Total Treg, CD25int mTreg, and naive Treg showed equal suppressive function after expansion in DMSO (Figure 4A); however, suppressive function of naive Treg expanded in TAC appeared to be partially abrogated compared with CD25int mTreg, when lower ratios of Treg-Teff were examined (Figure 4B; 1:16 ratio P < 0.05). When analyzing the suppressive capacity of Treg generated from 6 donors independently, a clear trend was observed that TAC reduced suppression of the naive Treg subpopulation in a larger proportion of donors compared with the CD25int mTreg cell subpopulation (2-sided Fisher exact test: P = 0.08). Tacrolimus impaired suppressive capacity of CD25int mTreg in only 1 of 6 donors (Figure 1A, SDC, http://links.lww.com/TP/B286); in contrast, in 4 of 5 donors, naive Treg showed reduced suppressive activity after expansion with TAC (sufficient numbers of naive Treg expanded with TAC could not be obtained from 1/6 donors) (Figure 1B, SDC, http://links.lww.com/TP/B286). An examination of the ratio of suppression by Treg expanded in DMSO and TAC showed that the naive Treg population was more susceptible to TAC mediated reduction of suppression than the CD25int mTreg population (Figure 4C).