1. Department of Medicine, Division of Infectious Diseases, Massachusetts General Hospital, Cambridge, Massachusetts, USA
2. Department of Microbiology and Immunobiology, Harvard Medical School, Boston, Massachusetts, USA
3. Present address: Department of Pediatrics, Massachusetts General Hospital, Boston, Massachusetts, USA.
4. Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts, USA
5. Present address: Department of Anesthesiology, The Hardin Memorial Hospital Foundation, Inc., Elizabethtown, Kentucky, USA.