1. Department of Veterinary Microbiology, Graduate School of Agricultural and Life Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan
2. Department of Cell Regulation, Medical Research Institute, Tokyo Medical and Dental University, Bunkyo-ku, Tokyo 113-8510, Japan
3. Department of Virology, Nagoya University Graduate School of Medicine, 65 Tsurumai-cho, Showa-ku, Nagoya 466-8550, Japan
4. PRESTO, Japan Science and Technology Corporation, Tachikawa, Tokyo 190-0012, Japan