Natural Killer Cells in Viral Hepatitis

Natural killer (NK) cells are traditionally regarded as first-line effectors of the innate immune response, but they also have a distinct role in chronic infection. Here, we review the role of NK cells against hepatitis C virus (HCV) and hepatitis B virus (HBV), two agents that cause acute and chronic hepatitis in humans. Interest in NK cells was initially sparked by genetic studies that demonstrated an association between NK cell–related genes and the outcome of HCV infection. Viral hepatitis also provides a model to study the NK cell response to both endogenous and exogenous type I interferon (IFN). Levels of IFN-stimulated genes increase in both acute and chronic HCV infection and pegylated IFNα has been the mainstay of HCV and HBV treatment for decades. In chronic viral hepatitis, NK cells display decreased production of antiviral cytokines. This phenotype is found in both HCV and HBV infection but is induced by different mechanisms. Potent antivirals now provide the opportunity to study the reversibility of the suppressed cytokine production of NK cells in comparison with the antigen-induced defect in IFNγ and tumor necrosis factor-α production of virus-specific T cells. This has implications for immune reconstitution in other conditions of chronic inflammation and immune exhaustion, such as human immunodeficiency virus infection and cancer.