Effects of Daratumumab (Anti-CD38) Monoclonal Antibody Therapy on Flow Cytometry Analysis in Multiple Myeloma
Authors: Germans Sharon Koorse , Kahlow Christine Wamsley , Chen Weina , Fuda Franklin 
Published: February 2025

Background: Plasma cell myeloma is an incurable malignancy of clonal plasma cells. Recent success in immunotherapeutic strategies has altered the landscape of myeloma treatment. Daratumumab is an anti-CD38 IgG kappa monoclonal antibody that has shown great efficacy in the treatment of myeloma. However, Daratumumab brought with it new challenges in post-therapeutic laboratory assessment, including therapeutic antibody interference with serum protein electrophoresis and serum immunofixation electrophoresis assays. In this study, we highlight the interference identified in post-therapeutic flow cytometry analysis related to bound Daratumumab on normal hematopoietic cells. We also highlight the methods of detection of residual plasma cell neoplasm, post-Daratumumab therapy.: A total of 28 patients with refractory plasma cell myeloma who received Daratumumab (2016–2018) were included in this study. Flow cytometry was performed using 4- or 10-color antibody panels (BD FASC Canto) and analyzed by cluster analysis (Cytopaint Classic software) using four tube panels including VS38c for measurable residual disease (MRD) testing. Pretreatment and post-Daratumumab follow-up bone marrow flow cytometry samples were analyzed. In addition, 10 multiple myeloma patient samples were reflexed to multi-epitope CD38 analysis by flow cytometric analysis of post-Daratumumab residual disease. When discussing CD38 expression, we will refer to CD38 as being detected by conventional reagents. Results: All post-Daratumumab-treated cases (100%) showed negative staining for CD38 using conventional reagents on all plasma cells in the specimens. MRD testing successfully identified small clonal plasma cell populations using VS38C and multi-epitope CD38 (meCD38) antibodies, despite the absence of demonstrable CD38 expression. Additionally, all cases exhibited weak kappa light chain staining on hematogones, attributed to the binding of Daratumumab kappa monoclonal antibody. This interaction can create the appearance of a CD10+ monotypic B-cell population. We also noted diminished CD38 staining on myeloblasts, resulting in an atypical CD34/CD38 staining pattern. This alteration could potentially be misinterpreted as indicative of a myelodysplastic neoplasm (MDS). Furthermore, decreased staining of CD38 was noted on T cells, natural killer (NK) cells, basophils, monocytes, and plasmacytoid dendritic cells. Conclusions: With the emergence of successfully targeted immunotherapies, such as anti-CD38 antibodies, it is important to understand and correctly interpret variations in flow cytometry that may arise from the therapy. Hematogones exhibit high-intensity levels of CD38 expression; thus, Daratumumab binds to them, creating the appearance of kappa expression on all hematogones. Stage I/early stage II hematogones normally lack surface immunoglobulin light chain expression, but in the presence of Daratumumab, they appear to be a CD10(+) monotypic population of B cells. The misinterpretation of these normal cells as a CD10(+) B-cell clone can lead to inaccurate assessment, unnecessary bone marrow immunohistochemical evaluation, and unwarranted anxiety. Additionally, artefacts on various other hematopoietic cells can result in inaccurate assessments of immunophenotypic aberrancy due to binding of the drug. This may lead to the false interpretation of a secondary/therapy-related myeloid neoplasm. This study highlights in detail the interferences that must be considered when assessing residual disease in the era of targeted drug therapies.