Cytotoxic T lymphocytes (CTLs) in tumours are often dysfunctional. Bréart et al. investigated the association between cytokines and CTLs in human tumours and found a correlation between IL27 expression and CTL infiltration. In syngeneic mouse models, treatment with an IL-27-encoding plasmid or half-life-enhanced IL-27, with or without anti-PD-L1, improved tumour rejection without overt toxicity. Single-cell RNA sequencing of mouse CTLs showed that IL-27 induces the expression of cytotoxicity genes, such as Gzmb, and downregulates genes linked to exhaustion, such as Tox. In patients with metastatic urothelial bladder carcinoma or non-small-cell lung cancer, high IL-27 expression was associated with improved responses to PD-L1-targeted therapy, and in vitro exhaustion assays with human CTLs showed that IL-27 promotes the differentiation of cytotoxic and effector memory T cells while suppressing dysfunction-associated markers. Overall, IL-27 seems to support CTL fitness and cytotoxicity and may be an attractive therapeutic target in cancer.