Anti-CD25-specific mAbs promote tumour immunity through binding to CD25+ regulatory cells. (a) Percentage of CD25+ cells in anti-CD25 mAb-treated mice as a proportion of CD25+ cells in control mice. Mice treated twice with 1 mg of anti-CD25 mAbs were bled at intervals and stained with FITC-conjugated anti-CD25 antibodies (7D4, Pharmingen). CD25+ cells in the blood of a group of three mice treated with anti-CD25 mAbs was calculated as a percentage of the mean of CD25+ cells present at the same time point in a group of 3 mice treated with an isotype control antibody. (b) Anti-CD25 mAb levels in the serum of antibody-treated mice. Sera, collected from the blood of untreated or anti-CD25 mAb-treated mice, were used to stain CD4+ cells purified from the spleen of a naive B6 mouse. The percentage of total CD4+CD25+ cells that could be stained using serum collected from an untreated mouse (A), a mouse treated with antibody 11 and 13 days previously (B) and from a mouse treated with antibody 19 and 21 days previously (C) is shown. (c) Tumour rejection in mice with undetectable serum levels of anti-CD25 antibodies. Mice were treated with either anti-beta-gal mAbs (solid line, n=10) or anti-CD25 mAbs (dashed line, n=20) as described previously. Twenty-one days later mice with undetectable levels of anti-CD25 mAbs in their serum were injected with 2 x 10⁴ B16F10 cells and tumour growth was subsequently monitored. Using a chi-squared test the effect of prior treatment with CD25-specific mAbs on tumour growth was significant (P<0.01).