Natural killer cells ameliorate liver fibrosis by killing activated stellate cells in NKG2D-dependent and tumor necrosis factor-related apoptosis-inducing ligand-dependent manners.
OBJECTIVE
Viral hepatitis infection, which is a major cause of liver fibrosis, is associated with activation of innate immunity. However, the role of innate immunity in liver fibrosis remains obscure.


METHODS
Liver fibrosis was induced either by feeding mice with the 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) diet or by injecting them with carbon tetrachloride. The Toll-like receptor 3 ligand, polyinosinic-polycytidylic acid, was used to activate innate immunity cells and mediators, including natural killer cells and interferon gamma.


RESULTS
In the mouse model of DDC-induced liver fibrosis, natural killer cell activation by polyinosinic-polycytidylic acid induced cell death to activated hepatic stellate cells and attenuated the severity of liver fibrosis. Polyinosinic-polycytidylic acid treatment also ameliorated liver fibrosis induced by carbon tetrachloride. The observed protective effect of polyinosinic-polycytidylic acid on liver fibrosis was diminished through either depletion of natural killer cells or by disruption of the interferon gamma gene. Expression of retinoic acid early inducible 1, the NKG2D ligand, was undetectable on quiescent hepatic stellate cells, whereas high levels were found on activated hepatic stellate cells, which correlated with the resistance and susceptibility of quiescent hepatic stellate cells and activated hepatic stellate cells to natural killer cell lysis, respectively. Moreover, treatment with polyinosinic-polycytidylic acid or interferon gamma enhanced the cytotoxicity of natural killer cells against activated hepatic stellate cells and increased the expression of NKG2D and tumor necrosis factor-related apoptosis-inducing ligand on liver natural killer cells. Blocking NKG2D or tumor necrosis factor-related apoptosis-inducing ligand with neutralizing antibodies markedly diminished the cytotoxicity of polyinosinic-polycytidylic acid-activated natural killer cells against activated hepatic stellate cells.


CONCLUSIONS
Our findings suggest that natural killer cells kill activated hepatic stellate cells via retinoic acid early inducible 1/NKG2D-dependent and tumor necrosis factor-related apoptosis-inducing ligand-dependent mechanisms, thereby ameliorating liver fibrosis.
Section on Liver Biology, Laboratory of Physiologic Studies, National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, Bethesda, Maryland 20892, USA.
Svetlana
S
Radaeva
Rui
R
Sun
Barbara
B
Jaruga
Van T
VT
Nguyen
Zhigang
Z
Tian
Bin
B
Gao
D000818:Animals; D017209:Apoptosis / Q000276:immunology*; D017931:DNA Primers; D004195:Disease Models, Animal; D005434:Flow Cytometry; D006524:Hepatitis, Viral, Animal / Q000276:immunology; D007694:Killer Cells, Natural / Q000276:immunology*; D008024:Ligands; D008106:Liver Cirrhosis, Experimental / Q000276:immunology*; D008297:Male; D008562:Membrane Glycoproteins / Q000172:deficiency / Q000235:genetics; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D055655:NK Cell Lectin-Like Receptor Subfamily K; D054353:Perforin; D052899:Pore Forming Cytotoxic Proteins; D011971:Receptors, Immunologic / Q000235:genetics / Q000502:physiology*; D055607:Receptors, Natural Killer Cell; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014212:Tretinoin / Q000502:physiology
D017931:DNA Primers; C527086:Klrk1 protein, mouse; D008024:Ligands; D008562:Membrane Glycoproteins; D055655:NK Cell Lectin-Like Receptor Subfamily K; D052899:Pore Forming Cytotoxic Proteins; D011971:Receptors, Immunologic; D055607:Receptors, Natural Killer Cell; D054353:Perforin; D014212:Tretinoin