Multiple myeloma with 1q gain/amplification exhibits reduced CD38 expression via interleukin‐6 receptor overexpression

Summary Multiple myeloma (MM) with chromosome 1q21 gain/amplification (1q+) has been reported to respond poorly to daratumumab. We aimed to explore the mechanism of daratumumab resistance in 1q+ MM. Our findings revealed significantly lower CD38 expression in patients with 1q+ MM than in those with 1q wild type (WT) MM. Next, we focused on the interleukin‐6 receptor (IL6R) located in the 1q21 region because a previous report shows that interleukin‐6 (IL‐6) reduces CD38 expression via the IL‐6/Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway activation in MM. Indeed, IL6R expression was significantly higher in 1q+ MM than in 1q WT MM. We verified that the 1q+ human myeloma cell lines (HMCLs) expressed higher IL6R levels than the 1q WT HMCLs. IL‐6 treatment induced CD38 downregulation in both the 1q+ HMCLs and primary bone marrow (BM) samples but not in their 1q WT HMCLs and BM samples. IL‐6 also resulted in the upregulation of phosphorylated STAT3 in 1q+ HMCLs but not in the 1q WT HMCLs. Furthermore, inhibition of the IL‐6/JAK/STAT pathway by treatment with ruxolitinib or tocilizumab restored CD38 expression in the 1q+ HMCLs and BM samples. These findings elucidate the mechanisms underlying daratumumab resistance in 1q+ MM and provide insights for future therapeutic strategies.