Influence of absolute numbers of surface TCRs and costimulation on sensitivity to inhibition by AF.

(A) Absolute numbers of TCRs determine sensitivity to inhibition by AF. Jurkat cells expressing low or high levels of TCRs were preparatively sorted and stimulated with solid-phase OKT3 and anti-CD28 mAbs in the presence or absence of AF (25%). Histograms on the right show the relative CD3 immunostaining of the sorted cell populations. Comparable results were obtained in 2 repeat experiments in which cell populations were sorted independently. (B) Costimulation lowers the threshold number of TCRs required for IL-2 production and renders cells less sensitive to inhibition by AF. Jurkat cells were stimulated with varying concentrations of PHA, with (▵) or without (▪) addition of B7-1–transfected CHO cells. Results in panels A and B are presented as a percentage of inhibition of IL-2 secretion.