The Immunosuppressive Activities Of Myeloid Derived Suppressor Cells Can Be Reduced By Activated NKT Cells And Contribute To Inflammation In Fulminate Hepatitis Mice

The liver as a lymphoid organ which receives blood from both the systemic circulation and the intestine has a unique immunity.On one side,innate lymphocytes,including both natural killer cells and natural killer T cells,and adopt immunity cells such as antigen-presenting cells(APCs) and T cells can initiate immune response to exogenous antigens;on the other side,the exposure of liver cells to antigens,and to microbial products derived from the intestinal bacteria,has induced the production of immunosuppressive cytokines and inhibitory cell surface ligands and often results in tolerance,which is called "liver tolerance".Because of complex microenvironment, precious immune regulation is necessary in liver.Recently,a kind of inhibitory or regulatory cell populations called myeloid-derived suppressor cells(MDSCs) sharing the Gr-1 and CD11b markers were found related to acute liver injury progression:the number of MDSCs in different tissues was markedly and dramatically increased in acute liver injury mice.Though an increased production of MDSCs was also reported in a number of pathological conditions including infections,traumatic stress and cancers and remarkable suppress immune responses,we did not find noticeable immunosuppressive abilities of MDSCs in liver injury mice.In liver there are abundant of NKT cells,and different from conventional naive T cells,NKT cells can rapidly produce both Th1-type cytokines(IFN-γand IL-2) and Th2-type cytokines(IL-4,IL-13 and IL-10) within 1-2 h after activation.Previous studies also found NKT cells can enhance MDSC suppressive activity by secreting IL-13,however,activated NKT cells also can facilitate the conversion of immunosuppressive MDSCs into immunogenic APCs.Through co-injection of LPS and D-GalN inducing hepatitis,we investigated whether MDSCs negative regulated the immune responses in liver injury mice,and the interaction between MDSCs and NKT cells and its mechanism in Fulminant Hepatitis.After the i.p.co-injection of LPS(5μg/kg of body weight ) and D-GaIN(400 mg/kg of body weight) in mice,it induced liver injury that the serum ALT and AST elevated dramatically,and caused the mice death at 6.5h firstly,between 7h and 8h most mice died,and at 9h the mortality of mice was 100%.Along acute liver injury progression, the number of MDSCs in different tissues such as liver,spleen,bone marrow and MLN was markedly and dramatically increased.Examined the changes of two MDSCs subsets in different tissue,we found the monocytic MDSCs(Ly6G-Ly6C^high) accumulated in bone marrow,spleen and liver,and the granulocytic MDSCs (Ly6G⁺Ly6C^low) reduced in bone marrow,whereas increased in spleen and liver. MDSCs from liver and spleen in liver injury mice were expressed higher level of CCR2, CX₃CR1,CXCR2 and CD62L than those from BM,and because neutrophils expressed chemokine receptors CXCR2,whereas macrophages expressed CCR2 and CX₃CR1, showing that these factor induced the recruitment of MDSCs from BM to peripheral inflammation tissues,such as liver and spleen,and through the changes of CCR2, CX₃CR1 and CXCR2 expression,two MDSCs subsets recruited to different tissues. Interestingly,in LPS and D-GaIN induced hepatitis mice MDSCs were differentiating in liver or spleen rather than in BM,and MDSCs from inflamed tissues reduced the suppressive enzymes expression such as Agrl and iNOS,the abilities to inhibit the T-cell proliferation and promote the development of Treg cells,and appeared to produce pro-inflammation cytokines which could aggravate the hepatitis.It was noticeable that the NKT cells activation and cytokines production in our results were mainly confirmed the changes of STAT1 and STAT6 in MDSCs.Particularly at liver injury induced 6h,BM MDSCs had low differentiation and immunosuppressive abilities,whereas inflammation tissues MDSCs were differentiated,had reduced suppressive activities,and appeared to produce pro-inflammation cytokines,showing that NKT cells were associated with the immune functions of MDSCs.After we co-cultured MDSCs with different original NKT cells,we noticed that compared with no NKT cells co-culture,inflammation tissues NKT cells could inhibit MDSCs to express Argl,induce the cells to secrete of IL-12p40,and reduce their suppressive activities such as inhibition of T-cell proliferation.The transwell system shown that NKT cells from inflamed tissues could induce MDSCs to mature myeloid cells mainly depended on cell-cell interaction.To determine the effects of inflammation tissues MDSCs on the number of NKT cells, the percentage of NKT cells in the liver and spleen were measured at different time points.Percentages of NKT cells were decreased significantly in the liver and spleen in liver injury mice,and using a linear regression analysis,we found the decrease of NKT cells in the liver and spleen was inversely proportional related to the increased MDSCs numbers.We further proved in vitro that NKT cells were activated and MDSCs could induce NKT cells apoptosis mainly through Fas-FasL pathway.In a word,along acute liver injury progression,MDSCs accumulated in different tissues such as liver,spleen,bone marrow and MLN,and through the changes of CCR2, CX₃CR1 and CXCR2 expression,two MDSCs subsets recruited to different tissues.At inflamed tissues,NKT cells could markedly induce the differentiation of MDSCs which recruited from BM mainly depending on cell-cell interaction,restore their suppressive activities and induce MDSCs to produce pro-inflammation cytokines.And these pro-inflammation cytokines producing MDSCs further activated NKT cells,and finally induced the apoptosis of NKT cells.Our study provides new understanding about the function of innate immune cells such as NKT cells and regulatory cell such as MDSCs in liver injury,which may be useful for control and therapy innate immune cells induced liver diseases.