GABA treatment modulates APC function. Mice were immunized with bCII peptide and given plain water or water with GABA. Nine days later, their popliteal lymph node T cells and splenic APC were purified. The purified T cells from control mice (C-T), or GABA treated (G-T), were co-cultured with purified splenic APC from control (C-APC) or GABA-treated (G-APC) mice and challenged with the indicated concentrations of bCII peptide to assess T-cell proliferation. Data shown are mean CPM ± SEM for each group of mice (N = 5 per group). The cells in medium alone had 580–820 CPM and stimulated with anti-CD3 had 22,000–28,000 CPM. *p < 0.05 vs. the C-T/G-APC group, #p < 0.05 vs. the G-T/C-APC group by Student’s t-test.