The impact of duration versus extent of TCR occupancy on T cell activation: A revision of the kinetic proofreading model

The widely accepted kinetic proofreading theory proposes that rapid TOR dis sociation from a peptide/MHC ligand allows for stimulation of early but not late T cell activation events, explaining why low-affinity TCR ligands are poor agonists. We identified a low-affinity TCR ligand which stimulated la te T cell responses but, contrary to predictions from kinetic proofreading, inefficiently induced early activation events. Furthermore, responses indu ced by this ligand were kinetically delayed compared to its high-affinity c ounterpart. Using peptide/MHC tetramers, we showed that activation characte ristics could be dissociated from TOR occupancy by the peptide/MHC ligands. Our data argue that T cell responses are triggered by a cumulative signal which is reached at different time points for different TCR ligands.