Abstract: Objective To investigate the pathological changes of liver and spleen of mice infected with Schistosoma japonicum and the changes of T follicular helper（Tfh） cells and surface molecules after praziquantel treatment. Methods Fifteen female C57BL/6 mice （6-8 weeks） were randomly assigned into the praziquantel treated infection group （treated group）, infection control group（untreated group） and uninfected group （n=5 in each group）. The mice in the treated group and untreated group were each infected with 20 S. japonicum cercariae through the abdominal skin, and mice in the treated group were further administered with intragastric praziquantel ［200 mg/（kg·d）］ at week 6 post-infection for 3 consecutive days. Mice were sacrificed at week 4 after treatment to observe the morphological changes of liver and spleen and calculate the worm reduction rate and the liver egg reduction rate. The Tfh cell to CD4+ T cell ratio, as well as the expression of inducible T-cell costimulator （ICOS） and programmed cell death protein 1（PD-1） on peripheral blood and spleen, were determined by flow cytometry. Schistosome soluble egg antigen（SEA） specific IgG antibodies in serum were detected by ELISA. Results The pathological changes of liver and spleen in the treated group were less severe compared with those of the untreated group, with a worm reduction rate of 84.1% and liver egg reduction rate of 69.1%. Flow cytometry showed that the percent of Tfh cells in peripheral blood and spleen was significantly higher in the treated group（14.7%-18.0%, 15.6%-25.0%） and the untreated group（13.7%-16.7%, 12.4%-18.2%） than that in the uninfected group（2.5%-6.8%, 4.9%-8.0%）, but there was no significant difference between the treated and untreated groups. The expression of ICOS in the peripheral blood and the spleen was significantly higher in untreated group（1.3%-3.2%, 4.1%-7.0%） than in the treated group（0.7%-1.1%, 1.8%-6.8%） and the uninfected group（0.2%-0.3%, 0.5%-0.8%）（P<0.01）, The expression of ICOS in the spleen was significantly higher in the treated group than in the uninfected group（P<0.01）, while this difference was not found for ICOS expression in the peripheral blood. The PD-1 expression in the peripheral blood and spleen was significantly higher in the untreated group（0.8%-1.9%, 4.1%-10.7%） than in the uninfected group（0.4%-0.8%, 1.2%-1.8%）（P<0.01）, while there was no significant difference between the treated group（0.5%-1.5%, 4.5%-8.9%） and the untreated group（P>0.05）. In addition, there was no significant difference in the level of SEA specific IgG between the treated group（2.015±0.061） and the untreated group（1.969±0.038） at 4 weeks after praziquantel treatment. Conclusion Praziquantel treatment can significantly alleviate the lesions of the liver and the spleen and decrease the ICOS expression by Tfh cells in the peripheral blood and spleen.

Key words: Schistosoma japonicum, Praziquantel, T follicular helper cells, Mouse