Expansion of peripheral NTreg compartment due to increased proliferation of Foxp3⁺ CD4 T cells. (A) Mice were thymectomized before FLT3L treatment (10 μg intraperitoneal injection daily for 10 days). Histograms show percentages of Foxp3⁺ among CD4 T cells (left) and absolute numbers (right) in the spleen of control or treated mice. Each bar shows mean ± SEM. **P < .01. (B,C) CD45.1 WT mice were irradiated (100 rad) before intravenous injection of 2 × 10⁶ sorted CD4⁺Foxp3⁻ cells from CD45.2 Foxp3EGFP reporter mice. After 2 weeks, mice were treated with FLT3L (10 μg intraperitoneal injection daily for 10 days) or left as control. (B) Absolute numbers of CD4⁺Foxp3⁺ cells in the spleen of control or treated mice. Histogram shows mean ± SEM. *P < .05. (C) Histograms show percentage of Foxp3⁺ cells within CD45.2⁺D4⁺ donor cell population in control or treated animals. (D) Histograms show percentages of Ki67⁺ cells among Foxp3⁺ (top panels) or Foxp3⁻ (bottom panels) CD4 T cells in control or treated animals. (E) Histograms show incorporation of BrdU in Foxp3⁺ (top panels) or Foxp3⁻ (bottom panels) CD4 T cells in control or treated animals pulsed with BrdU.