1. Immunology Division and Division of Molecular Virology, Jichi Medical School, Tochigi-Ken 329-0498, Japan
2. Japanese Red Cross Saitama Blood Center, Saitama-Ken 338-0001, Japan
3. Institute of Swine Industry, Ibaraki Prefectural Livestock Research Center, Ibaraki-Ken 300-0508, Japan
4. Department of Medical Sciences, Toshiba General Hospital, Tokyo 140-8522, Japan