Eosinophils promote an immunosuppressive bone marrow microenvironment in multiple myeloma

Multiple Myeloma (MM) is an incurable malignancy characterized by the expansion of plasma cells within the bone marrow, leading to structural disruption, osteolytic lesions, and profound immune dysregulation. Although therapeutic advances have improved initial responses, nearly all patients eventually develop relapsed or refractory disease. Understanding the cellular mechanisms that drive tumor immune escape and MM progression remains a critical unmet need, particularly as patients become candidates for cellular therapies (CAR-T) or T-cell engagers (TCEs), which currently achieve a median progression-free survival of only ~1 year. This limited durability is largely attributed to the MM-induced immunosuppressive microenvironment and T-cell exhaustion. Identifying cellular mediators of immunosuppression within the bone marrow is therefore essential to improving the efficacy of immunotherapies.To address this gap in knowledge we leveraged spatial transcriptomics (10x Visium) to interrogate the MM bone marrow microenvironment in an unbiased manner and revealed that eosinophils are enriched within the tumor bed of MM-bearing mice. Eosinophils have not previously been investigated in MM and are underrepresented in scRNA-seq datasets due to their labile nature and high RNAase content. To test the functional role of eosinophils, we used a syngeneic MM model (5TGM1 cells in immunocompetent C57BL/6-KaLwRij mice) that recapitulates key features of human disease. We found that depletion of eosinophils (alpha-SiglecF antibody; IP) prior to the establishment of MM significantly reduces tumor growth in vivo . We then compared KaLwRij versus C57BL/6 RAG2 (B and T cell deficient) mice and observed that eosinophil-mediated MM progression requires an intact adaptive immune system. Using flow cytometry, we determined that eosinophil depletion decreases CD8⁺ T-cell exhaustion (TIM-3⁺ PD-1⁺) while enhancing T-cell activation (CD69⁺ CD25⁺). In vitro , we used primary eosinophils and found that exposure to 5TGM1 MM cells is sufficient to induce eosinophil PD-L1 expression. We also observed that MM-educated eosinophils can induce exhaustion (PD1+ CTLA4+) in pre-activated CD8 T cells.Together, our findings identify eosinophils as previously unrecognized mediators of immunosuppression in the MM bone marrow microenvironment and highlight them as a potential therapeutic target to enhance the durability of existing immunotherapies.