Interferon Regulatory Factor 4: An Alternative Marker for Plasma Cells in Daratumumab‐Treated Patients With Multiple Myeloma

IntroductionAnti‐CD38 therapeutic modalities (e.g., daratumumab) can impede classical CD38 and CD138 gating use for plasma cell (PC) detection in multiple myeloma (MM) patients with minimal residual disease (MRD). We assessed the applicability of CD229, CD269, and interferon regulatory factor (IRF‐4) for PC detection in MM MRD patients.MethodsBone marrow samples were collected from patients with MM. Through multiparameter flow cytometry, we evaluated the suitability of CD229, CD269, and IRF‐4 for distinguishing PCs from other hematopoietic cells and compared their expression pattern on normal PCs (nPCs) and aberrant PCs (aPCs). We also assessed IRF‐4 expression stability after sample storage under different conditions. A 10‐color MRD antibody panel was used to determine whether IRF‐4 is an alternative primary PC‐gating marker for MM MRD assessment.ResultsIRF‐4 was expressed specifically on all PCs; its mean fluorescence intensity (MFI) was highest on PCs among all hematopoietic cells. This MFI did not decrease even after sample storage at 4°C or 25°C for 72 h. In all 42 MRD assessment samples, except for samples (n = 10) with no PCs, the use of IRF‐4 enabled accurate nPC (n = 12), aPC (n = 13), and nPC + aPC (n = 7) identification. Even samples from daratumumab‐treated patients had high IRF‐4 MFI, with no difference between pre‐treatment and post‐treatment (n = 7; p = 0.610).ConclusionsIRF‐4 demonstrates high MFI on PCs, and it is not expressed on other leukocytes. In MM patients with MRD, daratumumab treatment does not affect IRF‐4 expression. IRF‐4 is a promising marker for PC identification in MRD assessment of MM patients undergoing anti‐CD38 therapy.