IL-21, and to a lesser extent IL-15, inhibits differentiation of antigen-primed CD8 T cells and promotes their homeostasis and anti-tumour activity. Here, we investigated molecular mechanisms behind tumour-specific responses of primary murine T lymphocytes engineered to express a TCR directed against human gp100/HLA-A2 following short-term exposure to IL-15 and/or IL-21. We demonstrated that IL-15 + IL-21, and to a lesser extent IL-21, enhanced antigen-specific T-cell cytotoxicity, which was related to enhanced expression of granzymes A and B, and perforin 1. Furthermore, IL-15 + IL-21 synergistically enhanced release levels and kinetics of T-cell IFNgamma and IL-2, but not IL-10. Enhanced secretion of IFNgamma was accompanied by increased gene expression and cytosolic protein content, and was restricted to effector memory T cells. To summarize, we show that IL-15 + IL-21 improves antigen-specific responses of TCR-transduced effector T cells at multiple levels, which provides a rationale to treat T cells with a combination of these cytokines prior to their use in adoptive TCR gene therapy.