The cell-intrinsic innate immune response to DNA damage in human cells

The innate immune response has evolved to detect DNA from viruses and intracellular bacteria and differentiate this from our almost identical self-DNA. DNA detected in the cytoplasm is thought to be identified as foreign due to its location, and bacterial DNA in endosomes in recognised by its hypomethylated motifs. However, it is not clear how foreign DNA in the nucleus is differentiated from self-DNA, or what happens when the appearance of self-DNA is altered as it is in the case of DNA damage, a regular cellular occurrence. DNA damage has been shown to induce an immune response in tissues exposed to chemical mutagens – this has implications in the clearance of cancer cells after chemotherapy-induced DNA damage, with evidence that the immune response mounted by patients may determine chemotherapy responsiveness. To investigate the immune events following DNA damage, we analysed human keratinocytes, fibroblasts, and monocytes, and found that Etoposide, an inducer of double strand breaks, induced an early innate immune response, characterised by Type-I interferon and inflammatory cytokine production. This innate immune response to DNA damage was particularly potent in keratinocytes. This response required components of the cytoplasmic DNA sensing pathway, the predominantly nuclear sensor IFI16 and the cytoplasmic adaptor protein STING, but was independent of the enzyme cGAS, which is essential for the immune response to cytosolic DNA. The transcriptional and signalling profiles of the Etoposide-induced response differ from those of the classical pathogen sensing pathway, indicating that these two stimuli have overlapping but distinct pathways. This cell-intrinsic innate immune response also involved DNA damage response factors ATM and p53, suggesting that DDR components may interact with immune components to alert the cell to damage. The innate immune response that we observe may play a role in the inflammatory phenotype and immune clearance of cancer cells after DNA damage-inducing chemotherapy