Search

A vast body of evidence provides support to a central role of exaggerated production of interferon-γ (IFN-γ) in causing hypercytokinemia and signs and symptoms of hemophagocytic lymphohistiocytosis (HLH). In this chapter, we will describe briefly the roles of IFN-γ in innate and adaptive immunity and in host defense, summarize results from animal models of primary HLH and secondary HLH with particular emphasis on targeted therapeutic approaches, review data on biomarkers associated with activation of the IFN-γ pathway, and discuss initial efficacy and safety results of IFN-γ neutralization in humans., (© 2024. The Author(s), under exclusive license to Springer Nature Switzerland AG.)

Multi-system Inflammatory Syndrome in Children (MIS-C) is a major complication of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection in pediatric patients. Weeks after an often mild or asymptomatic initial infection with SARS-CoV-2 children may present with a severe shock-like picture and marked inflammation. Children with MIS-C present with varying degrees of cardiovascular and hyperinflammatory symptoms. Here we perform a comprehensive analysis of the plasma proteome of more than 1400 proteins in children with SARS-CoV-2. We hypothesize that the proteome would reflect heterogeneity in hyperinflammation and vascular injury, and further identify pathogenic mediators of disease. We show that protein signatures demonstrate overlap between MIS-C, and the inflammatory syndromes macrophage activation syndrome (MAS) and thrombotic microangiopathy (TMA). We demonstrate that PLA2G2A is an important marker of MIS-C that associates with TMA. We find that IFNγ responses are dysregulated in MIS-C patients, and that IFNγ levels delineate clinical heterogeneity., (© 2021. The Author(s).)

Objective: Familial hemophagocytic lymphohistiocytosis (FHLH) is a complex cytokine storm syndrome caused by genetic abnormalities rendering CD8+ T cells and natural killer cells incapable of cytolytic killing. In murine models of FHLH, interferon-γ (IFNγ) produced by CD8+ T cells has been identified as a critical mediator of disease, and an IFNγ-blocking antibody (emapalumab) has recently been approved by the Food and Drug Administration. However, development of hemophagocytic lymphohistiocytosis (HLH)/macrophage activation syndrome (MAS) in patients who are genetically unresponsive to IFNγ questions the absolute necessity of IFNγ in driving disease. This study was undertaken to determine the necessity of IFNγ in driving HLH., Methods: IFNγ -/- Prf1 -/- mice were infected with lymphocytic choriomeningitis virus (LCMV), and HLH immunopathologic features, including survival, weight loss, cytopenias, cytokine profiles, and immune cell phenotypes, were assessed. Mixed bone marrow chimeras were created to determine the immune cell-intrinsic role of IFNγ receptor signaling. CD8+ T cell depletion and interleukin-33 (IL-33)/ST2 blockade were performed using monoclonal antibodies., Results: LCMV infection of IFNγ -/- Prf1 -/- mice resulted in severe HLH-like disease. CD8+ T cells and the IL-33/ST2 axis remained essential mediators of disease; however, IFNγ-independent HLH immunopathology correlated with a 10-15-fold increase in neutrophilia (P < 0.001) and an altered cytokine milieu dominated by IL-6, IL-1β, and granulocyte-macrophage colony-stimulating factor (GM-CSF) (P < 0.05). Furthermore, IFNγ regulated CD8+ T cell expression of GM-CSF and neutrophil survival., Conclusion: IFNγ is not necessary for the development of fulminant HLH, requiring physicians to consider case-by-case treatment strategies. Use of therapies that target upstream activators of CD8+ T cells, such as IL-33/ST2 signaling, may be more universally applicable treatment options that ameliorate both IFNγ-dependent and -independent manifestations of HLH/MAS., (© 2019, American College of Rheumatology.)

Objective: Macrophage activation syndrome (MAS) is a life-threatening cytokine storm syndrome that occurs in patients with underlying rheumatic diseases. Preclinical and clinical data suggest that interferon-γ (IFNγ) is pathogenic in MAS, but how IFNγ may be linked to disease pathogenesis remains unknown. This study was undertaken to determine whether IFNγ signals synergize with systemic innate immune responses to drive the cytokine storm in a murine model of MAS., Methods: IFNγ-deficient mice were treated with 5 doses of the Toll-like receptor 9 (TLR-9) agonist CpG 1826, IFNγ, or a combination of the 2 stimuli over the course of 10 days. Immunopathologic features of MAS, including cytopenias, hepatitis, hepatosplenomegaly, and induction of inflammatory myelopoiesis, were assessed. Mixed bone marrow chimeras were created to determine whether TLR-9- and IFNγ receptor 1 (IFNγR1)-dependent signals induce enhanced myelopoiesis in a cell-intrinsic or cell-extrinsic manner., Results: IFNγ-deficient mice did not develop features of MAS when treated with repeated doses of either the TLR-9 agonist or IFNγ alone. In contrast, IFNγ-deficient mice treated with both the TLR-9 agonist and IFNγ developed cytopenias, hepatitis, and hepatosplenomegaly, reproducing major clinical features of MAS. TLR-9- and IFNγR1-dependent signals synergized to enhance myeloid progenitor cell function and induce myelopoiesis in vivo, which occurred through cell-extrinsic mechanisms and correlated with the induction of disease., Conclusion: These findings demonstrate that TLR-9-driven signals potentiate the effects of IFNγ to initiate murine MAS, and provide evidence that induction of inflammatory myelopoiesis is a common TLR-9- and IFNγ-dependent pathway that may contribute to the pathogenesis of MAS., (© 2018, American College of Rheumatology.)

Systemic inflammatory response syndrome describes a heterogeneous group of cytokine storm disorders, with different immunogens and cytokines leading to variations in organ pathology. The severe inflammation generated by the cytokine storm results in widespread organ pathology including alterations in T- and B-lymphocyte counts. This study explores the roles of TLR9 and IFN-γR stimulation in decreasing T- and B-cell lymphopoiesis in a mouse model of hyperinflammation. We demonstrate that early B-cell lymphopoiesis is severely compromised during TLR9- and IFN-γ-driven hyperinflammation from the Ly-6D(+) common lymphoid progenitor stage onwards with different effects inhibiting development at multiple stages. We show that TLR9 signaling directly decreases in vitro B-cell yields while increasing T-cell yields. IFN-γ also directly inhibits B-cell and T-cell differentiation in vitro as well as when induced by TLR9 in vivo. Microarray and RT-PCR analysis of Ly-6D(-) common lymphoid progenitors point to HOXa9 and EBF-1 as transcription factors altered by TLR9-induced inflammation. Our work demonstrates both cellular and molecular targets that lead to diminished B-cell lymphopoiesis in sustained TLR9- and IFN-γ-driven inflammation that may be relevant in a number of infectious and autoimmune/inflammatory settings., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Objective: Macrophage activation syndrome (MAS) is a devastating cytokine storm syndrome complicating many inflammatory diseases and characterized by fever, pancytopenia, and systemic inflammation. It is clinically similar to hemophagocytic lymphohistiocytosis (HLH), which is caused by viral infection of a host with impaired cellular cytotoxicity. Murine models of MAS and HLH illustrate that interferon-γ (IFNγ) is the driving stimulus for hemophagocytosis and immunopathology. This study was undertaken to investigate the inflammatory contributors to a murine model of Toll-like receptor 9 (TLR-9)-induced fulminant MAS., Methods: Wild-type, transgenic, and cytokine-inhibited mice were treated with an IL-10 receptor blocking antibody and a TLR-9 agonist, and parameters of MAS were evaluated., Results: Fulminant MAS was characterized by dramatic elevations in IFNγ, IL-12, and IL-6 levels. Increased serum IFNγ levels were associated with enhanced IFNγ production within some hepatic cell populations but also with decreased numbers of IFNγ-positive cells. Surprisingly, IFNγ-knockout mice developed immunopathology and hemophagocytosis comparable to that seen in wild-type mice. However, IFNγ-knockout mice did not become anemic and had greater numbers of splenic erythroid precursors. IL-12 neutralization phenocopied disease in IFNγ-knockout mice. Interestingly, type I IFNs contributed to the severity of hypercytokinemia and weight loss, but their absence did not otherwise affect MAS manifestations., Conclusion: These data demonstrate that both fulminant MAS and hemophagocytosis can arise independently of IFNγ, IL-12, or type I IFNs. They also suggest that IFNγ-mediated dyserythropoiesis, not hemophagocytosis, is the dominant cause of anemia in fulminant TLR-9-induced MAS. Thus, our data establish a novel mechanism for the acute anemia of inflammation, but suggest that a variety of triggers can result in hemophagocytic disease., (Copyright © 2013 by the American College of Rheumatology.)

Hemophagocytic lymphohistiocytosis (HLH) and macrophage activation syndrome (MAS) are 2 similar diseases characterized by a cytokine storm, overwhelming inflammation, multiorgan dysfunction, and death. Animal models of HLH suggest that disease is driven by IFN-γ produced by CD8⁺ lymphocytes stimulated by persistent antigen exposure. In these models and patients with "primary" HLH, the antigen persists due to genetic defects, resulting in ineffective cytotoxic responses by CD8⁺ T cells and poor pathogen clearance. However, infectious triggers are often not identified in patients with MAS, and some patients with HLH or MAS lack defects in cytotoxic T cell killing. Herein, we show that repeated stimulation of TLR9 produced an HLH/MAS-like syndrome on a normal genetic background, without exogenous antigen. Like previous HLH models, TLR9-induced MAS was IFN-γ dependent; however, unlike other models, disease did not require lymphocytes. We further showed that IL-10 played a protective role in this model and that blocking IL-10 signaling led to the development of hemophagocytosis. IL-10 may therefore be an important target for the development of effective therapeutics for MAS. Our data provide insight into MAS-like syndromes in patients with inflammatory diseases in which there is chronic innate immune activation but no genetic defects in cytotoxic cell function.