P235 Positive T-cell and negative B-cell flow cytometry crossmatch due to HLA-B donor-specific antibodies

Abstract

Aim We describe here three cases with T-cell positive and B-cell negative flow cytometric crossmatch (FCXM) in the presence of HLA-B donor-specific antibodies (DSA). Methods A standard three-color FCXM was performed. FCXM reactivity was assessed as a ratio of test serum to negative control (Fluorescence Index = FI). The result was considered positive if the FI was > 1.5 for T-FCXM and > 2.0 for B-FCXM. HLA antibody specificity was determined by single antigen beads (Luminex or FlowPRA). Results Case 1: A kidney transplant candidate with a negative alloantibody status had an unexpected positive T-FCXM (FI = 2.4) with deceased cadaver donor. The B-FCXM was negative (FI = 1.23). An auto FCXM was performed but no auto-antibody reactivity with T (FI = 1.1) and B (FI = 1.3) cells was observed. The patient had a history of alloimmunization (4 pregnancies and 5 transfusions). Therefore, the positive allogeneic T-FCXM was interpreted as most likely due to donor-specific alloreactivity, which was proven by the detection of DSA in the new serum against the mismatched HLA-B56 (MFI = 1541). Case 2: The current serum of a patient was positive for T-FCXM (FI = 1.75) and negative for B-FCXM (FI = 1.27). Historic serum, containing weak HLA-B49 DSA (MFI = 1832) also demonstrated T-cell alloreactivity (FI = 1.65). Case 3: A patient was evaluated for living-donor kidney transplantation. DSA were not identified during the routine testing, although HLA class I antibodies were detected. The T-FCXM was positive with the current serum (FI = 1.66) and negative with a historic serum (FI = 1.06). The B-FCXMs were negative. The new serum contained antibodies against the mismatched HLA-B18 antigen. Conclusions According to our policy a positive T-FCXM is a contraindication for transplantation. Therefore, it is very important to clarify for each patient the reasons for an unexpected T-cell positive and B-cell negative FCXM. Acknowledgements Partly supported by grant 8509/12.2016, Medical University, Sofia, Bulgaria.