Human DNA-PK activates a STING-independent DNA sensing pathway Recognition of foreign nucleic acids is critical for antiviral defense. Detection of DNA is mediated by the cGAS-STING pathway, which activates a potent type I interferon response. This pathway is broadly required for antiviral defense across cell types and species, and its relevance in context of infection, cancer, and autoimmunity has been thoroughly established. However, we have discovered an additional, STING-independent DNA sensing pathway (SIDSP) in human cells. Using STING KO human cell lines, we find that the induction of interferon is indeed abrogated at early timepoints, but surprisingly, at later timepoints, we observe a robust DNAinduced interferon response. Here we identify DNA-PK as the sensor for the SIDSP and demonstrate that its kinase activity is required for the antiviral response. We show that a heat shock protein HSPA8/HSC70 is phosphorylated after DNA stimulation and acts as a marker for the SIDSP. Finally, we explore how DNA viruses antagonize both the cGAS-STING pathway and the SIDSP. Our work highlights the importance of nucleic acid sensing for both host and virus and has implications for modulating DNA sensing in order to improve therapies for cancer or autoimmunity.

Detection of intracellular DNA by the cGAS-STING pathway activates a type I interferon-mediated innate immune response that protects from virus infection and can be harnessed to promote anti-tumor immunity. Whether there are additional DNA sensing pathways, and how such pathways might function, remains controversial. We show here that humans -but not mice -have a second, potent, STINGindependent DNA sensing pathway that is blocked by the E1A viral oncogene of human adenovirus 5.We identify human DNA-PK as the sensor of this pathway and demonstrate that DNA-PK kinase activity drives a robust and broad antiviral response. We discover that the heat shock protein HSPA8/HSC70 is a unique target of DNA-PK. Finally, we demonstrate that detection of foreign DNA and DNA damage trigger distinct modalities of DNA-PK activity. These findings reveal the existence, sensor, unique target, and viral antagonists of a STING-independent DNA sensing pathway (SIDSP) in human cells. IntroductionThe cGAS-STING DNA sensing pathway has emerged as a key component of the innate immune response that is important for antiviral immunity (1), contributes to specific autoimmune diseases (2), and mediates important aspects of antitumor immunity (3). cGAS binds to double-stranded DNA and catalyzes the formation of cyclic GMP-AMP (4, 5), a diffusible cyclic dinucleotide that activates the endoplasmic adaptor protein STING (6). Activated STING then serves as a platform for the inducible recruitment of the TBK1 kinase, which phosphorylates and activates the transcription factor IRF3, leading to the induction of the type I interferon mediated antiviral response (7).Nearly all studies on the cGAS-STING pathway involve the use of mice and mouse cells.Knockouts of cGAS (Mb21d1) and STING (Tmem173) have clearly demonstrated that both are essential for the transcriptional and cell biological responses to foreign intracellular DNA, and that they mediate the pathology of specific autoimmune diseases (2,8,9). However, numerous additional sensors of intracellular DNA have been proposed, all of which are thought to act upstream of STING (10). Whether STING-independent DNA sensing exists is currently unknown.Here, we report the unexpected finding that the E1A oncogene of human adenovirus 5 blocks two distinct DNA sensing pathways in human cells: the well-known cGAS-STING pathway (11), and a second, STING-independent DNA sensing pathway (SIDSP). We identify the DNA damage response protein DNA-PK as the sensor of the SIDSP, along with the heat shock protein HSPA8 as a unique SIDSP target. We show that the SIDSP is potently activated in human and primate cells, but it is weak or absent from mouse cells. Our findings demonstrate that human cells have a second DNA sensing pathway, with implications for host defense, autoimmunity, and anti-tumor immunity. Results Human adenovirus 5 E1A blocks two DNA sensing pathways in human cellsWe previously demonstrated that the viral oncogenes of the DNA tumor viruses are potent antagonists of the cGAS-STING DNA sensing pathway ...