Comparison of IgG Antibody Levels in Mice Immunized with SARS-CoV-2 RBD Proteins Produced from Two Expression Systems / 中国生物化学与分子生物学报

ABSTRACT

The COVID-19 pandemic is still spreading around the world,posing a severe challenge to public health security and economic development in China and beyond. The pivotal process of SARS-COV-2 invasion is that the receptor binding domain (RBD) of Coronavirus spike protein binds with the angioten-sin converting enzyme 2 (ACE2) receptor on host cells. Therefore, RBD is closely related to our major anti-epidemic strategies such as rapid pathogen detection and development of vaccines and antiviral drugs. In this study, we aim to compare the anti-RBD IgG by immunizing mice with the recombinant RBDs of novel Coronavirus spike protein, and evaluate the antibody titers and duration elicited by RBD produced from different host cells and used in different doses, which can be used for vaccine evaluation, drug development, and pathogen detection. SDS-PAGE and mass spectrometry analysis showed that the molecular weight of bRBD (produced by insect cells) and hRBD (produced by mammalian cells) were slightly different, which are mainly caused by different glycosylation modification. Flow cytometry revealed that the binding rates of bRBD and hRBD to HEK293-ACE2-overexpressing cells were 88. 5% and 92. 7%, respectively, indicating that both antigens have favorable bioactivity. Balb/ c mice are immunized intramuscularly with 10 μg and 20 μg per mouse of RBD proteins, which were mixed with Quick Antibody adjuvant previously, and blood samples collected from the tail at 2, 4, 6, 8, 12 and 24 weeks after the primary immunization. Enzyme-linked immunosorbent assay (ELISA) showed that both RBD immunization with 10 μg or 20 μg could induce a rapid immune response to produce IgG antibodies. The antibody titers reached the peak at 4-6 weeks post first immunization, and persisted over a long time up to 6 months. No significant difference was observed in the induced antibody titers by bRBD and hRBD as well as their different doses. Virus Neutralization Assays showed that specific antibodies induced by RBD-immunized mice could inhibit the infection of ACE2-positive cells by SARS-CoV-2 pseudovirus. These results suggest that both RBD proteins through commendable adjuvant inoculation can rapidly induce an effective humoral immune response and generate specific neutralizing antibodies. It is expected to be helpful for rapid preparation of antibodies against the future various RBD mutants during the pandemic.