Assessment of a possible in vivo modulating activity of anti-CD28 antibodies. Anti-CD28 mAb (8 mg/kg) was infused intravenously into macacus fascicularis on days 0 and 2. Blood samples were collected before injection and every 2 days for 10 days. PBMCs were then analyzed by flow cytometry after gating of CD2⁺ cells. Mean fluorescence intensity (MFI) of CD28 expression (A) as well as percent of CD2⁺CD28⁺ double-positive cells within mononuclear cells (B) are represented; ⋄ indicates CD28.1; ▴, CD28.2; ▪, CD28.3; •, CD28.6; and ○, isotype control. Similar data were found in 2 monkeys per antibody, one of them being shown here.