Department of Vaccine Immunology, Graduate School of Medicine, Hokkaido University, Kita 15, Nishi 7, Kita-ku, Sapporo 060-8638, Japan; Nebuta Research Institute for Life Sciences, and Center for Brain and Health Sciences, Aomori University, Kohbata 2-3-1, Aomori, 030-0943, Japan. Electronic address: seya-tu@pop.med.hokudai.ac.jp.