Abstract 1725: Short-term administration of high dose mIL-2/CD25 fusion protein elicits CD8+ T cell-driven anti-tumor immunity and memory

Abstract Recombinant IL-2 (rIL-2) promotes the differentiation, proliferation, and survival of antigen-activated T cells and induces potent anti-tumor activity. High dose (HD) rIL-2 broadly targets T effector (Teff) and NK cells. However, rIL-2 cancer therapy requires long-term infusions of high doses due to its short half-life and is associated with severe toxicities which limit efficacy. To circumvent these limitations, our lab has developed and is testing a new IL-2-based biologic, mIL-2/CD25, which can be used at high doses to promote anti-tumor immunity. mIL-2/CD25 is a fusion protein (FP) composed of mouse IL-2 fused to the mouse CD25 extracellular domain via a non-cleavable glycine-serine linker with selectivity toward the high affinity IL-2R. This FP has a much longer half-life (16 hr) than rIL-2 (15 min). At a low dose, mIL-2/CD25 selectively expands Tregs, but at a HD, Teff cells are also engaged. These properties allow for HD FP to readily expand activated endogenous tumor-reactive Teff cells, but not memory-phenotypic CD8+ T cells and NK cells. In the current study, we tested the extent short-term HD FP therapy activated tumor-reactive Teff cells to elicit efficacious anti-tumor immunity responses despite Treg stimulation. HD FP prolonged survival in mice bearing B16.F10 melanoma, MC-38 colon adenocarcinoma, or CT26 colon carcinoma. In the CT26 model, a significant increase in both the number and the frequency of CD8+ T cells was detected in the tumor microenvironment (TME) of treated mice. FP-dependent Treg expansion was observed in secondary lymphoid tissues but not the TME. This therapeutic approach, therefore, led to a higher CD8+ Teff to Treg ratio specifically in the TME. After FP treatment, CD8+ T cells isolated from the TME expressed lower levels of TOX, PD-1, and LAG-3, suggesting that they were less exhausted. T cell depletion studies using anti-CD8 revealed that CD8+ T cells are the major driver of the FP-induced anti-tumor response. Rechallenge of animals that had previously cleared CT26 due to FP treatment led to successful tumor rejection, showing that the FP supported anti-tumor memory. These findings demonstrate that mIL-2/CD25 amplifies recently activated tumor-reactive CD8+ Teff cells to generate efficacious anti-tumor responses that limit Treg-dependent negative regulation in the TME and lead to anti-tumor memory. Citation Format: Kathryn LaPorte, Rosmely Hernandez, Thomas Malek. Short-term administration of high dose mIL-2/CD25 fusion protein elicits CD8+ T cell-driven anti-tumor immunity and memory [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1725.