T-cell proliferative response of healthy controls and rheumatoid arthritis patients stimulated with peptides. Peripheral blood mononuclear cells (PBMC) from 20 rheumatoid arthritis (RA) patients and six healthy controls were incubated with 0, 3 or 30 μg/ml citrullinated (cit) and native peptides or 4 Lfi/ml tetanus toxoid, as shown, for 5 days. T-cell proliferation was assessed by uptake of [3H]thymidine. Each dot represents one individual. ***P < 0.001 for RA patients and **P < 0.01 for healthy controls comparing multiple means (Kruskal-Wallis test). **P < 0.01 comparing healthy controls and RA patient PBMC for the response to tetanus toxoid (Mann-Whitney test). *P < 0.05 comparing RA patients' responses to citrullinated and native aggrecan peptides (Mann-Whitney test). SI, stimulation indices.