Dual-Fluorescence Labeling Pseudovirus for Real-Time Imaging of Single SARS-CoV-2 Entry in Respiratory Epithelial Cells

Design and Application of Biosafe Coronavirus Engineering Systems without Virulence

In the last twenty years, three deadly zoonotic coronaviruses (CoVs)-namely, severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and SARS-CoV-2-have emerged. They are considered highly pathogenic for humans, particularly SARS-CoV-2, which caused the 2019 CoV disease pandemic (COVID-19), endangering the lives and health of people globally and causing unpredictable economic losses. Experiments on wild-type viruses require biosafety level 3 or 4 laboratories (BSL-3 or BSL-4), which significantly hinders basic virological research. Therefore, the development of various biosafe CoV systems without virulence is urgently needed to meet the requirements of different research fields, such as antiviral and vaccine evaluation. This review aimed to comprehensively summarize the biosafety of CoV engineering systems. These systems combine virological foundations with synthetic genomics techniques, enabling the development of efficient tools for attenuated or non-virulent vaccines, the screening of antiviral drugs, and the investigation of the pathogenic mechanisms of novel microorganisms.

Pseudoviruses, a safer toolbox for vaccine development against enveloped viruses

INTRODUCTION

Pseudoviruses are recombinant, replication-incompetent, viral particles designed to mimic the surface characteristics of native enveloped viruses. They are a safer, and cost-effective research alternative to live viruses. With the potential emergence of the next major infectious disease, more vaccine scientists must become familiar with the pseudovirus platform as a vaccine development tool to mitigate future outbreaks.

AREAS COVERED

This review aims at vaccine developers to provide a basic understanding of pseudoviruses, list their production methods, and discuss their utility to assess vaccine efficacy against enveloped viral pathogens. We further illustrate their usefulness as wet-lab simulators for emerging mutant variants, and new viruses to help prepare for current and future viral outbreaks, minimizing the need for gain-of-function experiments with highly infectious or lethal enveloped viruses.

EXPERT OPINION

With this platform, researchers can better understand the role of virus-receptor interactions and entry in infections, prepare for dangerous mutations, and develop effective vaccines.

The development and application of pseudoviruses: assessment of SARS-CoV-2 pseudoviruses

Although most Coronavirus disease (COVID-19) patients can recover fully, the disease remains a significant cause of morbidity and mortality. In addition to the consequences of acute infection, a proportion of the population experiences long-term adverse effects associated with SARS-CoV-2. Therefore, it is still critical to comprehend the virus's characteristics and how it interacts with its host to develop effective drugs and vaccines against COVID-19. SARS-CoV-2 pseudovirus, a replication-deficient recombinant glycoprotein chimeric viral particle, enables investigations of highly pathogenic viruses to be conducted without the constraint of high-level biosafety facilities, considerably advancing virology and being extensively employed in the study of SARS-CoV-2. This review summarizes three methods of establishing SARS-CoV-2 pseudovirus and current knowledge in vaccine development, neutralizing antibody research, and antiviral drug screening, as well as recent progress in virus entry mechanism and susceptible cell screening. We also discuss the potential advantages and disadvantages.

Altered hACE2 binding affinity and S1/S2 cleavage efficiency of SARS-CoV-2 spike protein mutants affect viral cell entry

SARS-CoV-2 variants are constantly emerging, hampering public health measures in controlling the number of infections. While it is well established that mutations in spike proteins observed for the different variants directly affect virus entry into host cells, there remains a need for further expansion of systematic and multifaceted comparisons. Here, we comprehensively studied the effect of spike protein mutations on spike expression and proteolytic activation, binding affinity, viral entry efficiency and host cell tropism of eight variants of concern (VOC) and variants of interest (VOI). We found that both the full-length spike and its receptor-binding domain (RBD) of Omicron bind to hACE2 with an affinity similar to that of the wild-type. In addition, Alpha, Beta, Delta and Lambda pseudoviruses gained significantly enhanced cell entry ability compared to the wild-type, while the Omicron pseudoviruses showed a slightly increased cell entry, suggesting the vastly increased rate of transmission observed for Omicron variant is not associated with its affinity to hACE2. We also found that the spikes of Omicron and Mu showed lower S1/S2 cleavage efficiency and inefficiently utilized TMPRSS2 to enter host cells than others, suggesting that they prefer the endocytosis pathway to enter host cells. Furthermore, all variants' pseudoviruses we tested gained the ability to enter the animal ACE2-expressing cells. Especially the infection potential of rats and mice showed significantly increased, strongly suggesting that rodents possibly become a reservoir for viral evolution. The insights gained from this study provide valuable guidance for a targeted approach to epidemic control, and contribute to a better understanding of SARS-CoV-2 evolution.

Diagnostics and analysis of SARS-CoV-2: current status, recent advances, challenges and perspectives

The disastrous spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has induced severe public healthcare issues and weakened the global economy significantly. Although SARS-CoV-2 infection is not as fatal as the initial outbreak, many infected victims suffer from long COVID. Therefore, rapid and large-scale testing is critical in managing patients and alleviating its transmission. Herein, we review the recent advances in techniques to detect SARS-CoV-2. The sensing principles are detailed together with their application domains and analytical performances. In addition, the advantages and limits of each method are discussed and analyzed. Besides molecular diagnostics and antigen and antibody tests, we also review neutralizing antibodies and emerging SARS-CoV-2 variants. Further, the characteristics of the mutational locations in the different variants with epidemiological features are summarized. Finally, the challenges and possible strategies are prospected to develop new assays to meet different diagnostic needs. Thus, this comprehensive and systematic review of SARS-CoV-2 detection technologies may provide insightful guidance and direction for developing tools for the diagnosis and analysis of SARS-CoV-2 to support public healthcare and effective long-term pandemic management and control.

Confocal interferometric scattering microscopy reveals 3D nanoscopic structure and dynamics in live cells

Bright-field light microscopy and related phase-sensitive techniques play an important role in life sciences because they provide facile and label-free insights into biological specimens. However, lack of three-dimensional imaging and low sensitivity to nanoscopic features hamper their application in many high-end quantitative studies. Here, we demonstrate that interferometric scattering (iSCAT) microscopy operated in the confocal mode provides unique label-free solutions for live-cell studies. We reveal the nanometric topography of the nuclear envelope, quantify the dynamics of the endoplasmic reticulum, detect single microtubules, and map nanoscopic diffusion of clathrin-coated pits undergoing endocytosis. Furthermore, we introduce the combination of confocal and wide-field iSCAT modalities for simultaneous imaging of cellular structures and high-speed tracking of nanoscopic entities such as single SARS-CoV-2 virions. We benchmark our findings against simultaneously acquired fluorescence images. Confocal iSCAT can be readily implemented as an additional contrast mechanism in existing laser scanning microscopes. The method is ideally suited for live studies on primary cells that face labeling challenges and for very long measurements beyond photobleaching times.

Pseudotyped Viruses for Coronaviruses

Seven coronaviruses have been identified that can infect humans, four of which usually cause mild symptoms, including HCoV-229E, HCoV-NL63, HCoV-OC43, and HCoV-HKU1, three of which are lethal coronaviruses, named severe acute respiratory syndrome coronavirus, Middle East respiratory syndrome coronavirus, and severe acute respiratory syndrome coronavirus 2. Pseudotyped virus is an important tool in the field of human coronavirus research because it is safe, easy to prepare, easy to detect, and highly modifiable. In addition to the application of pseudotyped viruses in the study of virus infection mechanism, vaccine, and candidate antiviral drug or antibody evaluation and screening, pseudotyped viruses can also be used as an important platform for further application in the prediction of immunogenicity and antigenicity after virus mutation, cross-species transmission prediction, screening, and preparation of vaccine strains with better broad spectrum and antigenicity. Meanwhile, as clinical trials of various types of vaccines and post-clinical studies are also being carried out one after another, the establishment of a high-throughput and fully automated detection platform based on SARS-CoV-2 pseudotyped virus to further reduce the cost of detection and manual intervention and improve the efficiency of large-scale detection is also a demand for the development of SARS-CoV-2 pseudotyped virus.

Construction of ratiometric Si-Mn:ZnSe nanoparticles for the immunoassay of SARS-CoV-2 spike protein

The continuing global spread of Coronavirus Disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) infection, has led to an unprecedented global health crisis. Effective and affordable methods are needed to diagnose SARS-CoV-2 infection. In this work, a ratiometric fluorescence probe, Si-Mn:ZnSe nanoparticles, was constructed through the electrostatic interaction between Si dots and Mn:ZnSe QDs, and the fluorescence of Mn:ZnSe QDs has a specifical response to H₂O₂. An immunocomplex was formed by the recognition of capture antibody/spike (S) protein/spike neutralizing antibody/biotinylated second antibody/streptavidin/biotinylated catalase (CAT). In the presence of S protein, CAT effectively catalyzed the decomposition of H₂O₂ in the system, and the fluorescence of Mn:ZnSe QDs was not specifically quenched. Based on this principle, a ratiometric immunoassay of SARS-CoV-2 S protein was established. The sensitivity of the proposed ELISA method was comparable to that of the commercial kit. In addition, this method can effectively distinguish the pseudo-SARS-CoV-2 virus and other pseudovirus. Therefore, this method provided a reliable and potential direction for diagnosing SARS-CoV-2 infection.

Immunomolecular assay based on selective virion capture by spike antibody and viral nucleic acid amplification for detecting intact SARS-CoV-2 particles

Background

Effective therapeutics and vaccines for coronavirus disease 2019 (COVID-19) are currently lacking because of the mutation and immune escape of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Based on the propagation characteristics of SARS-CoV-2, rapid and accurate detection of complete virions from clinical samples and the environment is critical for assessing infection risk and containing further COVID-19 outbreaks. However, currently applicable methods cannot achieve large-scale clinical application due to factors such as the high viral load, cumbersome virus isolation steps, demanding environmental conditions, and long experimental periods. In this study, we developed an immuno molecular detection method combining capture of the viral spike glycoprotein with monoclonal antibodies and nucleic acid amplification via quantitative reverse transcription PCR to rapidly and accurately detect complete virions.

Results

After constructing a novel pseudovirus, screening for specific antibodies, and optimizing the detection parameters, the assay achieved a limit of detection of 9 × 10² transduction units/mL of viral titer with high confidence (~ 95%) and excellent stability against human serum and common virus/pseudovirus. The coefficients of variation were 1.0 ~ 2.0% for intra-assay and inter-assay analyses, respectively. Compared with reverse transcription-PCR, the immunomolecular method more accurately quantified complete virions. SARS-CoV-2/pseudovirus was more stable on plastic and paper compared with aluminum and copper in the detection of SARS-CoV-2 pseudovirus under different conditions. Complete virions were detected up to 96 h after they were applied to these surfaces (except for copper), although the titer of the virions was greatly reduced.

Conclusion

Convenient, inexpensive, and accurate complete virus detection can be applied to many fields, including monitoring the infectivity of convalescent and post-discharge patients and assessing high-risk environments (isolation rooms, operating rooms, patient living environments, and cold chain logistics). This method can also be used to detect intact virions, including Hepatitis B and C viruses, human immunodeficiency virus, influenza, and the partial pulmonary virus, which may further improve the accuracy of diagnoses and facilitate individualized and precise treatments.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01558-8.

SARS-CoV-2 spike protein receptor-binding domain N-glycans facilitate viral internalization in respiratory epithelial cells

N-glycosylation plays an important role in the pathogenesis of viral infections. However, the role of SARS-CoV-2 RBD N-glycosylation in viral entry remains elusive. In this study, we expressed and purified N331 and N343 N-glycosite mutants of SARS-CoV-2 RBD. We found that de-glycosylation at N331 and N343 drastically reduces the RBD binding to ACE2. More importantly, based on qualitative and quantitative virology research methods, we show that the mutation of RBD N-glycosites interfered with SARS-CoV-2 internalization rather than attachment potentially by decreasing RBD binding to the receptors. Also, the double N-glycosites mutant (N331 + N343) showed significantly increased sensitivity against the designated RBD neutralizing antibodies. Taken together, these results suggest that N-glycosylation of SARS-CoV-2 RBD is not only critical for viral internalization into respiratory epithelial cells but also shields the virus from neutralization. It may provide new insights into the biological process of early-stage SARS-CoV-2 infection with potential therapeutic implications.