An Extended Lymphocytotoxicity Test for Patients Treated with Lymphocytotoxic Antibodies

Abstract Background and Objectives: After organ transplantation, HLA antibodies (HLA‐Ab) stimulated by transplant or transfusion are usually indistinguishable from passive therapeutic lymphocytotoxic antibodies (LyAb), e.g. antilymphocyte globulin (ALG), antithymocyte globulin (ATG) and OKT3, by standard lymphocytotoxicity tests (LCTs). Herein, an extended technique capable of distinguishing between these two types of antibodies is described. Materials and Methods: Serum samples from 11 patients who received therapeutic antibodies were screened for HLA‐Ab by LCTs. The administered LyAb were murine OKT3 (n = 2), equine ALG (n = 3), rabbit ATG (n = 2), and combinations of ALG and OKT3 (n = 1), ATG and ALG (n = 1), and ATG and OKT3 (n = 2). To discriminate between therapeutically applied antibodies and active HLA alloimmunization, the sera were preincubated with species‐specific anti‐Fc lgG, thus blocking the therapeutic antibodies. Results: The sera of all patients treated showed strong positive lymphocytotoxic reactions [panel reactive antibodies (PRA) 50–100%]. After incubation with the corresponding Fc antibodies, in 9 of the 11 samples the amount of PRA decreased to the level present before transplantation (0–18%). In the remaining cases, new‐onset alloimmunization was detected. Conclusion: This extended LCT allows the screening of serum samples for active HLA‐Ab even in the presence of therapeutic LyAb.