sars-cov-2 pseudotype virus production and serum neutralization assay

Assessing Serum Neutralization Against SARS-CoV-2 with Pseudotyped Viruses

Pseudotyped viruses (PVs) are molecular tools used in microbiology to study host-virus and pathogen-pathogen interactions1,2,3,4. PVs consist of an inner part, the viral core that protects the viral genome, and an outer part, the envelope glycoproteins on the surface of the virus that defines the tropism5. A pseudovirus is replication-incompetent in the target cell because it does not contain all the genetic information to generate new viral particles. This combination of peculiar features makes PVs a safe alternative to a wildtype virus. Wildtype viruses, on the other hand, are highly pathogenic and cannot be used in BSL 2 laboratories for analysis6.
The infectivity of PVs can be monitored by a reporter gene, usually coding for a fluorescent protein (GFP, RFP, YFP) or an enzyme that produces chemiluminescent products (luciferase). This is contained in one of the plasmids used for PV production and incorporated in the genome of the pseudovirus7.
Several types of PV cores currently exist, including lentiviral-derived particles based on the HIV-1 genome. The great advantage of HIV-1-based PVs over other platforms is their intrinsic integration process in the target cell genome8. Although HIV-1 is a highly contagious virus and is the causative agent of AIDS, these lentiviral vectors are safe to use because of the extensive optimization steps over the years. Optimal safety conditions were achieved with the introduction of 2nd-generation lentiviral vectors, in which viral genes were depleted without influencing transduction capabilities9. The 3rd and 4th generations contributed to the increased safety of lentiviral vector handling with the further splitting of the viral genome into separate plasmids10, 11. The latest generations of PVs are generally employed to produce lentiviral vectors for gene therapy.
PVs can be used to study interactions between viruses and host cells, during both the production and the infection phases. PVs are especially employed in pseudovirus neutralization assays (PVNA). PVNAs are widely validated to assess the neutralization potential of serum or plasma by targeting the viral glycoprotein on the PV&#39;s envelope12,13. Neutralization activity, expressed as the inhibitory concentration 50 (IC50), is defined as the dilution of serum/plasma that blocks 50% of viral particle entry14. In this protocol, we described the set-up of a PVNA to test the antibody activity against Severe Acute Respiratory Syndrome - Coronavirus 2 (SARS-CoV-2) in sera collected before and after receiving a booster vaccine dose.

Author Spotlight: Studying Host-Virus Interactions with Pseudotyped Viruses 

Pseudotyped Viruses As a Molecular Tool to Monitor Humoral Immune Responses Against SARS-CoV-2 Via Neutralization Assay

Pseudotyped viruses are molecular tools that can be used to study host-virus interactions and to test neutralizing activity of serum samples. In the work we present today, a general methodology to produce pseudotyped viruses, based on free plasmid, will be presented. For neutralization assays, Y-type viruses are generally used, but they are highly pathogenic and cannot be handled in a BSL-2 laboratories, rendering this system purely applicable on large scale.The technology of pseudo viruses can be a safer alternative compared to live virus to allow the study of pathogen to host interaction and to test neutralizing activity of antibodies. Future experiments will investigate the role of specific antibodies in the maintenance of immunity after vaccination using the technology of pseudo viruses.

Watch this Scientific Journal Video about SARS-CoV-2 Pseudotype Virus Production and Serum Neutralization Assay at JoVE.com

SARS-CoV-2 Pseudotype Virus Production and Serum Neutralization Assay  

To begin, in a six-well plate, seed one milliliter of human embryonic kidney 293T cells. Add two milliliters of complete DMEM. The next day, replace the medium with fresh antibiotic-free medium to prepare the cells for transfection.To transfect the adherent cells, first, prepare mix A by adding plasmid DNA to 100 microliters of Opti-MEM. Next, add 17.5 microliters of polyethylenimine to 100 microliters of Opti-MEM. Mix the contents of the mix A and B, then incubate.During incubation, gently flick the tube every three to four minutes to enhance mixing. Add the whole volume of the mixture to the plate with the HEK 293T cells. After 16 to 20 hours of transfection, replace the culture medium with fresh complete DMEM.Incubate the plate at 37 degrees Celsius under 5%carbon dioxide to produce the pseudotype viruses. After 72 hours of transfection, harvest the supernatant into a collection tube. Centrifuge the supernatant at 1, 600 G for seven minutes at room temperature.Then filter the supernatant through a 0.45 micrometer cellulose acetate filter. Distribute 50 microliters of complete DMEM in the wells of a 96-well plate. Leave row A empty.Add 100 microliters of the pseudovirus stock to the wells in row A.Next, pipette 50 microliters of the solution from row A to B.Repeat this process up to row G to obtain serial dilutions. Remove the exhausted medium from a plate with cultured susceptible HEK 293T ACE2 cells, and wash the cells with four milliliters of DPBS. Then detach the cells with one milliliter of trypsin EDTA in DPBS saline.Now add 50 microliters of the susceptible cell suspension into each well containing the pseudotype viruses. Incubate the plate at 37 degrees Celsius under 5%carbon dioxide for 48 hours. Add 100 microliters of the luciferase reagent to each well.After incubation, transfer the contents of each well into a black 96-well plate. Then read the plate in a plate reader. In a 96-well plate, add 50 microliters of fresh complete DMEM to columns one to 10 from rows B to H.Add 95 microliters of fresh complete DMEM to the corresponding wells of row A.Now add 50 microliters of complete DMEM to the wells in column 11 and 100 microliters of DMEM to column 12.Pipette five microliters of heat-inactivated serum samples to row A.With a multi-channel pipette, mix the samples in the first row and transfer 50 microliters of medium-containing serum from row A to B up to row H.Distribute 50 microliters of dilute pseudovirus containing medium to each well from columns one to 11. Incubate the plate at 37 degrees Celsius under 5%carbon dioxide for one hour. Prepare a suspension of susceptible HEK293T ACE2 cells in five milliliters.Add 50 microliters of the cell suspension to each well, then incubate before performing the luciferase assay. Lower serum dilution resulted in decreased viral entry into the cells. This is confirmed by the increased percentage of neutralization.

sars-cov-2-pseudotype-virus-production-and-serum-neutralization-assay

Research

JoVE Journal

Immunology and Infection

174 Views.  University of Verona. To begin, in a six-well plate, seed one milliliter of human embryonic kidney 293T cells. Add two milliliters of complete DMEM. The next day, replace the medium with fresh antibiotic-free medium to prepare the cells for transfection.To transfect the adherent cells, first, prepare mix A by adding plasmid DNA to 100 microliters of Opti-MEM. Next, add 17.5 microliters of polyethylenimine to 100 microliters of Opti-MEM. Mix the contents of the mix A and B, then incubate.During incuba...