Progressive enhancement of kinetic proofreading in T cell antigen discrimination from receptor activation to DAG generation

T cells use kinetic proofreading to discriminate antigens by converting small changes in antigen binding lifetime into large differences in cell activation, but where in the signaling cascade this computation is performed is unknown. Previously, we developed a light-gated immune receptor to probe the role of ligand kinetics in T cell antigen signaling. We found significant kinetic proofreading at the level of the signaling lipid diacylglycerol (DAG) but lacked the ability to determine where the multiple signaling steps required for kinetic discrimination originate in the upstream signaling cascade (Tischer and Weiner, 2019). Here we uncover where kinetic proofreading is executed by adapting our optogenetic system for robust activation of early signaling events. We find the strength of kinetic proofreading progressively increases from Zap70 recruitment to LAT clustering to downstream DAG generation. These data suggest a distributed kinetic proofreading mechanism, with proofreading steps both at the receptor and at downstream signaling events. Leveraging the ability of our system to rapidly disengage ligand binding, we measure slower reset rates for downstream signaling events. Our observations of distributed kinetic proofreading and slowed resetting of downstream steps suggest a basis of cooperativity between multiple active receptors with implications in tissue homeostasis, autoimmunity, and immunotherapy off-target effects.