1. Department of Pathology and Immunology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110, USA.
2. Howard Hughes Medical Institute, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110, USA.
3. Departments of Medicine and Molecular Microbiology, 660 South Euclid Avenue, St. Louis, MO 63110, USA.