1. Department of Virology, Research Institute for Microbial Diseases, Osaka University, Suita, Osaka 565-0871, Japan
2. Section of Immunology, Department of Medical Science, National Institute of Health, Nonthaburi 11000, Thailand
3. Department of Veterinary Science, National Institute of Infectious Diseases, Toyama, Shinjuku, Tokyo 162-8640, Japan