1. Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA 02115, USA.
2. Vaccine Research Center, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
3. Institute of Human Genetics, Biozentrum, Am Hubland, D-97074 Würzburg, Germany.
4. Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02115, USA.