1. Partners AIDS Research Center, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129
2. Division of Infectious Diseases, The Children's Hospital, Boston, Massachusetts 02115
3. Oxford Transplant Centre, Churchill Hospital, Oxford OX3 7LJ, United Kingdom
4. Laboratoire d'Immunologie, Institut de Recherches Cliniques de Montreal, Montreal, Quebec H2W 1R7, Canada