Aging microenvironment induces CD8⁺ T cell exhaustion by suppressing hepatic β-hydroxybutyrylate synthesis

The metabolic mechanisms by which aging blunts CD8⁺ T cell antitumor and pathogen defense remain unknown. We demonstrate that the aged microenvironment induces CD8⁺ T cell exhaustion by reducing β-hydroxybutyrate (3HB) bioavailability. Aging represses hepatic BDH1-dependent 3HB synthesis, restricting SLC16A1-mediated 3HB uptake. Hepatic BDH1 ablation recapitulates age-associated CD8⁺ T cell dysfunction, compromising antiviral and antitumor immunity, whereas 3HB supplementation reverses these deficits via protein β-hydroxybutyrylation. Using a 3HB-derived chemical probe, 3Halk, together with functional screening, we identify PRKAR1B as a primary effector of 3HB signaling. PRKAR1B β-hydroxybutyrylation inhibits the transcription factor cyclic AMP (cAMP)-responsive element modulator (CREM), which activates T cell exhaustion-related gene expression. Age-associated 3HB depletion enhances CREM-dependent transcription, sustaining CD8⁺ T cell exhaustion. Consistently, the aged microenvironment compromises chimeric antigen receptor (CAR) T antitumor activity, which is substantially restored by 3HB treatment. Collectively, this study uncovers a hepatic metabolism-derived 3HB-CREM axis governing CD8⁺ T cell immunosenescence, highlighting 3HB as a viable immunorestorative strategy to improve immunotherapy outcomes in aged individuals.