Mice PD-1-KO-N10 (strain: BALB/cJ) mice were kindly provided by Prof. T. Honjo (Department of Immunology and Genomic Medicine, Kyoto University, Japan). The WT control mice were purchased from the Animal Center of Beijing University School of Medicine. All mice were maintained in micro-isolator cages and housed in the animal colony at the Animal Center, Third Military Medical University, standard laboratory chow diet and water was supplied ad libitum. Mice were used in experimental analysis at age of six weeks and at an average weight of 17 g (range: 16,18 g).