested by docking analysis) as well as cellular proteins, such as NF-κB, PKCs, cathepsins and topoisomerase 2, that we have previously identified in our work. Thus, this combined double action of virus inhibition and anti-inflammatory activity may enhance the overall effectivity of DTBN. The promising results from this proof-of-concept in vitro and in vivo preclinical study should encourage future studies to optimize the use of DTBN and/or its molecular derivatives against this and other related viruses.

Turning high-throughput structural biology into predictive inhibitor design.

A common challenge in drug design pertains to finding chemical modifications to a ligand that increases its affinity to the target protein. An underutilized advance is the increase in structural biology throughput, which has progressed from an artisanal endeavor to a monthly throughput of hundreds of different ligands against a protein in modern synchrotrons. However, the missing piece is a framework that turns high-throughput crystallography data into predictive models for ligand design. Here, we designed a simple machine learning approach that predicts protein-ligand affinity from experimental structures of diverse ligands against a single protein paired with biochemical measurements. Our key insight is using physics-based energy descriptors to represent protein-ligand complexes and a learning-to-rank approach that infers the relevant differences between binding modes. We ran a high-throughput crystallography campaign against the SARS-CoV-2 main protease (MPro), obtaining parallel measurements of over 200 protein-ligand complexes and their binding activities. This allows us to design one-step library syntheses which improved the potency of two distinct micromolar hits by over 10-fold, arriving at a noncovalent and nonpeptidomimetic inhibitor with 120 nM antiviral efficacy. Crucially, our approach successfully extends ligands to unexplored regions of the binding pocket, executing large and fruitful moves in chemical space with simple chemistry.

Alternate atxA and acpA dependent response of Bacillus anthracis to serum, HCO3- and CO2.

Bacillus anthracis overcomes host immune responses by producing capsule and secreting toxins. Production of these virulence factors in response to entering the host environment was shown to be regulated by atxA, the major virulence regulator, known to be activated by HCO3- and CO2. While toxin production is regulated directly by atxA, capsule production is independently mediated by two regulators; acpA and acpB. In addition, it was demonstrated that acpA has at least two promotors, one of them shared with atxA. We used a genetic approach to study capsule and toxin production under different conditions. Unlike previous works utilizing NBY, CA or R-HCO3- medium under CO2 enriched conditions, we used a sDMEM-based medium. Thus, toxin and capsule production can be induced in ambient or CO2 enriched atmosphere. Using this system, we could differentiate between induction by 10% NRS, 10% CO2 or 0.75% HCO3-. In response to high CO2, capsule production is induced by acpA based response in an atxA-independent manner, with little to no toxin (protective antigen PA) production. atxA based response is activated in response to serum independently of CO2, inducing toxin and capsule production in an acpA or acpB dependent manner. HCO3- was also found to activate atxA based response, but in non-physiological concentrations. Our findings may help explain the first stages of inhalational infection, in which spores germinating in dendritic cells require protection (by encapsulation) without affecting cell migration to the draining lymph-node by toxin secretion.

Cap-independent translation and a precisely located RNA sequence enable SARS-CoV-2 to control host translation and escape anti-viral response.

Translation of SARS-CoV-2-encoded mRNAs by the host ribosomes is essential for its propagation. Following infection, the early expressed viral protein NSP1 binds the ribosome, represses translation, and induces mRNA degradation, while the host elicits an anti-viral response. The mechanisms enabling viral mRNAs to escape this multifaceted repression remain obscure. Here we show that expression of NSP1 leads to destabilization of multi-exon cellular mRNAs, while intron-less transcripts, such as viral mRNAs and anti-viral interferon genes, remain relatively stable. We identified a conserved and precisely located cap-proximal RNA element devoid of guanosines that confers resistance to NSP1-mediated translation inhibition. Importantly, the primary sequence rather than the secondary structure is critical for protection. We further show that the genomic 5'UTR of SARS-CoV-2 drives cap-independent translation and promotes expression of NSP1 in an eIF4E-independent and Torin1-resistant manner. Upon expression, NSP1 further enhances cap-independent translation. However, the sub-genomic 5'UTRs are highly sensitive to eIF4E availability, rendering viral propagation partially sensitive to Torin1. We conclude that the combined NSP1-mediated degradation of spliced mRNAs and translation inhibition of single-exon genes, along with the unique features present in the viral 5'UTRs, ensure robust expression of viral mRNAs. These features can be exploited as potential therapeutic targets.

Revisiting SARS-CoV-2 environmental contamination by patients with COVID-19: The Omicron variant does not differ from previous strains.

SARS-CoV-2 Omicron strain emergence raised concerns that its enhanced infectivity is partly due to altered spread/contamination modalities. We therefore sampled high-contact surfaces and air in close proximity to patients who were verified as infected with the Omicron strain, using identical protocols applied to sample patients positive to the original or Alpha strains. Cumulatively, for all 3 strains, viral RNA was detected in 90 of 168 surfaces and 6 of 49 air samples (mean cycle threshold [Ct]=35.2±2.5). No infective virus was identified. No significant differences in prevalence were found between strains.

A Novel Quantitative Multi-Component Serological Assay for SARS-CoV-2 Vaccine Evaluation.

A multi-component microarray, applying a novel analysis algorithm, was developed for quantitative evaluation of the SARS-CoV-2 vaccines' immunogenicity. The array enables simultaneous quantitation of IgG, IgM, and IgA, specific to the SARS-CoV-2 spike, receptor binding domain, and nucleocapsid proteins. The developed methodology is based on calculating an apparent immunoglobulin signal from the linear range of the fluorescent read-outs generated by scanning the microarray slides at different exposure times. A dedicated algorithm, employing a rigorous set of embedded conditions, then generates a normalized signal for each of the unique assays. Qualification of the multi-component array performance (evaluating linearity, extended dynamic-range, specificity, precision, and accuracy) was carried out with an in-house COVID-19, qRT-PCR positive serum, as well as pre-pandemic commercial negative sera. Results were compared to the WHO international standard for anti-SARS-CoV-2 immunoglobulins. Specific IgG, IgM, and IgA signals obtained by this array enabled successful discrimination between SARS-CoV-2 q-RT-PCR positive (seroconverted SARS-CoV-2 patients) and negative (naïve) samples. This array is currently used for evaluation of the humoral response to BriLife, the VSV-based Israeli vaccine during phase I/II clinical trials.

Development of an immunofluorescence assay for detection of SARS-CoV-2.

SARS-CoV-2, the etiologic agent of the COVID-19 pandemic, emerged as the cause of a global crisis in 2019. Currently, the main method for identification of SARS-CoV-2 is a reverse transcription (RT)-PCR assay designed to detect viral RNA in oropharyngeal (OP) or nasopharyngeal (NP) samples. While the PCR assay is considered highly specific and sensitive, this method cannot determine the infectivity of the sample, which may assist in evaluation of virus transmissibility from patients and breaking transmission chains. Thus, cell-culture-based approaches such as cytopathic effect (CPE) assays are routinely employed for the identification of infectious viruses in NP/OP samples. Despite their high sensitivity, CPE assays take several days and require additional diagnostic tests in order to verify the identity of the pathogen. We have therefore developed a rapid immunofluorescence assay (IFA) for the specific detection of SARS-CoV-2 in NP/OP samples following cell culture infection. Initially, IFA was carried out on Vero E6 cultures infected with SARS-CoV-2 at defined concentrations, and infection was monitored at different time points. This test was able to yield positive signals in cultures infected with 10 pfu/ml at 12 hours postinfection (PI). Increasing the incubation time to 24 hours reduced the detectable infective dose to 1 pfu/ml. These IFA signals occur before the development of CPE. When compared to the CPE test, IFA has the advantages of specificity, rapid detection, and sensitivity, as demonstrated in this work.

Coupling immuno-magnetic capture with LC-MS/MS(MRM) as a sensitive, reliable, and specific assay for SARS-CoV-2 identification from clinical samples.

Recently, numerous diagnostic approaches from different disciplines have been developed for SARS-CoV-2 diagnosis to monitor and control the COVID-19 pandemic. These include MS-based assays, which provide analytical information on viral proteins. However, their sensitivity is limited, estimated to be 5 × 104 PFU/ml in clinical samples. Here, we present a reliable, specific, and rapid method for the identification of SARS-CoV-2 from nasopharyngeal (NP) specimens, which combines virus capture followed by LC-MS/MS(MRM) analysis of unique peptide markers. The capture of SARS-CoV-2 from the challenging matrix, prior to its tryptic digestion, was accomplished by magnetic beads coated with polyclonal IgG-α-SARS-CoV-2 antibodies, enabling sample concentration while significantly reducing background noise interrupting with LC-MS analysis. A sensitive and specific LC-MS/MS(MRM) analysis method was developed for the identification of selected tryptic peptide markers. The combined assay, which resulted in S/N ratio enhancement, achieved an improved sensitivity of more than 10-fold compared with previously described MS methods. The assay was validated in 29 naive NP specimens, 19 samples were spiked with SARS-CoV-2 and 10 were used as negative controls. Finally, the assay was successfully applied to clinical NP samples (n = 26) pre-determined as either positive or negative by RT-qPCR. This work describes for the first time a combined approach for immuno-magnetic viral isolation coupled with MS analysis. This method is highly reliable, specific, and sensitive; thus, it may potentially serve as a complementary assay to RT-qPCR, the gold standard test. This methodology can be applied to other viruses as well.

Preliminary nonclinical safety and immunogenicity of an rVSV-ΔG-SARS-CoV-2-S vaccine in mice, hamsters, rabbits and pigs.

rVSV-ΔG-SARS-CoV-2-S is a clinical stage (Phase 2) replication competent recombinant vaccine against SARS-CoV-2. To evaluate the safety profile of the vaccine, a series of non-clinical safety, immunogenicity and efficacy studies were conducted in four animal species, using multiple doses (up to 108 Plaque Forming Units/animal) and dosing regimens. There were no treatment-related mortalities or any noticeable clinical signs in any of the studies. Compared to unvaccinated controls, hematology and biochemistry parameters were unremarkable and no adverse histopathological findings. There was no detectable viral shedding in urine, nor viral RNA detected in whole blood or serum samples seven days post vaccination. The rVSV-ΔG-SARS-CoV-2-S vaccination gave rise to neutralizing antibodies, cellular immune responses, and increased lymphocytic cellularity in the spleen germinal centers and regional lymph nodes. No evidence for neurovirulence was found in C57BL/6 immune competent mice or in highly sensitive type I interferon knock-out mice. Vaccine virus replication and distribution in K18-human Angiotensin-converting enzyme 2-transgenic mice showed a gradual clearance from the vaccination site with no vaccine virus recovered from the lungs. The nonclinical data suggest that the rVSV-ΔG-SARS-CoV-2-S vaccine is safe and immunogenic. These results supported the initiation of clinical trials, currently in Phase 2.

A Serological Snapshot of COVID-19 Initial Stages in Israel by a 6-Plex Antigen Array.

The first case of SARS-CoV-2 was discovered in Israel in late February 2020. Three major outbreaks followed, resulting in over 800,000 cases and over 6,000 deaths by April 2021. Our aim was characterization of a serological snapshot of Israeli patients and healthy adults in the early months of the COVID-19 pandemic. Sera from 55 symptomatic COVID-19 patients and 146 healthy subjects (early-pandemic, reverse transcription-quantitative PCR [qRT-PCR]-negative), collected in Israel between March and April 2020, were screened for SARS-CoV-2-specific IgG, IgM, and IgA antibodies, using a 6-plex antigen microarray presenting the whole inactivated virus and five viral antigens: a stabilized version of the spike ectodomain (S2P), spike subunit 1 (S1), receptor-binding-domain (RBD), N-terminal-domain (NTD), and nucleocapsid (NC). COVID-19 patients, 4 to 40 days post symptom onset, presented specific IgG to all of the viral antigens (6/6) in 54 of the 55 samples (98% sensitivity). Specific IgM and IgA antibodies for all six antigens were detected in only 10% (5/55) and 4% (2/55) of the patients, respectively, suggesting that specific IgG is a superior serological marker for COVID-19. None of the qRT-PCR-negative sera reacted with all six viral antigens (100% specificity), and 48% (70/146) were negative throughout the panel. Our findings confirm a low seroprevalence of anti-SARS-CoV-2 antibodies in the Israeli adult population prior to the COVID-19 outbreak. We further suggest that the presence of low-level cross-reacting antibodies in naive individuals calls for a combined, multiantigen analysis for accurate discrimination between naive and exposed individuals. IMPORTANCE A 6-plex protein array presenting the whole inactivated virus and five nucleocapsid and spike-derived SARS-CoV-2 antigens was used to generate a serological snapshot of SARS-CoV-2 seroprevalence and seroconversion in Israel in the early months of the pandemic. Our findings confirm a very low seroprevalence of anti-SARS-CoV-2 antibodies in the Israeli adult population. We further propose that the presence of low-level nonspecific antibodies in naive individuals calls for a combined, multiantigen analysis for accurate discrimination between naive and exposed individuals enabling accurate determination of seroconversion. The developed assay is currently applied to evaluate immune responses to the Israeli vaccine during human phase I/II trials.

LAMP-Seq enables sensitive, multiplexed COVID-19 diagnostics using molecular barcoding.

Frequent testing of large population groups combined with contact tracing and isolation measures will be crucial for containing Coronavirus Disease 2019 outbreaks. Here we present LAMP-Seq, a modified, highly scalable reverse transcription loop-mediated isothermal amplification (RT-LAMP) method. Unpurified biosamples are barcoded and amplified in a single heat step, and pooled products are analyzed en masse by sequencing. Using commercial reagents, LAMP-Seq has a limit of detection of ~2.2 molecules per µl at 95% confidence and near-perfect specificity for severe acute respiratory syndrome coronavirus 2 given its sequence readout. Clinical validation of an open-source protocol with 676 swab samples, 98 of which were deemed positive by standard RT-qPCR, demonstrated 100% sensitivity in individuals with cycle threshold values of up to 33 and a specificity of 99.7%, at a very low material cost. With a time-to-result of fewer than 24 h, low cost and little new infrastructure requirement, LAMP-Seq can be readily deployed for frequent testing as part of an integrated public health surveillance program.

Mice with induced pulmonary morbidities display severe lung inflammation and mortality following exposure to SARS-CoV-2.

Mice are normally unaffected by SARS coronavirus 2 (SARS-CoV-2) infection since the virus does not bind effectively to the murine version of the angiotensin-converting enzyme 2 (ACE2) receptor molecule. Here, we report that induced mild pulmonary morbidities rendered SARS-CoV-2-refractive CD-1 mice susceptible to this virus. Specifically, SARS-CoV-2 infection after application of low doses of the acute lung injury stimulants bleomycin or ricin caused severe disease in CD-1 mice, manifested by sustained body weight loss and mortality rates greater than 50%. Further studies revealed markedly higher levels of viral RNA in the lungs, heart, and serum of low-dose ricin-pretreated mice compared with non-pretreated mice. Furthermore, lung extracts prepared 2-3 days after viral infection contained subgenomic mRNA and virus particles capable of replication only when derived from the pretreated mice. The deleterious effects of SARS-CoV-2 infection were effectively alleviated by passive transfer of polyclonal or monoclonal antibodies generated against the SARS-CoV-2 receptor binding domain (RBD). Thus, viral cell entry in the sensitized mice seems to depend on viral RBD binding, albeit by a mechanism other than the canonical ACE2-mediated uptake route. This unique mode of viral entry, observed over a mildly injured tissue background, may contribute to the exacerbation of coronavirus disease 2019 (COVID-19) pathologies in patients with preexisting morbidities.

Role of acpA and acpB in Bacillus anthracis capsule accumulation and toxin independent pathogenicity in rabbits.

The poly- δ- d-glutamic acid capsule of Bacillus anthracis plays a major role in this bacterium pathogenicity. Capsule synthesis relies on a 5 gene operon; capB, C, A, D and E that are regulated by acpA and acpB, that respond to the major virulence regulator - atxA. We took a genetic approach to examine the involvement of acpA and acpB in capsule production in vitro and on B. anthracis virulence in vivo. To complement the effect of the mutations on capsule accumulation in vitro, we applied our toxin independent systemic infection method to study their effects in vivo. We found that though the roles of acpA and axpB are redundant in vitro, deleting acpA had a significant effect on pathogenicity, mainly on the time to death. As expected, deletion of both acpA and acpB resulted in loss of capsule accumulation in vitro and full attenuation in vivo, indicating that capsule production depends exclusively on acpA/B regulation. To identify additional effects of acpA and acpB on pathogenicity via non-capsule related virulence pathways, we bypassed acpA/B regulation by inserting the pagA promotor upstream to the cap operon, diverting regulation directly to atxA. This resulted in restoration of capsule accumulation in vitro and virulence (in intravenous or subcutaneous inoculation) in vivo. To test for additional pXO2-based genes involved in capsule production, we cloned the pagAprom-capA-E into the chromosome of VollumΔpXO2, which restored capsule accumulation. These results indicate that of the pXO2 genes, only capA-E and acpA are required for capsule production.

Spike vs nucleocapsid SARS-CoV-2 antigen detection: application in nasopharyngeal swab specimens.

Public health experts emphasize the need for quick, point-of-care SARS-CoV-2 detection as an effective strategy for controlling virus spread. To this end, many "antigen" detection devices were developed and commercialized. These devices are mostly based on detecting SARS-CoV-2's nucleocapsid protein. Recently, alerts issued by both the FDA and the CDC raised concerns regarding the devices' tendency to exhibit false positive results. In this work, we developed a novel alternative spike-based antigen assay, comprising four high-affinity, specific monoclonal antibodies, directed against different epitopes on the spike's S1 subunit. The assay's performance was evaluated for COVID-19 detection from nasopharyngeal swabs, compared to an in-house nucleocapsid-based assay, composed of novel antibodies directed against the nucleocapsid. Detection of COVID-19 was carried out in a cohort of 284 qRT-PCR positive and negative nasopharyngeal swab samples. The time resolved fluorescence (TRF) ELISA spike assay displayed very high specificity (99%) accompanied with a somewhat lower sensitivity (66% for Ct < 25), compared to the nucleocapsid ELISA assay which was more sensitive (85% for Ct < 25) while less specific (87% specificity). Despite being outperformed by qRT-PCR, we suggest that there is room for such tests in the clinical setting, as cheap and rapid pre-screening tools. Our results further suggest that when applying antigen detection, one must consider its intended application (sensitivity vs specificity), taking into consideration that the nucleocapsid might not be the optimal target. In this regard, we propose that a combination of both antigens might contribute to the validity of the results. Schematic representation of sample collection and analysis. The figure was created using BioRender.com.

Specific and Rapid SARS-CoV-2 Identification Based on LC-MS/MS Analysis.

SARS-CoV-2, the etiologic agent of the COVID-19 pandemic, emerged as the cause of a global crisis. Rapid and reliable clinical diagnosis is essential for effectively controlling transmission. The gold standard assay for SARS-CoV-2 identification is the highly sensitive real-time quantitative polymerase chain reaction (RT-qPCR); however, this assay depends on specialized reagents and may suffer from false results. Thus, additional assays based on different approaches could be beneficial. Here, we present a novel method for SARS-CoV-2 identification based on mass spectrometry. The approach we implemented combines a multistep procedure for the rational down-selection of a set of reliable markers out of all optional in silico derived tryptic peptides in viral proteins, followed by monitoring of peptides derived from tryptic digests of purified proteins, cell-cultured SARS-CoV-2, and nasopharyngeal (NP) swab matrix spiked with the virus. The marker selection was based on specificity to SARS-CoV-2 and on analytical parameters including sensitivity, linearity, and reproducibility. The final assay is based on six unique and specific peptide markers for SARS-CoV-2 identification. The simple and rapid (2.5 h) protocol we developed consists of virus heat inactivation and denaturation, tryptic digestion, and identification of the selected markers by liquid chromatography coupled to high-resolution mass spectrometry (LC-MS/MS). The developed assay enabled the identification of 104 PFU/mL SARS-CoV-2 spiked into buffer. Finally, the assay was successfully applied to 16 clinical samples diagnosed by RT-qPCR, achieving 94% concordance with the current gold standard assay. To conclude, the novel MS-based assay described here is specific, rapid, simple, and is believed to provide a complementary assay to the RT-qPCR method.