Background: In vivo studies have recently demonstrated that interleukin 21 (IL-21) enhances the anti-tumor function of T-cells and NK cells in murine tumor models, and the combined use of IL-21 and IL-15 has resulted in prolonged tumor regression and survival in mice with previously established tumors. However, the combined anti-tumor effects of IL-21 and low dose IL-2 have not been studied even though IL-2 has been approved for human use, and, at low dose administration, stimulates the proliferation of memory T cells, and does not significantly increase antigen-induced apoptosis or regulatory T cell (Treg) expansion. This study examined whether recombinant IL-21 alone or in combination with low-dose IL-2 could improve the in vivo anti-tumor function of naïve, tumor-antigen specific CD8+ T cells in a gp100(25-33) T cell receptor transgenic pmel murine melanoma model.
Methods: Congenic C57BL/6 (Ly5.2) mice bearing subcutaneous B16F10 melanoma tumors were sublethally irradiated to induce lymphopenia. After irradiation naive pmel splenocytes were adoptively transferred, and mice were immunized with bone marrow-derived dendritic cells pulsed with human gp100(25-33) (hgp100(25-33)). Seven days after vaccination groups of mice received 5 consecutive days of intraperitoneal administration of IL-2 alone (20 x 10(3) IU), IL-21 alone (20 microg) or IL-21 and IL-2. Control animals received no cytokine therapy.
Results: IL-21 alone and IL-2 alone both delayed tumor progression, but only IL-21 significantly augmented long-term survival (20%) compared to the control group. However, combination therapy with IL-21 and IL-2 resulted in the highest long-term (>150 days) tumor-free survival frequency of 46%. Animals that were tumor-free for > 150 days demonstrated tumor-specific protection after rechallenge with B16F10 melanoma cells. At peak expansion (21 days post vaccination), the combination of IL-21 plus IL-2 resulted in a 2- to 3-fold higher absolute number of circulating tumor antigen-specific pmel CD8+ T cells than was stimulated by IL-2 or IL-21 alone. Pmel CD8+ T cells were predominantly partitioned into central memory (CD62L+/CD127+) or effector-memory (CD62L-/CD127+) phenotypes by day 28-post vaccination in IL-21 + IL-2 treated mice.
Conclusion: These observations support the potential use of IL-21 and low-dose IL-2 therapy in combination with a tumor-antigen vaccine and lymphopenic conditioning in future cancer clinical trials to maintain high numbers of anti-tumor memory CD8+ T cells with the potential to sustain long term tumor regression and survival.