key: cord-0743639-r4vobu5w
authors: Wang, Jin; Jiang, Mengmeng; Chen, Xin; Montaner, Luis J.
title: Cytokine storm and leukocyte changes in mild versus severe SARS‐CoV‐2 infection: Review of 3939 COVID‐19 patients in China and emerging pathogenesis and therapy concepts
date: 2020-06-13
journal: J Leukoc Biol
DOI: 10.1002/jlb.3covr0520-272r
sha: e59c97508e8e173b24358e391688b8e1681b9f10
doc_id: 743639
cord_uid: r4vobu5w

Clinical evidence indicates that the fatal outcome observed with severe acute respiratory syndrome‐coronavirus‐2 infection often results from alveolar injury that impedes airway capacity and multi‐organ failure—both of which are associated with the hyperproduction of cytokines, also known as a cytokine storm or cytokine release syndrome. Clinical reports show that both mild and severe forms of disease result in changes in circulating leukocyte subsets and cytokine secretion, particularly IL‐6, IL‐1β, IL‐10, TNF, GM‐CSF, IP‐10 (IFN‐induced protein 10), IL‐17, MCP‐3, and IL‐1ra. Not surprising, therapies that target the immune response and curtail the cytokine storm in coronavirus 2019 (COVID‐19) patients have become a focus of recent clinical trials. Here we review reports on leukocyte and cytokine data associated with COVID‐19 disease in 3939 patients in China and describe emerging data on immunopathology. With an emphasis on immune modulation, we also look at ongoing clinical studies aimed at blocking proinflammatory cytokines; transfer of immunosuppressive mesenchymal stem cells; use of convalescent plasma transfusion; as well as immunoregulatory therapy and traditional Chinese medicine regimes. In examining leukocyte and cytokine activity in COVID‐19, we focus in particular on how these levels are altered as the disease progresses (neutrophil NETosis, macrophage, T cell response, etc.) and proposed consequences to organ pathology (coagulopathy, etc.). Viral and host interactions are described to gain further insight into leukocyte biology and how dysregulated cytokine responses lead to disease and/or organ damage. By better understanding the mechanisms that drive the intensity of a cytokine storm, we can tailor treatment strategies at specific disease stages and improve our response to this worldwide public health threat.

Since December 2019, the frequent incidence of pneumonia has become a distinctive feature of the infection caused by a novel coronavirus (severe acute respiratory syndrome-coronavirus-2, SARS-CoV-2) in Wuhan, Hubei province, China. 1 CD8 + T cell counts, yet lacked any evidence of pulmonary changes upon imaging. 8 The most common symptoms observed in COVID-19 patients are malaise, dry cough, and high fever. Symptoms of diarrhea, hemoptysis, and headache are not uncommon. 9 Recently the Centers for Disease Control and Prevention (USA) expanded target symptoms of COVID-19 to include chills, repeated shaking with chills, muscle pain, headache, sore throat, and loss of taste or smell. Disease can be mild or progress toward dyspnea and/or hypoxemia, or even acute respiratory distress syndrome (ARDS) and septic shock, which then can lead to multiple organ dysfunction syndrome (MODS). 10 The main reason for death outcomes following SARS-CoV-2 infection is respiratory failure, with changes in heart and liver function as a secondary or related consequence of disease. [11] [12] [13] Epidemiologic data does suggest certain groups are at particular risk for severe disease outcomes (i.e., elderly, select preexisting conditions). Although we know there is a link between disease severity, viral production, and a cytokine storm or cytokine release syndrome (CRS), it is still unclear which molecular triggers fuel the onset of the cytokine storm and why it can quickly advance to ARDS or MODS, with a fatal outcome in a subset of patients. 14, 15 It remains unknown if the observed cytokine storm and resulting leukocyte changes impact high-and low-risk individuals in the same way, and how these factors can turn an otherwise natural protective cytokine response against infection into a lethal pathogenic process.

In this article, we summarize reports on blood cytokine levels and leukocyte activation to examine for differences in immunopathogenesis between mild and severe cases of COVID-19 in patients from

China. Data were obtained from reports from April 2019 to April 2020 (Supporting Information Fig. S1 ). We also discuss emerging concepts on the interphase between leukocyte biology and disease pathogenesis. Last, we summarize findings from ongoing clinical trials in China and United States that seek to control the onset or damage caused by the cytokine storm to reduce both morbidity and mortality.

The release of cytokines in response to infection can lead to mild or severe clinical manifestations. The hallmarks of a mild/nonlethal cytokine release response to infection include increased local temperature (heat), myalgia, arthralgia, nausea, rash, depression, and other mild flu-like symptoms. Concurrent to immune activation, the body launches compensatory-repair processes to restore tissue and organ function. The term "cytokine storm" was first coined in 1993 to describe a graft-vs.-host disease. 16 The term has since been extended to describe the similar sudden cytokine releases associated with autoimmune, hemophagocytic lymphohistiocytosis, sepsis, cancers, acute immunotherapy responses, and infectious diseases [17] [18] [19] [20] Cytokine storm occurs when an immune system is overactivated by infection, drug, and/or some other stimuli, leading to high levels of cytokines (IFN, IL, chemokines, CSF, TNF, etc.) being released into circulation with a widespread and detrimental impact on multiple organs. 21 The severe inflammatory responses induced by a cytokine storm start locally and spread systemically, causing collateral damage in tissues. 22, 23 To date, it is unclear what determinants of the host response to infection are responsible for triggering the inflammatory sequence leading to the clinical syndrome associated with high cytokine release. In general, it is believed to be caused by an imbalance in immune-system regulation (i.e., increase in immune cell activation via TLR or other mechanism, decrease in anti-inflammatory response, 25 In addition to NK cells, once myeloid cells such as resident macrophages are activated by IFN-, it amplifies subsequent TLR-mediated stimulation. This includes the release of high levels of TNF, IL-12, and IL-6, which, in turn, can further modulate NK cells. 26 Although IL-12 acts to increase NK IFN-secretion, high IL-6 levels also may limit the immune response by its effects on the cytotoxic activity of NK cells via the down-regulation of intracellular perforin and granzyme B levels. 27 As disease progresses, T cell and antibody responses give rise to additional cytokine responses, leading to greater or sustained antigen release and added TLR ligands from viral-induced cytotoxicity. 28 Once these responses are in motion, other host-or pathogen-related factors (i.e., decreases in pathogen load, anti-inflammatory responses, genetics) work together to prevent a dysregulated response or a CRS that, if allowed to develop, could itself cause tissue damage and organ failure. For example, a lack of a negative feedback mechanism by IL-10 and IL-4 would be expected to increase the severity of cytokine responses toward a pathogenic CRS or cytokine storm. 21 On the other hand, targeting treatment to disrupt the formation of cytokine storm by using pharmacologic agents, such as tocilizumab (anti-IL-6), may stabilize the advanced cases from transitioning to a more critical state. 27, 29 At the onset of SARS-CoV-2 infection, there typically is a preferential infection of the respiratory track as a consequence of dropletbased viral transfer. However, a recent study 30 supports the theory that SARS-CoV-2 also could potentially infect intestine enterocytes directly through angiotensin-converting enzyme 2 (ACE2). ACE2 is highly expressed on differentiated enterocytes and may help to explain why diarrhea occurs with acute infection as well as the observed fecal shedding. Consequently, having a broader infection footprint may impact the source of inflammatory cascades to include tissues other than the lung.

In COVID-19 disease, a cytokine storm is common in patients with severe-to-critical symptoms; at the same time, lymphocytes and NK cell counts are sharply reduced with elevations in levels of D-dimer, C-reactive protein (CRP), ferritin, and procalcitonin. 31 Consequences of a lethal cytokine storm exhibit diffuse alveolar damage characterized by hyaline membrane formation and infiltration of interstitial lymphocytes. 32, 33 The collateral tissue damage, organ failure, and poor outcomes of people with COVID-19 and its accompanying uncontrolled inflammatory responses share similarities with SARS and MERS. In severe SARS patients, the serum levels of IFN-, IL-1, IL-6, IL-12, TGF-, MCP-1, and IL-8 were higher than patients with mild-to-moderate symptoms. 34,35 IL-1 , IL-6, and IL-8 levels also were increased in the patients severely infected by MERS-CoV. 36 Leukocyte and cytokine changes with severe-to-critical SARS-CoV-2 infection are further detailed below.

Developing criteria to predict and diagnose a cytokine storm in COVID-19 patients with surrogate biomarkers is key because the peak levels of circulating cytokines are not routinely monitored for a change in kinetics. In addition to CRP and ferritin, increases in D-dimer and procalcitonin have also been associated with a higher likelihood of developing or continuing a cytokine storm in COVID-19 patients. 37 Of interest, elevated CRP and ferritin levels are associated with the onset of a cytokine storm in patients receiving chimeric antigen receptor T cell therapy. 38 Further study of the changes that occur in leukocyte and cytokine parameters, as described later, may help identify biomarkers for a cytokine storm in COVID-19 patients. In the following sections, we review 28 recently published studies and examine the changes in circulating leukocytes and cytokine profiles of 3939 COVID patients (see schematic for information on the process used for selecting literature in Supporting Information Fig. S1 ).

In humans, both monocytes and macrophages express ACE2 and consequently can be infected by SARS-CoV and SARS-CoV-2, 39 which results in the activation and transcription of proinflammatory genes. 40 Intriguingly, infection caused by SARS-CoV-2 appears to markedly down-regulate the expression of ACE2 on peripheral blood (PB) monocytes on a per cell basis, which may be a secondary outcome to viral binding. 41 46 Wen et al. 47 also observed that an abundance of inflammatory CD14 ++ IL1 + and IFN-activated monocytes existed in the PB of COVID-19 patients. In addition, SARS-CoV-2 was found to trigger macrophages through ACE2, generating IL-6 expression in the spleen and lymph nodes and IL-6, TNF, IL-10, and PD-1 expression from the alveolar macrophages. 47, 48 This added mechanism might promote lymphocytopenia and contribute to a cytokine storm, initiating in the lung as viral levels rise. 42, 48 Autopsy reports indicated that inflammatory macrophages accumulated in the lungs of COVID-19 patients. 49 in turn, reduced the duration of elevated blood levels for the inflammatory markers IL-6 and CRP. 54 However, the mechanism underlying SARS-CoV-2 ′ s ability to dampen initial innate responses at acute infection and suppress Type-I IFNs to maximize viral release still needs additional study.

As COVID-19 progresses, the number of neutrophils in circulation gradually increase; thus, elevated neutrophil levels may be useful for predicting the severity of disease. 55 Zhang et al. 56 reported that the neutrophil-to-lymphocyte ratio (NLR) combined with IgG might be a better predictor than neutrophil count alone in predicting the severity of COVID-19. Neutrophil extracellular traps (NETs), which are extracellular webs of DNA/histones released by neutrophils to control infections, also are known to exacerbate inflammation. 57, 58 Previous studies have revealed that aberrant NETs might contribute to ARDS, cystic fibrosis, excessive thrombosis, and cytokine storm (IL-1 ). [59] [60] [61] [62] In severe cases of COVID-19, elevated levels of NETosis with cell-free DNA and myeloperoxidase (MPO)-DNA have been noted frequently. 63 Indeed, when neutrophils from uninfected persons were exposed to serum from COVID-19 patients in vitro, it triggered NETosis, indicating that patient sera may have the capacity to promote NETosis in neutrophils. 63, 64 Apart from contributing to a cytokine storm, increased NETosis likely also impacts the onset of venous and arterial thrombosis in COVID-19 patients. 65 This finding raises the hypothesis that NETs may be a central dysregulated pathologic mechanism driving cytokine release and multi-organ damage, ultimately leading to respiratory failure and coagulopathy. Moreover, the transcriptional analysis 51 

Data on the NK cell response in COVID-19 is limited. In SARS, NK cells were found to be useful in predicting disease severity and CD158b + NK cells were associated with the presence of anti-SARS-CoV-specific antibodies. 66 

Despite the role of dendritic cells and T cells in respiratory infections, 70, 71 there is currently no available evidence to show a link between SARS-CoV2 infection and the modulation of myeloid or plasmacytoid dendritic cells nor reports of how T cells may impact the disease. 72

Compelling evidence exists that COVID-19 is characterized by a marked decrease in circulating T and B lymphocytes, particularly in severe and critical stages of COVID-19 infection. 47, 68, 73 Of note, such lymphopenia in patients with severe COVID-19 frequently occurs along with aberrant activation of monocytes/macrophages and an increase of neutrophils. 74 Changes in leukocyte subsets associated with mild vs. severe infection outcomes are described in the following sections and summarized in Figure 1 . 

T cell subsets change as COVID-19 infection progresses from mild to severe. A single-cell sequencing study found target inflammatory genes were highly expressed by CD4 T cells, and CD8 CTLs underwent clonal expansion in the recovery stage of COVID-19 patients. 47 However, in patients advancing to severe disease, the number of T lymphocytes remained low as compared with mild-stage patients, with the levels of helper T cell subsets, including Th1, Th2, and Th17, at below-normal levels. 68 in CD4 + and CD8 + T cells was associated with an increase in CD38 + (CD8 39.4%) HLA-DR + (CD4 3.47%) T cells. 32 Zheng et al. 81 and Chen et al. 83 further reported that CD8 + T cells in the PB of COVID-19

patients exhibited an overactivated phenotype, which is indicative of a sustained adaptive immune response (whether effective or not) in addition to an innate immune response. Of interest, activated CD8 + T cell responses appear to be ineffective as patients advance to severe stages of disease, as evidenced by the exhaustive phenotype of CD8

T cells (HLADR + TIGIT + CD8 + T cells). 81 Consistent with an onset of T cell exhaustion, Chen et al. 83 found that T cells in the PB of severe COVID-19 patients expressed high levels of PD-1 and Tim-3. The expression levels of PD-1 and Tim-3 on T cells also was positively correlated with disease severity.

Activated T cells could also add fuel to the cytokine storm by further stimulating inflammatory responses from innate immune cells.

For example, it was shown that the expression of IL-1 , CSF1, and CSF2 on T cells may bind to the IL-1R and colony stimulating factor receptor (CSFR) expressed on monocytes, and further stimulate the activation of monocytes. 47 Th1 cells in patients with severe COVID-19 were reported to stimulate the production of IL-6 by inflammatory monocytes, 39 which, together with Th17 cells, 84 may directly join innate immune cells in sparking the release of proinflammatory cytokines further contributing to the cytokine storm and subsequent organ damage. 47, 84 Regulatory T cells (Tregs) play a critical role in dampening an excessive inflammatory response as well as in antiviral immune responses. 85 Therefore, Tregs may be central to maintaining a balance between antiviral immunity and the harmful cytokine storm. To date, the reports regarding the Treg cells in COVID-19 patients remain inconsistent.

Some studies found up-regulated Treg cells in severe illness, whereas others reported that the number of Tregs was reduced or unchanged in COVID-19 patients. 68, 77, 86 Therefore, the role of Treg cells in the pathogenesis of COVID-19 needs to be further clarified.

Circulating B cells appear to be restored to normal levels in patients upon recovery from COVID-19 and convalescent plasma (CP) infusion (further discussed later) has been shown to be a potentially effective treatment as reported in 6 severe COVID-19 patients. 87 These findings support the hypothesis that humoral immune responses that boost SARS-CoV-2-specific antibodies are important in the host's resolution of SARS-CoV-2 infection and likely would help protect against reinfection. 51, 88, 89 Of interest, this ability to develop neutralizing antibody production (and memory) after infection may not be the same in all recovered patients. Recent data suggests that as many as 30% of recovered patients who had a milder disease course may develop low titers of neutralizing antibodies, which may convey a higher risk for reinfection. 90 Indeed, several studies have detected higher antibody titers after recovery from more severe disease when compared with milder cases (i.e., higher antibody titer in recovered elderly patients as compared with younger patients that often have mild or asymptomatic disease). This suggests a disease course with a stronger immune response may also lead to greater protection against reinfection. 90, 91 It is important to note that binding antibodies can also play a role in antibody-mediated phagocytosis and antibody-dependent cytotoxicity. It remains to be determined if patients with low total or neutralizing antibody titers will have higher reinfection rates.

Similar to CD4 + T, CD8 + T cells, and NK cells, B cells were also markedly decreased in severe COVID-19 patients, as compared with mild patients, and the counts of B lymphocytes were negatively associated with viral burden. 51 Aside from protection, it also remains undetermined if prior exposure may prime the body's response, leading to antibody-dependent enhancement (ADE) effects for greater infection or an amplification of inflammation cascades. Previously,

thus, ADE, if present, may hinder the host's ability to manage inflammation in lung and other tissues. 52 An analysis of 173 COVID-19

patients, analyzed for SARS-CoV-2-specific IgM and IgGs, found that those with severe or critical disease had a high titer of total antibodies as well as a high IgG response, which were associated with a poor outcome and prognosis. 92 Whether the increase in antibody titers during disease represents a secondary outcome of the host's response to high viral titers or whether an ADE mechanism could contribute to a sudden rise in viral load and onset of a cytokine storm remains undetermined.

Many proinflammation cytokines, such as IL-6, TNF, IL-1, IL-2, IL-17, IFN-, G-CSF, MCP-1 (macrophage inflammatory protein 1), IP-10 (IFN--induced protein 10), and others, were found to be significantly elevated in severe COVID-19 patients (Table 1 and Fig. 1 ), a profile that is similar to that found in patients with SARS and MERS. 36, 93, 94 As shown in Table 1 , the elevation of IL-6 was most frequently measured and detected in severe cases of SARS-CoV-2 infection.

Zhou et al. 39 reported that the pathogenic GM-CSF-producing Th1 cells in severe COVID-19 patients induced IL-6 production from CD14 + CD16 + monocytes and, thus, accelerated the inflammation associated with a cytokine storm. Elevated levels of IL-6 also were found in patients with exacerbating disease progression as evidenced by chest CT. 95 Moreover, Chen et al. 96 reported that the serum SARS-CoV-2 viral load was closely associated with IL-6 levels in critical patients (R = 0.902). High levels of IL-6 may also contribute to an increase in neutrophil cells and decrease in lymphocytes. Clearly, IL-6 may impact the development of ARDS in COVID-19 patients 97 and a rise in IL-6 may be a useful marker for severe disease onset. Furthermore, as lung-centric coagulopathy can also play an important role in the pathophysiology in the severe COVID-19 patients, 98 

Increase:

Increase: IL-6 (0-16 d);

IL-10 (0-13 d); IL-2 and IFN-(4-6 d) 78 Yunnan In column 3, N is the number of cases with available data. In addition, in columns 3 and 5, "n" is the number of cases in which leukocyte or cytokines changed overall, according to the corresponding report.

TNF is a master proinflammatory cytokine that is involved in the pathogenesis of autoimmune diseases such as rheumatoid arthritis (RA); consequently, anti-TNF biologics have become a first-line treatment for such diseases. 104 High levels of TNF often are produced in the initial response to infectious disease. 105 A recent study shows that the level of TNF (60-130 pg/mL) was higher than that of IL-6 (10-50 pg/mL) in the plasma of severe COVID-19 patients. 14 Therefore, further investigation is warranted to define how TNF impacts immunopathology as well as the effectiveness of anti-TNF therapy in severe COVID-19 patients.

In addition to examining IL-6 and TNF, Yang et al. 93 A Th17-type cytokine storm caused by a mobilization of Th17 responses has also been observed in both SARS and MERS patients. 109, 110 It was reported that a high number of CCR4 + CCR6 + Th17 cells, which at least partially attributed to this immunopathology, was also present in COVID-19 patient with ARDS. 84 Markedly elevated cytokines (i.e., IL-1, IL-17, TNF, and GM-CSF) in COVID-19

patients have been associated with Th17 responses. 107 These findings suggest that the Th17-type cytokine storm may lead or be associated with the onset of organ damage commonly observed in severe COVID-19 patients. 84 Taken together, staging of SARS-CoV-2 infection are commonly divided into four stages (mild/common/severe/critical), all of which may have different magnitudes of cytokine release. It is also possible that the qualitative features of the cytokine storm between disease stages may point to distinct host or pathogen triggers (e.g., cytokine storm with or without concomitant bacterial pneumonia). Figure 2 summarizes the combined contribution of blood and lung tissue infiltrates, and shows how added factors such as an accompanying bacterial pneumonia may exacerbate cytokine responses.

Because of its central role in the pathogenesis of SARS-CoV-2 infection, the cytokine storm and its accompanying excessive inflammatory responses have become a therapeutic target in the treatment of COVID-19 patients. Immunotherapy strategies aimed at curtailing the cytokine storm are under investigation in several countries ( Table 2) .

Additional therapies to indirectly reduce viral burden and secondarily reduce the incidence of a cytokine storm are summarized in Supporting Information Tables S1-S4. Strategies to date do not address a specific type of cytokine storm but assume a "one size fits all" approach in treating severe disease in all patients.

Clinical reports showing that elevated levels of IL-6 are associated with the immunopathology and disease severity of COVID-19 111 have provided a strong scientific rationale for examining the effects of IL-6 or its receptor antagonists (e.g., siltuximab and clazakizumab or sarilumab and tocilizumab). In fact, tocilizumab has been recommended in China to treat COVID-19 patients with bilateral pulmonary damage and severe symptoms. 112 Early clinical reports of tocilizumab showed that fevers subsided in 20 severe patients within 1 d after the treatment and 95% of these patients achieved recovery sufficient to allow them to be released from the hospital within 2 wk. 113 Guo et al. 114 showed that tocilizumab treatment (400 mg once through an i.v. drip 115 patients and restored the CD4/CD8 ratio, leading to a marked reduction in SARS-CoV-2 plasma viremia. 106 Deng and colleagues have also suggested that upstream targets, such as cyclic guanosine monophosphate (GMP) -adenosine monophosphate (AMP) synthase (cGAS), anaplastic lymphoma kinase (ALK), and stimulator of interferon genes (STING), may help reduce cell activation and cytokine release. 118 Likewise, JAK-STAT signaling inhibitors (Baricitinib and Ruxolitinib) have also been proposed for preventing CRS 119 ( where treated patients saw reductions in their levels of IL-6 as well as a shorter duration in viral shedding. 54 IFN-beta-1b has also been noted as beneficial when added to antiviral regimen that includes lopinavir and ritonavir. 120

MSCs are adult stem cells that have the ability to self-replicate and which show potential for differentiation into multiple cell types. 121 MSCs have the potential to impact anti-inflammatory activities by producing immunosuppressive cytokines and by directly interacting with and inhibiting the activation of immune cells. 121 (Table 2 and Supporting Information Table S1 ). (Table 2 and Supporting Information Table S3 ). clinics. [136] [137] [138] [139] [140] [141] [142] There also is evidence that the immunosuppressive features of TCM may convey beneficial effects in the prevention and treatment of a cytokine storm. 114 A retrospective study reported that matrine and sodium chloride Injection could markedly improve lymphopenia in COVID-19 patients, possibly by inhibiting pulmonary inflammatory cytokines. 143, 144 In an in vitro experiment, Zhu et al. 145 found that liquiritin could markedly inhibit replication of SARS-CoV- inhibitors and, consequently, suggested that herbal medicines which included these agents might be most useful in dampening a cytokine storm in patients. 146 We anticipate additional TCM strategies will be identified for the management of cytokine storm in COVID-19

patients. For example, ulinastatin, a natural anti-inflammatory substance from fresh human urine is believed to inhibit cytokine release and subsequent tissue injury 147 and is currently under investigation as a treatment of COVID-19 patients. 148 In addition to TCM, corticosteroid treatment has been shown to dampen the inflammatory cascade from a cytokine storm. 149 A recent study found that early and short-term administration of methylprednisolone could improve the clinical outcome in moderate-to-severe COVID-19 patients. 150 Although useful in critical patients as a tool for managing host inflammatory responses, corticosteroids are not recommended in high doses nor for prolonged periods of time as they may dampen normal immune responses that could help contain the viral infection. 146, 151, 152 Nevertheless, there are a number of ongoing clinical trials (Table 2 and Supporting Information Table S4 ) that may show if and when safe and beneficial corticosteroid-based treatment is indicated for treating in COVID-19.

Last, several immunomodulatory therapies, including recombinant human IFN-alpha, rhG-CSF, i.v. immunoglobulin, antibody, vaccine, nutritional supplements (zinc, vitamin C, and vitamin D3), colchicine, and cellular therapy are also under investigation for inhibiting a cytokine storm (Table 2 and Supporting Information Table S4 ).

It is generally believed that the cytokine storm triggered by SARS- Defining shared or population-specific "triggers and amplifiers" of a cytokine storm at specific disease stages will advance novel precision medicine strategies.

Other factors known to contribute to disease pathogenesis (such as host genetics and epigenetics, microbiome, mucosal infection apart from lung, immunoregulatory networks, aging, co-occurring disease, exosomes, complement, neurologic, endocrine, and polydrug environments, etc.) have yet to be fully investigated. For example, Fogarty et al. 98 reported a potential 3-4-fold higher thrombotic risk in Caucasians, which underscores the potential impact that race and ethnicity may have on disease outcomes. Although coagulopathy seems to be a severe complication in COVID-19 infection, it may not necessarily be related to disease severity or exclusively present during or after a cytokine storm. 156 It remains undetermined how the onset of coagulopathy in the absence of compromised lung capacity (ventilation/perfusion) or CRS may be related to early press reports of higher stroke incidence in young and middle-aged infected patients in the United States. Indeed, it has been proposed that, in predisposed individuals, alveolar viral damage may trigger an underlying inflammatory reaction promoting a microvascular pulmonary thrombosis or endothelial thromboinflammatory syndrome affecting microvascular beds beyond the lung (e.g., brain and other vital organs). 157 How immune factors identified in this review (e.g., NETosis, IL-6, macrophage activation, antibody response, etc.) may contribute to damage to microvascular beds or how this damage predisposes incidence of a CRS remains to be determined. Therefore, there is a need to identify leading molecular triggers of multi-organ failure after a cytokine storm in order to prevent added lethal outcomes.

Regarding therapy, the timing and the best ways to combine various treatments also remains to be determined (e.g., early antiviral therapy followed by anti-CRS therapy when that occurs). 158 There is also a need to extend immunopathogenesis studies beyond adults. Based on studies of COVID-19 clinical features in pregnancy, it is known that neonates can be different from adults. 159, 160 For example, children rarely progress with a disease course requiring ICU care. Regrettably, the limited clinical trial activity focusing on children remains a missed opportunity to better address how to treat and limit transmission in this group. 161 Greater advances in all of these areas will allow for selective treatments both to target key pathogenic components of the cytokine storm and further advance highly targeted precision medicine strategies. 

The authors declare no conflicts of interest.

https://orcid.org/0000-0002-2628-4027

Luis J. Montaner https://orcid.org/0000-0001-5799-6759

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Treatment of 5 critically ill patients with COVID-19 with convalescent plasma

Neutralizing antibody responses to SARS-CoV-2 in a COVID-19 recovered patient cohort and their implications

Neutralizing antibodies responses to SARS-CoV-2 in COVID-19 inpatients and convalescent patients

Antibody responses to SARS-CoV-2 in patients of novel coronavirus disease 2019

Exuberant elevation of IP-10, MCP-3 and IL-1ra during SARS-CoV-2 infection is associated with disease severity and fatal outcome

Characterization of cytokine/chemokine profiles of severe acute respiratory syndrome

The potential role of IL-6 in monitoring severe case of coronavirus disease 2019

Detectable serum SARS-CoV-2 viral load (RNAaemia) is closely correlated with drastically elevated interleukin 6 (IL-6) level in critically ill COVID-19 patients

Clinical and laboratory profiles of 75 hospitalized patients with novel coronavirus disease

COVID-19 coagulopathy in Caucasian patients

Interleukin 6 and haemostasis

Analysis of coagulation parameters in patients with COVID-19 in Shanghai, China. Biosci Trends. 2020

Hypercoagulability of COVID-19 patients in intensive care unit. A report of thromboelastography findings and other parameters of hemostasis

Clinical characteristics of 36 non-survivors with COVID-19 in Wuhan

Use of tocilizumab for COVID-19-induced cytokine release syndrome: a cautionary case report

Drugs for autoimmune inflammatory diseases: from small molecule compounds to Anti-TNF biologics

Acute-phase proteins during inflammatory reaction by bacterial infection: fish-model

Disruption of the CCL5/RANTES-CCR5 pathway restores immune homeostasis and reduces plasma viral load in critical COVID-19

Elevated plasma level of selective cytokines in COVID-19 patients reflect viral load and lung injury

Innate immune response of human alveolar type II cells infected with severe acute respiratory syndrome-coronavirus

Distinct immune response in two MERS-CoV-infected patients: can we go from bench to bedside?

Cell host response to infection with novel human coronavirus EMC predicts potential antivirals and important differences with SARS coronavirus

Can we use interleukin-6 (IL-6) blockade for coronavirus disease 2019 (COVID-19)-induced cytokine release syndrome (CRS)

Advance in cognition of revised and updated 'guidelines of diagnosis and treatment of COVID-19

China optimizes treatment for COVID-19

Tocilizumab treatment in severe COVID-19 patients attenuates the inflammatory storm incited by monocyte centric immune interactions revealed by single-cell analysis

Effective treatment of severe COVID-19 patients with tocilizumab

Tocilizumab treatment in COVID-19: a single center experience

Induction of pro-inflammatory cytokines (IL-1 and IL-6) and lung inflammation by Coronavirus-19 (COVI-19 or SARS-CoV-2): anti-inflammatory strategies

Regulation of interferon production as a potential strategy for COVID-19 treatment

COVID-19: combining antiviral and anti-inflammatory treatments

Triple combination of interferon beta-1b, lopinavir-ritonavir, and ribavirin in the treatment of patients admitted to hospital with COVID-19: an open-label, randomised, phase 2 trial

Mesenchymal stromal cells: clinical challenges and therapeutic opportunities

The efficacy of convalescent plasma for the treatment of severe influenza

Clinical observation of 6 severe COVID-19 patients treated with plasma exchange or tocilizumab

Should chloroquine and hydroxychloroquine be used to treat COVID-19? A rapid review. BJGP Open. 2020

New insights on the antiviral effects of chloroquine against coronavirus: what to expect for COVID-19?

The lysosomotropic amines, chloroquine and hydroxychloroquine: a potentially novel therapy for graft-versushost disease

Mode of action of hydroxychloroquine in RA-evidence of an inhibitory effect on toll-like receptor signaling

Azithromycin during acute chronic obstructive pulmonary disease exacerbations requiring hospitalization (BACE). A multicenter, randomized, doubleblind, placebo-controlled trial

Hydroxychloroquine and azithromycin as a treatment of COVID-19: results of an open-label non-randomized clinical trial

A pilot study of hydroxychloroquine in treatment of patients with common coronavirus disease-19

Efficacy of hydroxychloroquine in patients with COVID-19: results of a randomized clinical trial

Hydroxychloroquine application is associated with a decreased mortality in critically ill patients with COVID-19

Clinical course of coronavirus disease 2019 (COVID-19) in a series of 17 patients with systemic lupus erythematosus under long-term treatment with hydroxychloroquine

The QT interval in patients with SARS-CoV-2 infection treated with hydroxychloroquine/azithromycin

Traditional Chinese medicine in the treatment of patients infected with 2019-new coronavirus (SARS-CoV-2): a review and perspective

Effects of shuanghuanglian and qingkailing, two multi-components of traditional Chinese medicinal preparations, on human leukocyte function

Tetrandrine inhibits differentiation of proinflammatory subsets of T helper cells but spares de novo differentiation of iTreg cells

Triptolide, a constituent of immunosuppressive Chinese herbal medicine, is a potent suppressor of dendritic-cell maturation and trafficking

Chemokines and chemokine receptors as novel therapeutic targets in rheumatoid arthritis (RA): inhibitory effects of traditional Chinese medicinal components

Shikonin, a component of chinese herbal medicine, inhibits chemokine receptor function and suppresses human immunodeficiency virus type 1

Tannic acid is an inhibitor of CXCL12 (SDF-1alpha)/CXCR4 with antiangiogenic activity

Regulatory effects of deoxycholic acid, a component of the anti-inflammatory traditional Chinese medicine Niuhuang, on human leukocyte response to chemoattractants

Clinical efficacy of matrine and sodium chloride injection in treatment of 40 cases of COVID-19

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Effect of matrine sodium chloride injection on mouse model of combination of disease and syndrome of pulmonary syndrome of human coronavirus pneumonia virus virus infection

An artificial intelligence system reveals liquiritin inhibits SARS-CoV-2 by mimicking type I interferon

Adjuvant corticosteroid therapy for critically ill patients with COVID-19

Ulinastatin inhibits the inflammation of LPS-induced acute lung injury in mice via regulation of AMPK/NF-kappaB pathway

Chinese Clinical Trial Register, 2020 A clinical trial for Ulinastatin Injection in the treatment of patients with severe novel coronavirus pneumonia (COVID-19)

The cytokine storm of severe influenza and development of immunomodulatory therapy

Early short course corticosteroids in hospitalized patients with COVID-19

Clinical evidence does not support corticosteroid treatment for 2019-nCoV lung injury

Risk factors associated with acute respiratory distress syndrome and death in patients with coronavirus disease 2019 Pneumonia in Wuhan, China

A machine learning model reveals older age and delayed hospitalization as predictors of mortality in patients with COVID-19

Hypertension in patients hospitalized with COVID-19 in Wuhan, China: a single-center retrospective observational study

Genomic diversity of SARS-CoV-2 in coronavirus disease 2019 patients

Large-vessel stroke as a presenting feature of Covid-19 in the young

Microvascular COVID-19 lung vessels obstructive thromboinflammatory syndrome (Micro-CLOTS): an atypical acute respiratory distress syndrome working hypothesis. Crit Care Resusc

Don't forget the host: cOVID-19 cytokine storm

Clinical and CT features in pediatric patients with COVID-19 infection: different points from adults

COVID-19 in children, pregnancy and neonates: a review of epidemiologic and clinical features

A call for pediatric COVID-19 clinical trials

Clinical characteristics of 2019 novel coronavirus infection in China

Analysis of clinical features of 29 patients with 2019 novel coronavirus pneumonia

Correlation analysis between disease severity and inflammation-related parameters in patients with COVID-19 pneumonia

Clinical features of 69 cases with coronavirus disease 2019 in Wuhan, China

Epidemiological and clinical features of 2019-nCoV acute respiratory disease cases in Chongqing municipality, China: a retrospective, descriptive, multiple-center study

Clinical characteristics of 138 hospitalized patients with 2019 novel coronavirus-infected pneumonia in Wuhan

Clinical features of severe pediatric patients with coronavirus disease 2019 in Wuhan: a single center's observational study

Down-regulated gene expression spectrum and immune responses changed during the disease progression in COVID-19 patients

Clinical manifestations and sero-immunological characteristics of 155 patients with COVID-19

Value of various inflammatory markers combined with lymphocyte subsets on clinical diagnosis of different clinical types of COVID-19

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Risk factors for severity and mortality in adult COVID-19 inpatients in Wuhan

Cytokine storm and leukocyte changes in mild versus severe SARS-CoV-2 infection: Review of 3939 COVID-19 patients in China and emerging pathogenesis and therapy concepts