The mark of T reg cells

Regulatory T (T reg) cells found in the synovial fluid of inflamed joints can be identified by the coexpression of CD4, CD25, and CD27, according to Ruprecht and colleagues on page 1793.

Suppression of damaging T cell responses by CD4⁺ T reg cells is critical for the prevention of autoimmune disease. But studying these cells in the context of disease has been problematic, largely because CD4⁺ T reg cells and activated CD4⁺ T cells express many of the same surface molecules and are thus difficult to distinguish. Although coexpression of CD4 and the high affinity interleukin (IL)-2 receptor (CD25) identifies T reg cells in the circulation, CD25 cannot distinguish between T reg cells and local effector T cells, which up-regulate this...