Emerging areas for therapeutic discovery in SLE

Abstract

Recent advances in the field of autoimmunity have identified numerous dysfunctional pathways in Systemic Lupus Erythematosus (SLE), including aberrant clearance of nucleic-acid-containing debris and immune complexes, excessive innate immune activation leading to overactive type I IFN signalling, and abnormal B and T cell activation. On the background of genetic polymorphisms that reset thresholds for immune responses, multiple immune cells contribute to inflammatory amplification circuits. Neutrophils activated by immune complexes are a rich source of immunogenic nucleic acids. Identification of new B subsets suggests several mechanisms for induction of autoantibody producing effector cells. Disordered T cell regulation involves both CD4 and CD8 cells. An imbalance in immunometabolism in immune cells amplifies autoimmunity and inflammation. These new advances in understanding of disease pathogenesis provide fertile ground for therapeutic development.

Figure 1.. Innate immune mechanisms in SLE

1A. An intact pathway for safe recognition and digestion of dead cells by macrophages includes a functional sensing mechanism, upregulation of scavenger receptors and activation of an alternative autophagy pathway called lysosome activated phagocytosis (LAP) that recruits part of the autophagy machinery, including LC3, to single membrane phagosomes and induces the release of regulatory cytokines such as IL-10 and TGFβ. Absence of LAP, but not of canonical autophagy in macrophages results in inefficient clearance of phagosomal material and release of inflammatory cytokines; this is associated with a mild spontaneous form of SLE in mouse models [58]. 1B. Extracellular particles containing nucleic acids within immune complexes, microparticles, biofilms and neutrophil NETs are internalized, often via receptor mediated phagocytosis, and encounter endosomal Toll like receptors, TLRs 7, 8 and 9 within phagosomes. These TLRs use the adaptor molecular MyD88 and induce either inflammatory cytokines or Type 1 interferons. Absence of MyD88, IRAK4 and IRF5 all prevent SLE in mouse models. A role for DNAse1L3 in clearance of DNA associated with extracellular microparticles has recently been described [59]. 1C. Intracellular nucleic acids engage a different set of sensors. The intracellular DNAse Trex1 clears cytoplasmic DNA; complete deficiency causes an interferonopathy called Aicardi-Goutieres syndrome. Cyclic GMP-AMP synthase (cGAS) catabolizes the formation of the cyclic dinucleotide cGAMP from excess digested cytoplasmic DNA. cGAMP binds to and activates the stimulator of IFN genes (STING), located in the endoplasmic reticulum, resulting in phosphorylation of NFκB and IRFs and the production of inflammatory cytokines. Similarly, cytoplasmic RNA sensors, RIG-I and MDA5 bind to MAVS, a mitochondrial membrane protein that similarly phosphorylates NFκB and IRFs to induce inflammatory cytokines needed for the anti-viral response. An immunogenic role has been defined for oxidized mitochondrial DNA released from activated neutrophils that do not perform mitophagy efficiently. Both TLR9 and STING have been implicated in recognition of neutrophil derived oxidized mitochondrial DNA [15,16]. Mitochondrial DNA released into the cytoplasm during apoptosis may also trigger the activation of cGAS.

Figure 2.. The intersection of innate and adaptive immunity in regulating autoantibody production in SLE

Pathways of B cell differentiation and some of the mediators that drive pathway specific differentiation. Upon activation of naïve B cells, TLR9 engagement, CD40 ligation and higher BCR affinity favor the extrafollicular route leading to short term plasma cell differentiation, whereas lower BCR affinity favors the germinal center route. The spontaneous formation of germinal centers is a feature of many mouse models of SLE; recent studies have shown the B cell intrinsic requirement for TLR7, IFNγ receptor and IL6 in their formation [21,60,61]. If B cells enter the germinal center, lower affinity BCR interactions are associated with lower expression of integrins and higher expression of Bcl6; this results in less T cell help and favors the memory cell or recycling GC fate. Long-lived plasma cell differentiation is enhanced when higher BCR affinity facilitates stable interactions with follicular T helper cells in the light zones and is associated with lower expression of Bcl6 and upregulation of BLIMP1 [62]. Surprisingly, in mouse models, excess Type I interferon enhances germinal center reactions but skews plasma cells to a short-lived fate [63]. Orange labels: Stages of B cell differentiation that are influenced by innate immune mediators intrinsic to B cells or interacting with receptors on B cells. Red arrows: Stages of development at which innate signals preferentially induce autoantibodies. Blue arrows: Autoreactive B cells arise routinely but are regulated at every stage of the B cell developmental pathway. Dashed arrows: Potential pathways not fully confirmed. MZ: marginal zone; GC: germinal center; LZ: light zone; DZ: dark zone; ABC: age associated B cell; PB: plasmablast; PC: plasma cell.

Figure 3.. Metabolic pathways are fertile ground for therapeutic interventions in SLE

Metabolic pathways influence effector function of multiple cell types. Glycolysis is typically associated with inflammatory functions of immune cells whereas oxidative phosphorylation is associated with suppressive or reparative functions. Activated CD4 T cells in SLE have enhanced glycolysis and oxidative phosphorylation. Metformin inhibits mitochondrial complex 1, thereby inhibiting oxidative phosphorylation, but also acts via the mTOR pathway to increase fatty acid oxidation and glycolysis. The latter is inhibited by 2 deoxy-glucose (2DG), explaining why the combination of metformin and 2DG can reverse active SLE in mouse models, whereas neither drug can do this alone [51].