1. Center of Infectious Diseases, West China Hospital of Sichuan University, Chengdu, Sichuan 610041, P.R. China
2. West China School of Medicine, Sichuan University, Chengdu, Sichuan 610041, P.R. China
3. Graduate Program in Cellular and Molecular Physiology, Graduate School of Biomedical Sciences, Tufts University School of Medicine, Boston, MA 02111, USA