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Severe COVID-19 associated pneumonia patients may exhibit features of systemic hyper-inflammation designated under the umbrella term of macrophage activation syndrome (MAS) or cytokine storm, also known as secondary haemophagocytic lymphohistocytosis (sHLH). This is distinct from HLH associated with immunodeficiency states termed primary HLH -with radically different therapy strategies in both situations. COVID-19 infection with MAS typically occurs in subjects with adult respiratory distress syndrome (ARDS) and historically, non-survival in ARDS was linked to sustained IL-6 and IL-1 elevation. We provide a model for the classification of MAS to stratify the MAS-like presentation in COVID-19 pneumonia and explore the complexities of discerning ARDS from MAS. We discuss the potential impact of timing of anti-cytokine therapy on viral clearance and the impact of such therapy on intra-pulmonary macrophage activation and emergent pulmonary vascular disease., (Crown Copyright © 2020. Published by Elsevier B.V. All rights reserved.)

Hemophagocytic lymphohistiocytosis (HLH) and macrophage activation syndrome (MAS) are life-threatening hyperferritinemic systemic inflammatory disorders. Although profound cytotoxic impairment causes familial HLH (fHLH), the mechanisms driving non-fHLH and MAS are largely unknown. MAS occurs in patients with suspected rheumatic disease, but the mechanistic basis for its distinction is unclear. Recently, a syndrome of recurrent MAS with infantile enterocolitis caused by NLRC4 inflammasome hyperactivity highlighted the potential importance of interleukin-18 (IL-18). We tested this association in hyperferritinemic and autoinflammatory patients and found a dramatic correlation of MAS risk with chronic (sometimes lifelong) elevation of mature IL-18, particularly with IL-18 unbound by IL-18 binding protein, or free IL-18. In a mouse engineered to carry a disease-causing germ line NLRC4 T337S mutation, we observed inflammasome-dependent, chronic IL-18 elevation. Surprisingly, this NLRC4 T337S -induced systemic IL-18 elevation derived entirely from intestinal epithelia. NLRC4 T337S intestines were histologically normal but showed increased epithelial turnover and upregulation of interferon-γ-induced genes. Assessing cellular and tissue expression, classical inflammasome components such as Il1b , Nlrp3, and Mefv predominated in neutrophils, whereas Nlrc4 and Il18 were distinctly epithelial. Demonstrating the importance of free IL-18, Il18 transgenic mice exhibited free IL-18 elevation and more severe experimental MAS. NLRC4 T337S mice, whose free IL-18 levels were normal, did not. Thus, we describe a unique connection between MAS risk and chronic IL-18, identify epithelial inflammasome hyperactivity as a potential source, and demonstrate the pathogenicity of free IL-18. These data suggest an IL-18-driven pathway, complementary to the cytotoxic impairment of fHLH, with potential as a distinguishing biomarker and therapeutic target in MAS.

The term macrophage activation syndrome (MAS) defines a severe, potentially fatal disorder characterized by overwhelming inflammation and multiorgan involvement. Interleukin-18 (IL-18) is a proinflammatory cytokine belonging to the IL-1 family, the activity of which is regulated by its endogenous inhibitor IL-18 binding protein (IL-18BP). Elevated IL-18 levels have been reported in patients with MAS. Herein, we show that on repeated toll-like receptor 9 (TLR9) stimulation with unmethylated cytosine guanine dinucleotide containing single-stranded DNA (CpG), IL-18BP -/- mice display severe MAS manifestations, including increased weight loss, splenomegaly, anemia, thrombocytopenia, hyperferritinemia, and bone marrow hemophagocytosis as compared with wild-type mice. Serum-free IL-18 was detected in CpG-treated IL-18BP -/- mice only. Levels of interferon-γ (IFN-γ) and of IFN-γ signature genes, such as the chemokine Cxcl9 or the transcription factor CIIta , were significantly increased in IL-18BP -/- mice. Blocking IL-18 receptor signaling attenuated the severity of MAS and IFN-γ responses in IL-18BP -/- mice. Blocking IFN-γ had comparable effects to IL-18 inhibition on most MAS manifestations. Our data indicate that endogenous IL-18BP exerts a protective role in CpG-induced MAS and that IL-18, which acts upstream of IFN-γ, is involved in the severity of MAS., (© 2018 by The American Society of Hematology.)

Systemic juvenile idiopathic arthritis (sJIA) is considered as a polygenic autoinflammatory disease. The prominent systemic clinical features, the marked elevation of inflammatory markers, and the absence of autoantibodies make this disease very different from the other juvenile idiopathic arthritis (JIA) forms. Innate immune mechanisms appear to play a central role: the overproduction of inflammatory cytokines of innate immunity is a typical feature of sJIA. Increased understanding of the role of these cytokines has been translated into therapeutic approaches. Indeed, remarkable results have been observed with interleukin-1 (IL-1) and interleukin-6 (IL-6) inhibitors both in clinical trials and in real life. Other inhibitors of these cytokines will be available in the near future, thereby increasing the therapeutic armamentarium. A better understanding of the pathophysiology of sJIA is still needed to identify IL-1 or IL-6 responders, define a potential window of opportunity for early cytokine blockade, and identify a targeted treatment for macrophage activation syndrome. Additional therapeutic targets are needed for a small proportion of patients who do not respond to either IL-1 or IL-6 inhibition., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Systemic-onset juvenile idiopathic arthritis (SoJIA) is a systemic inflammatory disease which has up to now been classified as a category of juvenile idiopathic arthritis. However, in this context, systemic inflammation has been associated with dysregulation of the innate immune system, suggesting that it may rather be part of the spectrum of autoinflammatory disorders. The disease is in fact unique with regard to the other JIA categories, in terms of clinical manifestations, prognosis, and response to conventional immunosuppressant therapies. It is characterized clinically by fever, lymphadenopathy, arthritis, rash, and serositis. IL-1 and IL-6 play a major role in the pathogenesis of SoJIA, and treatment with IL-1 and IL-6 inhibitors has shown to be highly effective. However, complications of SoJIA, including macrophage activation syndrome, limitations in functional outcome by arthritis and long-term damage from chronic inflammation continue to be a major issue in patients' care. Recent advances on the pathogenesis and treatment have revolutionized the care and prognosis of this potentially life-threatening pediatric condition., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Macrophage activation syndrome (MAS) is an episode of overwhelming inflammation that occurs most commonly in children with systemic juvenile idiopathic arthritis (SJIA). It is characterized by expansion and activation of T lymphocytes and hemophagocytic macrophages and bears great similarity to hemophagocytic lymphohistiocytosis (HLH). This disorder has substantial morbidity and mortality, and there is frequently a delay in recognition and initiation of treatment. Here, we will review what is known about the pathogenesis of MAS and, in particular, its similarities to HLH. The development of MAS is characterized by a cytokine storm, with the elaboration of numerous pro-inflammatory cytokines. We will examine the evidence for various cytokines in the initiation and pathogenesis of MAS and discuss how new biologic therapies may alter the risk of MAS. Finally, we will review current treatment options for MAS and examine how cytokine-directed therapy could serve as novel treatment modalities., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Systemic juvenile idiopathic arthritis (sJIA) is a rare, systemic inflammatory disease classified as a subtype of JIA. Besides arthritis, it is characterised by systemic features such as spiking fever, skin rash, hepatosplenomegaly or serositis. It is becoming clear now that abnormalities in the innate immunity (cytokines such as interleukin (IL)-1, IL-6 and IL-18, and neutrophils and monocytes/macrophages rather than lymphocytes) play a major role in the pathogenesis of sJIA, distinguishing it from other JIA subtypes. Another distinctive feature of sJIA is its strong association with macrophage activation syndrome (MAS). Based on this, consensus is emerging that sJIA should be viewed as an autoinflammatory syndrome rather than a classic auto-immune disease. As a consequence of the progression in understanding the underlying mechanisms of sJIA, major changes in the management are evolving. So far, treatment has been based on glucocorticosteroids in combination with disease-modifying drugs such as methotrexate. Recently, remarkable improvement has been observed with IL-1 and IL-6 targeted therapies. These therapies might also change the long-term outcome of this disease. However, controlled trials set up in international collaboration are needed to determine the optimal treatment strategies for all sJIA patients.

Multi-organic failure in the context of autoimmune diseases is a multi-factorial condition where different pathways concur to produce a global system breakdown. Some of these pathways include the coagulation, fibrinolysis, kinin and complement cascades which in normal conditions work together to provide a comprehensive response to injury. In pathologic conditions these regulatory mechanisms are replaced by positive feed-back loops. The common response pattern is the activation of the immune system via endothelium activation. Furthermore, these different plasma-driven mechanisms may induce standardised endothelial cell responses of which the most relevant are the activation of p38, JNK, NF-kbeta and IRF-3 pathways. In this paper we review the common points between these major pathways and how they become activated, contributing to a global clinical picture. We present two examples of apparently different clinical settings, caused by the same global dysfunction: the Macrophage Activation Syndrome and the iatrogenic "cytokine storm" triggered by the administration of anti-CD28 monoclonal antibody TGN1412 in a phase 1 trial.