Granzyme B-expressing regulatory B cells share the same origin as conventional blood B cells

Regulatory B cells (Bregs) play a pivotal role in various dysimmune conditions. We have previously demonstrated that granzyme B-expressing (GZMB⁺) B cells are more numerous in patients with graft tolerance following kidney transplantation and that these cells display regulatory properties through GZMB. However, their ontogeny remains poorly understood. To address this gap, we analyzed the B cell receptor repertoire of GZMB⁺ Bregs and GZMB^- B cells sorted from healthy donors and drug-free tolerant kidney-graft recipients. This revealed that both subsets exhibited similar repertoire features in terms of diversity, V/J gene usage, CDR3 profile and somatic hypermutation pattern, suggesting a common origin and recent divergence. Furthermore, there was limited overlap between GZMB⁺ and GZMB^- clonotypes within individual donors. Taken together, although these observations should be interpreted with caution given the limited cohort size and considered primarily descriptive, these findings suggest that GZMB⁺ Bregs likely arise from the conventional mature B cell pool and represent a differentiation state shaped by environmental cues rather than constituting a distinct clonal lineage.