Prednisolone modulates CD8⁺ and regulatory T-cell activity to dampen response to immune checkpoint inhibitor therapy in melanoma

Immune checkpoint inhibitors (ICIs) have transformed the treatment of advanced melanoma, yet their efficacy is limited by high-grade immune-related adverse events that often require treatment with systemic corticosteroids. Although corticosteroids are widely used, their impact on anti-tumor immunity remains poorly defined. Using an ICI-responsive murine melanoma model, we show that tapered systemic prednisolone administered after three cycles of combined anti-CTLA4 and anti-PD1 therapy compromises ICI-mediated tumor control, leading to delayed progression in one-third of initially responding animals. Mechanistically, prednisolone selectively suppressed CD8+ effector T-cell activation in tumor-draining lymph nodes and in the circulation, while expanding activated regulatory T-cells. These changes increased the Treg:CD8+ effector ratio, reduced cytotoxic T-cell function and blocked the early ICI-mediated induction of cytokines, including IL-2, IFNγ, VEGF, CCL3/4, IL-13, IL-3, and GM-CSF. Importantly, despite these early immunosuppressive effects, long-term tumor-specific memory responses were preserved. Autologous melanoma:T-cell cocultures validated these findings. Overall, systemic prednisolone disrupts early CD8+ T-cell-mediated anti-tumor activity but spares durable immunity, highlighting the critical importance of timing and context in the introduction of corticosteroids during ICI therapy.