IFN-γ production by human natural killer cells in response to HCV-infected hepatoma cells is dependent on accessory cells.

BACKGROUND & AIMS: Interferon-γ (IFN-γ), a cytokine produced by activated natural killer cells (NK) and T lymphocytes, is an important regulator of innate and adaptive immunity during hepatitis C virus (HCV) infection. However, the cellular sources and mechanisms of IFN-γ induction in HCV-infection are not fully understood.
METHODS: We cultured normal human peripheral blood mononuclear cells (PBMCs) with different populations of immune cells and JFH-1 HCV-infected HuH7.5 (JFH-1/HuH7.5) cells.
RESULTS: We found that PBMCs produced large amounts of IFN-γ after co-culture with JFH-1/HuH7.5 cells. Using intracellular cytokine staining, we confirmed that NK cells and NKT cells (to a lesser extent) were the major IFN-γ producers within PBMCs. Purified NK/NKT cells did not produce IFN-γ in response to JFH-1/HuH7.5 cells and depletion of accessory (HLA-DR(+)) cells prevented IFN-γ induction in PBMCs. Through selective cell depletion of dendritic cells or monocytes from PBMCs, we determined that plasmacytoid dendritic cells (pDCs) were indispensable for NK-IFN-γ induction and the presence of monocytes was needed for maximal NK-IFN-γ induction. We further revealed that NK-IFN-γ induction depended on pDC-derived IFN-α while other IFN-γ inducing cytokines, IL-12, and IL-18, played minimal roles. Close contact between JFH-1/HuH7.5 cells and NK cells was required for IFN-γ production and monocyte-derived IL-15 significantly augmented IFN-γ induction.
CONCLUSIONS: We discovered a novel mechanism where NK cells interact with pDCs and monocytes, efficiently producing IFN-γ in response to HCV-infected cells. This indicates that co-operation between NK cells and accessory cells is critical for IFN-γ production and regulation of immunity during HCV infection.