Timosaponin AIII enhances CAR-T cell potency and prevents relapse through impairing CAR-Tregs.

Chimeric antigen receptor (CAR)-T cell therapy has transformed treatment of relapsed/refractory DLBCL, yet resistance driven by regulatory T cells (Tregs) limits its efficacy. Here we identify Timosaponin AIII (TAIII), a clinical-stage natural product, as an effective modulator of CAR-T function that depletes CAR-Tregs while enhancing effector activity. Mechanistically, TAIII acts as an allosteric A2AR inhibitor by competing with cholesterol, suppressing CREB-dependent FoxP3 transcription and disrupting the A2AR-Treg axis. Ablation of A2AR or Tregs in vitro and in vivo abolishes TAIII activity, confirming specificity. Furthermore, TAIII reduces intratumoral Tregs, increases CD8⁺ T cells infiltration, and potentiates PD-1 blockade in solid tumor models. Importantly, TAIII promotes central memory T-cell formation and enhances CAR-T cytotoxic cytokine secretion. Combining or pretreating CAR-T cells with TAIII markedly improves antitumor efficacy and prevents late relapse across preclinical models. These findings establish TAIII as a combinatorial strategy to deplete CAR-Tregs, enhance CAR-T activity, and extend therapeutic durability.