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The application of chimeric antigen receptor (CAR) or T cell receptor (TCR)-engineered T cells to solid tumours is challenged by the immunosuppressive local microenvironment, which suppresses T cell activation. Although the administration of inflammatory cytokines such as interleukin-2 (IL-2) — which plays a critical role in T cell activation and expansion — could help reverse tumour suppression, systemic IL-2 delivery is highly toxic. Now, Allen et al. have engineered therapeutic human T cells bearing synthetic cytokine circuits, in which tumour-specific anti-CD19 synthetic Notch (synNotch) receptors locally induce production of IL-2 at the site of the tumour. In vitro, the engineered T cells were demonstrated to drive their own proliferation, as well as that of other cocultured non-engineered T cells. When injected intravenously in a mouse model, the engineered T cells locally expanded within the target tumour. Furthermore, the synthetic IL-2 circuit potently enhanced CAR T cell infiltration and clearance of immune-excluded tumours in mouse pancreatic cancer and melanoma models, without systemic toxicity.