Enhancement of the neutralization ability resulting from a single amino acid change in the light chain of a chimeric antibody against SARS-CoV-2

Objective·To screen the human-mouse chimeric antibodies that neutralize the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and analyse the key amino acid site changes in their light chain.Methods·Antibody heavy chain gene-humanized mice were immunized with the spike protein of SARS-CoV-2. Enzyme-linked immunosorbent assay (ELISA) and pseudovirus neutralization assay were performed to analyse the antibody titer in serum level. Flow cytometry was used to sort plasma cells and germinal center (GC) B cells from the mouse spleen and lymph nodes, and the proportions of different B cells were analysed. RNA was extracted to construct a mouse-origin antibody light chain gene library, which was paired with the knocked-in human-origin antibody heavy chain gene for antibody expression. ELISA was employed to screen for high-affinity chimeric antibodies. Amino acid changes in the light chain of high-affinity chimeric antibodies were analysed by comparing them with mouse-origin germline antibody light chain genes. Chimeric antibodies with single amino acid changes in the light chain were constructed and expressed, and their affinity and neutralization abilities were tested through ELISA and pseudovirus neutralization experiments. By comparing the affinity and neutralization abilities of the chimeric antibodies with or without single amino acid changes in the light chain, the key single amino acid change was analysed.Results·Compared to unimmunized mice, the immunized mice showed an increased titer of specific antibodies against SARS-CoV-2 on the serum level, along with an elevated proportion of plasma cells and GC B cells in the spleen and lymph nodes. ELISA showed that a high-affinity chimeric antibody was screened from GC B cells in the mouse spleen. The light chain of the antibody had a 76th serine-to-isoleucine substitution and a 98th phenylalanine deletion. Another ELISA showed that the chimeric antibody exhibited high affinity, the antibody without amino acid change on the light chain and the antibody with only the 98th phenylalanine deletion showed low affinity, and the antibody with only the 76th serine-to-isoleucine substitution demonstrated intermediate affinity between the chimeric antibody and the antibody without amino acid change on the light chain. Pseudovirus neutralization experiments revealed that the chimeric antibody had a half-maximal inhibitory concentration (IC50) of 0.995 ng/μL; the antibody with only the 76th serine-to-isoleucine substitution had an IC50 of 1.724 ng/μL, indicating a slight decrease in neutralization ability; the antibody with only the 98th phenylalanine deletion had an IC50 of 71.05 ng/μL; the antibody without amino acid change on the light chain had an IC50 of 42.06 ng/μL, suggesting weak neutralization ability.Conclusion·For the screened human-mouse chimeric antibodies that neutralize SARS-CoV-2 in this study, the key amino acid change determining the neutralization ability of the chimeric antibody is the 76th serine-to-isoleucine substitution.