1. Department of Molecular Biology, Graduate School of Pharmaceutical Sciences, Hokkaido University Sapporo 060‐0812 Japan
2. CREST, Japan Science and Technology Corporation Saitama 332‐0012 Japan
3. Department of Immunology, Graduate School of Pharmaceutical Sciences, Hokkaido University Sapporo 060‐0812 Japan
4. College of Medical Technology, Hokkaido University Sapporo 060‐0812 Japan