Project Summary The long-term goals of this project are to determine the role of the cGAS-STING pathway and the messenger cyclic dinucleotide cGAMP, as a local and systemic immunotransmitter of the IFN response following skin exposure to UVB light. In addition, we seek to determine the significance of these observations in a relevant mouse model of SLE. SLE patients characteristically have a type I interferon (IFN-I) signature in peripheral blood cells and this same signature is prominent in lesional and non-lesional skin. While it is generally assumed that systemic immune activation leads to dissemination of the IFN-I response to tissues, we observed that, following a single exposure to ultraviolet light (UVB), UVB induces an IFN signature not only in the skin but also in the blood and kidneys of wild type mice. Since we observed that IFN-I produced soon after UV exposure requires the DNA activated cytoplasmic sensor cGAS, we hypothesize that cGAMP, the cyclic dinucleotide synthetic product of cGAS, is itself an immunotransmitter that is responsible for spreading the IFN-I response locally and systemically. In this proposal, we explore how UVB stimulated cGAMP production leads to spreading of the IFN signature in the skin, examine the effects on local immune response in the draining lymph nodes, and look for cGAMP dependent tissue inflammation in both wild type and lupus-prone strains. To achieve these Aims, we take advantage of genetically modified mice that are deficient in cGAS or ENPP1 (ectonucleotide pyrophosphatase phosphodiesterase-1 that hydrolyzes cGAMP after export from cells) and utilize reagents that can be exploited to manipulate the cGAS-STING pathway. We specifically interrogate the cGAMP transporters: connexins, SLC19A1 and the Volume Regulated Anion Channel (VRAC) LRRC8, to determine which transporters control cGAMP spreading and under what conditions. The significance of these studies are that they will help define the genetic and molecular mechanisms responsible for UVB induction of IFN-I, define how the signal spreads and, possibly, indicate how lupus flares occur following exposure to UVB. In addition, the research focuses on an enzyme, ENPP1, that has not previously been studied in the context of autoimmunity and, if shown to be important, could be harnessed therapeutically to abort cGAMP spreading. These studies could reverse the paradigm that systemic IFN-I causes the skin IFN signature. Since we have detected cGAMP in blood of a subset of patients with systemic lupus erythematosus (SLE), the results of these studies will have strong translational significance.