Wild-type mice treated with anti-CD25 antibodies develop a more severe form of arthritis. In three experiments, wild-type DBA/1 mice were immunised on day 0 with collagen type II in complete Freund's adjuvant. From day 11 (c) or 13 (b) after immunisation onwards, mice were treated every second day with 0.25 mg of depleting anti-CD25 monoclonal antibody (N = 7) or with 0.25 mg control rat IgG (N = 7). (a) Depletion of the CD25+ cell population was checked in the blood twice a week by flow-cytometric analysis with anti-CD4 and anti-CD25 antibodies. A representative staining pattern on day 27 is shown. The percentages of CD4+CD25+ cells in control-treated mice (left plot) and anti-CD25-treated mice (right plot) are shown. (b, c) Cumulative incidence of arthritis (and mean day of disease onset) and the mean limb score of the arthritic mice in female (b) and male (c) wild-type mice treated with anti-CD25 or control IgG are shown (the maximum score per limb is 4). Error bars indicate SEM. The data from the female mice are representative of two independent experiments. The data of the three experiments were pooled and the percentage of limbs with each limb score on days 27 and 40 after immunisation is shown in (d). The mean limb score of the arthritic mice in the two groups is also indicated for the two time points and is significantly higher in the treated mice (P < 0.05; Mann–Whitney U-test) than in those receiving control IgG. (e, f) Representative pictures of the most severe case of collagen-induced arthritis on day 25 after immunisation of a mouse treated with anti-CD25 (e) and a mouse treated with control IgG (f). (g) Haematoxylin-stained paraffin section of the joint of an anti-CD25-treated mouse on day 42 after immunisation. Hyperplasia and infiltration of immunocompetent cells in the synovium (s) and pannus formation (p) that penetrates into the bone (b) can be seen. Note the presence of osteoclast-like multinucleated giant cells (arrow). *P < 0.05 for comparison with control IgG1-treated mice (Mann–Whitney U-test).