CD4⁺CD25⁺ T cells from OVA-sensitized mice can inhibit CD4⁺CD25^– T cell–induced experimental asthma in Rag1^–/– mice. (A–F) Cotransfer of CD4⁺CD25⁺ and CD4⁺CD25^– spleen T cells into Rag1^–/– mice. CD4⁺CD25⁺ and CD4⁺CD25^– T cells were isolated from the spleens of OVA-sensitized and -challenged mice given IgG-control antibodies or anti–IL-6R antibodies. CFSE-labeled CD4⁺CD25^– spleen T cells from OVA-sensitized mice (5 × 10⁵; CSFE-labeled indicated with asterisks) were cotransferred i.p. with either 5 × 10⁵ unlabeled CD4⁺CD25^– T cells (A and D) or CD4⁺CD25⁺ T cells (B, C, E, and F) into immunocompromised Rag1 knockout mice. Cotransfer of CD4⁺CD25⁺ T cells from anti–IL-6R–treated (C and F) or IgG-treated (B and E) mice suppressed allergic airway inflammation induced by transfer of CD4⁺CD25^– T cells from OVA-sensitized mice. Magnification, ×100 (A–C), ×400 (D–F). Mice receiving only CD4⁺CD25^– T cells showed CFSE-positive cells in the lung (H), whereas mice receiving CD4⁺CD25⁺ T cells as well showed a marked decrease in the number of CFSE-positive cells (G and I), suggesting a CD4⁺CD25⁺ T cell–mediated suppression of effector CD4⁺CD25^– T cell proliferation. Results in G were obtained by calculating the average number of CFSE-positive cells per high power field (HPF) (n = 30). ***P = 0.001.