Collagen type II–specific type 1 regulatory T cells reduce the severity of ongoing collagen-induced arthritis. At day 28 after induction of arthritis, bovine collagen II–immunized DBA/1 mice received injections of either 1 or 3 × 106 collagen type II–specific type 1 regulatory T (Col-Treg) cells, and clinical and biological monitoring of arthritis were continued until day 56. (A) Graphed mean ± SEM data of arthritis severity scores from day 24 of saline-injected mice (closed circles; n = 7) or mice injected with 3 × 106 Col-Treg cells (closed triangles; n = 6) or 1 × 106 Col-Treg cells (open triangles; n = 6). *P < 0.05 (Mann–Whitney U test) indicates statistically significant difference in severity scores between mice injected with Col-Treg cells vs. saline buffer. (B) Graph describing disease incidence in each group of mice. (C) Bovine collagen II–specific total immunoglobulin G (IgG) levels in sera collected at the time the mice were killed. Values are expressed in arbitrary units (A.U) and represent the mean ± SEM. Similar results were obtained in three independent experiments. Asterisks represent statistically significant differences in IgG levels between arthritic mice and mice injected with saline buffer. Differences were analyzed by Mann–Whitney U test (*P < 0.05, **P < 0.005, ***P < 0.0005 with 95% confidence intervals).