Schistosomiasis is a zoonotic parasitic disease. Schistosoma japonicum eggs deposited in the liver tissue induce egg granuloma formation and liver fibrosis, seriously threatening human health. Natural killer (NK) cells kill activated hepatic stellate cells (HSCs) or induce HSC apoptosis and inhibit the progression of liver fibrosis. However, the function of NK cells in liver fibrosis caused by S. japonicum infection is significantly inhibited. The mechanism of this inhibition remains unclear. Twenty mice were percutaneously infected with S. japonicum cercariae. Before infection and 2, 4, 6, and 8 weeks after infection, five mice were euthanized and dissected at each time point. Hepatic NK cells were isolated and transcriptome sequenced. The sequencing results showed that Tigit expression was high at 4–6 weeks post infection. This phenomenon was verified by reverse transcription quantitative PCR (RT-qPCR) and flow cytometry. NK cells derived from Tigit^-/- and wild-type (WT) mice were co-cultured with HSCs. It was found that Tigit^-/- NK cells induced apoptosis in a higher proportion of HSCs than WT NK cells. Schistosomiasis infection models of Tigit^-/- and WT mice were established. The proportion and killing activity of hepatic NK cells were significantly higher in Tigit^-/- mice than in WT mice. The degree of liver fibrosis in Tigit^-/- mice was significantly lower than that in WT mice. NK cells were isolated from Tigit^-/- and WT mice and injected via the tail vein into WT mice infected with S. japonicum. The degree of liver fibrosis in mice that received NK cell infusion reduced significantly, but there was no significant difference between mice that received NK cells from Tigit^-/- and WT mice, respectively. Our findings indicate that Tigit knockout enhanced the function of NK cells and reduced the degree of liver fibrosis in schistosomiasis, thus providing a novel strategy for treating hepatic fibrosis induced by schistosomiasis.

Background and Aims: Schistosomiasis japonica is a widespread human zoonotic disease, and in China, there are many patients with schistosomiasis suffering from liver fibrosis. Many studies have shown that natural killer (NK) cells could reduce the progression of hepatic fibrosis by directly killing hepatic stellate cells (HSCs). However, NK cells could not inhibit the progress of liver fibrosis induced by Schistosoma japonicum infection. We aimed to investigate the function of NK cells in schistosomiasis. Methods: BALB/c mice were infected with S. japonicum cercariae. The receptors and their proportions expressed on NK cells in the liver and spleen from infected mice were detected using flow cytometry. Levels of IFN-γ, perforin, and granzyme of NK cells, and collagen I, III, and α-SMA of hepatic tissue, were detected using quantitative real-time PCR. Changes in cytokine levels in sera were detected using a cytometric bead array. Liver fibrosis was evaluated using hematoxylin and eosin and Masson staining. NK function in the schistosomiasis model was analyzed. Results: From 2 to 4 weeks post-infection, NK cells were activated, with significantly increased levels of effector molecules (IFN-γ, perforin, and granzyme) that peaked at 4 weeks after infection. The proportion of NK cells increased in the liver and spleen from 6 to 10 weeks post-infection. However, the function of NK cells was inhibited from 6 to 10 weeks post-infection with significantly decreased levels of activated receptors (AR), inhibitory receptors (IR), and effector molecules. The levels of IFN-γ, IL-12, and IL-6 in mouse serum peaked at 6 weeks post-infection, and IL-10 and IL-21 levels peaked at 8 weeks post-infection. Hepatic fibrosis markers increased significantly at 6 weeks after infection. Conclusion: Our study suggested that NK cells were activated from 2 to 4 weeks post-infection and participated in inflammation in the mouse model. After the S. japonicum laid their eggs, NK cells became inhibited, with decreased levels of both activating and inhibitory NK cell receptors, as well as cytotoxic molecules. In addition, liver fibrosis formed. In mice infected with S. japonicum, the process of liver fibrosis might be alleviated by removing the functional inhibition of NK cells.

Background: The main symptoms of schistosomiasis are granuloma and fibrosis, caused bySchistosomaeggs. Numerous types of cells and cytokines are involved in the progression ofSchistosomainfection. As a class of innate immune cells, γδ T cells play critical roles in the early immune response. However, their role in modulating granuloma and fibrosis remains to be clarified.Methods: Liver fibrosis in wild-type (WT) mice and T cell receptor (TCR) δ knockout (KO) mice infected withSchistosoma japonicumwas examinedviaMasson’s trichrome staining of collagen deposition and quantitative reverse transcriptase-PCR (RT-PCR) of fibrosis-related genes. Granuloma was detected by hematoxylin-eosin (H&E) staining and quantified. Flow cytometry was used for immune cell profiling and for detecting cytokine secretion. The abundance of the related cytokines was measured using quantitative RT-PCR.Results: The livers ofS. japonicum-infected mice had significantly increased proportions of interleukin (IL)-17A producing γδ T cells and secreted IL-17A. Compared with the WT mice, TCR δ deficiency resulted in reduced pathological impairment and fibrosis in the liver and increased survival in infected mice. In addition, the profibrogenic effects of γδ T cells in infected mice were associated with enhanced CD11b⁺Gr-1⁺cells, concurrent with increased expression of transforming growth factor (TGF)-β in the liver.Conclusions: In this mouse model ofSchistosomainfection, γδ T cells may promote liver fibrosis by recruiting CD11b⁺Gr-1⁺cells. These findings shed new light on the pathogenesis of liver pathology in murine schistosomiasis.

Background: CD4⁺T helper (Th) cells play critical roles in both host humoral and cellular immunity against parasitic infection and in the immunopathology of schistosomiasis. T follicular helper (Tfh) cells are a specialized subset of Th cells involved in immunity against infectious diseases. However, the role of Tfh cells in schistosome infection is not fully understood. In this study, the dynamics and roles of Tfh cell regulation were examined. We demonstrated that granulocytic myeloid-derived suppressor cells (G-MDSC) can suppress the proliferation of Tfh cells.Methods: The levels of Tfh cells and two other Th cells (Th1, Th2) were quantitated at differentSchistosoma japonicuminfection times (0,3, 5, 8, 13 weeks) using flow cytometry. The proliferation of Tfh cells stimulated by soluble egg antigen (SEA) and soluble worm antigen (SWA) in vivo and in vitro were analyzed. Tfh cells were co-cultured with MDSC to detect the proliferation of Tfh cells labelled by 5(6)-carboxyfluorescein diacetateN-succinimidyl ester. We dynamically monitored the expression of programmed cell death protein 1 (PD-1) on the surface of Tfh cells and programmed cell death ligand 1 (PD-L1) on the surface of MDSC at different infection times (0, 3, 5, 8 weeks). Naïve CD4⁺T cells (in Tfh cell differentiation) were co-cultured with G-MDSC or monocytic MDSC in the presence, or in the absence, of PD-L1 blocking antibody.Results: The proportion of Tfh cells among CD4⁺T cells increased gradually with time ofS. japonicuminfection, reaching a peak at 8 weeks, after which it decreased gradually. Both SEA and SWA caused an increase in Tfh cells in vitro and in vivo. It was found that MDSC can suppress the proliferation of Tfh cells. The expression of PD-1 on Tfh cells and PD-L1 from MDSC cells increased with prolongation of the infection cycle. G-MDSC might regulate Tfh cells through the PD-1/PD-L1 pathway.Conclusions: The reported study not only reveals the dynamics of Tfh cell regulation duringS. japonicuminfection,but also provides evidence that G-MDSC may regulate Tfh cells by PD-1/PD-L1. This study provides strong evidence for the important role of Tfh cells in the immune response toS. japonicuminfection.Graphical

Schistosomiasis, which is caused by infection with Schistosoma spp., is characterized by granuloma and fibrosis in response to egg deposition. Pattern recognition receptors are important to sense invading Schistosoma, triggering an innate immune response, and subsequently shaping adaptive immunity. Cyclic GMP-AMP synthase (cGAS) was identified as a major cytosolic DNA sensor, which catalyzes the formation of cyclic GMP-AMP (cGAMP), a critical second messenger for the activation of the adaptor protein stimulator of interferon genes (STING). The engagement of STING by cGAMP leads to the activation of TANK-binding kinase 1 (TBK1), interferon regulatory factor 3 (IRF3), and the subsequent type I interferon (IFN) response. cGAS is suggested to regulate infectious diseases, autoimmune diseases, and cancer. However, the function of cGAS in helminth infection is unclear. In this study, we found that Cgas deficiency enhanced the survival of mice infected with S. japonicum markedly, without affecting the egg load in the liver. Consistently, Cgas deletion alleviated liver pathological impairment, reduced egg granuloma formation, and decreased fibrosis severity. In contrast, Sting deletion reduced the formation of egg granulomas markedly, but not liver fibrosis. Notably, Cgas or Sting deficiency reduced the production of IFNβ drastically in mice infected with S. japonicum. Intriguingly, intravenous administration of recombinant IFNβ exacerbated liver damage and promoted egg granuloma formation, without affecting liver fibrosis. Clodronate liposome-mediated depletion of macrophages indicated that macrophages are the major type of cells contributing to the induction of the type I IFN response during schistosome infection. Moreover, cGAS is important for type I IFN production and phosphorylation of TBK1 and IRF3 in response to stimulation with S. japonicum egg- or adult worm-derived DNA in macrophages. Our results clarified the immunomodulatory effect of cGAS in the regulation of liver granuloma formation during S. japonicum infection, involving sensing schistosome-derived DNA and producing type I IFN. Additionally, we showed that cGAS regulates liver fibrosis in a STING-type I–IFN-independent manner.