Mechanism of FLT3L-induced Treg proliferation is TCR independent. (A) PKH26-labeled CD4⁺Foxp3⁺ cells sorted from Foxp3EGFP reporter mice were incubated with WT BMDCs (filled histogram, left panel), MHC class II KO BMDCs (filled histogram, right panel), or alone (empty histogram) in presence of extra IL-2. Histograms show PHK26 level on CD4⁺Foxp3⁺ cells after 5 days of incubation. (B-G) WT mice were lethally irradiated and reconstituted with MHC class II KO bone marrow. (B) CD4 versus CD8 expression in thymus of MHC class II KO → WT chimera. (C) CD4 versus CD3 expression in spleen of MHC class II KO → WT chimera. (D,E) MHC II expression in the spleen of control or FLT3L-treated MHC class II KO → WT chimera (black line) versus WT control (gray line). (F,G) Percentages of Foxp3⁺ cells among CD4 T cell or absolute number of Foxp3⁺ cells in spleen of treated versus control MHC class II KO → WT chimera. Each histogram shows mean ± SEM. *P < .05.