Antigen-specific regulatory T cells eliminate cognate CD4 effector T cells through perforin, granzyme B, and Bim.

T cells (Tregs) maintain immune tolerance, yet how they selectively eliminate cognate CD4+ effector T cells remains unclear. We propose a co-activation-dependent elimination mechanism, conceptually akin to synthetic lethality. Using in vitro co-cultures, we show that simultaneous peptide-MHC II recognition by Tregs and responders is required to trigger apoptosis-a "double-lock" condition. Dual activation induces perforin and granzyme B in Tregs, while responders become susceptible via TCR-driven Bim upregulation. Conditional deletion of Gzmb, Prf1, or Bcl2l11 abrogates killing. In an OVA-driven airway inflammation model, wild-type but not granzyme B-deficient Tregs reduce lung effector T cells and inflammation, with increased cleaved caspase-3 in targets. This effect requires cognate antigen and occurs without Treg conversion. Human assays recapitulate these findings. Our work defines stringent molecular requirements for Treg-mediated cytotoxicity, informing therapeutic strategies for allergy, autoimmunity, and cancer.