The E3 deubiquitinating enzyme ubiquitin-specific proteolytic enzyme 21 (USP21) plays vital roles in physiological activities and is required for Treg-cell-mediated immune tolerance. Using a murine model infected with Schistosoma japonicum, we observed that there were more cercariae developed into adults and more eggs deposited in the livers of the USP21fl/flFOXP3Cre (KO) mice. However, immunohistochemistry showed that the degree of egg granuloma formation and liver fibrosis was reduced. In USP21fl/flFOXP3Cre mice, levels of IFN-gamma, IL-4, anti-soluble egg antigen (SEA) IgG and anti-soluble worm antigen preparation (SWAP) IgG increased in blood, as determined using ELISAs and multiplex fluorescent microsphere immunoassays, while the levels of IL-10, lL-17A, IL-23, IL-9, and anti-SEA IgM decreased. In addition, the levels of the USP21 protein and mRNA in the liver and spleen of KO mice decreased. We further observed increased Th1 responses amplified by Tregs (regulatory T cells) and compromised Th17 responses, which alleviated the liver immunopathology. We speculated that these changes were related to polarization of Th1-like Tregs. Our results revealed the roles of USP21 in Treg-cell-mediated regulation of immune interactions between Schistosoma and its host. USP21 may have potential for regulating hepatic fibrosis in patients with schistosomiasis.

34USP21, an E3 de-ubiquitin enzyme playing vital roles in 35 physiological activities, is important for Treg cells to maintain immune 36 homeostasis and control immune tolerance. To understand its diverse 37 functions and potential mechanism is essential for disease development. 38 We, using the USP21 gene-conditional knock-out mice model of 39 Schistosoma Japonicum infection, found more cercariae developed into 40 adults, and more eggs deposited in the liver in KO mice. However, 41 immunohistochemistry showed the degree and the area of egg 42 granuloma and liver fibrosis were both reduced. This suggested 43 knock-out USP21 did affect the immunoregulation between 44 schistosomes and the host. In KO mice the content of IFN-gamma and 45 IL-4, and the expression of anti-SEA IgG and anti-SWAP IgG both 46 increased in the liver, spleen and blood by flow 47 cytometry , while the content of IL-10, lL-17A, IL-23, IL-9 and the 48 expression of USP21 and anti-SEA IgM decreased. This indicated 49 USP21-knockout-Tregs promoted both Th1-type and Th2-type 50 immunity and inhibited other immunities during schistosomes infection, 51 which disordered the host immunity. This study revealed the 52 immunomodulatory of USP21 and preliminarily suggests it might be 53 essential to regulate the complex immune network between the host and 54 schistosomes. USP21 provides a new possible target for schistosomiasis 55 treatment in the future . 56 57 Author summary 58 Schistosomiasis is a common neglected tropical disease that 59 affects more than 230 million people worldwide. Therefore, the study on 60 the mechanism of immune interaction between schistosomas and the host 61 is not only helpful for the understanding of immune homeostasis, but also 62 helpful for the further development of the treatment of schistosomiasis. 63 Ubiquitin Specific Protease 21(USP21) has been shown to be involved in 64 the regulation of many biological processes, such as maintaining immune 65 homeostasis and regulating cell growth. Here, we observed that the 66 specific deletion of USP21 led to the decrease of mice's ability to resist 67 schistosomes infection and promoted the survival of schistosomes. It 68 was also proved that unstable regulatory T cells would produce 69 polarization phenomenon and promote differentiation to helper T cells, 70 which would lead to disorder of immune response in mice. However, this 71 process reduced the serious immune pathological damage caused by egg 72 granuloma. Our findings reveal that USP21 may be an important 73 molecule regulating immune interaction between Schistosoma japonicum 74 and the host. 75 76 Introduction 77 Schistosomiasis is an acute and chronic parasitic disease caused 78 by blood flukes (trematode worms) of the genus Schistosoma. According 79 to the WHO Report 2017, Schistosomiasis transmission was reported 80 from 78 countries, and at least 220.8 million people required preventive 81 treatment in 2017. The estimates of death due to Schistosomiasis 82 varies between 24 072 and 200 000 globally per yea...