To "Lump" or to "Split" in Macrophage Activation Syndrome and Hemophagocytic Lymphohistiocytosis

New discoveries in the genetics and genomics of systemic juvenile idiopathic arthritis

INTRODUCTION

Systemic juvenile idiopathic arthritis (sJIA) is a severe inflammatory condition with onset in childhood. It is sporadic, but elements of its stereotypical innate immune responses are likely genetically encoded by both common variants with small effect sizes and rare variants with larger effects.

AREAS COVERED

Genomic investigations have defined the unique genetic architecture of sJIA. Identification of the class II HLA locus as the strongest sJIA risk factor for the first time brought attention to T lymphocytes and adaptive immune mechanisms in sJIA. The importance of the human leukocyte antigen (HLA) locus was reinforced by recognition that HLA-DRB1*15 alleles are strongly associated with development of drug reactions and sJIA-associated lung disease (sJIA-LD). At the IL1RN locus, genetic variation relates to both risk of sJIA and may also predict non-response to anakinra. Finally, rare genetic variants may have critical roles in disease complications, such as homozygous LACC1 mutations in families with an sJIA-like illness, and hemophagocytic lymphohistiocytosis (HLH) gene variants in some children with macrophage activation syndrome (MAS).

EXPERT OPINION

Genetic and genomic analysis of sJIA holds great promise for both basic discovery of the course and complications of sJIA, and may help guide personalized medicine and therapeutic decision-making.

Is severe COVID-19 a cytokine storm syndrome: a hyperinflammatory debate

PURPOSE OF REVIEW

The COVID-19 pandemic is a global public health crisis with considerable mortality and morbidity. A role for cytokine storm and therapeutic immunomodulation in a subgroup of patients with severe COVID-19 was proposed early in the pandemic. The concept of cytokine storm in COVID-19 has been criticised, given the lack of clear definition and relatively modest cytokinaemia (which may be necessary for viral clearance) compared with acute respiratory distress syndrome and bacterial sepsis. Here we consider the arguments for and against the concept of cytokine storm in COVID-19.

RECENT FINDINGS

Several criteria have been proposed to identify the subgroup of COVID-19 patients exhibiting a cytokine storm. The beneficial effects of corticosteroids and interleukin-6 inhibition suggest that inflammation is a modifiable pathogenic component of severe COVID-19. The presence of genetic polymorphisms and pathogenic auto-autoantibodies in severe COVID-19 also suggests a significant contribution of immune dysregulation to poor outcomes.

SUMMARY

Hyperinflammation is a key component of severe COVID-19, residing underneath the cytokine storm umbrella term, associated with poor outcomes. Better understanding of the aetiopathogenesis, with identification of biomarkers to predict treatment responses and prognosis, will hopefully enable a stratified and ultimately precision medicine approach.

Hemophagocytic Lymphohistiocytosis in the Emergency Department: Recognizing and Evaluating a Hidden Threat

Background

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hematologic disorder resulting from an ineffective and pathologic activation of the immune response system that may mimic common emergency department presentations, including sepsis, acute liver failure, disseminated intravascular coagulation, and flu-like illnesses such as coronavirus disease 2019 (COVID-19).

Objective

This narrative review provides a summary of the disease and recommendations for the recognition and diagnostic evaluation of HLH with a focus on the emergency clinician.

Discussion

Though the condition is rare, mortality rates are high, ranging from 20% to 80% and increasing with delays in treatment. Importantly, HLH has been recognized as a severe variation of the cytokine storm associated with COVID-19. Common features include a history of infection or malignancy, fever, splenomegaly or hepatomegaly, hyperferritinemia, cytopenias, coagulopathies, abnormal liver enzymes, and hypertriglyceridemia. Using specific features of the history, physical examination, laboratory studies, and tools such as the HScore, HLH-2004/2009, and hyperferritinemia thresholds, the emergency clinician can risk-stratify patients and admit for definitive testing. Once diagnosed, disease specific treatment can be initiated.

Conclusion

This review describes the relevant pathophysiology, common presentation findings, and a framework for risk stratification in the emergency department.

Inflammation Unrestrained by SIRPα Induces Secondary Hemophagocytic Lymphohistiocytosis Independent of IFN-γ

A hallmark of secondary hemophagocytic lymphohistiocytosis (sHLH), a severe form of cytokine storm syndrome, is the emergence of overactivated macrophages that engulf healthy host blood cells (i.e., hemophagocytosis) and contribute to the dysregulated inflammation-driven pathology. In this study, we show that depleting SIRPα (SIRPα^-/-) in mice during TLR9-driven inflammation exacerbates and accelerates the onset of fulminant sHLH, in which systemic hemophagocytosis, hypercytokinemia, consumptive cytopenias, hyperferritinemia, and other hemophagocytic lymphohistiocytosis hallmarks were apparent. In contrast, mice expressing SIRPα, including those deficient of the SIRPα ligand CD47 (CD47^-/-), do not phenocopy SIRPα deficiency and fail to fully develop sHLH, albeit TLR9-inflamed wild-type and CD47^-/- mice exhibited hemophagocytosis, anemia, and splenomegaly. Although IFN-γ is largely considered a driver of hemophagocytic lymphohistiocytosis pathology, IFN-γ neutralization did not preclude the precipitation of sHLH in TLR9-inflamed SIRPα^-/- mice, whereas macrophage depletion attenuated sHLH in SIRPα^-/- mice. Mechanistic studies confirmed that SIRPα not only restrains macrophages from acquiring a hemophagocytic phenotype but also tempers their proinflammatory cytokine and ferritin secretion by negatively regulating Erk1/2 and p38 activation downstream of TLR9 signaling. In addition to TLR9 agonists, TLR2, TLR3, or TLR4 agonists, as well as TNF-α, IL-6, or IL-17A, but not IFN-γ, similarly induced sHLH in SIRPα^-/- mice but not SIRPα⁺ mice. Collectively, our study suggests that SIRPα plays a previously unappreciated role in sHLH/cytokine storm syndrome pathogenesis by preventing macrophages from becoming both hemophagocytic and hyperactivated under proinflammation.

Silencing the cytokine storm: the use of intravenous anakinra in haemophagocytic lymphohistiocytosis or macrophage activation syndrome

The term cytokine storm syndromes describes conditions characterised by a life-threatening, fulminant hypercytokinaemia with high mortality. Cytokine storm syndromes can be genetic or a secondary complication of autoimmune or autoinflammatory disorders, infections, and haematological malignancies. These syndromes represent a key area of interface between rheumatology and general medicine. Rheumatologists often lead in management, in view of their experience using intensive immunosuppressive regimens and managing cytokine storm syndromes in the context of rheumatic disorders or infection (known as secondary haemophagocytic lymphohistiocytosis or macrophage activation syndrome [sHLH/MAS]). Interleukin (IL)-1 is pivotal in hyperinflammation. Anakinra, a recombinant humanised IL-1 receptor antagonist, is licenced at a dose of 100 mg once daily by subcutaneous injection for rheumatoid arthritis, systemic juvenile idiopathic arthritis, adult-onset Still's disease, and cryopyrin-associated periodic syndromes. In cytokine storm syndromes, the subcutaneous route is often problematic, as absorption can be unreliable in patients with critical illness, and multiple injections are needed to achieve the high doses required. As a result, intravenous anakinra is used in clinical practice for sHLH/MAS, despite this being an off-licence indication and route of administration. Among 46 patients admitted to our three international, tertiary centres for sHLH/MAS and treated with anakinra over 12 months, the intravenous route of delivery was used in 18 (39%) patients. In this Viewpoint, we describe current challenges in the management of cytokine storm syndromes and review the pharmacokinetic and safety profile of intravenous anakinra. There is accumulating evidence to support the rationale for, and safety of, intravenous anakinra as a first-line treatment in patients with sHLH/MAS. Intravenous anakinra has important clinical relevance when high doses of drug are required or if patients have subcutaneous oedema, severe thrombocytopenia, or neurological involvement. Cross-speciality management and collaboration, with the generation of international, multi-centre registries and biobanks, are needed to better understand the aetiopathogenesis and improve the poor prognosis of cytokine storm syndromes.

Continuous Intravenous Anakinra Infusion to Calm the Cytokine Storm in Macrophage Activation Syndrome

OBJECTIVE

The objective of this study was to report the benefit of a therapeutic approach consisting of intravenous (IV) continuous anakinra (recombinant human interleukin-1 receptor antagonist) infusions in treating severely ill adult patients with secondary hemophagocytic lymphohistiocytosis or macrophage activation syndrome (MAS).

METHODS

A retrospective chart review of five patients treated at Regions Hospital from 2016 to 2019 was conducted. Demographic, clinical, and laboratory characteristics and outcomes were recorded.

RESULTS

Continuous IV anakinra infusions up to 2400 mg/d resulted in rapid serologic, then clinical response in 4 of 5 severely ill patients who were refractory to all other therapies, including subcutaneous anakinra. Subsequently, 3 of 5 patients have been maintained on anakinra or canakinumab, with no recurrence of MAS.

CONCLUSION

Continuous infusion of IV anakinra may result in rapid serologic and subsequent clinical improvement in adult patients with MAS. This method for treating cytokine storm should be considered in the current COVID-19 pandemic in the subgroup of patients with severe disease who have a cytokine storm presentation.

The genetics of macrophage activation syndrome

Macrophage activation syndrome (MAS), or secondary hemophagocytic lymphohistiocytosis (HLH), is a cytokine storm syndrome associated with multi-organ system dysfunction and high mortality rates. Laboratory and clinical features resemble primary HLH, which arises in infancy (1 in 50,000 live births) from homozygous mutations in various genes critical to the perforin-mediated cytolytic pathway employed by NK cells and cytotoxic CD8 T lymphocytes. MAS/secondary HLH is about ten times more common and typically presents beyond infancy extending into adulthood. The genetics of MAS are far less defined than for familial HLH. However, the distinction between familial HLH and MAS/secondary HLH is blurred by the finding of heterozygous perforin-pathway mutations in MAS patients, which may function as hypomorphic or partial dominant-negative alleles and contribute to disease pathogenesis. In addition, mutations in a variety of other pathogenic pathways have been noted in patients with MAS/secondary HLH. Many of these genetically disrupted pathways result in a similar cytokine storm syndrome, and can be broadly categorized as impaired viral control (e.g., SH2P1A), dysregulated inflammasome activity (e.g., NLRC4), other immune defects (e.g., IKBKG), and dysregulated metabolism (e.g., LIPA). Collectively these genetic lesions likely combine with states of chronic inflammation, as seen in various rheumatic diseases (e.g., still disease), with or without identified infections, to result in MAS pathology as explained by the threshold model of disease. This emerging paradigm may ultimately support genetic risk stratification for high-risk chronic and even acute inflammatory disorders. Moving forward, continued whole-exome and -genome sequencing will likely identify novel MAS gene associations, as well as noncoding mutations altering levels of gene expression.