Immune Cells in the Tumor Microenvironment: Mechanisms of Immune Resistance and Therapeutic Advances

Abstract: The Tumor Microenvironment (TME) is a complex and dynamic network composed of immune cells, stromal cells, and the Extracellular Matrix (ECM), which collectively influence tumor progression, metastasis, and resistance to various therapies, ultimately shaping clinical outcomes. Crosstalk between tumor cells and these microenvironmental components fosters immune evasion, metabolic reprogramming, and resistance to immunotherapy. Accumulating evidence indicates that Tumor-Associated Macrophages (TAMs) often polarize toward immunosuppressive M2 phenotypes, secreting cytokines like IL-10 and TGF-β that dampen immune responses. Regulatory T cells (Tregs) inhibit effector T-cell activation, while Myeloid-Derived Suppressor Cells (MDSCs) impair T-cell and NK-cell functions via the release of suppressive mediators such as arginase and reactive oxygen species. Additionally, Natural Killer (NK) cells frequently lose cytotoxic potential in the TME. These immunosuppressive mechanisms collectively contribute to resistance against immune checkpoint inhibitors and other immunotherapies. To counteract this, emerging strategies aim to reprogram the TME and enhance anti-tumor immunity. These include the use of cytokine modulators to reverse immunosuppression, adoptive cell therapies such as CAR-T and CAR-NK cells to target tumors directly, and ECM-targeted therapies to improve immune infiltration. This review integrates the current understanding of how the TME mediates immune resistance and explores innovative therapeutic interventions aimed at overcoming these barriers to restore effective anti-tumor immune responses.