Julia Pollmann, Jana-Julia Götz, Daniel Rupp, Otto Strauss, Markus Granzin, Oliver Grünvogel, Pascal Mutz, Catharina Kramer, Felix Lasitschka, Volker Lohmann, Niklas K. Björkström, Robert Thimme, Ralf Bartenschlager, Adelheid Cerwenka

Natural killer (NK) cells are found at increased frequencies in hepatitis C virus (HCV)-infected patients. NK cell activation was shown to correlate with HCV clearance and to predict favourable treatment response. The aim of our study was to dissect mechanisms leading to NK cell activation and proliferation in response to HCV. NK cell phenotype, proliferation, and function were assessed after six-day co-culture of human peripheral blood mononuclear cells (PBMCs) with HCV-replicon containing Huh6 hepatoblastoma cells or HCV-infected Huh7.5 cells. Obtained results were confirmed by immunohistochemistry of liver biopsies from HCV patients and HCV-negative controls. In HCV-containing co-cultures, a higher frequency of NK cells upregulated expression of the high-affinity IL-2 receptor chain CD25, proliferated more rapidly and produced higher amounts of IFN-γ compared to control co-cultures. This NK cell activation was dependent on IL-2, cell-cell contact-mediated signals and HCV replicon-exposed monocytes. The TNF-receptor superfamily member OX40 became induced on the activated CD25(±) NK cell subset and induction was abrogated by depletion of CD14(+) monocytes. Moreover, OX40L was upregulated on CD14+ monocyte-derived cells co-cultured with HCV-containing cells and observed in liver biopsies from HCV patients. Importantly, blocking of the OX40/OX40L interaction abolished NK cell activation and proliferation. Our data uncover a previously unappreciated cell-cell contact mediated mechanism of NK cell activation and proliferation in response to HCV mediated by monocyte-derived cells and the OX40/OX40L axis. These results reveal a novel mode of cross-talk between innate immune cells during viral infection. Using a cell-culture model of hepatitis C virus infection our study reveals that Natural Killer cells become activated and proliferate when they are co-cultured with HCV-containing liver cells. The mechanism of this activation involves a cross-talk with other innate immune cells and a cell/cell contact interaction mediated by the cell surface molecules OX40 and OX40L. Our study uncovers a novel pathway leading to NK cell proliferation and activation towards virus-infected cells that might be of high relevance in anti-viral immunity.