However, the level was reduced in the low avidity cells. Averaged data are shown in Fig. 4(b). The total phospho-ERK1/2 level in unstimulated cells was similar between the lines. The kinetics of ERK phosphorylation in high and low avidity CTL suggested that high avidity CTL undergo more rapid phosphorylation of ERK1/2 compared with low avidity CTL. However, at 60 min, the amount of phospho-ERK present in high and low avidity cells was similar when evaluated under conditions where the threshold stimulatory peptide concentration was used (10−6 m for low avidity cells and 10−12 m

for high AP24534 clinical trial avidity cells). By 6 hr post-stimulation, the phospho-ERK1/2 signal had returned to baseline in both cell types (data not shown). The marked peptide concentration-dependent see more differences in ERK1/2 phosphorylation and calcium flux between the lines suggested that differences in the peptide sensitivity of high

versus low avidity cells was controlled at a more membrane proximal step in the TCR signal transduction cascade. The transmembrane adaptor protein LAT provides a central signalling nexus for activation through initiation of signalosome formation. This complex controls recruitment and activation of phospholipase C-γ1, phophoinositide 3 kinase, and Ras.6,7 We first determined whether total protein levels of LAT in high and low avidity CTL differed and found that this protein was present at equal levels in both CTL lines (Fig. 5a). To evaluate LAT activation, the high and low avidity CTL were stimulated with titrated concentrations of peptide. Phosphorylation of LAT at tyrosine 191 was quantified by intracellular staining. This analysis revealed a pattern similar to that for other molecules analysed in that high avidity CTL were able to induce phosphorylation at all concentrations of peptide used, whereas low avidity CTL exhibited statistically significant increases in LAT phosphorylation Terminal deoxynucleotidyl transferase compared

with stimulation with APC in the absence of peptide only following exposure to APC pulsed with the highest amount of peptide (Fig. 5b,c, for clarification, the significance (*) shown on figure is comparing −5M and −9MCTL). These data suggested that differences in ERK1/2 signalling in high versus low avidity cells arose at a more membrane proximal step in TCR signalling. Tyrosine phosphorylation of ITAMs on the TCR-associated CD3 chains is one of the initial biochemical events detectable in T cells after TCR ligation.3 Phosphorylation at these sites allows ZAP-70 binding and activation, which then becomes competent for phosphorylation of LAT.37,38 To assess CD3ζ phosphorylation in high or low avidity CTL, cells were stimulated with APC bearing titrated concentrations of Ova257–264 peptide and CD3ζ immunoprecipitated at 10 or 60 min post-stimulation. The immunoprecipitates were subjected to SDS–PAGE and immunoblotted with anti-phosphotyrosine antibody. As evident from Fig.