Flt3-ITD facilitates antigen-driven and homeostatic T cell proliferation. (A–C) MLRs were set up with CD11c⁺ splenocytes from B6xFVB Flt3^+/+ or Flt3^ITD/+ littermates (n = 3 per genotype) as stimulators and CTV-labeled allogeneic T cells from B6 FoxP3^GFP mice as responders. The fractions of GFP⁻ conventional (T conv) and GFP⁺ regulatory (T reg) responder T cells were analyzed 4 d later. Bars indicate mean values of three biological replicates (circles); representative of three independent experiments. *, P < 0.05; **, P < 0.01; ***, P < 0.001, using unpaired, two-tailed Student’s t test. (A) Absolute numbers of responder T cell populations from the MLRs above. (B) The proportion of responder T reg cells out of responder T cells from the MLRs above. (C) Representative staining profiles and fractions of proliferating responder T cells that have diluted the CTV dye in the MLRs above. (D and E) CTV-labeled lymphocytes from FoxP3^GFP mice were transferred into Rag1-deficient Flt3^+/+ or Flt3^ITD/+ mice. Adoptively transferred T cell populations were analyzed in the spleens of recipient animals 5 d later. (D) Numbers of GFP⁻ conventional (T conv) and GFP⁺ regulatory (T reg) T cells in 10–12 individual recipient mice (circles) per group and their averages (lines) pooled from two independent experiments. (E) Representative histograms of CTV dye levels in T conv cells, with the proportion of dye-diluting proliferative T cells indicated.