Introduction
Serum neutralization corresponds protection against viral infection after vaccination or natural infection. However, high levels of mutation observed in viral glycoprotein of different variants could prevent effective protection provided by neutralization serum. Therefore, immediate determination of serum neutralization potency against different variants is crucial (Wu et al., 2020). Particularly, new variants have begun to emerge for disease caused by global infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) since 2019. It has been reported that mutations on the major neutralization target of these variants alter serum-neutralizing activity induced by early strains or vaccines (Kuzmina et al., 2021; Robbiani et al., 2020). Some of these mutations can provide an escape from immune response by decreasing monoclonal antibody (mAb) neutralization (Liu et al., 2021).
Due to the high risk of SARS-CoV-2 infection, cultivation of authentic viruses requires laboratories with at least level 3 biological safety (BSL3) equipped with negative pressure systems. This limitation prevents to perform neutralization assays in many research centers, which only has BSL2 laboratories. Pseudotyped virus neutralization assay is an alternative method based on packaging a reliable replication-defective pseudovirus with Spike protein to mimic the entry of the authentic virus. Thereby, pseudoviruses can be contained in BSL2 conditions to perform neutralization assays (Chen and Zhang, 2021; Li et al., 2018). In the broad extent, pseudoviruses can be utilized in seroepidemiological studies, vaccine and monoclonal antibody development, screening of viral entry inhibitors, and fundamental virological methods (Ou et al., 2020). Several convenient pseudotyped virus for SARS-CoV-2 have been reported, such as human immunodeficiency virus (HIV)-based lentiviral particles (Donofrio et al., 2021), murine leukemia virus (MLV)-based retroviral particles (Chen and Zhang, 2021), or vesicular stomatitis virus (VSV)-based systems (Nie et al., 2020; Salazar-García et al., 2022) with higher performance for evaluating the efficacy of therapeutic drugs and vaccines (Salazar-García et al., 2022). The results of such pseudovirus neutralization tests are very closely correlated to those of authentic virus measurements. Also, tracking the changes in Spike glycoprotein using pseudovirus neutralizing assays is relatively easy (Schmidt et al., 2020).
From the beginning of the pandemic, DNA sequencing data of virus glycoproteins is utilized to identify individual mutations in SARS-CoV-2 (Schrörs et al., 2021). One of the first variants called B.1.1.7 was first emerged in the United Kingdom and had multiple mutations in the target regions of neutralizing antibodies such as receptor binding domain and N-terminal domain of Spike. SubsequentlyB.1.351 variant has been identified in South Africa with additional mutations. B.1.1.7 and B.1.351 variants share key mutations in the RBD (N501Y, D614G), but B.1.351 has additional changes causing widespread escape from mAbs (E484K, K417N) (Harvey et al., 2021; Zhou et al., 2021).
In this paper, we evaluated human convalescent plasma with different serum neutralizing activities using pseudotyped VSV-ΔG virus carrying Spike variants (Wuhan strain, B.1.1.7, and B.1.351). First, we made a series of point mutations in Spike sequence of SARS-CoV-2 to obtain global variants. Second, we generated pseudotyped viruses usingSpike of ancestoral Wuhan strain and two variants and utilized to evaluate neutralization activity of human serum samples. Neutralization assay using Spike pseudotyped VSV-ΔG virus was found to correlate with plaque assay using SARS-CoV-2 authentic virus. Additionally, Spike pseudotyped VSV-ΔG virus was sufficient to discriminate serum responses against different variants of the virus.