1. Departments of Medicine and Immunology, Duke Human Vaccine Institute, Duke University School of Medicine, Durham, NC 27710, USA.
2. Department of Immunology and Microbiology, The Scripps Research Institute, La Jolla, CA 92037, USA.
3. Ragon Institute of MGH, MIT and Harvard, Cambridge, MA 02139, USA.
4. Vaccine Research Center, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20014, USA.