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Abstract
Systemic Lupus Erythematosus (SLE) is a chronic inflammatory autoimmune disease in which type I interferons (IFN) play a key role. The IFN response can be triggered when oxidized DNA engages the cytosolic DNA sensing platform cGAS-STING, but the repair mechanisms that modulate this process and govern disease progression are unclear. To gain insight into this biology, we interrogated the role of oxyguanine glycosylase 1 (OGG1), which repairs oxidized guanine 8-Oxo-2′-deoxyguanosine (8-OH-dG), in the pristane-induced mouse model of SLE. Ogg1−/− mice showed increased influx of Ly6Chi monocytes into the peritoneal cavity and enhanced IFN-driven gene expression in response to short-term exposure to pristane. Loss of Ogg1 was associated with increased auto-antibodies (anti-dsDNA and anti-RNP), higher total IgG, and expression of interferon stimulated genes (ISG) to longer exposure to pristane, accompanied by aggravated skin pathology such as hair loss, thicker epidermis, and increased deposition of IgG in skin lesions. Supporting a role for type I IFNs in this model, skin lesions of Ogg1−/− mice had significantly higher expression of type I IFN genes (Isg15, Irf9, and Ifnb). In keeping with loss of Ogg1 resulting in dysregulated IFN responses, enhanced basal and cGAMP-dependent Ifnb expression was observed in BMDMs from Ogg1−/− mice. Use of the STING inhibitor, H151, reduced both basal and cGAMP-driven increases, indicating that OGG1 regulates Ifnb expression through the cGAS-STING pathway. Finally, in support for a role for OGG1 in the pathology of cutaneous disease, reduced OGG1 expression in monocytes associated with skin involvement in SLE patients and the expression of OGG1 was significantly lower in lesional skin compared with non-lesional skin in patients with Discoid Lupus. Taken together, these data support an important role for OGG1 in protecting against IFN production and SLE skin disease.
Highlights
Systemic Lupus Erythematosus (SLE), a chronic systemic inflammatory autoimmune disease, occurs in about 1.8–7.6 people per 100,000 in the United States, with a 9:1 female to male ratio [1]
Analysis of peritoneal lavage demonstrated that pristane treatment resulted in increased recruitment of inflammatory CD11b+Ly6Chi monocytes and expression of Siglec1, an IFN inducible gene, in Ogg1−/− mice compared to WT mice, whereas neutrophil (CD11b+Ly6G+) recruitment was similar across both genotypes (Figures 1A–D)
In order to assess the level of lupus-like systemic inflammatory responses induced by pristane, we measured circulating levels of anti-dsDNA and anti-RNP autoantibodies, and found that all were substantially higher in Ogg1−/− mice than WT mice 10 months after pristane exposure (Figures 2D,E), whereas total IgG, IgG1, and IgG2a and other autoantibodies showed no difference between genotypes (Supplementary Figures 1A–D)
Summary
SLE, a chronic systemic inflammatory autoimmune disease, occurs in about 1.8–7.6 people per 100,000 in the United States, with a 9:1 female to male ratio [1]. Patients with SLE have a diverse range of immunological abnormalities that contribute to disease progression and pathology. One source of extracellular DNA that initiates and drives type I IFN production is genomic DNA released from dead and dying cells, which is recognized by TLR9 in endosomes, driving IFNα expression. Oxidized DNA from mitochondria (driven by ROS production) has been recognized as a trigger for type I IFN production. In SLE, low density neutrophils/granulocytes are an important source of oxidized mitochondrial DNA (mtDNA), which is released from activated neutrophils as they undergo NETosis. Oxidized DNA, including 8-OH-dG, is sensed by the cytosolic DNA sensor cyclic GMP-AMP synthase (cGAS), which has recently emerged as an important contributor to elevated type I IFN in SLE [6, 7]. CGAS generates 2′ 3′ -cyclic-dinucleotides which bind and activate the ER-resident adaptor protein stimulator of IFN genes (STING), and drive TBK1-IRF3 dependent induction of IFNβ [7,8,9,10]
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