Mitogenic activity of conventional and superagonistic anti-CD28 mAb. (A) In vitro proliferation of CFSE-labeled rat lymph node T cells induced by TCR stimulation alone, TCR plus CD28 costimulation, or by conventional or superagonistic CD28–specific mAb alone. (B) Cell divisions in transferred CFSE-labeled T cells after intravenous injection of conventional or superagonistic CD28–specific mAbs. Numbers indicate cell divisions of boxed populations.