Suppressor activity and potency among regulatory T cells is discriminated by functionally active CD44

CD4CD25⁺ regulatory T cells are fundamental to the maintenance of peripheral tolerance and have great therapeutic potential. However, efforts in this regard have been hampered by limiting cell numbers in vivo, an anergic phenotype in vitro, and a rudimentary understanding of the molecular basis for the functional state of CD4CD25⁺ regulatory T cells (Treg cells). Here we show heterogeneity of suppressor activity among activated CD4CD25 ⁺ Treg cells and that, within this population, the functionally active, hyaluronan-binding form of CD44 (CD44^act) is strikingly correlated with superior suppressor activity. Within 16 hours after in vitro activation, CD44^act can discriminate enhanced suppressive function in in vitro proliferation assays and in an in vivo bone marrow engraftment model. The expression of other surface markers and that of Foxp3 are similar irrespective of hyaluronan binding and associated degree of suppressor potency. Furthermore, CD44^act is induced on resting CD4CD25⁺ cells in vivo by allogeneic stimulation, with similar functional consequences. These results reveal a cell-surface marker that delineates functional activity within a population of activated CD4CD25⁺ regulatory T cells, thereby providing a potential tool for identifying regulatory activity and enriching for maximal suppressor potency.