An immunohistochemical atlas of necroptotic pathway expression.

Necroptosis is a lytic form of regulated cell death reported to contribute to inflammatory diseases of the gut, skin and lung, as well as ischemic-reperfusion injuries of the kidney, heart and brain. However, precise identification of the cells and tissues that undergo necroptotic cell death in vivo has proven challenging in the absence of robust protocols for immunohistochemical detection. Here, we provide automated immunohistochemistry protocols to detect core necroptosis regulators - Caspase-8, RIPK1, RIPK3 and MLKL - in formalin-fixed mouse and human tissues. We observed surprising heterogeneity in protein expression within tissues, whereby short-lived immune barrier cells were replete with necroptotic effectors, whereas long-lived cells lacked RIPK3 or MLKL expression. Local changes in the expression of necroptotic effectors occurred in response to insults such as inflammation, dysbiosis or immune challenge, consistent with necroptosis being dysregulated in disease contexts. These methods will facilitate the precise localisation and evaluation of necroptotic signaling in vivo.

A common human MLKL polymorphism confers resistance to negative regulation by phosphorylation.

Across the globe, 2-3% of humans carry the p.Ser132Pro single nucleotide polymorphism in MLKL, the terminal effector protein of the inflammatory form of programmed cell death, necroptosis. Here we show that this substitution confers a gain in necroptotic function in human cells, with more rapid accumulation of activated MLKLS132P in biological membranes and MLKLS132P overriding pharmacological and endogenous inhibition of MLKL. In mouse cells, the equivalent Mlkl S131P mutation confers a gene dosage dependent reduction in sensitivity to TNF-induced necroptosis in both hematopoietic and non-hematopoietic cells, but enhanced sensitivity to IFN-ß induced death in non-hematopoietic cells. In vivo, MlklS131P homozygosity reduces the capacity to clear Salmonella from major organs and retards recovery of hematopoietic stem cells. Thus, by dysregulating necroptosis, the S131P substitution impairs the return to homeostasis after systemic challenge. Present day carriers of the MLKL S132P polymorphism may be the key to understanding how MLKL and necroptosis modulate the progression of complex polygenic human disease.

Administration of an LXR agonist promotes atherosclerotic lesion remodelling in murine inflammatory arthritis.

Objectives: The leading cause of mortality in patients with rheumatoid arthritis is atherosclerotic cardiovascular disease (CVD). We have shown that murine arthritis impairs atherosclerotic lesion regression, because of cellular cholesterol efflux defects in haematopoietic stem and progenitor cells (HSPCs), causing monocytosis and impaired atherosclerotic regression. Therefore, we hypothesised that improving cholesterol efflux using a Liver X Receptor (LXR) agonist would improve cholesterol efflux and improve atherosclerotic lesion regression in arthritis. Methods: Ldlr -/- mice were fed a western-type diet for 14 weeks to initiate atherogenesis, then switched to a chow diet to induce lesion regression and divided into three groups; (1) control, (2) K/BxN serum transfer inflammatory arthritis (K/BxN) or (3) K/BxN arthritis and LXR agonist T0901317 daily for 2 weeks. Results: LXR activation during murine inflammatory arthritis completely restored atherosclerotic lesion regression in arthritic mice, evidenced by reduced lesion size, macrophage abundance and lipid content. Mechanistically, serum from arthritic mice promoted foam cell formation, demonstrated by increased cellular lipid accumulation in macrophages and paralleled by a reduction in mRNA of the cholesterol efflux transporters Abca1, Abcg1 and Apoe. T0901317 reduced lipid loading and increased Abca1 and Abcg1 expression in macrophages exposed to arthritic serum and increased ABCA1 levels in atherosclerotic lesions of arthritic mice. Moreover, arthritic clinical score was also attenuated with T0901317. Conclusion: Taken together, we show that the LXR agonist T0901317 rescues impaired atherosclerotic lesion regression in murine arthritis because of enhanced cholesterol efflux transporter expression and reduced foam cell development in atherosclerotic lesions.

G-CSF drives autoinflammation in APLAID.

Missense mutations in PLCG2 can cause autoinflammation with phospholipase C gamma 2-associated antibody deficiency and immune dysregulation (APLAID). Here, we generated a mouse model carrying an APLAID mutation (p.Ser707Tyr) and found that inflammatory infiltrates in the skin and lungs were only partially ameliorated by removing inflammasome function via the deletion of caspase-1. Also, deleting interleukin-6 or tumor necrosis factor did not fully prevent APLAID mutant mice from autoinflammation. Overall, these findings are in accordance with the poor response individuals with APLAID have to treatments that block interleukin-1, JAK1/2 or tumor necrosis factor. Cytokine analysis revealed increased granulocyte colony-stimulating factor (G-CSF) levels as the most distinct feature in mice and individuals with APLAID. Remarkably, treatment with a G-CSF antibody completely reversed established disease in APLAID mice. Furthermore, excessive myelopoiesis was normalized and lymphocyte numbers rebounded. APLAID mice were also fully rescued by bone marrow transplantation from healthy donors, associated with reduced G-CSF production, predominantly from non-hematopoietic cells. In summary, we identify APLAID as a G-CSF-driven autoinflammatory disease, for which targeted therapy is feasible.

MLKL deficiency protects against low-grade, sterile inflammation in aged mice.

MLKL and RIPK3 are the core signaling proteins of the inflammatory cell death pathway, necroptosis, which is a known mediator and modifier of human disease. Necroptosis has been implicated in the progression of disease in almost every physiological system and recent reports suggest a role for necroptosis in aging. Here, we present the first comprehensive analysis of age-related histopathological and immunological phenotypes in a cohort of Mlkl-/- and Ripk3-/- mice on a congenic C57BL/6 J genetic background. We show that genetic deletion of Mlkl in female mice interrupts immune system aging, specifically delaying the age-related reduction of circulating lymphocytes. -Seventeen-month-old Mlkl-/- female mice were also protected against age-related chronic sterile inflammation in connective tissue and skeletal muscle relative to wild-type littermate controls, exhibiting a reduced number of immune cell infiltrates in these sites and fewer regenerating myocytes. These observations implicate MLKL in age-related sterile inflammation, suggesting a possible application for long-term anti-necroptotic therapy in humans.

CD98 defines a metabolically flexible, proinflammatory subset of low-density neutrophils in systemic lupus erythematosus.

BACKGROUND: Low-density neutrophils (LDN) are a distinct subset of neutrophils rarely detected in healthy people but appear in the blood of patients with autoimmune diseases, including systemic lupus erythematosus (SLE), and are mobilised in response to granulocyte colony-stimulating factor (G-CSF). The aim of this study was to identify novel mechanisms responsible for the pathogenic capacity of LDN in SLE. METHODS: Neutrophils were isolated from donors treated with G-CSF, and whole-cell proteomic analysis was performed on LDN and normal-density neutrophils. RESULTS: CD98 is significantly upregulated in LDN from G-CSF donors and defines a subset of LDN within the blood of SLE patients. CD98 is a transmembrane protein that dimerises with L-type amino acid transporters. We show that CD98 is responsible for the increased bioenergetic capacity of LDN. CD98 on LDN mediates the uptake of essential amino acids that are used by mitochondria to produce adenosine triphosphate, especially in the absence of glucose. Inhibition of CD98 reduces the metabolic flexibility of this population, which may limit their pathogenic capacity. CD98+ LDN produce more proinflammatory cytokines and chemokines than their normal density counterparts and are resistant to apoptosis, which may also contribute to tissue inflammation and end organ damage in SLE. CONCLUSIONS: CD98 provides a phenotypic marker for LDN that facilitates identification of this population without density-gradient separation and represents a novel therapeutic target to limit its pathogenic capacity.

Mechanistic Target of Rapamycin Inhibition Prevents Coronary Artery Remodeling in a Murine Model of Kawasaki Disease.

OBJECTIVE: Remodeling of the coronary arteries is a common feature in severe cases of Kawasaki disease (KD). This pathology is driven by the dysregulated proliferation of vascular fibroblasts, which can lead to coronary artery aneurysms, stenosis, and myocardial ischemia. We undertook this study to investigate whether inhibiting fibroblast proliferation might be an effective therapeutic strategy to prevent coronary artery remodeling in KD. METHOD: We used a murine model of KD (induced by the injection of the Candida albicans water-soluble complex [CAWS]) and analyzed patient samples to evaluate potential antifibrotic therapies for KD. RESULTS: We identified the mechanistic target of rapamycin (mTOR) pathway as a potential therapeutic target in KD. The mTOR inhibitor rapamycin potently inhibited cardiac fibroblast proliferation in vitro, and vascular fibroblasts up-regulated mTOR kinase signaling in vivo in the CAWS mouse model of KD. We evaluated the in vivo efficacy of mTOR inhibition and found that the therapeutic administration of rapamycin reduced vascular fibrosis and intimal hyperplasia of the coronary arteries in CAWS-injected mice. Furthermore, the analysis of cardiac tissue from KD fatalities revealed that vascular fibroblasts localizing with inflamed coronary arteries up-regulate mTOR signaling, confirming that the mTOR pathway is active in human KD. CONCLUSION: Our findings demonstrate that mTOR signaling contributes to coronary artery remodeling in KD, and that targeting this pathway offers a potential therapeutic strategy to prevent or restrict this pathology in high-risk KD patients.

How toxic is an old friend? A review of the safety of hydroxychloroquine in clinical practice.

Hydroxychloroquine (HCQ) and its close relative chloroquine (CQ) were initially used as antimalarial agents but are now widely prescribed in rheumatology, dermatology and immunology for the management of autoimmune diseases. HCQ is considered to have a better long-term safety profile than CQ and is therefore more commonly used. HCQ has a key role in the treatment of connective tissue diseases including systemic lupus erythematosus (SLE), where it provides beneficial immunomodulation without clinically significant immunosuppression. HCQ can also assist in managing inflammatory arthritis, including rheumatoid arthritis (RA). Debate around toxicity of HCQ in COVID-19 has challenged those who regularly prescribe HCQ to discuss its potential toxicities. Accordingly, we have reviewed the adverse effect profile of HCQ to provide guidance about this therapeutic agent in clinical practice.

Intimal macrophages develop from circulating monocytes during vasculitis.

Objective: Vasculitis is characterised by inflammation of the blood vessels. While all layers of the vessel can be affected, inflammation within the intimal layer can trigger thrombosis and arterial occlusion and is therefore of particular clinical concern. Given this pathological role, we have examined how intimal inflammation develops by exploring which (and how) macrophages come to populate this normally immune-privileged site during vasculitis. Methods: We have addressed this question for Kawasaki disease (KD), which is a type of vasculitis in children that typically involves the coronary arteries. We used confocal microscopy and flow cytometry to characterise the macrophages that populate the coronary artery intima in KD patient samples and in a mouse model of KD, and furthermore, have applied an adoptive transfer system to trace how these intimal macrophages develop. Results: In KD patients, intimal hyperplasia coincided with marked macrophage infiltration of the coronary artery intima. Phenotypic analysis revealed that these 'intimal macrophages' did not express markers of resident cardiac macrophages, such as Lyve-1, and instead, were uniformly positive for the chemokine receptor Ccr2, suggesting a monocytic lineage. In support of this origin, we show that circulating monocytes directly invade the intima via transluminal migration during established disease, coinciding with the activation of endothelial cells lining the coronary arteries. Conclusions: During KD, intimal macrophages develop from circulating monocytes that infiltrate the inflamed coronary artery intima by transluminal migration.

G-CSF Receptor Deletion Amplifies Cortical Bone Dysfunction in Mice With STAT3 Hyperactivation in Osteocytes.

Bone strength is determined by the structure and composition of its thickened outer shell (cortical bone), yet the mechanisms controlling cortical consolidation are poorly understood. Cortical bone maturation depends on SOCS3-mediated suppression of IL-6 cytokine-induced STAT3 phosphorylation in osteocytes, the cellular network embedded in bone matrix. Because SOCS3 also suppresses granulocyte-colony-stimulating factor receptor (G-CSFR) signaling, we here tested whether global G-CSFR (Csf3r) ablation altereed bone structure in male and female mice lacking SOCS3 in osteocytes, (Dmp1Cre :Socs3f/f mice). Dmp1Cre :Socs3f/f :Csf3r-/- mice were generated by crossing Dmp1Cre :Socs3f/f mice with Csf3r-/- mice. Although G-CSFR is not expressed in osteocytes, Csf3r deletion further delayed cortical consolidation in Dmp1Cre :Socs3f/f mice. Micro-CT images revealed extensive, highly porous low-density bone, with little true cortex in the diaphysis, even at 26 weeks of age; including more low-density bone and less high-density bone in Dmp1Cre :Socs3f/f :Csf3r-/- mice than controls. By histology, the area where cortical bone would normally be found contained immature compressed trabecular bone in Dmp1Cre :Socs3f/f :Csf3r-/- mice and greater than normal levels of intracortical osteoclasts, extensive new woven bone formation, and the presence of more intracortical blood vessels than the already high levels observed in Dmp1Cre :Socs3f/f controls. qRT-PCR of cortical bone from Dmp1Cre :Socs3f/f :Csf3r-/- mice also showed more than a doubling of mRNA levels for osteoclasts, osteoblasts, RANKL, and angiogenesis markers. The further delay in cortical bone maturation was associated with significantly more phospho-STAT1 and phospho-STAT3-positive osteocytes, and a threefold increase in STAT1 and STAT3 target gene mRNA levels, suggesting G-CSFR deletion further increases STAT signaling beyond that of Dmp1Cre :Socs3f/f bone. G-CSFR deficiency therefore promotes STAT1/3 signaling in osteocytes, and when SOCS3 negative feedback is absent, elevated local angiogenesis, bone resorption, and bone formation delays cortical bone consolidation. This points to a critical role of G-CSF in replacing condensed trabecular bone with lamellar bone during cortical bone formation. © 2022 American Society for Bone and Mineral Research (ASBMR).

Targeting necroptosis in muscle fibers ameliorates inflammatory myopathies.

Muscle cell death in polymyositis is induced by CD8+ cytotoxic T lymphocytes. We hypothesized that the injured muscle fibers release pro-inflammatory molecules, which would further accelerate CD8+ cytotoxic T lymphocytes-induced muscle injury, and inhibition of the cell death of muscle fibers could be a novel therapeutic strategy to suppress both muscle injury and inflammation in polymyositis. Here, we show that the pattern of cell death of muscle fibers in polymyositis is FAS ligand-dependent necroptosis, while that of satellite cells and myoblasts is perforin 1/granzyme B-dependent apoptosis, using human muscle biopsy specimens of polymyositis patients and models of polymyositis in vitro and in vivo. Inhibition of necroptosis suppresses not only CD8+ cytotoxic T lymphocytes-induced cell death of myotubes but also the release of inflammatory molecules including HMGB1. Treatment with a necroptosis inhibitor or anti-HMGB1 antibodies ameliorates myositis-induced muscle weakness as well as muscle cell death and inflammation in the muscles. Thus, targeting necroptosis in muscle cells is a promising strategy for treating polymyositis providing an alternative to current therapies directed at leukocytes.

Emerging roles for IL-11 in inflammatory diseases.

Interleukin-11 (IL-11) is a cytokine that has been strongly implicated in the pathogenesis of fibrotic diseases and solid malignancies. Elevated IL-11 expression is also associated with several non-malignant inflammatory diseases where its function remains less well-characterized. Here, we summarize current literature surrounding the contribution of IL-11 to the pathogenesis of autoimmune inflammatory diseases, including rheumatoid arthritis, multiple sclerosis, diabetes and systemic sclerosis, as well as other chronic inflammatory conditions such as periodontitis, asthma, chronic obstructive pulmonary disease, psoriasis and colitis.

Hydroxychloroquine inhibits the mitochondrial antioxidant system in activated T cells.

Although hydroxychloroquine (HCQ) has long been used to treat autoimmune diseases, its mechanism of action remains poorly understood. In CD4 T-cells, we found that a clinically relevant concentration of HCQ inhibited the mitochondrial antioxidant system triggered by TCR crosslinking, leading to increased mitochondrial superoxide, impaired activation-induced autophagic flux, and reduced proliferation of CD4 T-cells. In antigen-presenting cells, HCQ also reduced constitutive activation of the endo-lysosomal protease legumain and toll-like receptor 9, thereby reducing cytokine production, but it had little apparent impact on constitutive antigen processing and peptide presentation. HCQ's effects did not require endo-lysosomal pH change, nor impaired autophagosome-lysosome fusion. We explored the clinical relevance of these findings in patients with celiac disease-a prototypic CD4 T-cell-mediated disease-and found that HCQ limits ex vivo antigen-specific T cell responses. We report a T-cell-intrinsic immunomodulatory effect from HCQ and suggest potential re-purposing of HCQ for celiac disease.

G-CSF - A double edge sword in neutrophil mediated immunity.

Neutrophils are vital for the innate immune system's control of pathogens and neutrophil deficiency can render the host susceptible to life-threatening infections. Neutrophil responses must also be tightly regulated because excessive production, recruitment or activation of neutrophils can cause tissue damage in both acute and chronic inflammatory diseases. Granulocyte colony stimulating factor (G-CSF) is a key regulator of neutrophil biology, from production, differentiation, and release of neutrophil precursors in the bone marrow (BM) to modulating the function of mature neutrophils outside of the BM, particularly at sites of inflammation. G-CSF acts by binding to its cognate cell surface receptor on target cells, causing the activation of intracellular signalling pathways mediating the proliferation, differentiation, function, and survival of cells in the neutrophil lineage. Studies in humans and mice demonstrate that G-CSF contributes to protecting the host against infection, but conversely, it can play a deleterious role in inflammatory diseases. As such, neutrophils and the G-CSF pathway may provide novel therapeutic targets. This review will focus on understanding the role G-CSF plays in the balance between effective neutrophil mediated host defence versus neutrophil-mediated inflammation and tissue damage in various inflammatory and infectious diseases.

Differential requirement for the Polycomb repressor complex 2 in dendritic cell and tissue-resident myeloid cell homeostasis.

Dendritic cells (DCs) and macrophages are at the forefront of immune responses, modifying their transcriptional programs in response to their tissue environment or immunological challenge. Posttranslational modifications of histones, such as histone H3 lysine-27 trimethylation (H3K27me3) by the Polycomb repressive complex 2 (PRC2), are tightly associated with epigenetic regulation of gene expression. To explore whether H3K27me3 is involved in either the establishment or function of the mononuclear phagocyte system, we selectively deleted core components of PRC2, either EZH2 or SUZ12, in CD11c-expressing myeloid cells. Unexpectedly, EZH2 deficiency neither prevented the deposition and maintenance of H3K27me3 in DCs nor hindered DC/macrophage homeostasis. In contrast, SUZ12 deficiency markedly impaired the capacity of DCs and macrophages to maintain H3K27me3. SUZ12 ablation induced a rapid loss of the alveolar macrophage and Langerhans cell networks under both steady state and inflammatory conditions because these cells could no longer proliferate to facilitate their self-renewal. Despite the reduced H3K27me3, DC development and function were unaffected by SUZ12 ablation, suggesting that PRC2-mediated gene repression was dispensable for DC homeostasis. Thus, the role of SUZ12 highlights the fundamentally different homeostatic mechanisms used by tissue-resident myeloid cells versus DCs.

Extracellular Vesicles in Synovial Fluid from Rheumatoid Arthritis Patients Contain miRNAs with Capacity to Modulate Inflammation.

In rheumatoid arthritis (RA), extracellular vesicles (EVs) are associated with both the propagation and attenuation of joint inflammation and destruction. However, the specific EV content responsible for these processes is largely unknown. Investigations into identifying EV content are confounded by the challenges in obtaining high-quality EV preparations from synovial fluid. Implementing a size exclusion chromatography-based method of EV isolation, coupled with small RNA sequencing, we accurately characterised EV miRNAs in synovial fluid obtained from RA patients and investigated the differences between joints with high- and low-grade inflammation. Synovial fluid was obtained from the joints of 12 RA patients and, based on leukocyte counts, classified as either high (n = 7)- or low (n = 5)-grade inflammation. Using size exclusion chromatography, EVs were purified and small RNA was extracted and sequenced on a NextSeq 500. Sequencing reads were aligned to miRBase v21, and differences in miRNA profiles between RA patients with high- and low-grade joint inflammation were analysed. In total, 1972 distinct miRNAs were identified from RA synovial fluid EVs. miRNAs with less than five reads in fewer than five patients were filtered out, leaving 318 miRNAs for analysis. Analysis of the most abundant miRNAs suggested that they negatively regulate multiple genes relevant to inflammation, including signal transducer and activator of transcription 3 (STAT3), which lies downstream of IL-6 and has a pro-inflammatory role in RA. Synovial fluid from joints with high-grade inflammation contained 3.5-fold more EV miRNA per mL of synovial fluid (p = 0.0017). Seventy-eight EV miRNAs were differentially expressed between RA joints with high- and low-grade inflammation, and pathway analysis revealed that their target genes were commonly involved a variety of processes, including cellular apoptosis, proliferation and migration. Of the 49 miRNAs that were elevated in joints with high-grade inflammation, pathway analysis revealed that genes involved in cytokine-mediated signalling pathways were significantly enriched targets. In contrast, genes associated with reactive oxygen species signalling were significantly enriched as targets of the 29 miRNAs elevated in joints with low-grade inflammation. Our study identified an abundance of EV miRNAs from the synovial fluid of RA patients with the potential to modulate inflammation. In doing so, we defined potential mechanisms by which synovial fluid EVs may contribute to RA pathophysiology.

Blockade of the co-inhibitory molecule PD-1 unleashes ILC2-dependent antitumor immunity in melanoma.

Group 2 innate lymphoid cells (ILC2s) are essential to maintain tissue homeostasis. In cancer, ILC2s can harbor both pro-tumorigenic and anti-tumorigenic functions, but we know little about their underlying mechanisms or whether they could be clinically relevant or targeted to improve patient outcomes. Here, we found that high ILC2 infiltration in human melanoma was associated with a good clinical prognosis. ILC2s are critical producers of the cytokine granulocyte-macrophage colony-stimulating factor, which coordinates the recruitment and activation of eosinophils to enhance antitumor responses. Tumor-infiltrating ILC2s expressed programmed cell death protein-1, which limited their intratumoral accumulation, proliferation and antitumor effector functions. This inhibition could be overcome in vivo by combining interleukin-33-driven ILC2 activation with programmed cell death protein-1 blockade to significantly increase antitumor responses. Together, our results identified ILC2s as a critical immune cell type involved in melanoma immunity and revealed a potential synergistic approach to harness ILC2 function for antitumor immunotherapies.

BCL-XL antagonism selectively reduces neutrophil life span within inflamed tissues without causing neutropenia.

Neutrophils help to clear pathogens and cellular debris, but can also cause collateral damage within inflamed tissues. Prolonged neutrophil residency within an inflammatory niche can exacerbate tissue pathology. Using both genetic and pharmacological approaches, we show that BCL-XL is required for the persistence of neutrophils within inflammatory sites in mice. We demonstrate that a selective BCL-XL inhibitor (A-1331852) has therapeutic potential by causing apoptosis in inflammatory human neutrophils ex vivo. Moreover, in murine models of acute and chronic inflammatory disease, it reduced inflammatory neutrophil numbers and ameliorated tissue pathology. In contrast, there was minimal effect on circulating neutrophils. Thus, we show a differential survival requirement in activated neutrophils for BCL-XL and reveal a new therapeutic approach to neutrophil-mediated diseases.

Natural killer cells in inflammatory autoimmune diseases.

Natural killer (NK) cells are a specialised population of innate lymphoid cells (ILCs) that help control local immune responses. Through natural cytotoxicity, production of cytokines and chemokines, and migratory capacity, NK cells play a vital immunoregulatory role in the initiation and chronicity of inflammatory and autoimmune responses. Our understanding of their functional differences and contributions in disease settings is evolving owing to new genetic and functional murine proof-of-concept studies. Here, we summarise current understanding of NK cells in several classic autoimmune disorders, particularly in rheumatoid arthritis (RA), multiple sclerosis (MS), systemic lupus erythematosus (SLE) and type 1 diabetes mellitus (T1DM), but also less understood diseases such as idiopathic inflammatory myopathies (IIMs). A better understanding of how NK cells contribute to these autoimmune disorders may pave the way for NK cell-targeted therapeutics.