Stimulation of T cells by antigen-presenting cells is kinetically controlled by antigenic peptide binding to major histocompatibility complex class II molecules.

Activation of CD4+ T cells by antigenic peptide involves the interaction of major histocompatibility complex (MHC) class II-peptide complexes on the surface of antigen-presenting cells (APCs) with T-cell receptors. This report describes the kinetics of T-cell triggering by exogenous antigenic peptides in the presence of APCs. A rapid specific increase in extracellular acidification rate is observed within minutes upon exposure of A.E7 T cells (restricted for IEk and moth cytochrome c peptide containing residues 88-103) and 4R3.9 T cells (restricted for IAk and myelin basic protein peptide containing residues 1-14 [AcMBP-(1-14)]) to their cognate peptides in the presence of CH27 cells bearing both IAk and IEk MHC class II molecules. Pretreatment of cloned T cells, but not APCs, with herbimycin A resulted in complete inhibition of triggering events, indicating that the acidification response is mediated by T-cell second messenger pathways. This rapid assay for 4R3.9 T-cell stimulation showed increased T-cell triggering activity for AcMBP-(1-14)-A4 and MBP-(1-14)-M4 peptides compared to the native AcMBP-(1-14)-K4. By using the previously determined kinetic constants for MBP-(1-14)-A4 reactions with IAk, it is possible to show that at the lowest peptide concentrations the kinetics of T-cell triggering are limited by the kinetics of the peptide binding to MHC class II molecules.