Inhibition of peptide-stimulated proliferation of CD28⁻ cells by anti-CD137L antibody. Replicate cultures of purified CD28⁻CD8⁺ T cells were stimulated with irradiated peptide-pulsed autologous PBMC in the presence or absence of antibody. Duplicate cultures were harvested at intervals and the total cell count per well determined. (A) ♦, no antibody; ■, control irrelevant antibody anti-CD4; ▴, anti-CD137L. (B) ♦, no antibody; ■, control irrelevant antibody anti-CD4; ▴, anti-CD275. (C) ♦, no antibody; ■, control irrelevant antibody anti-CD4; ▴, anti-CD80; □, anti-CD86; ▵, both anti-CD80 and anti-CD86. The experiment was performed on 4 independent donors, of which this is a representative experiment. The results from all 4 donors were antibodies similar to CD80, CD86, and ICOSL were unable to block proliferation, whereas anti-41BBL was (within the range 40%-60%, depending on donor). Anti-CD4 was an irrelevant antibody control in that this antibody is able to bind the antigen-presenting cells and demonstrates that steric hindrance by an irrelevant antibody was not able to block costimulation and T-cell proliferation. Anti-ICOSL, -CD80 and -CD86 antibodies were all the same isotype (IgG1) as the blocking 41BBL antibody and act as internal isotype controls. Error bars represent SD.