IL-2/CD25 acts within the tumor microenvironment to reprogram tumor-specific cytotoxic CD8+ T cells while restraining regulatory T-cell accumulation.

Abstract

Background: Targeting the high-affinity interleukin-2 receptor (IL-2R) on CD8⁺ T effector cells elicits potent antitumor responses that are augmented when co-administered with programmed cell death protein 1 (PD-1) blockade.

Methods: Using the mouse IL-2/CD25 fusion protein (mIL-2/CD25) at a high dose (HD) and anti-PD-1 checkpoint blockade, with the CT26 and MC38 colon carcinomas as models, the current study investigated how antitumor responses occurred when regulatory T cells (Tregs) are also a target of HD mIL-2/CD25. Single-cell RNA sequencing (scRNA-seq) and T-cell receptor (TCR) repertoire analysis of tumor-associated immune cells were used to identify how combination therapy reshaped the tumor microenvironment (TME). These data were cross-referenced with published datasets to (1) corroborate TCR specificity and (2) identify how CD8⁺ T-cell differentiation is affected by different IL-2/PD-1 strategies.

Results: Here, we show that effective antitumor immunity by the combination therapy is a result of HD mIL-2/CD25-dependent reprogramming of CD8⁺ tumor-specific T cells that increases their function and supports their expansion within the TME, the latter also shaped by PD-1 blockade. This therapy supports antitumor responses by chemokines and inflammatory cytokines that selectively favor the migration of CD8⁺ T cells but not Tregs deep within the tumor. After HD mIL-2/CD25 treatment, Tregs within the TME showed impaired downstream IL-2R signaling, reflected by failed upregulation of forkhead box P3 and CD25, impaired development into effector Tregs, and enhanced cell death, the latter which likely offsets Treg proliferation. Many similarities were noted after direct comparison of these scRNA-seq findings with another study where IL-2 activity was targeted to PD-1⁺ CD8⁺ T cells. Notably, HD mIL-2/CD25 and anti-PD-1 combination promotes effector and 'stem-like' CD8⁺ T-cell subsets, perhaps even to a greater extent than the PD-1-targeted IL-2 strategy.

Conclusions: The efficient differentiation of exhausted CD8⁺ T cells into highly functional cells and the resulting potent antitumor responses by HD mIL-2/CD25 plus anti-PD-1 suggest that this approach may augment durable response rates more than currently achieved with checkpoint blockade monotherapy.