1. Division of Immune Cell Biology, MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK.
2. Division of Molecular Immunology, MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK.
3. Department of Experimental Pathology, Institute for Frontier Medical Sciences, Kyoto University, Kyoto 606-8507, Japan.