Engineering persistent interleukin-2 for cancer immunotherapy

Abstract

Mobilizing the immune system to recognize and destroy tumor cells is a promising strategy for treating cancer. In contrast to standard therapeutic approaches such as surgery, radiation, and chemotherapy, immunotherapy offers the possibility of systemic yet tumor-specific cell killing as well as long-lasting cancer protection. A significant mode of tumor rejection is direct tumor cell killing by immune cells, such as cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. These cell types are stimulated to proliferate by the cytokine interleukin-2 (IL-2). Consequently, IL-2 has been actively pursued as an agent for immunotherapy, either alone or in combination with other therapeutic strategies. IL-2 is characterized by rapid systemic clearance, with a fast-phase serum half-life of 13 minutes and a slow-phase half-life of 85 minutes. We hypothesized that prolonging the persistence of IL-2 at the cell surface or extending its circulation lifetime would increase its immunostimulatory potency. Therefore, we evolved murine IL-2 to bind the alpha subunit of its receptor, known as IL-2Ra or CD25, with 500-fold higher affinity; tethered IL-2 to the surface of T cells via streptavidin sandwiches; and fused IL-2 to the antibody Fc fragment, designated Fc/ IL-2, which extended the slow-phase serum half-life by 15 hours. Compared to free IL-2, Fc/IL-2 fusions induced superior control of solid tumors in mice. Interestingly, combining Fc/IL-2 with an anti-tumor antibody led to potent suppression of tumor growth during treatment. Furthermore, combination therapy protected two of three mice from subsequent tumor re-challenge. Depletion of CTLs or NK cells completely or partially, respectively, abrogated treatment efficacy, suggesting these immune cell types contribute to the anti-tumor response. In the context of Fc fusion, increasing the affinity of IL-2 for CD25 did not further improve efficacy. Ablation of CD25 binding, however, significantly reduced efficacy and also increased treatment toxicity. Since we employed a mutant Fc with disrupted FcyR binding, and hence reduced effector function, and fused IL-2 to mutant Fc monovalently, the significant therapeutic benefit of Fc/IL-2 over free IL-2 likely results from the extension of IL-2 circulation lifetime. We hypothesize that long-circulating IL-2 would potently synergize with other anti-tumor antibodies for effective cancer immunotherapy.