IRXR4204 stabilizes Tregs. (A-D) Lethally irradiated BALB/c recipients were infused with 10⁷ CD45.1⁺ B6 NTCD BM cells, along with 0.9 × 10⁶ CD45.1⁺ CD25- T cells and 0.45 × 10⁶ flow cytometry–sorted RFP⁺ eGFP⁺ Tregs (A-B) from B6 CD45.2⁺ Rosa-RFP × Foxp3^{eGFP-Cre-ERT2} reporter mice. Recipients were treated with either vehicle or IRX4204. On day 18 after transplantation, the frequency of ex-Tregs (RFP⁺ eGFP⁻) was analyzed in SIs (A) and SPLs (B). (A-B) Representative lineage tracing of Tregs in vehicle- vs IRX4204-treated mice by flow cytometry. Cells depicted are gated on donor CD45.2⁺ CD4⁺ T cells. Bar graphs indicate the frequency of ex-Tregs. (C-D) SI LPLs and splenocytes and were restimulated with phorbol 12-myristate 13-acetate and ionomycin to determine the frequency of IFN-γ– and IL-17– producing donor CD45.2⁺ Tregs. One experiment was performed. **P < .01.