Expansion of antigen-experienced CD8 ⁺ T cells is critical for the success of tumour-infiltrating lymphocyte (TIL)-adoptive cell therapy (ACT) in patients with cancer ¹ . Interleukin-2 (IL-2) acts as a key regulator of CD8 ⁺ cytotoxic T lymphocyte functions by promoting expansion and cytotoxic capability ^2,3 . Therefore, it is essential to comprehend mechanistic barriers to IL-2 sensing in the tumour microenvironment to implement strategies to reinvigorate IL-2 responsiveness and T cell antitumour responses. Here we report that prostaglandin E2 (PGE ₂ ), a known negative regulator of immune response in the tumour microenvironment ^4,5 , is present at high concentrations in tumour tissue from patients and leads to impaired IL-2 sensing in human CD8 ⁺ TILs via the PGE ₂ receptors EP2 and EP4. Mechanistically, PGE ₂ inhibits IL-2 sensing in TILs by downregulating the IL-2Rγ _c chain, resulting in defective assembly of IL-2Rβ-IL2Rγ _c membrane dimers. This results in impaired IL-2-mTOR adaptation and PGC1α transcriptional repression, causing oxidative stress and ferroptotic cell death in tumour-reactive TILs. Inhibition of PGE ₂ signalling to EP2 and EP4 during TIL expansion for ACT resulted in increased IL-2 sensing, leading to enhanced proliferation of tumour-reactive TILs and enhanced tumour control once the cells were transferred in vivo. Our study reveals fundamental features that underlie impairment of human TILs mediated by PGE ₂ in the tumour microenvironment. These findings have therapeutic implications for cancer immunotherapy and cell therapy, and enable the development of targeted strategies to enhance IL-2 sensing and amplify the IL-2 response in TILs, thereby promoting the expansion of effector T cells with enhanced therapeutic potential.