1. Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305, USA.
2. Department of Pediatrics, Stanford University School of Medicine, Stanford, CA 94305, USA.
3. Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA 94305, USA.
4. Department of Medicine, Stanford University School of Medicine, Stanford, CA 94305, USA.
5. Istituto Ricerche Immunobiologiche Siena–Chiron Vaccines, via Fiorentina 1, 53100 Siena, Italy.