Antigen-specific type 1 regulatory T cells dampen the proliferation of effector T cells. BALB/c mice received injections of carboxyfluorescein diacetate succinimidyl ester (CFSE)–labeled, ovalbumin (ova)-specific effector CD4+ cells on the day before subcutaneous immunization with a mixture of ovalbumin and incomplete Freund’s adjuvant. At day 5, either phosphate-buffered saline (PBS) (n = 7) or 1 × 106 ova-specific type 1 regulatory T (ova-Treg) cells (n = 10) were injected intravenously. The mice received ova injections into their hind paws. Two days afterward, the proliferation of CFSE+KJ1.26+ cells was analyzed by flow cytometry. (A) Representative staining results for KJ1.26+ effector T cells in the draining lymph nodes (DLNs) are graphed. (B) Representative CFSE dilution due to the proliferation of effector KJ1.26+ T cells was analyzed using FlowJo software (TreeStar, Ashland, OR, USA). Graph shows the same numbers of CD4+KJ1.26+ cells isolated from mice that received injections of saline (gray) or ova-Treg cells (bold). (C) Graphed numbers of KJ1.26+ proliferating cells in the DLNs. (D) CD90.1 congenic BALB/c mice received injections of CFSE-labeled, ova-specific effector CD4+ cells and 1 × 107 ova-Treg cells (n = 8) or PBS (n = 5). The numbers of ova-Treg cells (CD90.2+) in the DLNs are shown. Differences were analyzed by Mann–Whitney U test with 95% confidence intervals.