Over a decade ago scientists discovered a population of suppressive CD4+ T cells expressing CD25. These cells seemed to have the ability to suppress T cell proliferation and cytokine production, thus providing protection from autoimmunity, transplant rejection, graft-versus-host disease, allergy, and tissue damage during infection. Conversely, these cells can also enable tumor cells to evade the host immune response. These specialized T cell populations are now known as Tregs.
Although these three markers are important in Treg identification, activated T cells also up-regulate CD25 during immune responses associated with infection, which causes difficulties in discriminating between Treg and activated T effector cells. Thus, Treg identification is often achieved using multiple antbodies.
NB: FOXP3 is a nuclear protein requiring cell fixation and permeabilization for detection by flow cytometry - Leucoperm™ cell permeabilization reagent is recommended for this purpose. Therefore, while FOXP3 is an important lineage marker it does not facilitate Treg purification for functional studies.