1. Howard Hughes Medical Institute, Departments of Medicine and of Microbiology and Immunology, University of California, San Francisco, CA 94143, USA.
2. Department of Pediatrics, University of California, San Francisco, CA 94143, USA.
3. Department of Pediatrics, University of Minnesota, Minneapolis, MN 55455, USA.
4. Department of Laboratory Medicine, University of California, San Francisco, CA 94143, USA.
5. Departments of Pathology and of Microbiology and Immunology, University of California, San Francisco, CA 94143, USA.