Engineering CAR-Tregs with Phage-Selected scFv Enables a New Paradigm for Immune Regulation

Regulatory T cells (Tregs) are central to immune tolerance, yet antigen-specific cell therapies lag behind cytotoxic CAR-T approaches. Most CAR-Treg programs still use single-chain variable fragments (scFvs) derived from monoclonal antibodies optimized for effector function, a bias that may promote CAR clustering, tonic signaling, and lineage instability. Here, we propose a phage-first binder discovery framework for CAR-Tregs and state testable hypotheses linking phage-display selection pressures to scFv biophysical properties and Treg fate. We hypothesize that multi-parameter selection and counter-selection can enrich scFvs with moderate affinity, low polyspecificity, and improved framework stability, thereby lowering tonic signaling in defined backbone architectures and preserving FOXP3/TSDR stability under inflammatory stress. We outline a falsifiable two-phase roadmap-bench triage followed by mechanistic and preclinical validation to determine how scFv properties, expression level, and intracellular signaling domains define an optimal tonic window for durable immune regulation.