# Targeted degradation of sICOSL reverses cytotoxic T cells dysfunction

## TL;DR

A soluble form of ICOSL weakens T cell function in cancer, and targeting its degradation could improve immunotherapy.

## Contribution

A nanobody-DPP4 fusion protein is developed to specifically degrade sICOSL and restore T cell function.

## Key findings

- Increased sICOSL causes T cell dysfunction and poor breast cancer survival.
- DPP4 degrades sICOSL, but chemotherapy drugs reduce DPP4 via EZH2 activation.
- Nanobody-DPP4 fusion proteins selectively target and degrade sICOSL in tumors.

## Abstract

Dysfunction of cytotoxic T cells (CTL) remains a major cause of tumor immune evasion and is correlated with poor cancer survival. Here, we found that increased soluble form of ICOSL (sICOSL) induced CTL dysfunction and was associated with shorter survival of patients with breast cancer. sICOSL emerged as a formidable adversary to CTLs, by directly triggering ICOS internalization and subsequent degradation—a critical blow to the co-stimulatory machinery essential for CTL activation. Our research shows that dipeptidyl peptidase-4 (DPP4) mainly breaks down sICOSL. Notably, certain chemotherapeutic drugs activate the histone methyltransferase Enhancer of zeste homolog 2 (EZH2), which in turn suppresses DPP4 expression. To address this issue, we have developed nanobody-DPP4 fusion proteins that can specifically degrade sICOSL, achieving substrate selectivity and tumor targeting. Overall, This work unveils that sICOSL orchestrates CTL dysfunction, and establishs targeted degradation of sICOSL as a new strategy for immunotherapy.

The online version contains supplementary material available at 10.1186/s40164-025-00692-x.