Residual serum rituximab induces false positive flow cytometric B cell crossmatch more than 6 months post-infusion

Abstract

Case 1 : 18 year old female patient was listed for transplant due to glomerulonephritis and kidney failure. She was diagnosed with microscopic polyangiitis since childhood. Her transplant workup indicated the absence of HLA class I and class II antibodies, but unexpectedly, she had a positive B cell crossmatch with a virtually matched deceased donor. Case 2 : 58 year old female patient with autoimmune glomerulonephritis was listed for transplant due to end stage renal disease. She was scheduled to receive a living donor kidney from her sister. Her transplant workup showed no detectable HLA class I or class II antibodies, but she had an unexpected positive B cell crossmatch. Further investigation revealed that both patients received rituximab treatment for their autoimmune disease 7, and 8 months earlier. With no HLA explanation, it seemed likely that the positive B cell crossmatch was caused by residual rituximab in the patients’ sera. Rituximab is a chimeric monoclonal antibody that binds to CD20 expressed on B cells, inducing depletion. Initially approved for the treatment of B cell lymphoma, it is increasingly used for the treatment of autoimmune diseases. Although the expected half-life of the drug is 14–21 days, significant variations have been reported, complicated by the lack of standardized dosing in various autoimmune diseases. Detected in the serum many months post-infusion, rituximab interferes with cytotoxic crossmatch due to inducing complement-dependent cytotoxicity, and with flow cytometric crossmatch due to being recognized by the anti-human antibody used in the assay. To confirm interference by the drug, the patients’ sera were pre-treated with an idiotype-specific antibody to rituximab, which resulted in negative B cell crossmatch. Both patients were successfully transplanted. Although rituximab was previously reported to interfere with B cell crossmatch 2–3 months after the dose, these cases demonstrate that false positive B cell crossmatch can occur as long as 7 or 8 months post-infusion. Thorough record of the patients’ medication history is essential for the transplant workup. Methodologies that eliminate cell surface CD20 such as pronase treatment, or remove rituximab by antibody absorption should be added to the routine procedure, when rituximab-containing serum is used for crossmatch.