Targeting intratumoral regulatory T cells by CD137 aptamer-shRNA chimeras

The cytokine CD137 (TNFRSF9, 4-1BB) is best known as a T cell costimulatory molecule. However, CD137 is also part of a negative feedback mechanism that limits T cell activation, and that is employed by regulatory T cells (Tregs) to prevent an overstimulation of T cells and subsequent autoimmune damage. CD137 has been identified as the gene that most significantly distinguishes intra- from extratumoral Tregs. CD137⁺ Tregs are more immunosuppressive than CD137^- ones, and CD137 levels on intratumoral Tregs correlate with a poor prognosis for cancer patients. In addition, CD137 is also ectopically expressed on several types of malignant cells that usurp this physiological feedback mechanism to evade immune surveillance. This suggests CD137 as a target for eliminating or changing intratumoral Tregs. Here we demonstrate that a murine CD137-specific aptamer can bind to CD137 on malignant cells and T cells and be internalized.

We fused the aptamer to short hairpin (sh) RNAs that are specific for Enhancer of zeste homolog 2 (EzH2) or neuropilin-1 (Nrp1), and demonstrated their uptake into CD137⁺ malignant cells and Tregs, and the subsequent downregulation or EzH2 and Nrp1.

This study confirms CD137 as a target for tumor immunotherapy and introduces CD137 aptamer-shRNA chimeras as novel tools to be evaluated for their usefulness in cancer treatment.