1. D. F. Tough and J. Sprent, Department of Immunology, IMM4, Scripps Research Institute, 10666 North Torrey Pines Road, La Jolla, California 92037, USA.
2. P. Borrow, Department of Neuropharmacology, IMM6, Scripps Research Institute, 10666 North Torrey Pines Road, La Jolla, California 92037, USA.