1. National Institute of Animal Health; Tsukuba Ibaraki 305 Japan
2. Department of Molecular Immunology, Faculty of Agriculture; The University of Tokyo; Bunkyo-ku Tokyo 113 Japan
3. Mitsui Pharmaceuticals Inc.; Mobara Chiba 297 Japan
4. Department of Biomedicine; Nihon University, Faculty of Agriculture and Veterinary Medicine; Fujisawa Kanagawa 251 Japan