The critical discriminatory event in the activation of T lymphocytes bearing αβ T cell receptors (TCRs) is their interaction with a molecular complex consisting of a peptide bound to a major histocompatibility complex (MHC)- encoded class I or class II molecule on the surface of an antigen-presenting cell. The kinetics of binding were measured of a purified TCR to molecular complexes of a purified soluble analog of the murine MHC class I molecule H-2L(d) (sH-2L(d)) and a synthetic octamer peptide p2CL in a direct, real-time assay based on surface plasmon resonance. The kinetic dissociation rate of the MHC-peptide complex from the TCR was rapid (2.6 x 10-2 second-1, corresponding to a half-time for dissociation of approximately 27 seconds), and the kinetic association rate was 2.1 x 105 M-1 second-1. The equilibrium constant for dissociation was approximately 10-7 M. These values indicate that TCRs must interact with a multivalent array of MHC-peptide complexes to trigger T cell signaling.
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