Background Type I interferon (IFN-I) is strongly implicated in the pathogenesis of Systemic Lupus Erythematosus (SLE) and “interferonopathies” such as Aicardi-Goutieres Syndrome (AGS). A recently discovered DNA-activated IFN-I pathway, cyclic GMP-AMP (cGAMP) synthase (cGAS), which is crucial in viral immunity, has been linked to AGS and mouse models of SLE. Whether this pathway plays a role in human SLE is unknown.
Objectives Given the pivotal role played by interferon in the pathogenesis of SLE, our initial aim was to determine whether cGAS expression and subsequent cGAMP production contribute to IFN-I production in SLE. A further objective was to establish whether there is a specific disease phenotype associated with activation of this pathway.
Methods There were three phases to this work. Initially, we evaluated cGAS mRNA expression by QPCR from stored cDNA in 51 SLE patients and 20 normal controls. Peripheral blood mononuclear cells (PBMC) were obtained from a second cohort comprising 17 SLE patients, 10 RA patients and 9 healthy volunteers. Extracts from these PBMC were tested for cGAMP by a Multiple Reaction Monitoring (MRM) assay developed on a mass spectrometer coupled with Ultra-Performance Liquid Chromatogram (UPLC). In a third cohort, cGAS and cGAMP were examined in the same SLE patients (n=31). Disease activity was determined by SELENA-SLEDAI and organ involvement, serology and medications recorded. Categorical variables were compared using Fischer's exact test, mean data was compared using a t-test or the Wilcoxon Sum-Rank test for normal or non-normally distributed data respectively.
Results Phase 1 revealed that cGAS expression in PBMC was significantly higher in SLE patients (n=51) when compared to normal controls (n=20) (p=0.0045). cGAS expression positively correlated with IFN response gene (ISG) expression in SLE patients (p=1.8 x 10–5). When PBMC from healthy controls were incubated with IFN-I, cGAS mRNA increased in a dose response manner consistent with it being an IFN stimulated gene. Targeted measurement of cGAMP by mass spectrometery detected cGAMP in 14.6% (7/48) SLE patients but not in any of the normal (n=19) or RA (n=22) controls. Disease activity was significantly higher in SLE patients with cGAMP detected versus those in which it was not detected (SLEDAI=7.0, SD 5.09 versus 3.52, SD 2.78, p=0.0102).
Conclusions Although cGAS appears to be a prominent cytosolic DNA sensor responsible for inducing IFN in response to infection, its role in human autoimmune diseases is unknown. Here, we describe for the first time, evidence of cGAS/cGAMP activation in human SLE. We demonstrated high disease activity associated with activation of this pathway. The inducing stimulus, cell type of production and overall importance in generating the IFN signature in SLE remain to be determined.
Disclosure of Interest None declared