Expression of CD39 on mouse Treg cells. (A) CD39 is constitutively expressed on CD4⁺CD25⁺ T cells. Lymphocytes isolated from naive mice were analyzed by FACS. CD39 expression is shown for CD4⁺CD25⁻ (left panel) and CD4⁺CD25⁺ subsets (right panel). Gray histogram represents isotype control (α-CD39L1, not expressed by lymphocytes). (B) CD39 is expressed by CD4⁺Foxp3⁺ cells. Mouse lymphocytes were stained for intracellular Foxp3 and gated for CD4⁺Foxp3⁻ and CD4⁺Foxp3⁺ cells (left panel). CD39 and CD73 expression for the 2 subsets are shown in costaining with α-CD25 (right panels). Numbers in end quadrant indicate percentage of total cells.