Expansion of the Treg compartment in untreated ADA^−/− mice. (A) Anti-CD3 and FoxP3 immunohistochemistry in thymi of 3-week-old ADA^+/+ and ADA^−/− mice (original magnification ×10). (B) TaqMan gene expression analysis for CD4 and FoxP3 on total thymi of ADA^−/− (n = 6) mice compared with ADA^+/+ (n = 6). ADA^+/+ gene expression was arbitrarily considered 1 as represented by the dashed line. Data are mean ± SD. (C) Representative FACS analysis for CD4⁺CD25⁺FoxP3⁺CTLA4⁺CD45RB^low Tregs in ADA^+/+ and ADA^−/− PB. (D) Percentage of CD25⁺FoxP3⁺CTLA4⁺CD45RB^low Tregs in CD4⁺ compartment in PB, lymph nodes (LN), thymus, and spleen of ADA^+/+ (n = 11) and ADA^−/− (n = 11) mice. Data are mean ± SD. **P = .005-.0005. ***P ≤ .0005. (E) ADA enzymatic activity in ADA^+/+ CD4⁺CD25⁻ T effector (n = 3) and CD4⁺CD25⁺ T regulatory cells (n = 3). Data are mean ± SD. **P = .008.