Cellular activation leads to enhanced FcγRI-mediated binding of opsonized tumor celllines. (A) Percentage of Ba/F3- FcγRI cells binding to Daudi cells opsonized with Rituximab (RTX), Ofatumumab (OFA) or unopsonized (unopson) Daudi cells. E/T ratio was 1/2. (B) Percentage of Ba/F3- FcγRI cells binding to Cetuximab (CTX) or unopsonized (unopson) A1207. E/T ratio was either 1/2 or 1/4. Data represent the means of at least 3 replicates. FcγRI-blocking mAb 10.1 was used to investigate FcγRI-dependent binding (not shown) *P < .05. Experiment was repeated 3 times yielding similar results. (C-D) Binding of IL-3 stimulated Ba/F3-FcγRI to Ofatumumab-opsonized Daudi cells (C) or Cetuximab-opsonized A1207 cells (D) determined with fluorescence microscopy. Ba/F3-FcγRI cells are shown in green, Daudi and A1207 cells in red. Arrowheads denote contacts between Ba/F3-FcγRI and tumor cells.