Early establishment of the naïve B-cell receptor repertoire with gradual antigen-experienced maturation after pediatric hematopoietic stem cell transplantation.

Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative treatment for diverse inherited and acquired immuno-hematological disorders. While quantitative B-cell recovery is well characterized, qualitative reconstitution of the B-cell receptor (BCR) repertoire, particularly in pediatric patients, remains incompletely defined. We delineated longitudinal reconstitution of naïve and antigen-experienced immunoglobulin heavy chain (IGH) repertoires during the first two years after pediatric HSCT. Peripheral blood mononuclear cells from 10 pediatric patients were analyzed at 3, 6, 12, and 24 months post-HSCT and compared with age-matched healthy donors. High-throughput sequencing of IGH transcripts was performed to assess V(D)J gene usage, repertoire diversity, junctional characteristics, somatic hypermutation (SHM), and clonal architecture across naïve and antigen-experienced compartments. The naïve IGHM repertoire demonstrated broad, unbiased IGHV, IGHD, and IGHJ gene usage as early as 3 months post-HSCT, with diversity and junctional features comparable to controls. In contrast, antigenexperienced repertoires (IGHG and IGHA) showed reduced SHM frequencies and diminished N-nucleotide insertions during the first year, normalizing by 24 months. Despite delayed SHM accumulation, targeting of activation-induced cytidine deaminase and polymerase η motifs and replacement-to-silent mutation ratios were preserved, indicating intact molecular affinity maturation. Clonal expansions were detectable in class-switched compartments but did not compromise overall repertoire diversity. These findings indicate early establishment of a diverse naïve IGH repertoire after pediatric HSCT, followed by gradual maturation of antigen-experienced repertoires, resembling key features of physiological B-cell development.