1. *Program in Immunology, Department of Pathology, Tufts University School of Medicine, Boston, MA 02111;
2. †Howard Hughes Medical Institute and Department of Genetics, University of Pennsylvania School of Medicine, Philadelphia, PA 19704;
3. ‡Department of Pathology, School of Medicine, University of New Mexico, Albuquerque, NM 87131; and
4. §Trudeau Institute, Saranac Lake, NY 12983