1. Department of Pediatrics, University of California, San Francisco, CA 94143-0110, USA.
2. Department of Pathology and Department of Microbiology and Immunology, University of California, San Francisco, CA 94143, USA.
3. Howard Hughes Medical Institute, Department of Medicine and Department of Microbiology and Immunology, University of California, San Francisco, CA 94143, USA.
4. Infectious Diseases Laboratory, Salk Institute, San Diego, CA 92186, USA.