High donor regulatory B cells (Bregs) and their rapid recovery are associated with protection from the development of acute graft-versus-host disease (aGVHD) following allogeneic hematopoietic stem cell transplantation.

This study aimed to evaluate the peripheral blood donor CD19+CD24hiCD38hi regulatory B-cell (Breg) content and the Breg recovery rates during 90 days following allogeneic hematopoietic stem cell transplantation (allo-HSCT) and their correlation with the development of acute graft-versus-host disease (aGVHD). Twenty-four donor-recipient pairs with hematological malignancies who underwent unmanipulated, fully HLA-matched allo-HSCT from related siblings were enrolled. The frequency of Bregs and their absolute number were significantly lower in donors whose recipients developed aGVHD (*p = 0.005 and *p = 0.016, respectively). Acute GVHD (+) patients had a significantly diminished percentages of Bregs recovered at days 60 and 90 compared to the aGVHD (-) ones (*p 1.85% decreased the risk of grades II-IV aGVHD (hazard ratio (HR) = 0.033, 95% confidence interval CI = 0.002-0.477; *p = 0.012). Our results revealed, for the first time, that high donor Breg content and their rapid recovery within 90 days after allo-HSCT were associated with a decreased risk of aGVHD.