key: cord-0695590-hiob2shn
authors: Zizzo, Gaetano; Cohen, Philip L
title: Imperfect storm: is interleukin-33 the Achilles heel of COVID-19?
date: 2020-10-09
journal: Lancet Rheumatol
DOI: 10.1016/s2665-9913(20)30340-4
sha: d4c47ace715e531caed63600b2a7e67ec198eb1c
doc_id: 695590
cord_uid: hiob2shn

The unique cytokine signature of COVID-19 might provide clues to disease mechanisms and possible future therapies. Here, we propose a pathogenic model in which the alarmin cytokine, interleukin (IL)-33, is a key player in driving all stages of COVID-19 disease (ie, asymptomatic, mild–moderate, severe–critical, and chronic–fibrotic). In susceptible individuals, IL-33 release by damaged lower respiratory cells might induce dysregulated GATA-binding factor 3-expressing regulatory T cells, thereby breaking immune tolerance and eliciting severe acute respiratory syndrome coronavirus 2-induced autoinflammatory lung disease. Such disease might be initially sustained by IL-33-differentiated type-2 innate lymphoid cells and locally expanded γδ T cells. In severe COVID-19 cases, the IL-33–ST2 axis might act to expand the number of pathogenic granulocyte–macrophage colony-stimulating factor-expressing T cells, dampen antiviral interferon responses, elicit hyperinflammation, and favour thromboses. In patients who survive severe COVID-19, IL-33 might drive pulmonary fibrosis by inducing myofibroblasts and epithelial–mesenchymal transition. We discuss the therapeutic implications of these hypothetical pathways, including use of therapies that target IL-33 (eg, anti-ST2), T helper 17-like γδ T cells, immune cell homing, and cytokine balance.

Intensive efforts are underway to unravel the immuno pathology of COVID19, caused by severe acute respiratory syndrome coronavirus 2 (SARSCoV2), and to control the pandemic. Given the public health emergency, scarcity of effective antiviral therapies, and rapid evolution of lung disease associated with COVID19, patients who are critically ill with COVID19 and have exuberant inflam mation, lifethreatening acute respiratory distress syn drome, and coagulopathy, are basically treated as if they had secondary haemophagocytic lympho histiocytosis or virusassociated macrophage activation syndrome (MAS). These treatments are focused on therapies that neutralise key cytokines driving classical MAS, such as interleukin6 ([IL]6; eg, tocilizumab) or interferon gamma (IFNγ; eg, emapa lu mab). 1,2 In fact, some fatal cases of COVID19 are accompanied not only by severe respiratory disease, but also by increased systemic inflammation as shown by higher ferritin concentrations. 2 However, in many aspects, COVID19 does not resemble typical MAS. We propose that the cytokine storm syn drome seen in COVID19 is dissimilar to that seen in canonical MAS and should be regarded as a distinct entity and approached in a novel way reflecting its unique qualities.

Whereas virusinduced MAS shows the classic hall marks of a Thelper (Th)1 profile, with high production of IFNγ, 1,3 COVID19 is instead characterised by circulating T cells that show an activated Th17 membrane phenotype (CD38 + HLADR + CD4 + CCR6 + ) 4 and express granulocytemacrophage colonystimulating factor (GMCSF) in part along with IFNγ. 5 Con centrations of both IL17 and IFNγ are increased in serum from patients with COVID19 in proportion with viral load and lung injury. 6 Simi larly, Middle East respiratory syndrome has been associated with a combined Th1-Th17 inflammatory response. 7 Notably, the cytokine storm composition induced by SARSCoV2 differs from that induced by severe acute respi ra tory syndrome corona virus (SARSCoV) and Middle East respiratory syndrome coronavirus, with lower pro duc tion of type 1 cytokines (eg, IL12p70, IL15), and high con centrations of type 2 cytokines (eg, IL4, IL9, IL10, trans forming growth factor β [TGFβ], IL13). [6] [7] [8] [9] [10] [11] These findings might provide important clues to the specific immuno pathology of COVID19.

Transcriptomic analyses of bronchoalveolar lavage fluid from patients with COVID19 have revealed a strong upregulation of IL33. 11 IL33 is a cytokine of the IL1 family that is expressed in barrier tissues and exerts pleiotropic functions. In the lungs, IL33 is promptly released, mainly by injured epithelial alveolar cells, following infection and cellular damage. 12 Among its functions, IL33 enhances TGFβmediated differentiation of Foxp3 + regulatory T (Treg) cells 13 and stimulates CD11c + myeloid dendritic cells to secrete IL2, which drives Treg cell expansion, thus ultimately promoting resolution of inflammation. 14 Indi vid uals infected with SARSCoV2 who develop milder symptoms tend to have large numbers of Treg cells 10 and alveolar macrophages showing a scavenger resolving (FABP4 + ) phenotype. 15 In the presence of an adequate immune response and virus clearance, IL33 might drive rapid Treg celldependent restoration of respiratory tissue homoeostasis, which probably accounts for the mild or asymptomatic forms of COVID19 seen in most individuals.

In susceptible individuals who develop symptomatic SARSCoV2 infection and COVID19 pneumonia (eg, in the presence of indivi dual cytokine or receptor polymorph isms), IL33 might abnormally upregulate expression of its own receptor ST2 (also known as IL1RL1) on Treg cells, resulting in increased expression of the canonical Viewpoint Th2 transcription factor GATAbinding factor 3 (GATA3), which impairs the suppressive function of Treg cells. The dysregulation of GATA3 + Foxp3 + Treg cells might result in impaired immunological tolerance and increased secretion of type 2 cytokines, thus promoting auto inflam matory lung dis ease. 16 TGFβ2, which is also increased in the broncho alveolar lavage fluid of patients with COVID19, 11 might further enhance ST2 expression in innate lymphoid cells, and IL33 is the key cytokine that drives these cells to differentiate into type 2 innate lymphoid cells (ILC2). 17 ILC2 subsequently elicit lung inflammation by releasing large amounts of IL9, which promotes their own survival and expands γδ T cells. 18, 19 IL9 is known to stimulate proliferation and expansion of Vγ9Vδ2 + T cells that have a predominantly effector memory phenotype and a com bined Th1-Th17 cytokine response profile. 19 When exposed to TGFβ, γδ T cells can also become an important source of IL9. 20 By acting in both autocrine and paracrine manners, IL33induced IL9 might sustain a proinflam matory ILC2-γδT cell axis in the lungs of patients with COVID19, thus initiating mildmoderate forms of pneumonia.

Both ILC2 and γδ T cells are centrally involved in lung homoeostasis and are rapidly activated in response to pathogens including viruses; 19, 21 in COVID19, IL4 is upregulated at early stages and in milder forms of the disease, 10 whereas IL9 and activated γδ T cells are observed more frequently in mildtomoderate disease, 9, 22 and IFNγ and IL17 progressively increase with disease sever ity. 6 Vγ9Vδ2 + T cells from patients with COVID19 have been found to express an effector memory pheno type three times more frequently than do conventional αβ T cells, 23 thus suggesting that this T cell subset is selec tively stimulated in COVID19. Because of significantly higher expression of the chemokine receptor CXCR3 compared with their αβ counterparts, 24 γδ T cells might be rapidly recruited into inflamed lungs of patients with COVID19 in response to the observed strong upregu lation of the CXCR3 ligands CXCL9 and CXCL10 (figure 1). 6,9,11,15,25-28

The cellular composition of lung infiltrates in patients with COVID19 pneumonia changes with the progression of disease. Infiltrates in patients with moderate pneumonia include mainly lym phoid and dendritic cells; whereas, severe forms of disease are characterised by massive infiltration of macrophages and neutrophils. 15 In patients with COVID19, expression of Tcell chemoattractants (eg, CXCL9, CXCL10) and their receptors (eg, CXCR3) pre cedes expression of monocyte and neutrophil chemo attrac tants (eg, CCL2, CCL3, CCL4, CCL7, CXCL8) and their corresponding receptors (eg, CCR1, CXCR2). 15 The composition and phenotypes of lung macrophages also change with disease severity. Resident alveolar (AFABP4 + ) macrophages, which show scavenger and lipid metabolic functions typical of antiinflammatory or resolving M2like cells (eg, macro phage receptor MARCO, PPARγ, ApoCI), pre dominate in mild and moderate forms, whereas CD14 + monocytederived macrophages (FCN1 high ) and chemo attractant (FCN1 low SPP1 + ) macrophages, which show highly inflammatory M1like profiles (eg, nuclear factorkappa B [NFκB], CCL2, CCL3), dominate tissue specimens from patients with severe forms of COVID19 and who are critically ill. 15 In the circulation of patients with COVID19, amounts of proinflammatory CD14 + CD16 + intermediate mono cytes increase with disease severity, and upregulation of GMCSF in CD4 + and CD8 + T cells might account for tissue recruit ment and activation of neutrophils and monocytederived macrophages in most severe forms of the disease. 5 Although described as Th1 cells, at least half of the GMCSFproducing T cells observed in the circulation of patients with severe COVID19 do not coexpress the IL-33 released from virus-damaged cells might induce dysregulated GATA3 + Foxp3 + Tregs and promote IL-2 production by dendritic cells, resulting in further expansion of Tregs. IL-33 might also elicit differentiation of ILC2, with TGFβ enhancing ST2 expression on these cells and facilitating production of IL-9. IL-9 in turn stimulates expansion of effector memory Vγ9Vδ2 + T cells with mixed Th1 and Th17 profiles that express CXCR3 and are recruited to the lungs by CXCL9 and CXCL10. IL-9 possibly induces its own transcription factor PU.1 and thus act in an autocrine and paracrine manner (along with TGFβ) to drive proliferation and survival of ILC2 and γδ T cells. Additional positive loops might be fed by IFNγ, which triggers production of CXCL9 and CXCL10 by macrophages. In severe forms of COVID-19, IL-33, along with IL-2 and IL-7 released by dendritic cells, might further stimulate T-cell expansion through STAT5 and induce production of large amounts of GM-CSF by γδ and T helper cells. At advanced stages of disease, aberrant activation of the MyD88-related NF-κB pathway and activation of the NLRP3 inflammasome might induce virus-exposed cells and infiltrating monocytes-macrophages to overproduce IL-1β, IL-23, and IL-6. IL-1β, IL-23, IL-6, and IL-7 act on STAT3 and RORC, thus promoting differentiation of CCR2 + T cells that are recruited to the lungs by CCL2 and CCL8 into γδT17 and Th17 cells producing IL-17 and GM-CSF. In turn, GM-CSF might further recruit and activate proinflammatory monocytes-macrophages. CCR=C-C motif chemokine receptor. CCL=C-C motif chemokine ligand. CXCL=C-X-C motif chemokine ligand. CXCR=C-X-C chemokine receptor. Viewpoint canonical Th1 cytokine IFNγ. 5 Lymphocytes from patients with COVID19 appear to be functionally exhausted, pro ducing lower amounts of IFNγ, IL2, and tumour necrosis factor (TNF), and having decreased cyto toxic func tion. 29 Many factors could possibly explain this lympho cyte dysfunction, in particular the upregulation of mul tiple coinhibitory receptors such as CD94, CD152 (cytotoxic Tlymphocyteassociated antigen 4), pro grammed cell death protein 1 (PD1), and Tcell immuno globulin mucin receptor 3 (TIM3). 29 How ever, suboptimal production of IFNγ, poor cytotoxic capabilities, a shorter lymphocyte lifespan, and lympho penia might also be attributable to a scarcity in type I and III interferons (IFNα, IFNβ, and IFNλ), in the blood as well as in the lungs of patients with COVID19. 27 Interferons are more highly sup pressed by SARSCoV2 than by SARSCoV infection, 27, 28 and this most likely accounts for the impaired antiviral responses and spon taneous apoptosis of dysfunctional lymphocytes. 11, 30 Lymphocyte impairment in COVID19 resembles the cytotoxic dysfunction of CD8 + cytotoxic T lymphocytes and natural killer cells observed in familial haemophago cytic lymphohistiocytosis, in which T cell dysfunction is the result of heterozygous mutations in genes affecting the expression of perforin or other proteins involved in the trafficking and docking of cytolytic granules, 1 and in patients who are predisposed to MAS, in whom IL6 overexpression can reduce perforin and granzyme B con centration inside granules. 31 The inability to kill infected or activated antigen presenting cells in patients with either MAS or COVID19 could result in persistent interactions between T cells and antigen presenting cells, culminating in hyper production of cytokines as a result of overstimulation of both cell types. 1 However, by contrast with COVID19, IFNγ is not impaired in MAS, and is a major driver of disease. In MAS, IFNγproducing CD8 + Tcell populations are elevated in primary and secondary lymphoid organs, leading to IFNγdriven macrophage hyperactivation and haemophagocytosis. 1,3

The effects of IFNγ deficiency have been investigated in an experimental model of haemophagocytic lymphohistio cytosis, which develops when perforin deficient (Prf1 / ) mice are infected with the lymphocytic choriomeningitis virus. Surprisingly, mice lacking both IFNγ and perforin (IFNγ / Prf1 / ) still develop a severe MASlike disease that requires the IL33-ST2 axis and is downstream mediated by GMCSFproducing CD8 + T cells. The inflammatory burden in infected IFNγ / Prf1 / mice is even higher than in Prf1 / mice, being characterised by a 10-15 times increase in neutrophils and stronger upregulation of IL1β and IL6. 32 The same interplay between IL33 and GMCSF might occur in patients with COVID19, which would initiate the cyto kine storm syndrome. Thus, severe forms of COVID19 might represent atypical MAS or MASlike reactions with incorporated interferon deficiencies.

Failure of lymphocytes to adequately respond to viral antigens and proapoptotic signals might induce dendritic cells to produce large amounts of the lymphocyte growth factors IL7 and IL2, thereby stimulating Tcell survival and expansion. High concen trations of IL2 and IL7 in serum are characteristic of severe COVID19 cases. 6,9 However, IL2 and IL7 might amplify ILC2 survival and differentiation induced by IL33, 33 expand γδ T cells, which produce IL17 through the signal transducer and activator of transcription (STAT)3, 34 and enhance IL33induced pathologic expan sion of T cells expressing GMCSF through STAT5. 35, 36 Patients infected with SARSCoV2 show an increase in circulating CD4 + γδ T cells that over express the IL2 receptor CD25 but not PD1, suggesting that these cells are not exhausted, but are specifically activated in response to IL2. 22 IL7 enables γδ T cells to fully differentiate into γδT17 cells 34 that coproduce IL17F along with IL17A, and rapidly migrate into inflamed tissues in response to CCR2 and CCR5 ligands such as CCL2 and CCL8. 37, 38 As shown for murine γδT17 cells, human Vδ2 + T cells that coexpress CCR2 and CCR5 also express the IL7 receptor and show a Th17like phenotype (CCR6 + CD161 + IL23R + ). 39 Transcrip tional analyses of respiratory cell populations in response to SARSCoV2 infection reveal strong upregula tion of CCL8, CCL2, CXCL9, CXCL10 and their respec tive receptors, 11,15,27 and global upregulation of IL17 and IL17Frelated pathways, 26 including the CCR6 ligand CCL20 and IL23. 27 IL23 and IL1β are required for GMCSF production by γδT17 cells and conventional αβ Th17 cells. 40, 41 In models of autoimmunity in which GMCSF is a key pathogenic molecule, such as experimental autoimmune encephalo myelitis, γδ T cells have been identified as the major source of GMCSF. 42 Whereas conventional αβ Th17 cells evolve to produce IFNγ during the development of the disease, γδT17 cells are less likely to produce IFNγ and will more likely evolve to produce GMCSF. 42 As for γδT17, recruitment of IL23driven, GMCSFproducing Th17 cells requires CCR2. 43 By promptly releasing multiple cytokines such as IL9, IL17, IL17F, TNF, IFNγ, and GMCSF, γδT17 might be instrumental in recruiting neutrophils and proinflam matory monocytes into the capillaries and alveoli of patients with COVID19. Moreover, activation of γδ T cells might be important in the cytokinedriven induction of procoagulant tissue factor in endothelial cells, 44 thus also having a potential role in vascular manifestations and pulmonary thromboses associated with COVID19 pneumonia. 2, 29 Cytotoxicity against virusinfected alveolar epithelial cells by γδ T cells has been shown for influenza virus 45 and might involve atypical pathways alternative to granzyme B and perforin, which are more commonly used by CD8 + T cells and natural killer cells and could be impaired in COVID19 and MAS. 1,29,31 Specifically, γδ T cells might exert cytotoxic effects through the TNFrelated apoptosis inducing ligand, Fas ligand, and granzyme K, 39, 45 which are all overexpressed in the lungs of patients with COVID19, 11,15 and might therefore explain how γδ T cells cause diffuse damage to the alveolar epithelium (figure 1).

Advanced stages of COVID19 are characterised by high circulating and pulmonary concentrations of IL1α, IL1β, and IL1 receptor antagonist (IL1RA). 6,15, 25 The increased production of these mole cules probably relates to high viral loads resulting in increased viroporins and subse quent activation of the NACHT, LRR, and PYD domains containing protein 3 (NLRP3) inflam masome. The strong expression of IL1α, IL1β, and IL1RA is also due to monocyte activation and intense lung infiltra tion of monocytederived macrophages at later stages, as sug gested by an abundance of CD14 + IL1β + monocytes in the circulation of patients with COVID19 in the early stages of recovery. 46 Active IL1β is produced following NLRP3 assembly and consequent caspase1 activation. By modu lat ing ion fluxes across host cell membranes, viroporins (in particular the ORF3a protein) have been shown to activate NLRP3 during SARSCoV infection, and a simi lar mechanism might be at play during SARSCoV2 infection. 47 An imbalance in signalling from tolllike receptor (TLR) pathways, with the myeloid differentiation primary response protein (MyD88) pathway predominating over the TIR domaincontaining adapter molecule 1 (TICAM1, also known as TRIF) pathways, might further increase NLRP3 activation. 48 Signalling downstream of IL1 family receptors, including the IL33 receptor ST2, and down stream of membrane TLRs, can activate MyD88 and elicit inflammation; whereas TRIFmediated pathways down stream of endo somal TLRs would be expected to mount antiviral inter feron responses and protect against corona viruses. 48, 49 Although coronaviruses are singlestranded RNA viruses that are predicted to bind directly to endo somal TLR7 and TLR8, and indirectly to TLR3 (using doublestranded RNA replication inter mediates), aberrant inflammation induced by corona viruses might instead involve membraneexpressed TLR2, as suggested by virus spike protein interactions with heparan sulphateenriched regions of TLR2 in studies of the mouse hepatitis coronavirus. 50 A predominance of MyD88 signalling over TRIF signalling would lead virusexposed cells to produce high amounts of IL1β, and NFκBinduced cytokines and chemokines (eg, TNF, IL8, IL6, IL12p40, IL23, CCL2) rather than interferons, IL12p35 and IL12p70. 30, 49, 50 High concentrations of MyD88related cytokines and reduced expression of TRIFrelated cytokines charac terise the cytokine milieu observed in the lungs of patients with severe and lifethreatening COVID19. 15,27,28 Such an altered cytokine environment would polarise the immune response towards detrimental (Th17sustained and GMCSFinduced) hyperinflammation 40,41 caused by monocytederived macrophages and neutrophils, in place of protec tive (Th1sustained and IFNinduced) antiviral responses exerted by cytotoxic T lymphocytes, natural killer cells, and B cells. 29, 30 Altogether, cor ona viruses seem to deceive and escape the immune sys tem by elicit ing a response that is generally more appropriate for extra cellular rather than intracellular pathogens.

In addition to NLRP3 stimulation and IL1 release, 47 substantial amounts of viroporins in patients with life threatening COVID19 might also account for exten sive injury of alveolar epithelial cells and overproduction of IL33. 51 IL33, IL1α, and GMCSF also stimulate each other's release by alveo lar type 2 pneumocytes. 52,53 Accord ingly, diffuse alveolar damage with alveolar denudation and reactive type 2 pneumo cyte hyperplasia are histo logical hallmarks of COVID19 with acute respiratory distress syndrome. 4 Feedforward loops might also engage mast cells, macrophages, endothelial cells, T cells, and neutrophils. 40, 54 Although whether mast cells and macrophages produce IL33 is still up for debate, 51 it is well established that mast cells, infiltrating neutrophils, and cytotoxic T lymphocytes secrete serine proteases (eg, tryptase, cathepsin G, elas tase, granzymes) that cleave IL33 released from damaged epithelial and endothelial barriers into a mature form of IL33 that is 10-30 times more active. 51 IL33 ampli fies lung inflammation by inducing various pro inflam matory cytokines (eg, GMCSF, IL1β, IL6, TNF, granulocyte colonystimulating factor [GCSF]), chemo kines (eg, CXCL1, CXCL2, CXCL6, CXCL8, CCL2, CCL20), and adhesion molecules (eg, Eselectin, ICAM1, VCAM1) in several target cells. 32, [54] [55] [56] [57] Conversely, by inhib iting type 1 interferons and IL12p35, IL33 might contribute to impaired antiviral cytotoxic responses. 58 In models of MASlike disease, IL33 is a crucial con tributor to the weight loss and hyperferritinaemia related to systemic hyperinflammation, and to the expan sion of GMCSF producing CD8 + T cells, upregulation of IL1β and IL6, and tissue neutrophilia. 32 These features are the same as key characteristics seen in patients with critical COVID19. 5,15, 26 IL33 has also been implicated in the formation of neutrophil extracellular traps during virusinduced asthma exacerbation. 58 Similarly, neutrophil priming with GMCSF might promote the production of neutrophil extracellular traps. 59 By releasing neutrophil elastase and other proteinases, neutrophil extracellular traps could in turn cleave and further activate IL33. These pathways might be relevant in patients with critical COVID19, since neutrophilia and the neutrophiltolymphocyte ratio are associated with poor prognosis, and high concentrations of neutrophil extracellular traps have been detected in patients with COVID19 admitted to hospital and receiving mechanical ventilation. 60

Neutrophil extracellular traps might propagate inflam ma tion and microvascular thrombosis in patients with COVID19 and severe acute respiratory distress syn drome. 60 Along with IL33, IL1, TNF, and other cytokines, neutrophil extracellular traps might increase endothelial permeability and induce a procoagulant phenotype in endo thelial tissues by inducing expression of tissue factor, 61-63 thus representing a possible link between hyper inflammation and hypercoagulability that could account for Ddimer elevation, pulmonary thrombosis, and micro vascular manifestations affecting the heart, kidneys, and small bowel seen in patients with critical COVID19. 64, 65 Endothelialitis and endothelial dysfunction would also account for predominant exudativephase diffuse alveolar damage characterised by hyaline mem branes and fibrin deposits typically observed in patients with COVID19 and severe acute respiratory distress syndrome. 4 IL33 has also been shown to stimulate expression of IL1β, IL6, CCL2, CXCL2, and GCSF by adipocytes. 57 Elevated circulating concentrations of soluble ST2 (mea sured more often than IL33 because of its higher concentration and stability) are associated with obesity, diabetes, hypertension, and acute cardiovascular diseases. High soluble ST2 concentrations also predict worse outcomes and are associated with extension of heart damage, heart failure, increased cardiovascular death, and allcause mortality. 54 Notably, diabetes, hypertension, and cardiovascular diseases are common comorbidities in patients with COVID19, and obesity has been inde pendently associated with increased severity and mortality among younger patients with COVID19. 66 Circulating concentrations of soluble ST2 correlate with the extent of tissue damage, and might represent an indicator in plasma of IL33 release and bioactivity in tissues. IL-33 might induce numerous cytokines and chemokines as well as its own receptor, ST2, in various cell types. In asymptomatic or paucisymptomatic patients, IL-33 might expand anti-inflammatory Foxp3 + Treg cells or induce IL-4 production by GATA3 + Foxp3 + Tregs and ILC2, thus stimulating mast cells, which might account for minor, allergy-like symptoms. In individuals with mild-to-moderate disease, IL-33 (along with TGFβ) might induce ILC2 to release large amounts of IL-9, driving local expansion of effector memory Vγ9Vδ2 + T cells in the lungs. In moderate-to-severe pneumonia, IL-33 combined with IL-2 and IL-7 from dendritic cells might further expand ILC2, γδT cells, and GM-CSF-producing T cells. In severe-critical COVID-19, IL-33, GM-CSF, and IL-1 might stimulate each other's release by acting on multiple cell types. IL-33 induction of cytokines, chemokines, adhesion molecules, tissue factor, and neutrophil extracellular traps might contribute to endothelialitis, thrombosis, and extrapulmonary involvement in patients with MAS-like disease. Neutrophil extracellular traps and mast cell degranulation could provoke protease-mediated cleavage of IL-33 into a 10-30 times more potent form, and IL-33-induced release of its soluble receptor ST2 might further polarise T cells and contribute to cardiovascular manifestations. In patients who survive, IL-33 might drive the post-acute fibrotic phase thorugh induction of IL-13 and TGFβ in M2-differentiated macrophages and ILC2, thereby stimulating myofibroblasts and eliciting the epithelial-to-mesenchymal transition of type 2 pneumocytes. Molecules inside brackets are part of self-amplifying proinflammatory loops fed by IL-33 and outside brackets indicate different factors possibly induced by IL-33. Question mark indicates the uncertainty of whether mast cells produce IL-33. bFGF=fibroblast growth factor. CCL=C-C motif chemokine ligand. CTGF=connective tissue growth factor. CXCL=C-X-C motif chemokine ligand. DIC=(systemic vascular thromboses mimicking) diffuse intravascular coagulation. EMT=epithelial-mesenchymal transition. Foxp=forkhead box protein. GATA=GATAbinding factor. G-CSF=granulocyte colony-stimulating factor. GM-CSF=granulocyte-macrophage colony-stimulating factor. ICU=intensive care unit. IFN=interferon. IL=interleukin. ILC2=type 2 innate lymphoid cell. MAS=macrophage activation syndrome. MOF=multiple organ failure. NET=neutrophil extracellular trap. PDGF=plateletderived growth factor. P/F ratio=arterial oxygen partial pressure to fractional inspired oxygen ratio. sST2=soluble ST2. ST2=ST2 receptor. TGF=transforming growth factor. TF-1=tissue factor-1. TNF=tumour necrosis factor. TRAIL=TNF-related apoptosis-inducing ligand. Treg=regulatory T cell. 

Parallels between COVID19 and rheumatic disorders can be made by referring to discrete autoinflammatory syndromes that share symp toms with COVID19 such as fever, frequent conjunc tivitis, and-most remarkablyvasculitic manifestations with neutrophilia, thrombosis, and aneurysmal dilations, involv ing coronary vessels (eg, Kawasaki disease in infants) or pulmonary vessels (eg, Behçet's disease in adults). Case series of children infected with SARSCoV2 who develop Kawasakilike disease with MAS features have been described. 68 GMCSF produced by cardiac fibroblasts is key in disease progression in mouse models of Kawasaki disease, and significantly increased soluble ST2, Eselectin, CXCL10, IL17F, and in some cases IL9, have been reported in the circulation of patients with acute Kawasaki disease compared with other children who are febrile. [69] [70] [71] Similarly, Behçet's disease has been associated with high concentrations of both soluble ST2 and IL33, as well as increased CXCL10 and CCL2, Vγ9Vδ2 Tcell expansion, IL17F gene polymorphisms, and intense recruitment of T cells producing IL9 and IL17 to the lungs. 72-76 Some patients with either Behçet's disease 77 or COVID19 78 also show positivity for antiphospholipid antibodies, which might further contribute to the coagulo pathy seen in both conditions.

Lung alveolar inflammation in COVID19 is accompanied by loose interstitial fibrosis and can result in widespread fibrotic changes. 79 IL33 could also be important at these later stages of the disease. In a bleomycininduced pul monary fibrosis mouse model, the IL33-ST2 axis is required to induce alternatively activated M2 macro phages and ILC2 to release key profibrotic cytokines. 80 IL33activated mast cells might also play a role in organ fib rosis. 81 Most remarkably, IL33 has been shown to induce epithelialtomesechymal transition of type 2 pneumocytes through TGFβ signalling. 82 IL33 concentrations are elevated in patients with sys temic sclerosis and correlate with the severity of pulmonary fibrosis, and patients with idiopathic pul monary fibrosis show increased serum concentrations of soluble ST2 when the disease is exacerbated. 82 IL33 can induce cyto kines (eg, TGFβ, IL13) and chemo kines (eg, CCL2, CXCL6) involved in pulmonary fibrosis, which are also increased in patients infected with SARSCoV2, 6,9,11,15,26 thus suggest ing addi tional roles for IL33 in driving the postacute fibrotic phase of COVID19. Growth factors such as vascular endothelial growth factor, plateletderived growth factor, and fibroblast growth factor are all involved in fibrotic processes and are overexpressed in patients with COVID19, 9 and γδ T cells exposed to TGFβ might produce connective tissue growth factor (figure 2). 83

Although more conclusive results are awaited from random ised controlled trials, encouraging preliminary results have been reported for the successful management of severe and critical COVID19 by therapeutic modulation of IL1α and IL1β with recombinant IL1RA (anakinra) 84, 85 and by blocking GMCSF using monoclonal antibodies (mavrilimumab, lenzilumab). 86, 87 Although some studies 9, 11 and the results with antiIL6 receptor antibodies (eg, tocilizumab, sarilumab) have been controversial. [88] [89] [90] Targeting IL33 (eg, by using antiST2 antibodies such as astegolimab), could be the key for controlling excessive lung inflammation. In a mouse model of influenza virusinduced asthma exacerbation, 58 administration of an antiST2 antibody significantly reduced airway hyper responsiveness and bodyweight loss, lowered inflam matory cell numbers in the lungs, and eliminated neutro phil extracellular traps in the airway lumen; more over, antiST2 treatment restored lung expression of IFNβ, IL12p35, and IL12p70, and reduced viral load. 58 There are several ongoing phase 2 trials using antiST2 therapy for inflammatory lung diseases such as chronic obstructive pulmonary disease and asthma. 12 In the IFNγdeficient mouse model of atypical MAS, blocking ST2 provided significant protection against weight loss, increased sur vival, reduced serum ferritin and sol uble CD25 con centrations, and lowered CD8+ Tcell frequencies and neutrophilia. 32 By contrast, individually blocking IL6, IL1β, or GMCSF did not provide major protection against disease, suggesting that dampening IL33-ST2 signalling, rather than individual downstream effector cytokines, might be more effective in treating either canonical MAS or atypical MASlike diseases, 32 such as COVID19. 2 A study evaluating the safety and efficacy of astegoli mab (MSTT1041A) in severe COVID19 pneumonia is recruiting participants (NCT04386616; EudraCT 202000271317). The antiIL33 monoclonal antibody, MEDI3506, has also been included among possible therapies for the treatment of hospitalised patients with COVID19 and is currently being tested in a phase 2 adaptive platform study. 91 Additional important cell targets to focus on include ILC2, IL17producing and GMCSFproducing γδT17 cells, proinflammatory monocytes-macrophages, extracellular trapproducing neutrophils, and mast cells. Different strategies could be set up to control aberrant activation of these cells, for instance by targeting IL9 (MEDI528), Vγ9Vδ2 Tcell activat ion (pentoxifylline), 74 IL17A, IL17F, and IL17E (bimekizumab, brodalumab), 92 and cyto kine balance (using colchicine, 93 NLRP3 inhibitors, 94 TLR2 inhibitors, 95 JAK2 inhibitors, 96 apremilast, 97 mast cells stabilisers, 55 and vitamin D). 98 Acting more directly on the Tcell homing by using antiCCR5 (leronlimab), 99 antiCCR2 (prozalizumab), CCR2 and CCR5 inhibitors (BMS813160), antiCCL2 (carlumab), or antiCCL8 antibodies, might be another crucial strat egy in the management of patients with COVID19. Additionally, antiCCR1 (AZD4818) and anti CXCR2 (AZD5069) antibodies, which selectively act on recruitment of monocytes and neutrophils, might be considered for patients at advanced stages of disease. In patients with postacute respiratory distress syndrome, diverse antifibrotic agents acting on TGFβ (fresolimu mab, pirfenidone), IL13 (lebrikizumab, dupilumab), 100 connective tissue growth factor (pamrevlumab), fibro blast growth factor, plateletderived growth factor, and vascu lar endothelial growth factor signalling (ninte danib), 79 might all represent viable therapeutic options (figure 3, table).

Upon increasing release of alarmin IL33 from injured respiratory cells, in the lack of interferon expression, and alongside efforts of the immune system to overcome inefficient natural killer cells, cytotoxic T lymphocytes, and Th1 antiviral responses, sequential compensatory secretion of IL2 family cytokines (ie. IL4, IL9, IL2, IL7) from dysregulated GATA3 + Treg cells, differentiated ILC2, and overstimulated antigenpres enting cells might account for the early expansion of polyfunctional (CXCR3 + ) Vγ9Vδ2 T cells and the later expansion of (CCR2 + CCR5 + ) GMCSFproducing lympho cytes, both recruited to the lungs by specific chemoattractants. These cells amplify alveolar damage and establish autoinflammatory lung disease. At advanced stages of COVID19, intense activa tion of the NLRP3 inflammasome and TLR2-MyD88-NFκB mediated pathways most likely create a cytokine environment enriched in IL1β, IL23, IL6, and TNF, which would further elicit Th17 differentia tion and GMCSF production by γδT17, Th17, and CD8 T cells ( figure 1) . Ultimately, the resulting cytokine and chemokine milieu could account for the hyperinflammatory state of tissues and vessels mediated by dysfunctional endo thelial cells, mast cells, monocytederived macrophages, and extra cellular trapproducing neutrophils. Endothelial release of tissue factor induced by IL33, activated γδT cells, and neutrophil extracellular traps might act to promote thrombotic manifestations. In patients who survive acute COVID19, IL33 might finally drive pulmonary fibrosis by activating M2 macrophages, ILC2, and mast cells to release TGFβ and IL13, which act in turn on fibroblasts and type 2 pneu mocytes to elicit an epithelialtomesenchymal

We searched PubMed and Google Scholar for articles published in English from Jan 1, 2020, to July 31, 2020, using the search terms "COVID-19", "coronavirus", "IL-33", "ST2", "type-2 innate lymphoid cells (ILC-2)", "gamma delta T cells", "T cells", "macrophages", "mast cells", "neutrophils", "endothelial cells", "adipocytes", "IL-17", "IL-7 ", "IL-9", "GM-CSF", "cytokines", "chemokines", "bronchoalveolar lavage fluid (BALF)", "lung", "heart", "hyperinflammation", "vasculitis", "thrombosis", "adult respiratory distress syndrome (ARDS)", "hemophagocytic lymphohistiocytosis (HLH)", "macrophage activation syndrome (MAS)", "obesity", and "treatment". Viewpoint transition (figure 2). As a result, different stages of COVID19 disease can be distinguished (ie, mildto moderate, severetocritical, chronictofibrotic), and we suggest that IL33 plays a central role in all of these pathogenic phases ( figure 3) . A preprint 101 that recently appeared online supports our model, revealing that SARS CoV2 peptide exposure elicits IL33 expression from patients who are virus seropositive, and IL33 production is correlated with Tcell activa tion and lung disease severity. Targeting the IL33-ST2 axis using monoclonal antibodies (or, alternatively, smallmolecule inhibitors) could prove to be an effective strategy for controlling the COVID19 pandemic.

IL33 dysregulates regulatory T cells and impairs established immunologic tolerance in the lungs

TGFβ induces ST2 and programs ILC2 development

Group 2 innate lymphoid cells utilize the IRF4IL9 module to coordinate epithelial cell maintenance of lung homeostasis

Interleukin (IL)9/IL9R axis drives γδ T cells activation in psoriatic arthritis patients

Human Vδ2 T cells are a major source of interleukin9

Lungresident γδ T cells and their roles in lung diseases

The phenotypic changes of γδ T cells in COVID19 patients

More bricks in the wall against SARSCoV2 infection: involvement of γ9δ2 T cells

Patterns of chemokine receptor expression on peripheral blood gamma delta T lymphocytes: strong expression of CCR5 is a selective feature of V delta 2/V gamma 9 gamma delta T cells

Plasma IP10 and MCP3 levels are highly associated with disease severity and predict the progression of COVID19

Heightened innate immune responses in the respiratory tract of COVID19 patients

Imbalanced host response to SARSCoV2 Drives development of COVID19

Comparative replication and immune activation profiles of SARSCoV2 and SARSCoV in human lungs: an ex vivo study with implications for the pathogenesis of COVID19

Immunology of COVID19: current state of the science

Impaired type I interferon activity and inflammatory responses in severe COVID19 patients

Inhibition of natural killer cell cytotoxicity by interleukin6: implications for the pathogenesis of macrophage activation syndrome

Genetic deficiency of interferonγ reveals interferonγindependent manifestations of murine hemophagocytic lymphohistiocytosis

Lung natural helper cells are a critical source of Th2 celltype cytokines in protease allergeninduced airway inflammation

Interleukin 7 (IL7) selectively promotes mouse and human IL17producing γδ cells

STAT5 programs a distinct subset of GMCSFproducing T helper cells that is essential for autoimmune neuroinflammation

Pleiotropic effects of IL33 on CD4 + T cell differentiation and effector functions

IL17producing γδ T cells switch migratory patterns between resting and activated states

The transcriptional repressor BLIMP1 curbs host defenses by suppressing expression of the chemokine CCL8

Heterogeneous yet stable Vδ2(+) Tcell profiles define distinct cytotoxic effector potentials in healthy human individuals

Inflammasomederived IL1β regulates the production of GMCSF by CD4(+) T cells and γδ T cells

The encephalitogenicity of T(H)17 cells is dependent on IL1 and IL23induced production of the cytokine GMCSF

Innately versatile: γδ17 T cells in inflammatory and autoimmune diseases

CCR2 defines in vivo development and homing of IL23driven GMCSFproducing Th17 cells

A novel prothrombotic pathway in systemic sclerosis patients: possible role of bisphosphonateactivated γδ T cells

Human Vγ9Vδ2T cells efficiently kill influenza virusinfected lung alveolar epithelial cells

Immune cell profiling of COVID19 patients in the recovery stage by singlecell sequencing

Viroporins and inflammasomes: a key to understand virusinduced inflammation

Enhanced IL1β production is mediated by a TLR2MyD88NLRP3 signaling axis during coinfection with influenza A virus and Streptococcus pneumoniae

Tolllike receptor 3 signaling via TRIF contributes to a protective innate immune response to severe acute respiratory syndrome coronavirus infection

Macrophage interleukin6 and tumour necrosis factoralpha are induced by coronavirus fixation to Tolllike receptor 2/heparan sulphate receptors but not carcinoembryonic cell adhesion antigen 1a

Interleukin33 (IL33): a nuclear cytokine from the IL1 family

Interleukin1α controls allergic sensitization to inhaled house dust mite via the epithelial release of GMCSF and IL33

A GMCSF/IL33 pathway facilitates allergic airway responses to subthreshold house dust mite exposure

Conflicting vascular and metabolic impact of the IL33/sST2 axis

Mast cells contribute to coronavirusinduced inflammation: new anti inflammatory strategy

IL33 induces Th17mediated airway inflammation via mast cells in ovalbuminchallenged mice

IL33 stimulates expression of the GPR84 (EX33) fatty acid receptor gene and of cytokine and chemokine genes in human adipocytes

IL33 drives influenza induced asthma exacerbations by halting innate and adaptive antiviral immunity

Viable neutrophils release mitochondrial DNA to form neutrophil extracellular traps

Neutrophil extracellular traps in COVID19

Tissue factor is induced by interleukin33 in human endothelial cells: a new link between coagulation and inflammation

Human IL6, IL17, IL1β, and TNFα differently regulate the expression of proinflammatory related genes, tissue factor, and swine leukocyte antigen class I in porcine aortic endothelial cells

Neutrophil extracellular traps induce endothelial cell activation and tissue factor production through interleukin1α and cathepsin g

Anticoagulant treatment is associated with decreased mortality in severe coronavirus disease 2019 patients with coagulopathy

Endothelial cell infection and endotheliitis in COVID19

Obesity could shift severe COVID19 disease to younger ages

ST2 blockade reduces sST2producing T cells while maintaining protective mST2expressing T cells during graftversushost disease

An outbreak of severe Kawasakilike disease at the Italian epicentre of the SARSCoV2 epidemic: an observational cohort study

GMCSF primes cardiac inflammation in a mouse model of Kawasaki disease

CXCL10/IP10 is a biomarker and mediator for Kawasaki disease

Cardiovascular biomarkers in acute Kawasaki disease

Serum level of interleukin33 and soluble ST2 and their association with disease activity in patients with Behcet's disease

Increased interleukin 33 in patients with neuroBehcet's disease: correlation with MCP1 and IP10 chemokines

Pentoxifylline inhibits Vgamma9/Vdelta2 T lymphocyte activation of patients with active Behçets disease in vitro

Interleukin17F gene polymorphisms in Korean patients with Behçet's disease

Th 9 cells in Behçet disease: possible involvement of IL9 in pulmonary manifestations

Prevalence of antiphospholipid antibodies in Behçet's disease: a systematic review and metaanalysis

Coagulopathy and antiphospholipid antibodies in patients with COVID19

Pulmonary fibrosis and COVID19: the potential role for antifibrotic therapy

IL33 promotes ST2dependent lung fibrosis by the induction of alternatively activated macrophages and innate lymphoid cells in mice

IL33/ST2 axis in organ fibrosis

TGFβ1smad signaling pathway participates in interleukin33 induced epithelialtomesenchymal transition of A549 cells

Human gamma deltaT lymphocytes express and synthesize connective tissue growth factor: effect of IL15 and TGFbeta 1 and comparison with alpha betaT lymphocytes

Interleukin1 blockade with highdose anakinra in patients with COVID19, acute respiratory distress syndrome, and hyperinflammation: a retrospective cohort study

Favorable anakinra responses in severe COVID19 patients with secondary hemophagocytic lymphohistiocytosis

GMCSF blockade with mavrilimumab in severe COVID19 pneumonia and systemic hyperinflammation: a singlecentre, prospective cohort study

First clinical use of lenzilumab to neutralize GMCSF in patients with severe COVID19 pneumonia

Impact of low dose tocilizumab on mortality rate in patients with COVID19 related pneumonia

Tocilizumab for treatment of severe COVID19 patients: preliminary results from SMAtteo COvid19 REgistry (SMACORE)

Interleukin6 blockade with sarilumab in severe COVID19 pneumonia with systemic hyperinflammation: an openlabel cohort study

ACCORD: a multicentre, seamless, phase 2 adaptive randomisation platform study to assess the efficacy and safety of multiple candidate agents for the treatment of COVID19 in hospitalised patients: a structured summary of a study protocol for a randomised controlled trial

COVID19: a case for inhibiting IL17?

The antiviral facet of antirheumatic drugs: lessons from COVID19

Efficacy and pharmacology of the NLRP3 inflammasome inhibitor CP456,773 (CRID3) in murine models of dermal and pulmonary inflammation

Discovery of a novel TLR2 signaling inhibitor with antiviral activity

TH17 responses in cytokine storm of COVID19: an emerging target of JAK2 inhibitor fedratinib

Apremilast, a cAMP phosphodiesterase4 inhibitor, demonstrates antiinflammatory activity in vitro and in a model of psoriasis

1,25Dihydroxyvitamin D3 and its analog TX527 promote a stable regulatory T cell phenotype in T cells from type 1 diabetes patients

Disruption of the CCL5/RANTESCCR5 pathway restores immune homeostasis and reduces plasma viral load in critical COVID19

Repurposing dupilumab may treat advanced COVID19 patients with severe acute respiratory syndrome by mitigating cytokine storm

IL33 expression in response to SARSCoV2 correlates with seropositivity in COVID19 convalescent individuals