Exploring multiple myeloma pathobiology and immune cell signatures: Evidence from bidirectional genetic analysis

The causal relationship between immune cell signatures and multiple myeloma (MM) pathobiology remains incompletely understood. This study aimed to explore the bidirectional causal associations between 731 circulating immune cell traits and MM risk using a two-sample, bidirectional Mendelian randomization (MR) approach. Two-sample MR analyses were conducted utilizing genome-wide association study (GWAS) summary statistics for 731 immune cell phenotypes and MM GWAS datasets. Sensitivity analyses were performed to assess the robustness and reliability of the findings. Furthermore, meta-analyses were conducted on specific immune cell traits identified through the primary MR analyses to reinforce causal inferences. We identified 11 immune cell traits across 3 immune profiles - absolute cell count (AC), relative cell (RC) frequency, and median fluorescence intensity (MFI) - that exhibited significant causal associations with MM. Three immune traits were associated with an increased risk of MM: CD39+ CD8br %CD8br (P < .001), CD20 on CD20- CD38- cells (P = .002), and CD38 on transitional cells (P < .001). Conversely, 8 immune traits were found to confer a protective effect against MM, including CD11c+ monocyte AC (P = .002), Im myeloid-derived suppressor cell (MDSC) %CD33dim HLA-DR- CD66b- (P = .002), CD8dim %T cell (P < .001), CD8dim %leukocyte (P < .001), CD24 on CD24+ CD27+ cells (P = .003), CD4 on naive CD4+ cells (P < .001), CD11b on Mo MDSCs (P = .002), and HLA-DR on CD33dim HLA-DR+ CD11b- cells (P < .001). The reverse MR analysis demonstrated a significant causal association between the expression of CD19 on CD24+ CD27+ B cells and CD16 on CD14+ CD16+ monocytes in MM. These findings were validated through sensitivity tests and meta-analyses, which confirmed the robustness of the results and the absence of significant heterogeneity. Our bidirectional MR analyses provide compelling evidence for causal relationships between specific immune cell phenotypes and MM risk. These insights deepen the understanding of immune system dysregulation in MM pathogenesis and highlight potential immune-related targets for future therapeutic interventions.