IL-21 enhances and sustains CD8⁺ T cell responses to achieve durable tumor immunity: Comparative evaluation of IL-2, IL-15, and IL-21

Cytokines that use the common receptor γ-chain for regulating CD8⁺ T cell responses to Ag include IL-2, IL-15, and the recently identified IL-21. The ability of these cytokines to regulate antitumor activity in mice has generated considerable interest in understanding their mode of action. In this study we compare the abilities of IL-2, IL-15, and IL-21 to stimulate immunity against tumors in a syngeneic thymoma model. Durable cures were only achieved in IL-21-treated mice. By monitoring both endogenous and adoptively transferred tumor Ag-specific CD8⁺ T cells, it was determined that IL-21 activities overlap with those of IL-2 and IL-15. Similar to IL-2, IL-21 enhanced Ag activation and clonal expansion. However, unlike IL-2 treatment, which induces activation-induced cell death, IL-21 sustained CD8⁺ T cell numbers long term as a result of increased survival, an effect often attributed to IL-15. These findings indicate that the mechanisms used by IL-21 to promote CD8⁺ T cell responses offer unique opportunities for its use in malignant diseases and infections.