Overexpressed tumor associated antigens (e.g. HER2 and EGFR) are attractive targets for therapeutic T cells, but toxic cross-reaction with normal tissues expressing low antigen levels has been observed with Chimeric Antigen Receptor (CAR) T cells targeting such antigens. Inspired by natural ultrasensitive response circuits, we engineer a two-step positive feedback circuit that allows T cells to discriminate targets based on a sigmoidal antigen density threshold. In this circuit, a low affinity SynNotch receptor for HER2 controls the expression of a high affinity CAR for HER2. Increasing HER2 density thus has cooperative effects on T cells ╌ it both increases CAR expression and activation ╌ leading to a sigmoidal response. T Cells with this circuit show sharp discrimination between target cells expressing normal and disease levels of HER2, both in vitro and in vivo.One Sentence SummaryA two-step positive feedback circuit generates engineered T cells capable of killing target cells with an ultrasensitive antigen density threshold.
