KINETICS AND EXTENT OF T-CELL ACTIVATION AS MEASURED WITH THE CALCIUMSIGNAL

We have characterized the calcium response of a peptide-major histocom patibility complex (MHC)-specific CD4+ T lymphocyte line at the single cell level using a variety of ligands, alone and in combination. We a re able to distinguish four general patterns of intracellular calcium elevation, with only the most robust correlating with T cell prolifera tion. Whereas all three antagonist peptides tested reduce the calcium response to an agonist ligand, two give very different calcium release patterns and the third gives none at all, arguing that (a) antagonism does not require calcium release and (b) it involves interactions tha t are more T cell receptor proximal. We have also measured the time be tween the first T cell-antigen-presenting cell contact and the onset o f tile calcium signal. The duration of this delay correlates with the strength of the stimulus, with stronger stimuli giving a more rapid re sponse. The dose dependence of this delay suggests that the rate-limit ing step in triggering the calcium response is not the clustering of p eptide-MHC complexes on the cell surface but more likely involves the accumulation of some intracellular molecule or complex with a half-lif e of a few minutes.