Altered peptide ligands markedly suppress IL-17 production by glucose-6-phosphate isomerase-primed CD4+ T cells. Altered peptide ligand (APL) 6, APL 7, APL 9, APL 12 and APL 13 markedly suppress IL-17 production by glucose-6-phosphate isomerase (GPI)-primed CD4+ T cells. Mice were sacrificed on day 8 after immunization. CD4+ T cells were purified from draining lymph node cells of recombinant human GPI (hGPI)-immunized DBA/1 mice, and CD11c+ antigen-presenting cells (APCs) were purified from spleen cells. (a) CD4+ T cells primed with hGPI and APCs were co-cultured with 10 μM synthetic peptide for 72 hours. The supernatants were assayed for IL-17 by ELISA. P, positive control (hGPI325--339). (b) CD11c+ APCs were cultured with a suboptimal concentration GPI325--339 (3 μM) for 2 hours, washed twice to remove unbound peptides, and 30 μM each antagonistic APL was added. After 18 hours of culture, CD4+ T cells (2 × 104/well) were added, and after an additional 72 hours of culture, the supernatants were assayed for IL-17 by ELISA. The inhibition ratio was calculated as stated in Pre-pulse assay. Data presented as average ± standard deviation of three culture wells. *P < 0.05, **P < 0.01 (Mann--Whitney U test). Representative data of two independent experiments.