Systemic Cysteine Elevation Sustains T-Cell Activation to Potentiate PD-1 Blockade.

Resistance to immune checkpoint inhibition remains a major barrier in pancreatic cancer treatment. Here, we show that concurrent administration of probiotics restores sensitivity to anti-PD-1 therapy in pancreatic cancer mouse models. Mice treated with the combination of anti-PD-1 and probiotics demonstrate robust tumor control, accompanied by enrichment of microbial pathways governing cysteine biosynthesis, elevated serum cysteine levels, and increased T cell function. Serum cysteine levels, rather than intratumoral cysteine concentrations, inversely correlate with tumor burden. Functionally, cysteine directly promotes T cell survival, activation, and cytotoxicity while its restriction induces uncoupled transcriptional-translational stress and impairs T cell function. Oral cysteine supplementation synergizes with anti-PD-1 therapy in pancreatic cancer mice, reducing tumor burden and enhancing intratumoral T cell activation, phenocopying probiotics-mediated immune restoration. These findings suggest systemic cysteine availability as a tractable metabolic target to enhance cancer immunotherapy.