Which cell type is a major immunosuppressive component of the tumor microenvironment?

• A. Myeloid-derived suppressor cells
• B. Regulatory T cells
• C. Tumor-associated macrophages
• D. Dendritic cells

Answer: (B)

Regulatory T cells are central to immunosuppression in the tumor microenvironment. They accumulate within many tumors and actively dampen anti-tumor immunity by inhibiting the function of cytotoxic CD8+ T cells and helper T cells. These cells express FoxP3 and high levels of CD25, and they use multiple brakes on the immune response: they secrete inhibitory cytokines such as IL-10 and TGF-β, they suppress effector T cells by consuming IL-2 through CD25, and they express CTLA-4 which reduces co-stimulation by antigen-presenting cells. They can also promote a tolerogenic environment through metabolic and cellular interactions, further blunting immune attack on tumor cells. The cumulative effect is a weaker anti-tumor response and easier tumor immune evasion, which is why higher Treg activity in tumors is often associated with poorer outcomes and why strategies that disrupt Treg function can enhance immunotherapy. While other cells like MDSCs and tumor-associated macrophages also contribute to suppression, regulatory T cells frequently act as the dominant, orchestrating force in the immunosuppressive tumor milieu.