We previously identified Ku70, a subunit of a DNA repair protein complex, as a cytosolic DNA sensor that induces the production of interferon-λ1 (IFN-λ1) by human primary cells and cell lines. IFN-λ1 is a type III IFN and has similar antiviral activity to that of the type I IFNs (IFN-α and IFN-β). We observed that human embryonic kidney (HEK) 293T cells, which are deficient in the innate immune adaptor protein STING (stimulator of IFN genes), did not produce IFN-λ1 in response to DNA unless they were reconstituted with STING. Conversely, parental HEK 293 cells produced IFN-λ1 after they were exposed to exogenous DNA; however, when STING was knocked out in the HEK 293 cells through the CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 genome editing system, they lost this response. Through confocal microscopy, we demonstrated that endogenous Ku70 was located in the nucleus and then translocated to the cytoplasm upon DNA exposure to form a complex with STING. Additionally, the DNA binding domain of Ku70 was essential for formation of the Ku70-STING complex. Knocking down STING in primary human macrophages inhibited their ability to produce IFN-λ1 in response to transfection with DNA or infection with the DNA virus HSV-2 (herpes simplex virus-2). Together, these data suggest that STING mediates the Ku70-mediated IFN-λ1 innate immune response to exogenous DNA or DNA virus infection.
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