Memory B‐Cell Assay Uncovers Prior HLA Sensitisation in Transplant Candidates With Prior Exposure to Non‐Self HLA

Memory B cells are critical in kidney transplantation for rapid anti‐HLA antibody production upon re‐exposure, yet pretransplant serum antibodies often remain undetectable despite prior sensitisation. Using R848 stimulation (a TLR7/8 agonist inducing polyclonal B‐cell activation), we tested 45 kidney recipients with past sensitisation events but no circulating antibodies. The assay identified 56 memory B‐cell‐derived HLA antibodies (HLAm) in 8 patients, primarily from pregnancy or transfusion sensitisation, with a 17‐year average latency between exposure and transplantation. This pilot demonstrates that latent memory B cells may reactivate post‐transplant, driving antibody production and adverse outcomes. Our findings advocate integrating memory B‐cell assays into pretransplant risk assessment to improve sensitivity in detecting subclinical HLA sensitisation, refining donor‐recipient matching strategies.