[1] Sci Univ Tokyo, Res Inst Biol Sci, Div Mol Biol, Noda, Chiba 278, Japan
[2] JST, PREST, Noda, Chiba 278, Japan
[3] Univ Tokyo, Dept Vet Med, Bunkyo Ku, Tokyo 113, Japan
[4] Univ Tokyo, Dept Vet Surg, Bunkyo Ku, Tokyo 113, Japan
[5] Juntendo Univ, Sch Med, Dept Immunol, Bunkyo Ku, Tokyo 113, Japan