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.
In addition to CD4 and CD25, Tregs are routinely characterized by their expression of the forkhead transcription factor FOXP3.
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.
Recently research has uncovered CD39 as an important Treg marker. The CD4+CD25hiCD39+ subset of Tregs are believed to be implicated in the suppression of IL-17 production. (Fletcher, J et al 2009).
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.
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