L33-specific T-cell clones respond specifically to HLA-DRB3*0101–positive monocytes pulsed with either synthetic peptide or HPA-1a–positive platelets. Proliferative responses induced by stimulation with peptide-pulsed or platelet-pulsed monocytes. Both clones proliferate specifically in response to (A) L33, but not Lol P1 and P33 control, peptides, and (B) HPA-1a–positive, but not HPA-1a–negative, platelets. (C) D7T1 and D7T4 T-cell clones proliferate in response to stimulation with HPA-1a–positive platelets (HPA-1aa or HPA-1ab), but not HPA-1a–negative (HPA-1bb) platelets, combined with HLA-DRB3*0101–positive, but not negative, monocytes. Representative data for clone D7T4 is shown.