Multiple myeloma (MM) is a cancer of plasma cells in the bone marrow (BM) and represents the second most common hematological malignancy in the world. The MM tumor microenvironment (TME) within the BM niche consists of a wide range of elements which play important roles in supporting MM disease progression, survival, proliferation, angiogenesis, as well as drug resistance. Together, the TME fosters an immunosuppressive environment in which immune recognition and response are repressed. Macrophages are a central player in the immune system with diverse functions, and it has been long established that macrophages play a critical role in both inducing direct and indirect immune responses in cancer. Tumor-associated macrophages (TAMs) are a major population of cells in the tumor site. Rather than contributing to the immune response against tumor cells, TAMs in many cancers are found to exhibit protumor properties including supporting chemoresistance, tumor proliferation and survival, angiogenesis, immunosuppression, and metastasis. Targeting TAM represents a novel strategy for cancer immunotherapy, which has potential to indirectly stimulate cytotoxic T cell activation and recruitment, and synergize with checkpoint inhibitors and chemotherapies. In this review, we will provide an updated and comprehensive overview into the current knowledge on the roles of TAMs in MM, as well as the therapeutic targets that are being explored as macrophage-targeted immunotherapy, which may hold key to future therapeutics against MM.
Multiple myeloma (MM) is the cancer of plasma cells within the bone marrow (BM) and remains incurable. Tumor-associated macrophages (TAMs) within the tumor microenvironment often display a pro-tumor phenotype and correlate with tumor proliferation, survival, and therapy resistance. Thus, TAMs have become an emerging target of interest. IL-10 is a key immunosuppressive cytokine that leads to recruitment and development of TAMs. In this study, we investigated the role of IL-10 in MM TAM development as well as the therapeutic application of IL-10/IL-10R signaling inhibition. We demonstrated that IL-10 is overexpressed in MM BM and mediates M2-like polarization of TAMs in patient BM, 3D co-cultures in vitro, and mouse models. In turn, TAMs promote MM proliferation and drug resistance, both in vitro and in vivo. Moreover, inhibition of IL-10/IL-10R pathway using a blocking IL-10R antibody prevented M2 polarization of TAMs and the consequent TAM-induced proliferation of MM, and re-sensitized MM to therapy, in vitro and in vivo. Therefore, our findings suggest that inhibition of IL-10/IL-10R axis is a novel immunotherapy strategy with monotherapy efficacy and can be further combined with current anti-MM therapy to overcome drug resistance. Future investigation is warranted to evaluate the potential of such therapy in MM patients.
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