Natural Killer Cell Responses in Chronic Hepatitis C Virus Infection
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BACKGROUND: Hepatitis C virus (HCV) has infected 170 million people worldwide and is a leading cause of chronic viral hepatitis, liver cirrhosis, and hepatocellular carcinoma. Liver injury and disease progression in hepatitis C virus infection is driven by the host immune response. Previous research has largely focused on the contribution of virus-specific T cells to this process. However, in chronic HCV infection, T cells are functionally exhausted and do not control viremia. Thus, it is likely that immune cells other than HCV-specific T cells contribute to disease pathogenesis. Natural killer (NK) cells constitute the major innate immune cell population in the liver. They are capable of producing cytokines and utilize multiple mechanisms of cytotoxicity. The high prevalence of NK cells in the liver, along with their potential for viral specificity, suggested we study NK cell function in the setting of chronic hepatitis C infection. An improved understanding of the endogenous mechanisms that regulate liver inflammation and disease pathogenesis in chronic hepatitis C infection may result in a new focus for therapeutic options to decrease the rate of disease progression in individuals who are unable to clear infection with antiviral agents. OBJECTIVE: Here we sought to characterize whether natural killer cells from individuals chronically-infected with hepatitis C virus respond in chronic HCV infection and to compare the frequencies of HCV-specific NK cell responses with corresponding virus-specific T cell responses. METHODS: We tested the NK cell function of chronically HCV-infected patients not currently on antiviral treatment using a whole blood activation assay. In this assay, heparin-anticoagulated whole blood from 39 anti-HCV positive HCV patients and 10 healthy blood donors was stimulated for 8 hours with pools of overlapping 18 amino acid peptides (OLPs) comprising the HCV envelope (E1, E2) and nonstructural (NS3) sequences. Specificity controls consisted of a pool of overlapping hepatitis D virus peptides and samples without peptide addition. As a readout for NK and T cell function, the frequency of IFNγ+ NK cells and T cells was assessed by multicolor flow cytometry. In addition, cytotoxicity was assessed by the frequency of NK or T cells expressing CD107a, a marker of cellular degranulation. Variance between responses in HCV-infected versus healthy donors and variance between NK versus T cell responses were analyzed by matched two-way ANOVA with multiple comparisons and Bonferroni post-test using Graphpad Prism software. RESULTS: This study demonstrates that NK cells from chronically HCV-infected individuals are activated in HCV infection in a whole blood peptide stimulation assay with both structural and nonstructural HCV proteins, including E1 and E2 envelope proteins and NS3 nonstructural protein. NK cells secrete cytokines assessed here by release of IFNγ, as well as NK cell degranulation, and degranulated, assessed by CD107a expression. As expected, there were minimal NK cell or T cell responses in healthy donor blood. NK cell cytokine responses were 4-6 times greater than HCV-specific T cell cytokine responses. CONCLUSION: This study demonstrates that NK cell cytokine production and degranulation is greater than that of corresponding T cells. Further research is needed to characterize the mechanism of these NK cell responses to understand antiviral mechanisms and factors influencing disease progression.