DNA-Mediated Cyclic GMP-AMP Synthase-Dependent and -Independent Regulation of Innate Immune Responses

Cytoplasmic DNA activates cyclic GMP AMP synthase (cGAS) to produce cyclic 2'-5'3'-5'GMP AMP dinucleotide (2'5 'cGAMP). The binding of 2'5'cGAMP to an adaptor protein, stimulator of IFN genes (STING), activates a transcription factor, IFN regulatory factor 3, leading to the induction of IFN and chemokine gene expression. In this study, we found that the 2'5'cGAMP-dependent STING activation induced highly upregulated CXCL10 gene expression. Formation of a distinct STING dimer, which was detected by native PAGE, was induced by 2'5'cGAMP, but not 3'-5'3'-5'cGAMP. Analysis of DNase mice, which constitutively produce IFN-beta and CXCL10, showed the accumulation of 2'5'cGAMP in their fetal livers and spleens, suggesting that the undigested DNA accumulating in DNase II-/- cells may have leaked from the lysosomes into the cytoplasm. The DNase II-/- mouse embryonic fibroblasts produced 2'5'cGAMP in a cGAS-dependent manner during apoptotic cell engulfment. However, cGAS deficiency did not impair the STING-dependent upregulation of CXCL10 in DNase II-/- mouse embryonic fibroblasts that was induced by apoptotic cell engulfment or DNA lipofection. These results suggest the involvement of a cGAS-independent additional DNA sensor(s) that induces the STING-dependent activation of innate immunity.