Antigen-driven Convergent Evolution of Polysaccharide-specific "DH-less" B Cells in Glycoconjugate Immunized Mice

Glycoconjugate vaccines elicit robust anti-polysaccharide Ab response by recruiting T-cell help. Multiple doses of glycoconjugate vaccine are required to induce long-lasting immunity. The characteristics of anti-polysaccharide Ab response have been reported previously. However, the effect of glycoconjugate booster immunization on anti-polysaccharide and anti-carrier protein Ab repertoire remains poorly understood. In this study, we used clinically relevant pneumococcal capsular polysaccharide type 14 (PCP14) conjugated with cross-reactive material 197 (CRM197) as a model glycoconjugate Ag (PCP14-CRM197). We performed a comprehensive sequence analysis of mouse mAbs generated against PCP14 and CRM197 following immunization with one or three doses of PCP14-CRM197. Analysis of the paired Ig H and L chain transcripts revealed that anti-PCP14 Ab repertoire is extremely restricted. The reoccurrence of five replacement mutations at identical positions in anti-polysaccharide mAbs generated from different mice provided evidence for Ag-driven selection in PCP14-specific B cells. Convergent evolution was observed wherein distinct V(D)J rearrangements resulted in identical or nearly identical CDR3 in anti-PCP14 mAbs. Abs that lacked DH encoded amino acids dominated the anti-PCP14 Ab response. In contrast, anti-CRM197 Ab response was quite diverse, with fewer mutations compared with the anti-PCP14 mAbs, suggesting that conjugation of the polysaccharide to a carrier protein interferes with the development of carrier protein-specific Ab responses. Our findings provide molecular insights into the maturation of Ab responses driven by booster doses of glycoconjugate. This has fundamental implications for the design of glycoconjugate vaccines, especially where the development of Ab response against the carrier protein is also crucial.