| Summary: | NK cells are involved in innate immune responses to viral infections either via direct cytotoxicity which destroys virus-infected cells or production of immunoregulatory cytokines which modulate adaptive immunity and directly inhibit virus replication. These functions are mediated by different NK subpopulations, with cytotoxicity being generally performed by CD56dim NK cells, whereas CD56bright NK cells are mainly involved in cytokine secretion. NK functional defects are usually combined so that impaired degranulation is often associated with deficient cytokine production. Innate immunity is thought to be relevant in the control of hepatitis virus infections such as HBV and HCV, and recent findings reproducibly indicate that NK cells in chronic viral hepatitis are characterized by a functional dichotomy, featuring a conserved or enhanced cytotoxicity and a reduced production of IFN-gamma and TNF-alfa. In chronic HCV infection this appears to be caused by altered IFN-alfa signaling resulting from increased STAT1 phosphorylation, which polarizes NK cells toward cytotoxicity, and a concomitantly reduced IFN-alfa induced STAT4 phosphorylation yielding reduced IFN-gamma mRNA levels. These previously unappreciated findings are compatible on the one hand with the inability to clear HCV and HBV from the liver and on the other they may contribute to understand why these patients are often resistant to interferon (IFN)alfa-based therapies.
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