Mice. Specific-pathogen-free C57BL/6 (B6) (C57BL/6NTac) and B6.SJL-Ptprca/BoyAiTac mice, which are C57BL/6 mice that express the congenic CD45.1 allele of the Ptprc (protein-tyrosine-phosphatase, receptor type c locus) gene, were purchased from Taconic Laboratories (Germantown, NY). Breeding pairs of the Stat4 Ϫ/Ϫ mice (13) on the B6 background were a gift from Mark H. Kaplan of Indiana University, B6 IfngR Ϫ/Ϫ mice (25) with control WT mice were purchased from the Jackson Laboratories (Bar Harbor, ME), and Ifnar Ϫ/Ϫ mice (24) on the B6 background were a gift from Murali-Krishna Kaja of the University of Washington. All mice except B6.SJL mice expressed the CD45.2 allele. Genetically mutated mice were bred and maintained in isolation facilities at Brown University through brother-sister mating. All mice used in experiments were 5 to 12 weeks old. Animals obtained from sources outside Brown University were housed in the animal care facility for at least 1 week before use. Handling of mice and experimental procedures were conducted in accordance with institutional guidelines for animal care and use.