局部發炎環境對肝臟T細胞反應及B型肝炎病毒感染之影響

Abstract

Liver is considered as a unique lymphoid organ that favors immune tolerance which are highly associated with the tolerogenic properties of hepatic antigen-presenting cells and immunoregulatory molecules. The unique hepatic regulatory mechanisms prevent induction of immunity against non-harmful foreign antigens, such as gut-derived nutrients or bacterial degradation products in the portal venous blood. However, liver tolerogenic environment may also contribute to the chronicity of viral infections in the liver. Interestingly, under conditions of local inflammation induced by some bactiral and acute viral infections, liver tolerance can be overcome to favor induction of immunity, including upregulation of both class I and class II major histocompatibility complexes (MHC) and costimulatory molecules, on hepatocytes and antigen presenting cells. The subsequent expression of adhesion molecules and chemokines recruit and activate immune cells to the liver to help clear infecting pathogens. In this study, we hypothesize that hepatic expression of class II MHC and the costimulatory molecule CD86 may create an inflammatory environment that could overcome liver tolergenic property and provid help for lymphocyte activation as well as viral clearance. To address this hypothesis, we used recombinant adeno-associated viral vector (rAAV) to deliver CIITA, which is the master regulator of class II MHC molecules, and recombinant adenoviral vector (rAd) to deliver CD86. Mice transduced with rAAV/CIITA led to expression of CIITA and class II MHC in most of the hepatocytes. Similarly, mice transduced with rAd/CD86 resulted in CD86 expression in most of the hepatocytes, which led to proliferation and activation of hepatic lymphocytes, in particular CD8+ T lymphocytes. Since a high dose of rAd/CD86 caused severe liver damage, dose titration experiments were performed to identify an optimal dose which can induce hepatic inflammation but avoid extensive damage of hepatocytes. We used two model systems, one expressing hepatitis B viral proteins and the other ovalbumin (OVA), to address how local inflammation, induced by class II MHC and CD86, affects establishment of chronic viral infections in the liver. In the firs model of HBV infection, mice were infected by AAV/HBVp-, which produced all HBV proteins except polymerase, and treated with CIITA alone, CD86 alone or CIITA plus CD86. Treatment of CIITA alone slightly reduced the serum level of HBeAg but had no effect on serum HBsAg and hepatic HBcAg. Treatment of CD86 significantly reduced all viral proteins, in particular HBeAg and HBcAg. Importantly, a combination treatment of CIITA and CD86 showed the most significant effect, not only reducing all viral proteins but also resulting in complete clearance of HBsAg and HBeAg in some treated mice. Correspondingly, treatment of CD86 alone or a combination treatment of CD86 and CIITA increased the numbers of both CD4+ and CD8+ T lymphocytes in the liver and the spleen. These T cells showed increased activation phenotype, which was stronger in CD8+ T cells than in CD4+ T cells. IFNg ELISpot assay was used to determine the role of virus-specific T cell in viral antigen suppression mediated by CIITA and/or CD86. However, we failed to detect virus-specific T cells in any of the treated groups. To analyze the activation phenotypes of antigen-specific T cells in the liver environment, we used another murine model system that express OVA in the liver and adoptively transferred OVA-specific CD8+ T cells (OT-I) and CD4+ T cells (OT-II), isolated from their respective T cell receptor transgenic mice. This approach allowed us to track the phenotypic changes of T cells that encounter their specific antigens in the liver under various inflammatory conditions. Our data showed that OT-I T cells exhibited extensive proliferation and activation in mice expressing hepatic OVA protein. Local inflammation induced by expression of CIITA and/or CD86 did not further enhance proliferation and activation of OT-I cells. In contrast, the proliferation and activation of OT-II T cells to hepatic antigens was much weaker than that of OT-I cells and was dependent upon the local inflammation environment generated by CD86 or CIITA plus CD86. Furthermore, the transferred OT-I cells distributed mainly in the liver while most of the OT-II cells were found in the spleen. These results suggest that in early hepatic viral infection antigen-specific CD8+ T cells in the liver can be readily induced once they encounter their cognate viral antigens while activation of antigen-specific CD4+ cells required additional signaling that can be provided by appropriate local inflammation. Together, this study demonstrated that an appropriate liver inflammatory environment can be induced by transfecting and upregulating expression of CD86 and CIITA in the liver. This inflammatory environment enhances proliferation and activation of intrahepatic lymphocyte, in particular CD8+ T cells, leading to suppression of viral antigens in the early stage of viral infection.