Specific pathogen-free C57BL/6 (B6) and B6/Thy1.1 mice were purchased from the National Cancer Institute, Bethesda, MD. Foxp3 gfp mice on a B6 background were kindly provided by Dr. A. Rudensky (Sloan-Kettering Institute) and were bred to a Thy1.1 background. IL-12Rb2 2/2 mice on a B6 background were obtained from Dr. J. Harty (University of Iowa). 