yijie guo department of pathogenic microbiology and immunology, school of basic medical sciences, xi’an jiao tong university, xi’an, china; graduate school of agricultural science, tohoku university, sendai, japan; department of pathogenic microbiology and immunology, school of basic medical sciences, xi’an jiao tong university, xi’an, china. tel: +86-2985267814, fax: +86-2985267814 cuiping ding department of pathogenic microbiology and immunology, school of basic medical sciences, xi’an jiao tong university, xi’an, china bo zhang graduate school of agricultural science, tohoku university, sendai, japan jun xu faculty of science and engineering, iwate university, iwate, japan meng xun department of pathogenic microbiology and immunology, school of basic medical sciences, xi’an jiao tong university, xi’an, china jiru xu department of pathogenic microbiology and immunology, school of basic medical sciences, xi’an jiao tong university, xi’an, china