Role of B7 on FL cells during restimulation of productively stimulated allogeneic T cells in a secondary MLR. Allogeneic T cells were stimulated primarily with CD40-FL cells for 5 to 6 days. Cells were then harvested, enriched for viable cells by density-centrifugation, and rested for 48 hours before restimulation. Cells were then restimulated with FL cells, FL cells plus control-Ig (10 μg/mL), FL cells plus CTLA4-Ig (10 μg/mL), or CD40-FL cells. T-cell proliferation was assessed after 3 days (shown here) or 6 days (data not shown) by thymidine incorporation for the last 16 hours of coculture. Approriate controls (T cells, FL cells, CD40-FL cells) were less than 3,000 cpm. Control-Ig or CTLA4-Ig were added to FL cells 30 minutes before coculture and were present throughout the culture period.