Upregulation of PD‐1 follows tumour development in the AOM/DSS model of inflammation‐induced colorectal cancer in mice.

Summary: Chronic inflammation may drive development of cancer as observed in inflammation‐induced colorectal cancer (CRC). Though immune cells can infiltrate the tumour microenvironment, cancer cells seem to evade anti‐tumour responses, which is one of the established hallmarks of cancer. Targeting the programmed cell death protein‐1 (PD‐1)/PD‐L1 signalling pathway is currently at the forefront in the development of anti‐tumour immunity‐based therapies for multiple malignancies. By blocking the immune‐checkpoint of activated T‐cells, it is possible to rewire the adaptive resistance induced by the PD‐1 ligands expressed in the tumour microenvironment. However, adverse immunotherapy‐modulated events could complicate the treatment of individuals with preexisting chronic inflammatory conditions. In this study, we investigated the expression of different systemic and mucosal T‐cell subsets during the course of azoxymethane (AOM)/dextran sulphate sodium (DSS)‐induced colitis and colitis‐associated CRC. In addition, we examined the expression of PD‐1 and its ligands PD‐L1 and PD‐L2 as well as other molecular targets related to T‐cell exhaustion. We found a significant increase in PD‐1 expression on all examined mucosal T‐cell subsets of the colon and the ileum, which correlated with disease progression. We also observed an upregulation of PD‐L1 and PD‐L2 mRNA expression throughout the AOM/DSS regime. Blocking PD‐1 signalling with an anti‐PD1 antibody did not affect the tumour burden in the AOM/DSS‐treated mice, but did potentiate the weight loss in the third DSS cycle, indicating possible immune‐mediated toxicity. This raises a concern for patients with colitis‐associated CRCs and should be further investigated. [ABSTRACT FROM AUTHOR]