The frequency of antigen-specific T_regs in transplant recipients decreases immediately after HSCT. Donor mice (Thy1.2^+/+) with a 10-day established tumor burden received 2.5 × 10⁶ CD4⁺-enriched HA-specific T cells (Thy1.1⁺/1.2⁺). At 19 days after T-cell transfer, donor mice were killed, and their spleens and LNs were harvested and T-cell–enriched to be transferred into transplant recipients. Recipients (Thy1.1^+/+) were challenged with 1 × 10⁶ A20HA intravenously 10 days prior to HSCT and underwent transplantation as described in “Methods.” Half of the transplant recipients received daily injections of 10 μg/mouse IL-2 intraperitoneally. The frequency of HA-specific CD4⁺ T cells (Thy1.1⁺1.2⁺) expressing Foxp3 was determined by flow cytometry in the graft and in recipients killed 1 and 2 weeks after transplantation. Data represent means plus or minus SE.