Recombinant interleukin-2 (rIL-2) is associated with objective responses in 15-20 % of patients with metastatic melanoma and renal cell carcinoma. More recently, rIL-2 has also demonstrated improved clinical activity in patients with melanoma. Given the toxicity of high-dose rIL-2 and the availability of many new immunotherapy agents, it has been suggested that lower doses of rIL-2 may be preferred for combination clinical studies. In order to determine the impact of low doses of rIL-2 on anti-tumor immunity and therapeutic effectiveness, we challenged C57BL/6 mice with poorly immunogenic B16-F10 melanoma and treated them with varying doses of rIL-2 (range 103-105 IU). Tumor growth at day 14 was significantly reduced when rIL-2 was administered at 10,000 (P < 0.02) and 100,000 (P < 0.02) IU doses, but tumor growth was significantly increased when mice were treated at 1000 IU rIL-2 (P < 0.02), as compared to placebo treatment. While the proportions of CD8+ and CD4+ T cells in the tumor were similar at all doses tested, the proportion of NK cells was decreased and the proportion of Tregs was increased in tumors exposed to low-dose rIL-2. The ratio of gp100-specific CD8+ to CD4+ regulatory T cells was increased in tumors treated at 10,000 and 100,000 IU of rIL-2 but was decreased at the 1000 IU dose compared to placebo-treated mice. These findings suggest that low-dose rIL-2 may impair host anti-tumor immunity and promote tumor growth. Early-phase adjuvant and combination clinical studies should include patient cohorts with higher doses of rIL-2.
Keywords: CD8+ T cell; Cancer vaccines; Immunotherapy; Interleukin-2; Natural killer cell; Regulatory T cell.