Specific competition for cell surface–binding sites. (A-D) The different cell preparations were preincubated with the indicated competitor proteins at the indicated concentrations. GzmB-Bio or TF-Bio was then added to a final concentration of 10 μg/mL. (A) Inhibition of GzmB-Bio binding to monocytes and neutrophil granulocytes by nonlabeled GzmB and elastase but not by iC3b. (B) GzmB-Bio (▪) binding to K562 cells is inhibited by free nonlabeled GzmB, elastase, and proteinase 3 (PR3). In contrast, TF-Bio (▦) is not inhibited by either of these proteases. (C) Binding of GzmB to K562 cells is efficiently competed by GzmK (×) or nonlabeled GzmB (+) but not by GzmA (▴). SA-PE alone is indicated by □. Panels A-C depict the means of triplicate measurements with their standard deviations (error bars). (D) Native glycosylated GzmB (○) inhibits GzmB-Bio binding to HL-60 cells in a fashion similar to that for inactive GzmB^S195A (□), whereas other basic proteins, such as trypsin-TLCK () and lysozyme (▴), display no or very low inhibition.