Abstract
Background Myeloid-derived suppressor cells (MDSCs), which include polymorphonuclear-MDSCs and monocytic MDSCs (M-MDSCs), play key roles in disease progression, resistance to immune-cell therapies, and poor clinical outcomes in multiple myeloma (MM). Among them, M-MDSCs are especially potent in suppressing anti-tumor immune responses, making them an attractive therapeutic target. Previous studies have shown that certain human myeloma cell lines (HMCLs) can induce M-MDSC differentiation through the secretion of important mediators like CCL5 and macrophage migration inhibitory factor (MIF), and that blocking these molecules significantly reduces M-MDSC induction. We have previously demonstrated that immunomodulatory drugs (IMiDs), such as lenalidomide (LEN) and pomalidomide (POM), are effective at inhibiting M-MDSC induction by decreasing CCL5 and MIF expression (Kuwahara-Ota S, Br J Haematol 2020); however, the effects of cereblon E3 ligase modulators (CELMoDs) such as iberdomide (IBER) and mezigdomide (MEZI)—which exhibit anti-tumor activity even in LEN- and POM-resistant MM—on M-MDSC induction remain unknown. This study aims to examine the cellular and molecular effects of CELMoDs on M-MDSC induction in the presence of myeloma cells.
Methods To evaluate how CELMoDs influence M-MDSC induction, we conducted 96-hour co-culture experiments with two HMCLs and peripheral blood mononuclear cells (PBMCs) from healthy donors. IBER, MEZI, and POM were added at the start of co-culture at concentrations known to be non-cytotoxic from MTT viability tests. We identified M-MDSC populations phenotypically and functionally, and analyzed subsequent molecular changes using RNA microarray gene expression profiling and cytokine assays.
Results While all three agents promoted degradation of Aiolos and Ikaros in HMCLs, IBER (7.5 nM), MEZI (2.5 nM), and POM (1 μM) maintained over 70% viability in HMCLs after 96 hours of treatment. Despite this, IBER (7.5 nM), MEZI (2.5 nM), and POM (1 μM) significantly decreased M-MDSC induction from PBMCs in a dose-dependent manner. Notably, IBER and MEZI inhibited M-MDSC induction at concentrations 150–1000 times lower than those required for POM, indicating their higher potency. Transcriptomic profiling revealed that co-culture with HMCLs significantly upregulated inflammatory response genes, including TNFα/NFκB and IL6–JAK–STAT pathways, in myeloma cells. Conversely, CD33⁺ PBMCs showed downregulation of interferon-α and -γ response gene sets, consistent with the development of an immunosuppressive phenotype. Interestingly, CELMoDs and POM had opposing effects on inflammation-related gene expression in the two cell populations: they suppressed proinflammatory signals in HMCLs, as shown by decreased CCL5 expression, but simultaneously increased the expression of inflammatory and interferon-related genes in CD33⁺ PBMCs, including those involved in TNFα/NFκB signaling. These findings suggest that CELMoDs modulate immune responses in a cell-type-specific manner within the tumor microenvironment. Further analyses demonstrated that all agents suppressed MIF expression in CD33⁺ PBMCs. Consistent with this, pharmacological inhibition of MIF abrogated M-MDSC induction, confirming its mechanistic involvement. Additionally, we observed that CELMoDs and POM reduced IL-10 secretion from HMCLs, a cytokine previously implicated in M-MDSC generation. Neutralization of IL-10 with a specific antibody significantly inhibited M-MDSC induction in co-cultures with IL-10–producing HMCLs but had no effect in IL-10–non-producing HMCLs, underscoring the central role of IL-10 in this process.
Conclusion This study demonstrates that CELMoDs and POM suppress M-MDSC induction by targeting key immunosuppressive mediators such as IL-10 and MIF. Notably, these agents exert opposing transcriptional effects in tumor and immune cells, attenuating proinflammatory signals in myeloma cells while enhancing immune activation in PBMCs. These dual effects likely contribute to remodeling the tumor microenvironment toward an immune-permissive state. Collectively, our findings provide a mechanistic rationale for using CELMoDs to target MDSCs and enhance anti-tumor immunity in MM.
