Serial TCR engagement and down-modulation by peptide : MHC molecule ligands: Relationship to the quality of individual TCR signaling events

In the present study, we examined the relationships among quantitative aspe cts of TCR engagement as measured by receptor down-modulation, functional r esponses, and biochemical signaling events using both mouse and human T cel l clones, For T cells from both species, ligands that are more potent in in ducing functional responses promote TCR down-modulation more efficiently th an weaker ligands, At low ligand density, the number of down-modulated TCR exceeds the number of available ligands by as much as 80-100:1 in the optim al human case, confirming the previous description of serial ligand engagem ent of TCR (Valitutti, et al, 1995, Nature 375:148-151), A previously unapp reciated relationship involving TCR down-modulation, the pattern of proxima l TCR signaling., and the extent of serial engagement was revealed by analy zing different ligands for the same TCR, Functionally, more potent ligands induce a higher proportion of fully tyrosine phosphorylated zeta-chains and a greater amount of phosphorylated ZAP-70 than less potent ligands, and th e number of TCR down-modulated per available ligand is higher with ligands showing this full agonist-like pattern. The large number of receptors showi ng partial zeta phosphorylation following exposure to weak ligands indicate s that the true extent of TCR engagement and signaling, and thus the amount of sequential engagement, is underestimated by measurement of TCR down-mod ulation alone, which depends on full receptor activation. These data provid e new insight into T cell activation by revealing a clear relationship amon g intrinsic ligand quality, signal amplification by serial engagement, func tional T cell responses, and observable TCR clearance from the cell surface .