Killing of CD8⁺ T cells by a D^b-SAP tetramer depends on tetramer dose and avidity. (A) Mean fluorescence intensity of P14 T cells stained with PE-labeled cognate tetramers of varying avidity. The gpC9M tetramer binds with the highest efficiency. (B) D^b-SAP tetramers kill target T cells in a dose-dependent fashion. Cytotoxicity was normalized to cultures treated with unlabeled tetramer alone. The EC₅₀ of the SAP-coupled gpC9M-D227K, which cannot bind the CD8 coreceptor and hence binds P14 T cells with much lower avidity than gpC9M or gp33, was significantly different (*P < .001; Table 1). Incubation with control PE- or SAP-coupled hyC2A (not shown) gave identical results to that obtained with gpK1S; EC₅₀ values could not be determined for these tetramers. Results represent 3 independent experiments.