Abstract:

Objective  To investigate the change of Treg/Th17 cell ratio in mice infected with Plasmodium yoelii 17XL in the early stage of infection.  Methods  Forty 6-8 week-old female BALB/c mice were randomly divided into infection group and Treg blockade group. All the mice were infected by intraperitoneal injection with 1×10⁶ P. yoelii 17XL-parasitized erythrocytes. Mice in Treg blockade group were intraperitoneally injected with 1 mg CD25 mAb before infection and on day 1 post-infection， respectively. Three healthy mice served as control. Tail vein blood samples were collected in two groups each with 10 mice during day 1-7 post-infection. The parasitemia was monitored daily by examining Giemsa-stained thin film. The survival rate of infected mice was calculated.  Before infection, on day 3 and 5 post-infection， the spleen was aseptically removed from each mouse in infection group （n=3） and Treg blockade group （n=3）， and splenocyte suspensions were prepared. The proportion of Treg and Th17 cells were analyzed by flow cytometry. The ratio of Treg/Th17 cells was calculated and the relative analysis of the percentage of Treg and Th17 cells was conducted.  Results  Parasitemia appeared on day 2 post-infection in infection group. On day 5， the infection rate of P. yoelii in erythrocytes increased to （49.8±4.7）% with a survival rate of 60.0%. All mice died on day 6 post-infection. In Treg blockade group， P. yoelii 17XL-parasitized erythrocytes appeared on day 2 post-infection; the parasitemia was（10.0±3.5）% on day 5， and increased rapidly to 60.0% on day 7; all mice died on day 8. The percentages of Treg and Th17 cells reached（11.1±1.3）% and （16.7±3.3） % respectively on day 3 post-infection in infection group， and increased to（26.1±4.5）% and （19.6±2.6） % on day 5， considerably higher than that of the control（P＜0.01）. The ratio of Treg/Th17 cells was 0.7±0.1， 1.3±0.4 on day 3 and 5 post-infection in infection group， respectively， which was much lower than that of normal control （2.1±0.5）（P＜0.01， P＜0.05）. On day 3 and 5 post-infection， the percentages of Treg cells in CD4+ T cells was（2.4±1.3）% and （4.3±2.9）%， respectively, and that of Th17 cells reached （26.1±2.2）% and （6.5±2.1）% in Treg blockade group. The ratio of Treg/Th17 cells in Treg blockade group was 0.1±0.1 and 0.7±0.4， respectively， which was considerably lower than that of the control（3.6±0.4）（P＜0.01）. The proportion of Treg cells in CD4+ T cells was positively correlated with that of Th17 cells（r=0.8166， P＜0.01）.  Conclusion  The ratio of Treg/Th17 cells significantly decreases in the early stage of P. yoelii 17XL infection.

Key words: Plasmodium yoelii, Ratio of Treg/Th17 cells, Early infection