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Lupus-like disease is driven by NETs enriched in mitochondrial DNA. Neutrophil extracellular traps (NETs) are implicated in autoimmunity, but how they are generated and their roles in sterile inflammation remain unclear. Ribonucleoprotein immune complexes (RNP ICs), inducers of NETosis, require mitochondrial reactive oxygen species (ROS) for maximal NET stimulation. After RNP IC stimulation of neutrophils, mitochondria become hypopolarized and translocate to the cell surface. Extracellular release of oxidized mitochondrial DNA is proinflammatory in vitro , and when this DNA is injected into mice, it stimulates type I interferon (IFN) signaling through a pathway dependent on the DNA sensor STING. Mitochondrial ROS are also necessary for spontaneous NETosis of low-density granulocytes from individuals with systemic lupus erythematosus. This was also observed in individuals with chronic granulomatous disease, who lack NADPH oxidase activity but still develop autoimmunity and type I IFN signatures. Mitochondrial ROS inhibition in vivo reduces disease severity and type I IFN responses in a mouse model of lupus. Together, these findings highlight a role for mitochondria in the generation not only of NETs but also of pro-inflammatory oxidized mitochondrial DNA in autoimmune diseases.

Infectious and autoimmune diseases are associated with anemia and thrombocytopenia. A severe form of inflammatory cytopenia called macrophage activation syndrome (MAS) may occur during rheumatological disorders and viral infections. Akilesh et al. show that monocyte recognition of self- or pathogen-derived nucleic acids via Toll-like receptors 7 and 9 (TLR7 and TLR9) drives MAS-like disease in mice. TLR7 or TLR9 signaling in monocytes causes these cells to differentiate into inflammatory hematophagocytes (iHPCs), which are similar to but distinct from red pulp macrophages. Preventing iHPC differentiation by depleting monocytes relieves MAS-like symptoms. When mice were subjected to a model of malarial anemia, MyD88- and endosomal TLR-dependent iHPC differentiation also occurred. Thus, iHPCs may play a role in both MAS-driven and malarial anemia, as well as thrombocytopenia. Science , this issue p. eaao5213 Immune receptor signaling underlies the process whereby inflammatory hemophagocytes drive anemia and thrombocytopenia. Cytopenias are an important clinical problem associated with inflammatory disease and infection. We show that specialized phagocytes that internalize red blood cells develop in Toll-like receptor 7 (TLR7)–driven inflammation. TLR7 signaling caused the development of inflammatory hemophagocytes (iHPCs), which resemble splenic red pulp macrophages but are a distinct population derived from Ly6C hi monocytes. iHPCs were responsible for anemia and thrombocytopenia in TLR7-overexpressing mice, which have a macrophage activation syndrome (MAS)–like disease. Interferon regulatory factor 5 (IRF5), associated with MAS, participated in TLR7-driven iHPC differentiation. We also found iHPCs during experimental malarial anemia, in which they required endosomal TLR and MyD88 signaling for differentiation. Our findings uncover a mechanism by which TLR7 and TLR9 specify monocyte fate and identify a specialized population of phagocytes responsible for anemia and thrombocytopenia associated with inflammation and infection.

Cells are equipped with numerous sensors that recognize nucleic acids, which probably evolved for defence against viruses. Once triggered, these sensors stimulate the production of type I interferons and other cytokines that activate immune cells and promote an antiviral state. The evolutionary conserved enzyme cyclic GMP–AMP synthase (cGAS) is one of the most recently identified DNA sensors. Upon ligand engagement, cGAS dimerizes and synthesizes the dinucleotide second messenger 2′,3′-cyclic GMP–AMP (cGAMP), which binds to the endoplasmic reticulum protein stimulator of interferon genes (STING) with high affinity, thereby unleashing an inflammatory response. cGAS-binding DNA is not restricted by sequence and must only be >45 nucleotides in length; therefore, cGAS can also be stimulated by self genomic or mitochondrial DNA. This broad specificity probably explains why the cGAS–STING pathway has been implicated in a number of autoinflammatory, autoimmune and neurodegenerative diseases; this pathway might also be activated during acute and chronic kidney injury. Therapeutic manipulation of the cGAS–STING pathway, using synthetic cyclic dinucleotides or inhibitors of cGAMP metabolism, promises to enhance immune responses in cancer or viral infections. By contrast, inhibitors of cGAS or STING might be useful in diseases in which this pro-inflammatory pathway is chronically activated.The cyclic GMP–AMP synthase (cGAS)–stimulator of interferon genes (STING) pathway not only is involved in host defence against infection but can lead to immune dysregulation. Here, the authors examine the biology and biochemistry of cGAS–STING and discuss its role in disease and potential approaches to therapeutic targeting.

B cell-activating factor (BAFF) contributes to the pathogenesis of autoimmune diseases including systemic lupus erythematosus (SLE). Although several anti-BAFF Abs and derivatives have been developed for the treatment of SLE, the specific sources of BAFF that sustain autoantibody (auto-Ab) producing cells have not been definitively identified. Using BAFF-RFP reporter mice, we identified major changes in BAFF-producing cells in two mouse spontaneous lupus models ( Tg mice and ), and in a pristane-induced lupus (PIL) model. First, we confirmed that similar to their wildtype Tg and mice counterparts, BAFF-RFP Tg mice and BAFF-RFP mice had increased BAFF serum levels, which correlated with increases in plasma cells and auto-Ab production. Next, using the RFP reporter, we defined which cells had dysregulated BAFF production. BAFF-producing neutrophils (Nphs), monocytes (MOs), cDCs, T cells and B cells were all expanded in the spleens of BAFF-RFP Tg mice and BAFF-RFP mice compared to controls. Furthermore, Ly6C inflammatory MOs and T cells had significantly increased BAFF expression per cell in both spontaneous lupus models, while CD8 DCs up-regulated BAFF expression only in the Tg mice. Similarly, pristane injection of BAFF-RFP mice induced increases in serum BAFF levels, auto-Abs, and the expansion of BAFF-producing Nphs, MOs, and DCs in both the spleen and peritoneal cavity. BAFF expression in MOs and DCs, in contrast to BAFF from Nphs, was required to maintain homeostatic and pristane-induced systemic BAFF levels and to sustain mature B cell pools in spleens and BMs. Although acting through different mechanisms, Nph, MO and DC sources of BAFF were each required for the development of auto-Abs in PIL mice. Our findings underscore the importance of considering the relative roles of specific myeloid BAFF sources and B cell niches when developing treatments for SLE and other BAFF-associated autoimmune diseases.

Sensing of cytosolic DNA by cyclic GMP–AMP synthase (cGAS) is central to the pathogenesis of a number of autoinflammatory syndromes and possibly some autoimmune diseases, such as systemic lupus erythematosus (SLE). Activation of cGAS signalling requires its deacetylation, so might aspirin have therapeutic potential to treat SLE by acetylating cGAS?

Photosensitivity to ultraviolet (UV) light affects up to ∼80% of lupus patients. Sunlight exposure can exacerbate local as well as systemic manifestations of lupus, including nephritis, by mechanisms that are poorly understood. Here, we report that acute skin exposure to UV light triggers a neutrophil-dependent injury response in the kidney characterized by upregulated expression of endothelial adhesion molecules as well as inflammatory and injury markers associated with transient proteinuria. We showed that UV light stimulates neutrophil migration not only to the skin but also to the kidney in an IL-17A–dependent manner. Using a photoactivatable lineage tracing approach, we observed that a subset of neutrophils found in the kidney had transited through UV light–exposed skin, suggesting reverse transmigration. Besides being required for the renal induction of genes encoding mediators of inflammation (vcam-1, s100A9, and Il-1b) and injury (lipocalin-2 and kim-1), neutrophils significantly contributed to the kidney type I interferon signature triggered by UV light. Together, these findings demonstrate that neutrophils mediate subclinical renal inflammation and injury following skin exposure to UV light. Of interest, patients with lupus have subpopulations of blood neutrophils and low-density granulocytes with similar phenotypes to reverse transmigrating neutrophils observed in the mice post-UV exposure, suggesting that these cells could have transmigrated from inflamed tissue, such as the skin.

Signaling through Toll-like receptor 7 (TLR7) drives the production of type I IFN and promotes the activation of autoreactive B cells and is implicated in the pathogenesis of systemic lupus erythematosus (SLE). While TLR7 has been extensively studied in murine lupus, much less is known about its role in the pathogenesis of human SLE. Genetic studies support a link between the TLR7 rs3853839 C/G polymorphism, which affects TLR7 mRNA turnover, and SLE susceptibility; however, the effects of this polymorphism on B cells have not been studied. Here we determined how changes in TLR7 expression affect peripheral B cells and auto-Ab production in SLE patients. High TLR7 expression in SLE patients driven by TLR7 rs3853839 C/G polymorphism was associated with more active disease and upregulation of IFN-responsive genes. TLR7hi SLE patients showed an increase in peripheral B cells. Most notably, the percentage and numbers of CD19+CD24++CD38++ newly-formed transitional (TR) B cells were increased in TLR7hi SLE patients as compared to HCs and TLR7norm/lo SLE patients. Using auto-Ab arrays, we found an increase and enrichment of auto-Ab specificities in the TLR7hi SLE group, including the production of anti-RNA/RNP-Abs. Upon in vitro TLR7 ligand stimulation, TR B cells isolated from TLR7hi but not TLR7norm/lo SLE patients produced anti-nuclear auto-Abs (ANA). Exposure of TR B cells isolated from cord blood to IFNα induced the expression of TLR7 and enabled their activation in response to TLR7 ligation in vitro . Our study shows that overexpression of TLR7 in SLE patients drives the expansion of TR B cells. High TLR7 signaling in TR B cells promotes auto-Ab production, supporting a possible pathogenic role of TR B cells in human SLE.

Most systemic lupus erythematosus (SLE) patients are photosensitive and ultraviolet B light (UVB) exposure worsens cutaneous disease and precipitates systemic flares of disease. The pathogenic link between skin disease and systemic exacerbations in SLE remains elusive. In an acute model of UVB-triggered inflammation, we observed that a single UV exposure triggered a striking IFN-I signature not only in the skin, but also in the blood and kidneys. The early IFN-I signature was significantly higher in female compared to male mice. The early IFN-I response in the skin was almost entirely, and in the blood partly, dependent on the presence of cGAS, as was skin inflammatory cell infiltration. Inhibition of cGAMP hydrolysis augmented the UVB-triggered IFN-I response. UVB skin exposure leads to cGAS-activation and both local and systemic IFN-I signature and could contribute to acute flares of disease in susceptible subjects such as patients with SLE.

Exactly how nucleic acids trigger type I interferon responses via certain Toll-like receptors has been uncertain. Now, a new pathway involving gene products previously linked to systemic lupus erythematosus but not known to interact has been unravelled, which could be of relevance to the female sex bias in this disease.

A number of cytokine pathways are important in the disease process of SLE, and several biological agents for SLE have been developed that target different cytokines or their receptors. This Review discusses the rationale for the use of anticytokine therapies in SLE, reviews the different agents tested to date, and presents future directions for therapy. Systemic lupus erythematosus (SLE) is a heterogeneous autoimmune disease involving most immune cells. Studies in both experimental animal models of lupus and patients with SLE have revealed a number of cytokine pathways that are important in the disease process. Among these are B-cell activating factor, which promotes B-cell survival and autoantibody production, interferon-α, which acts as an immune adjuvant, and tumor necrosis factor, which contributes to organ inflammation. This knowledge, in combination with the successful use of anti-TNF treatment in rheumatoid arthritis, has spurred the development of several biologic agents targeting different cytokines or their receptors in SLE. Consequently, many trials of anticytokine therapies for SLE are underway. Although most of these trials are small or in early phases, the results of some large studies have also been reported. In this Review, we discuss the rationale for anticytokine therapies in SLE and review agents currently in use, and those being developed and tested experimentally. We present the results from published trials and discuss the tentative conclusions that can be drawn regarding the efficacy of the new agents. Finally, we provide suggestions for the future of treatment for SLE, including new therapeutic strategies. Key Points Many cytokine pathways of importance in the pathogenesis of systemic lupus erythematosus (SLE) have been identified and clinical trials of biologic agents targeting a number of these are underway The clinical efficacy of belimumab, a human monoclonal antibody that binds B lymphocyte stimulator, has been demonstrated in two phase III studies Monoclonal antibodies against tumor necrosis factor, interferon-α, interleukin-6 and interferon-γ are in early-phase trials, and there are several other potential cytokine targets for SLE therapy Owing to the heterogeneous nature of SLE, the precise cytokine target will probably differ between individual patients and could also vary according to disease stage New and improved biomarkers are needed in order to optimize anticytokine therapy for established disease and also to prevent disease exacerbation