1. Section of Serology; Institute of Immunological Science, Hokkaido University; Kita-ku Sapporo, Hokkaido 060-0815 Japan
2. Department of Hematology and Oncology; Osaka University School of Medicine; Suita Osaka 565-0871 Japan
3. Department of Microbiology; Osaka University School of Medicine; Suita Osaka 565-0871 Japan
4. Department of Virology; Research Institute for Microbial Diseases, Osaka University; Suita Osaka 565-0871 Japan