In vivo antibody diversification targeting a conserved coronavirus epitope.

To explore the use of human B cell receptor (BCR) knock-in mice for broadening antibody responses, we diversified CR3022, a monoclonal antibody (mAb) originally identified in a 2003 severe acute respiratory syndrome coronavirus (SARS-CoV) convalescent patient. This mAb targets a conserved epitope on the coronavirus receptor-binding domain (RBD). We took advantage of high- and low-affinity CR3022 BCR knock-in mice and immunized them with SARS-CoV-2 Wuhan RBD trimers to expand the breadth of these antibodies toward this virus. The resulting antibodies retained the ability to neutralize SARS-CoV and exhibited enhanced affinity and neutralization against the SARS-CoV-2 WA1/2020 strain, as well as the Delta (B.1.617.2) and Omicron KP.3 variants. They also showed broadened reactivity to two bat coronaviruses: WIV1 and, to a lesser potency, BtKY72. Structural analysis revealed key mutations that enhanced binding and neutralization, highlighting the importance of epitope accessibility and variant-specific conformations in antibody diversification. These findings demonstrate that human BCR-expressing mouse models can generate effective antibodies with broad neutralizing activity against viral epitopes.