Breakdown Of Liver Tolerance And HBV-induced Hepatocellular Carcinoma

Hepatitis B virus(HB V)is widely recognized to induce chronic inflammation,cirrhosis,and eventually hepatocellular carcinoma(HCC).Virologists attribute HBV-mediated hepatocarcinogenesis to the integration of the viral DNA into the host DNA and the oncoprotein regulatory X protein(HBx).However,it has been increasingly accepted that HBV virus is a non-cytopathic virus and HBV pathogenesis lies mostly in its immune-mediated liver injury during the process of clearing HBV,which triggers the development of HCC.The liver is a unique immune organ that favors the induction of immune tolerance rather than immune activation.HBV carriers in clinic and HBV transgenic mice doesn't get hepatitis due to the facts that their hepatic CD8+ T cells express numerous co-inhibitory receptors.When the liver tolerance is broken,immune cells acquire cytotoxic capacity and promote the hepatic inflammatory development.The co-inhibitory receptor TIGIT(T cell immunoglobulin and immune receptor tyrosine-based inhibitory motif domain)is a characteristic marker of exhausted CD4+T and CD8+ T cells in tumor tissue.Moreover,it has been demonstrated that TIGIT is highly expressed on activated T cells.It could inhibit T cell functions after engagement with its ligand CD 155 on antigen-presenting cells.Our laboratory has already found that TIGIT is highly present on hepatic T cells.We are wondering whether TIGIT also contributes to HBV-mediated immune tolerance maintainance and whether TIGIT blockade could break liver tolerance and then mediate the development of HBV-related HCC.We have got several results listed as follow:1 TIGIT blockade or TIGIT deficiency leads to chronic hepatitis in HBs-tg mice.After TIGIT blockade,HBs-tg mice developed chronic hepatitis,as evidenced by gradually increased ALT levels that were significantly higher than those in rat IgG-treated mice.Obvious leukocyte infiltration was seen around the hepatic portal vein in liver sections and severe fibrogenesis was found in HBs-tg mice after the 3-month TIGIT blockade.TUNEL assay revealed that hepatocyte apoptosis was more obvious in anti-TIGIT-treated HBs-tg mice than in control mice.Moreover,HBs-tg Tigit-/-mice got persistently higher levels of ALT levels than control HBs-tg mice.2 TIGIT blockade or TIGIT deficiency induces HCC after HBsAg vaccinationTo stimulate more effective immune responses within liver,HBs-tg mice were vaccinated with HBsAg after TIGIT blockade.We found that there were tumor nodules on the liver surface in 63.64%of HBs-tg mice that were treated with anti-TIGIT mAb in combination with HBsAg vaccination.Consistently,61.54%of the HBs-tg Tigit-/-mice also developed HCC after HBsAg vaccination.3 Gradually increased expression of TIGIT on CD8+ T cells in the liver of HBs-tg mice.Flow cytometric analysis revealed that TIGIT expression on hepatic CD8+ T cells was significantly higher than that on splenic counterparts in HBs-tg mice.The frequency of hepatic TIGIT+CD8+ T cells progressively increased with age in HBs-tg mice and was higher than that in WT mice at the same age.Consistent with the expression pattern of TIGIT,PD-1 expression on hepatic CD8+ T cells also increased with age,with significantly higher levels in HBs-tg mice compared to WT mice.4 Functional tolerance of hepatic CD8+ T cells is broken after TIGIT blockade in HBs-tg mice.The ratio and absolute number of total T cells and CD8+ T cells in the liver of HBs-tg mice significantly increased after persistent blockade of TIGIT.Importantly,a higher proportion of HBsAg-specific cytotoxic T lymphocytes(CTLs)appeared in the liver of HBs-tg mice treated with anti-TIGIT mAb for 3 months compared to control HBs-tg mice.Hepatic CD8+ T increased CD69 expression.The frequency of CD8+ T cells with an effector memory phenotype(CD44hiCD62Llow)in liver also dramatically increased.While hepatic CD8+ T cells with a central memory phenotype(CD127+CD62L+)significantly decreased after TIGIT blockade.Functional evaluation revealed an increase in both IFN-y and CD107a expression in hepatic CD8+T cells in anti-TIGIT mAb-treated mice.For HBs-tg Tigit-/-mice,liver CD8+ T cells also displayed enhanced ability to secret IFN-y relative to those control HBs-tg mice.5 TIGIT blockade-induced chronic liver inflammation and HCC depend on CD8+ T cells.HBs-tg mice were depleted of CD8+ T cells during long-term TIGIT blockade.ALT levels significantly decreased and severe leukocyte infiltration in the liver was abolished.To determine the potential roles of CD8+ T cells in inflammation-triggered HCC development,CD8+ T cells were depleted in TIGIT-blocked HBs-tg mice during the period of HBsAg vaccination.None of these mice had liver tumor nodules after CD8+ T cell depletion.Conclusion:TIGIT expressed on CD8+ T maintains hepatic immune tolerance in HBV transgenic mice.TIGIT blockade or deficiency specifically reverses hepatic CD8+ T cell tolerance to HBsAg and therefore results in higher cytokine secretion levels and stronger cytotoxic capacity in HBs-tg mice.This mediated the apoptosis of hepatocytes and the initiation of chronic inflammation.Long term of chronic inflammation promotes the development of HCC.