Aim: Daratumumab is a CD38-directed cytolytic antibody used for the treatment of patients with multiple myeloma. It is a human immunoglobulin G (IgG) kappa monoclonal antibody that binds to CD38 surface expressing cells, including circulating T and B lymphocytes, and thus may interfere with crossmatch tests. Daratumumab-mediated positive reactivity may persist for up to 6 months after the last administration. The aim of this study was to circumvent this interference allowing interpretable flow cytometric crossmatch results (FCXM) for patients in need of solid organ and hematopoietic cell transplantation.
Method: Anti-CD38 was added to sera at concentrations of 0, 40 and 400 µg/ml to mimic therapeutic levels. Dosed sera were compared in three color flow cytometric crossmatches tests performed on a BD FACSCanto flow cytometer and analyzed with FACSDiva software. Target lymphocytes were isolated using immunomagnetic negative selection and tested with or without incubation with 0.05M Dithiothreitol (DTT).
Results: Table 1 shows median fluorescence channel values. Treatment of donor lymphocytes with 0.5M DTT for 10 minutes at 37°C effectively eliminated false-positive FCXM outcomes caused by interference from anti-CD38 therapeutic antibody without affecting the sensitivity or specificity of the assay. There was minimal impact on expression of HLA class I and II.
Conclusion: Anti-CD38 interference in FCXM can be abrogated using 0.05M DTT which cleaves off most of the cell surface CD38 eliminating the false-positive reactivity. This treatment does not impact the cell surface level expression of HLA while maintaining test specificity and sensitivity.
