Several lines of evidence indicate the instability of CD4+FoxP3+ regulatory T cells (Tregs). We have therefore investigated means of promoting the stability of Tregs. In this study, we found that the proportion of Tregs in mouse strains deficient in TNFR2 or its ligands was reduced in the thymus and peripheral lymphoid tissues, suggesting a potential role of TNFR2 in promoting the sustained expression of FoxP3. We observed that upon in vitro activation with plate-bound antiCD3 Ab and soluble anti-CD28 Ab, FoxP3 expression by highly purified mouse Tregs was markedly down-regulated. Importantly, TNF partially abrogated this effect of TCR stimulation and stabilized FoxP3 expression. This effect of TNF was blocked by anti-TNFR2 Ab, but not by anti-TNFR1 Ab. Furthermore, TNF was not able to maintain FoxP3 expression by TNFR2- deficient Tregs. In mouse colitis model induced by transfer of naïve CD4 cells into Rag1−/− mice, the disease could be inhibited by co-transfer of WT Tregs, but not by co-transfer of TNFR2- deficient Tregs. Furthermore, in the lamina propria of the colitis model, the majority of WT Tregs maintained FoxP3 expression. In contrast, increased number of TNFR2-deficient Tregs lost FoxP3 expression. Thus, our data clearly show that TNFR2 is critical for the phenotypic and functional stability of Treg in the inflammatory environment. This effect of TNF should be taken into account when designing future therapy of autoimmunity and GVHD by using TNF inhibitors.