aNuffield Department of Clinical Medicine, John Radcliffe Hospital, Oxford OX3 9DU, UK; bMedical Research Council Human Immunology Unit, Institute of Molecular Medicine, Oxford OX3 9DS, UK; and cThe Caldecot Centre, King's College Hospital, London, SE5 9RS, UK.
Sponsorship: This work was funded by the Wellcome Trust (R.E.P., A.K.S., S.J.D.), EMBO (A.O.), the Royal Society (T.T.) and the Medical Research Council (D.A.P.).
Received: 8 September 2000; accepted: 12 September 2000.
*These authors contributed equally.