The Negative Regulatory Functior Of Liver-resident NK Cells

The liver acts as not only a metabolic organ,but also a special immunological organ of great importance in the body.Owing to the unique blood supply system,the liver can continuously receive massive stimulation from gastrointestinal microbiota and food metabolites.Besides,it can also produce abundant antigens via metabolic activity,tending to induce immune tolerance,including transplantation tolerance,oral tolerance and chronic infection,instead of triggering persistent immune response.This characteristic is strongly linked to the physiological structure and cell composition in the liver.Comparing with traditional immune organs,the liver possesses large variety and amount of innate immune cells with special phenotypes and functions.These unique cells,in turn,could shape the hepatic immunologic microenvironment and endow the liver with the feature of predominant innate immunity.Natural killer(NK)cells,as a vital component of the innate immune system,can rapidly react to viral infected cells or tumor cells without prior priming,thus maintaining homeostasis.NK cells exert positive and negative regulatory roles during immune responses through direct cytotoxicity,cytokine production or interacting with numerous immune cells.The liver contains a higher proportion of NK cells than other organs.And recently,we have identified a unique subset of NK cells enriched in murine liver with memory and tissue-resident capability for the first time,termed liver-resident CD49a+CD49b-NK cells(LrNK).With development of the research,tissue-resident NK cells have been identified in various other organs,representing a heterogeneous population of regional distribution.However,LrNK cells and conventional CD49a-CD49b+NK cells(cNK)exhibit significant differences in terms of phenotype,different regulatory factors for their development or effector molecules.In particular,transcriptomics analyses reveal a significant enrichment in LrNK cells for genes involved in pathways of negative regulation of immune system process,tolerance induction and regulation of homeostatic process.Considering the immune tolerance characteristics of liver,there is an intriguing question that whether LrNK cells could participate in the construction of the tolerogenic liver environment.In this regard,we begin to exploit the function of LrNK cells.And our study launches from the following two parts:I Liver-resident NK cell negatively regulate T cells during antiviral responses1.The distribution and characteristics of LrNK cells and other ILC1 sAccording to the analysis of LrNK cells and other ILC1s in different organs,we found that the liver contains the highest proportion of ILC1s possessing the phenotype as CD49a+CD49b-CD200R+T-bet+Eomes-NK.Transcriptomics analyses reveal a significant enrichment ILCls for genes involved in pathways of tolerance induction,negative regulation of immune system process and regulation of homeostatic process.We then adopted flow cytometry analyses to confirm this trend.2.LrNK cells proliferate obviously during viral infectionTo explore whether LrNK cells are involved in shaping T cell responses,the mouse model of acute and chronic lymphocytic choriomeningitis virus(LCMV)infection were used.The number of LrNK cells increased when the adaptive immune response was initiated and LrNK cells proliferated obviously during viral infection.By using adoptive transfer experiments,we demonstrated that LrNK cells represent a phenotypically stable lineage during viral infection.In contrast,conventional NK cells could proliferate and up-regulate CD49a surface expression after infection.3.LrNK cells inhibit T cell responses during acute and chronic viral infectionWe employed LrNK cells specific deficient mice to vertify their regulatory function during viral infection,and found that the absence of these cells could promote T cells responses.And after transfering the cells into wild type mice,they could negatively regulate anti-viral T cell responses during acute LCMV or adenovirus and chronic LCMV infections.In contrast,conventional NK cells could promote antiviral T cell responses.4.LrNK cells negatively regulate T cell functions via PD-L1 checkpoint controlTo investigate the potential mechanisms by which LrNK cells regulate virus-specific T cell responses,surface expression of immunosuppressive molecules on LrNK cells during viral infections were analyzed.We found that LrNK cells up-regulated PD-L1 expression during different viral infections,and higher expression of PD-1,the receptor for PD-L1,was observed on hepatic T cells after infection.After blockade of PD-L1 on liver-resident NK prior to adoptive transfer,they failed to inhibit anti-viral CD8+ T cell responses.Besides,LrNK cells directly inhibited T cell proliferation in vitro via PD-L1.Conclusion I:LrNK cells could highly express molecules associated with tolerance induction and negative regulation of immune system process,and inhibit T cell functions via PD-L1 checkpoint control during antiviral responses.These results reveal a regulatory role of LrNK cells in maintaining liver tolerance and defending against excessive immune responses.While conventional NK cells could promote antiviral immune responses.II Liver-resident NK cell negatively regulate NKT cells during acute hepatic damage1.LrNK cells upregulate negative immune regulators during ConA induced hepatitis.We found that LrNK cells could up-regulate molecules associated with tolerance induction and negative regulation during ConA induced fulminant hepatitis,suggesting that they may participate in this process.2.LrNK cells inhibit NKT cells responses and ameliorate ConA induced hepatitisDuring ConA induced hepatitis,the transfer of LrNK cells could inhibit the production of cytokines by NKT cells,and ameliorate ConA induced hepatitis.While conventional NK cells do not have the capability.3.LrNK cells directly promte NKT cells apoptosis in ConA induced hepatitisDuring ConA induced hepatitis,LrNK cells could promote NKT cells apoptosis in vitro and in vivo thus acting as a negative regulator of NKT cells.However,conventional NK cells do not have the same ability.4.LrNK cells inhibit NKT cells responses and promote NKT cells apoptosis in a-GalCer induced hepatitisDuring a-GalCer induced hepatitis,the transfer of LrNK cells could inhibit the production of effector molecules by NKT cells,promote NKT cells apoptosis and ameliorate a-GalCer induced hepatitis.While conventional NK cells do not have the capability.Conclusion II:LrNK cell negatively regulate NKT cells during NKT cell mediated hepatic damage.On one hand,they can inhibit the production of effector molecules by NKT cells,on the other hand,they promote NKT cells apoptosis during acute hepatic damage.These results also reveal a regulatory role of LrNK cells in maintaining liver tolerance and defending against excessive immune responses.