1. Nuffield Department of Clinical Medicine, John Radcliffe Hospital, University of Oxford, Oxford OX3 9DU, United Kingdom
2. Infectious Disease Unit and AIDS Research Center, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02129
3. Medical Research Council Human Immunology Unit, John Radcliffe Hospital, University of Oxford, Oxford OX3 9DU, United Kingdom
4. Gastrointestinal Unit, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02129