The realization that T reg cells have a unique surface expression profile incorporating CD25, CD62L and specific CD45 isoforms 4- 6, together with the identification of the T reg cell–specific transcription factor Foxp3, catapulted T reg cells from a rare CD4 + T cell subset to what many regard as ‘master regulators’ of immune homeostasis 7- 9. Fortunately, as biochemistry and the molecular biology revolution rescued the field of cytokine biology by identifying genes and biochemical structures tied to the varied biologic activities, the identification of a constellation of cell surface, transcriptional and biochemical markers that uniquely mark ‘regulatory’ T cells (T reg cells) has made possible a rebirth of the suppressor T cell field over the past decade. This situation was not unlike the early years of cytokine biology, during which dozens of activities were found in sera and cell supernatants without consistent molecular or biochemical ‘signatures’. However, that phenomenology was cast into doubt as various labs presented unique, hard-to-reproduce systems, each with complexities and idiosyncrasies that raised credibility issues. For almost 40 years, immunologists have postulated the existence of suppressor T cells that police the immune system to avert unwanted immune responses 1- 3. However, there has long been a belief that the immune system must have peripheral mechanisms in place to deal with immune cells that ‘escape’ central tolerance. The first line of such ‘self-tolerance’ is the elimination of self-reactive T lymphocytes and B lymphocytes during negative selection in the thymus and bone marrow, respectively. Armed with the potential to destroy invading microorganisms and stop aberrant outgrowth of tumor cells, the immune system has extensive built-in mechanisms for preventing attack of healthy self tissues.

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