key: cord-344683-lr1xr2um
authors: Cabler, Stephanie; French, Anthony; Orvedahl, Anthony
title: A Cytokine Circus with a Viral Ringleader: SARS-CoV-2-Associated Cytokine Storm Syndromes
date: 2020-09-30
journal: Trends Mol Med
DOI: 10.1016/j.molmed.2020.09.012
sha: 
doc_id: 344683
cord_uid: lr1xr2um

An unbridled host immune response to SARS-CoV-2 infection likely underlies severe cases of the disease and has been labeled a “Cytokine Storm Syndrome”. We emphasize that categorization of syndromes triggered by a completely novel pathogen based on other seemingly similar, but potentially distinct, known entities is an inherently risky endeavor.

Infection with SARS-CoV-2, the etiologic agent of Coronavirus Disease 2019 (COVID- 19) , can lead to severe pneumonia, multi-organ failure, and death. An over-exuberant immune response may contribute to severe COVID- 19 which is temporally associated with the SARS-CoV-2 pandemic. Whether the SARS-CoV-2associated CSS (termed S-CSS for purposes of this review) and MIS-C fall within a spectrum of disorders familiar to us, or represent groups of novel clinical entities-a herd of zebras, in essence-remains undetermined. However, new evidence in this rapidly moving area indicates that SARS-CoV-2-associated inflammatory disorders may be unique entities triggered by this completely novel pathogen. Here we review the proposed pathogenesis of MIS-C within the context of S-CSS, and highlight the need to gain further understanding of these disorders to define optimal therapeutic targets.

CSS involve life-threatening immune activation triggered by genetic, infectious, or iatrogenic causes, with associated hypercytokinemia, hemodynamic compromise, and multiorgan dysfunction. While different forms of CSS share these features, varying underlying host risk factors and inciting triggers have important implications for the pathogenesis of the disease. CSS in general, with reference to COVID-19, has recently been reviewed [1] . Briefly, viral but also other infections may trigger primary (inherited) and secondary (acquired)

[4], while cure of some patients with HLH may require bone marrow transplantation [5] . While targeted therapies based on understanding of the pathogenesis of unique CSS are emerging (e.g. neutralization of IL-6 for CRS, or interferon-gamma neutralizing antibodies for pediatric HLH [6] ), and there may be overlap in the pathogenesis and therapy, it is clear that a "one-size fits all" approach does not work.

Initial reports of adult patients with COVID-19 described hemodynamic compromise, endovascular lesions, multi-organ dysfunction, and elevated markers of inflammation that could not be ascribed entirely to respiratory failure from viral pneumonia [7] . Biphasic onset of fever and clinical worsening after an initial period of stability invoke the "cytokine storm" (Figure 1 ). Commonly involved systems in S-CSS include severe pulmonary disease with Acute Respiratory Distress Syndrome (ARDS), cardiac impairment, liver dysfunction, acute kidney injury, and coagulopathy. Markers of more severe disease include lymphopenia [8] , and elevated C-reactive protein (CRP), Lactate Dehydrogenase (LDH), and ferritin which are nonspecific indicators of inflammation and cellular injury [9] [10] [11] . Elevated D-dimer levels and thrombocytopenia portend worse outcomes and reflect an associated coagulopathy [11, 12] .

Large cohorts of pediatric patients with severe COVID-19 and associated laboratory findings are uncommon. Though detailed descriptions of pediatric S-CSS are few, the available data suggests elevated ferritin and CRP and endovascular damage with elevated D-dimers are common in these patients as well [13] .

A dysregulated immune response with suppression of IFN responses and concurrent hyper-production of other cytokines (e.g., IL-6 and TNF) has been proposed to give rise to S-CSS [14, 15] . Cytokines correlating to disease severity and poor survival include IL-6, IL-8, and IL-10, among others [8] [9] [10] 12, 16] . One recent study found elevated markers of apoptosis on lymphocytes profiled from severe COVID-19 patients, indicating a potential route to their depletion [15] . Nevertheless, it remains undetermined if hyperproduction of cytokines in S-CSS is a cause or a consequence of the observed organ injury and immune cell abnormalities. Trials J o u r n a l P r e -p r o o f Journal Pre-proof with therapies that inhibit specific cytokines (e.g., IL-6 and IL-1) and more indiscriminate immunosuppressants (e.g., steroids) are underway and may provide further insights.

Cohorts of pediatric patients with MIS-C reveal overlapping but distinct pictures from S-CSS with respect to their clinical and laboratory profile ( Table 1) . The syndrome is defined by fever, multi-organ dysfunction, and laboratory evidence of inflammation ( Table 2) . Abdominal pain, vomiting and diarrhea, mucocutaneous findings (conjunctivitis and rash) and shock with cardiac involvement are more common in MIS-C than in S-CSS [17, 18] . Respiratory symptoms, while often present, are mild, and musculoskeletal and neurological findings are rare. The patients) [17] , which is a lower mortality rate than severe COVID-19.

Given the clinical and laboratory features, it is reasonable to consider MIS-C as a separate but related entity to the severe multi-organ dysfunction observed in patients with S-CSS (Figure 1 ). For example, MIS-C patients less commonly have severe respiratory, renal, and hepatic involvement and more often have cardiac dysfunction. Lymphopenia and thrombocytopenia are observed in a minority of MIS-C patients (40% and 41%, respectively, in Group 1 patients reported in [17] ), and while lower levels are associated with more severe COVID-19 [21, 22] , an association for these factors in outcomes in MIS-C remains to be Additionally, cardiac involvement with MIS-C most typically presents as ventricular dysfunction, with only a small percentage having coronary artery dilation [17] . It is important to note that the pathogenesis of KD itself remains enigmatic. KD is a medium vessel vasculitis that may involve antibodies to a persistent, as of yet unidentified, viral pathogen. In contrast, TSS results from superantigens elaborated by bacteria that lead to indiscriminate cytokine release from lymphocytes. Thus, MIS-C exhibits features of two entities with patently distinct disease mechanisms. It is also important to note that typical KD, TSS, and other similar disorders could occur in patients testing positive for acute or recent SARS-CoV-2 infection, and diagnostic inertia should be avoided so as not to detract from their appropriate diagnosis and management.

One critical unresolved question is the mechanism by which SARS-CoV-2 infection Higher SARS-CoV-2 qPCR Ct values in MIS-C patients suggests a lower viral load and also points towards a para-infectious etiology [19] . Interestingly, even within MIS-C patients, there are suggestions of different subtypes, with younger non-white patients presenting with conjunctivitis and abdominal pain having positive antibodies (sub-acute infection). This contrasts with a group of MIS-C patients that presents with respiratory symptoms and positive PCR testing (acute infection) [23] . It is also important to consider that initial criteria for MIS-C case identification were relatively broad in order to capture many cases, and it remains possible that patients meeting current criteria with primarily respiratory disease could represent an entity that is distinct from MIS-C.

Two recent studies provide immunological data suggesting that MIS-C and severe COVID-19 are distinct entities. Systematic profiling of serum cytokines showed distinct inflammatory profiles in adults with S-CSS versus children with MIS-C and KD, with elevated IL-8 and IL-7 defining the S-CSS group, and MIS-C appearing more similar to KD in this comparison [20] . Comparing groups of healthy children, those with SARS-CoV-2, MIS-C, or KD, revealed that IL-6 and IL-17A were significantly higher only in the KD group [20] . A second study showed pediatric patients with MIS-C exhibit distinct cytokine profiles from those with severe SARS-CoV-2 respiratory disease, exhibiting higher IL-10 and TNF [19] . It is not clear if differences observed in cytokines that define these groups are a result of different patient populations, testing strategies, or in the groups used for comparisons. As with S-CSS, additional study and identification of effective (and ineffective) therapies will help in the "taxonomic" classification of MIS-C and potential subgroups.

One lesson that will surely emerge from the COVID-19 pandemic is that the necessity to intervene in the setting of immense devastation must be tempered by a core principle of medical ethics: first, do no harm. Empiric treatments of inflammatory entities related to a novel disease with an incomplete molecular understanding of the virulence factors and host J o u r n a l P r e -p r o o f Journal Pre-proof response, increases the risk of violating this maxim. The risk increases further when demand for individual therapies exceeds supply and additional alternatives are sought. In this setting we face even higher chances of getting "burned" and are forced to walk a narrower tightrope.

As we gain a deeper understanding of the pathogenesis of S-CSS and MIS-C, and the results of randomized trials are published, a clearer picture will emerge and targeted therapies may be identified (see Outstanding Questions). Knowledge in this area is keeping pace with the spread of the virus with new insights arising on a continual basis. We may ultimately find that manifestations of S-CSS represent a circus of unique zebras in which the virus is orchestrating the chaos at center ring. As the tent over this complex host-pathogen interaction comes down, rigorous studies will surely tip the scales in our favor and lessons learned may improve our readiness for when the next circus inevitably comes to town.

CNS does not include headache, does include meningeal signs; Mucous membrane ("Muc. memb.") includes conjunctivitis; Cardiac includes echocardiographic findings, need for resuscitative medications, and elevated cardiac markers (troponin, BNP);

Gastrointestinal does not include isolated abdominal pain which was uncommonly reported as an isolated symptom among studies, but is present in up to 62% of reported patients [17] ; Renal includes acute kidney injury. Patients from initial cohort recognizing the 

What is the mechanism by which SARS-CoV-2 infection contributes to Multisystem Inflammation? Is the hyperproduction of cytokines in SARS-CoV-2 Cytokine Storm Syndrome (S-CSS) a cause or a consequence of the observed organ injury and immune cell abnormalities? Trials with therapies that inhibit specific cytokines (e.g., IL-6 and IL-1) and more indiscriminate immunosuppressants (e.g., steroids) should provide further insights. Is Multisystem Inflammatory Syndrome in Children (MIS-C) a distinct syndrome or does it exist along the spectrum of disease seen in S-CSS? In the absence of data on the community prevalence of SARS-CoV-2 infection (i.e. the denominator), it is not yet possible to determine the relative risk of developing MIS-C in children exposed to SARS-CoV-2, or the baseline probability of testing positive for the virus in the general population.

What are the optimal clinical characteristics and biomarkers to identify and classify cases of MIS-C, S-CSS, and other diseases associated with Cytokine Storm Syndrome? Precise disease classification may help to guide cohort selection for trials to identify optimal therapies. Do we need to re-consider the initial criteria for MIS-C case identification that were set relatively broad in order to capture many cases? This could help to better evaluate and treat patients.

J o u r n a l P r e -p r o o f

Cytokine Storms: Understanding COVID-19

Genetic and mechanistic diversity in pediatric hemophagocytic

HLH-2004: Diagnostic and therapeutic guidelines for hemophagocytic lymphohistiocytosis

Emapalumab in Children with Primary Hemophagocytic Lymphohistiocytosis

Clinical progression of patients with COVID-19 in Shanghai

Clinical and immunological features of severe and moderate coronavirus disease 2019

Hematologic, biochemical and immune biomarker abnormalities associated with severe illness and mortality in coronavirus disease 2019 (COVID-19): a meta-analysis

Prognostic value of interleukin-6, C-reactive protein, and procalcitonin in patients with COVID-19

Factors associated with death outcome in patients with severe coronavirus disease-19 (COVID-19): a case-control study

Epidemiology, clinical course, and outcomes of critically ill adults with COVID-19 in New York City: a prospective cohort study

Pediatric Critical Care and COVID19

Imbalanced Host Response to SARS-CoV-2 Drives Development of COVID-19

Impaired type I interferon activity and inflammatory responses in severe COVID-19 patients

Profiling serum cytokines in COVID-19 patients reveals IL-6 and IL-10 are disease severity predictors

COVID-19-Associated Multisystem Inflammatory Syndrome in Children -United States

Pediatric SARS-CoV-2: Clinical Presentation, Infectivity, and Immune Responses

Multisystem inflammatory syndrome in children and COVID-19 are distinct presentations of SARS-CoV-2

The Immunology of Multisystem Inflammatory Syndrome in Children with COVID-19

Lymphopenia in severe coronavirus disease-2019 (COVID-19): systematic review and meta-analysis

Thrombocytopenia is associated with severe Dehydrogenase; PMN, polymorphonuclear cell/ neutrophil; ALT, Alanine Aminotransferase; WBC, White blood cell count; CSF, cerebrospinal fluid

We thank Melanie Yarbrough, PhD, for helpful comments on the manuscript.

J o u r n a l P r e -p r o o f