1. Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
2. Department of Immunology and Microbiology, National Defense Medical College, Namiki 3-2, Tokorozawa 359-8513, Japan
3. Department of Physiology, National Defense Medical College, Namiki 3-2, Tokorozawa 359-8513, Japan