1. Department of Virology, Research Institute for Microbial Diseases; Osaka University; Suita Osaka 565-0871 Japan
2. Department of Molecular Immunology, School of Agricultural and Life Sciences; University of Tokyo; Bunkyo-ku Tokyo 113-8657 Japan
3. Department of Environmental Information and Bio-production Engineering, Faculty of Agriculture; Kobe University; Kobe Hyogo 657-8501 Japan