Preliminary analysis demonstrated a large variation in phenotype, suppressive function, expansion, and susceptibility to TAC-induced alteration of Treg function between CD4+CD25+CD127−/lo cells (total Treg cells) isolated from different healthy human donors (data not shown). It is possible that heterogeneity in total CD4+CD25+CD127−/lo Treg cells between donors accounted for these variations. Further experiments were therefore performed by costaining with CD45RA in an attempt to define a more homogeneous subpopulation of Treg present consistently in different donors. Total Treg cells were sorted into naive Treg cell, CD25int mTreg cells, and CD25hi mTreg cells as described in the Methods (Figure 1A).