CD28 and cancer: In vitro coculture of control HPAF-II cancer cells with nonadherent human PBMCs significantly suppressed the CD3/CD28-induced proliferation of both CD8 and CD4 T cells (Fig. 7A and B), and also reduced levels of IFNγ and GMCSF (Fig. 7C; Supplementary Fig. S7A), cytokines important in generating an active antitumor immune response (24, 25).