Involvement Of IFN-γ In Liver Repopulation And Immune Tolerance

Liver is a vital organ, not only for its physiological functions, but also for its role in immune system. It has many interesting and unique features. First, the liver has a strong regenerated ability after liver injury or partial hepatectomy. Hepatocytes can rapidly replicate to repopulate the physiological structure and restore the function of the liver. Emerging evidences indicated that the bone marrow cell fusion can also give rise to hepatocyte generation, providing a novel route for liver repopulation. Second, the liver tend to induce immune tolerance rather immunity when encounting sustained antigen situmulation, such as the stimulation from gut microbiota metobolites or dietary derived antigens. Moreover, persistent exposure of exogenous antigens/pathogens in liver can result in systemic tolerance against these specified antigens.These physiological/immunological features of liver are considered to be dependent on the liver’s unique structure, such as the dual blood supply and sinusoids, and the special compostion of immune cells. Comparing with the other immune organs, the liver is predominant in innate immunity. Innate immune cells in liver have dominant percentages over other organs, and possess special phenotypes and functions. On one hand, the unique microenvironment of liver instruct the development and differentiation of hepatic innate immune cells. On the other hand, these innate immune cells can in turn shape the hepatic immunologic microenvironment, regulate adaptive immune responses, and participate in the development of liver diseases.Our study is focus on the role of IFN-γ in regulating bone marrow supported liver regeneration and liver induced immune tolerance respectively, the results were divided into two major aspects:I. NK cells produced IFN-y improves bone marrow derived hepatocytes regeneration in liver. 1. NK cells are important for bone marrow supported liver regeneration.We established a mouse model of bone marrow supported liver regeneration by using lethally irradiated Fah-/- mice that are transplanted with wild type bone marrow cells and deprived of NTBC. A robust formation of bone marrow derived hepatocytes (BMDHs) can be seen in the liver of these mice. These BMDHs were likely generated by fusion of donor BM-derived myelomonocytes with resident Fah-/- hepatocytes. NK cells play an indispensable role in this process, as NK cell defecient or depleted bone marrow transfer led to decreased survival and BMDH generation Fah-/- recipient mice.2. NK cells promote hepatic reconsititution after BMT via IFN-γ.Hepatic NK cells became sustainedly activated and produced IFN-γ during BMDH generation. Serum IFN-γ elevation was abolished after NK cells deficiency or depletion, which indicated that NK cells were the major producers of IFN-γ. IFN-γ were also indispensable for BMDH generation, as IFN-γ deficient bone marrow lost the ability to restore liver function and rescue survival of Fah-/- mice. IFN-γR-/- BMCs also failed to rescue Fah-/- mice, which suggested that IFN-y-responding cells were BM-derived myelomonocytes rather than host hepatocytes.3. The IFN-γ-IFN-γR interaction contributes to the myelomonocyte-hepatocyte fusion process.Fah+ BMDH generation was dramatically reduced in IFN-γ or IFN-γR deficient BM-reconstituted Fah-/- mice, but there were no differences in the expansion of BMDH or differentiation of myelomonocytes. The IFN-γ-IFN-γR interaction contributed to the myelomonocyte-hepatocyte fusion process, as most of the BMDHs in mixed BM chimeric Fah-/- hosts transplanted with a 1:1 ratio of WT and IFN-γR-/- BM cells were of WT origin. The in vitro experiments also showed that IFN-γ dose-dependently promoted the fusion of myelomonocytes with Fah-/- hepatocytes due to a direct effect on myelomonocytes, which confirmed with the in vivo findings.Conclusion Ⅰ:We found that NK cells are essential for BMDH generation in Fah-/-mice. Moreover, this NK cell function is dependent on their production of IFN-γ, which directly promotes myelomonocyte-hepatocyte fusion.Ⅱ. IFN-γ chemoattracts T cell into liver and induces systemic tolerance1. IFN-γ deficiency leads to loss of systemic tolerance.Wild type HBV carrier mice developed systemic tolerance and lost responsiveness after HBsAg vaccination. But IFN-γ deficient HBV carrier mice possessed robust responses to HBsAg vaccine. Serum HBsAg, HBeAg along with HBV DNA titers were dramatically reduced, and antibody can be produced as anti-HBs were detectable. This was ascribed to the retainment of HBV specific CD4+T cells in the periphery of IFN-γ deficient HBV carrier mice.2. HBcAg stimulates hepatic CD4+T cells to secret IFN-y.IFN-γ production by hepatic CD4+T cells was significantly enhanced after HBV plasmid injection, and lasted for a few weeks. HBV induced IFN-γ production was mainly caused by HBcAg, as the elevated IFN-γ production by hepatic T cells was abolished when HBcAg deficient plasmid was injected.3. IFN-γ chemoattracts T cell into liver via CXCR3 pathway.HBsAg specific CD4+T cells expressed higher levels of CXCR3 and was significantly increased in livers of WT mice after HBV plasmid injection. Whereas there was no accumulation of HBsAg specific CD4+T cells in the livers of IFN-y deficient mice. Blockade of CXCR3 can partially inhibit the recruitment of HBsAg specific CD4+T cells into liver.4. The apoptosis of HBsAg specific CD4+T cells within liver contributes to systemic tolerance.We found that the apoptosis of HBsAg specific CD4+T cells were significantly increased in the livers of wild type HBV carrier mice as compared to IFN-γ deficient HBV carrier mice. Blockade of CTLA-4 could inhibit the apoptosis of T cells, promote the retainment of HBsAg specific CD4+T cells in the periphery and reverse HBV induced systemic tolerance.Conclusion Ⅱ:IFN-γ plays an important role in liver induced systemic tolerance. HBV persistence induces IFN-γ production derived from hepatic CD4+T cells via HBcAg, which promotes the recruitment of HBsAg specific CD4+T cells into liver in a CXCR3 dependent manner. The apoptosis of HBsAg specific CD4+T cells within liver leads to clonal deletion and subsequent systemic tolerance.