Effect of 4-1BB–Fc on cell-mediated cytotoxic activity to 4-1BB ligand–expressing target cells by subset III cells.

The 4-1BB–Fc inhibits the cell-mediated cytotoxic activity to 4-1BB ligand–expressing target cells by subset III cells. Levels of 4-1BB ligand on EBV-transformed B cells were measured by staining cells with 4-1BB–Fc followed by a PE-conjugated anti–human IgG Ab (Fc-specific), as shown (A) by the filled histogram. The open histogram shows staining with an isotype human IgG₁. Subset III cells were isolated by cell sorting from the CB CD8⁺ T cells stimulated with anti–4-1BB as described in Figure 3A and used as effectors in a standard ⁵¹Cr release assay at an effector cell–to–target cell ratio of 2:1. Increasing amounts of 4-1BB–Fc (at final concentrations of 0, 1, 5, and 10 μg/mL) were added to the target cells for 15 minutes at room temperature before the addition of effector cells (B). Human IgG₁ was used as a negative control. The data are presented as the percentage of specific lysis. These results are representative of 4 experiments.