State of the art review Are mesenchymal stem cells and derived extracellular vesicles valuable to halt the COVID-19 inflammatory cascade? Current evidence and future perspectives Marta Monguió-Tortajada,1,2 Antoni Bayes-­Genis,1,2,3,4 Antoni Rosell,3,5,6 Santiago Roura1,2 Clinical management of patients suffering from abnormalities, hypoalbuminaemia and capillary 1 ICREC Research Program, COVID-19 includes infection prevention, control leak) similar to other systemic, uncontrolled cyto- Health Science Research measures and supportive care, including supple- kine release syndromes (CRS)(figure 1).6 7 Thus, at Institute Germans Trias i Pujol mental oxygen and mechanical ventilatory support.1 2 present, management of the potential inflammatory (IGTP), Badalona, Catalunya, If not expertly and individually managed with consid- complications of COVID-19 by using appropriate Spain 2 CIBERCV, Instituto de Salud eration for vasocentric features,3 20% of COVID-19 immunosuppressive and immunomodulatory drugs is Carlos III, Madrid, Spain patients who exhibit acute respiratory distress being explored.8 3 Department of Medicine, syndrome (ARDS; 14.8% of hospitalised patients4) To date, a variety of novel and repurposed drugs Autonomous University of may eventually progress to multiorgan failure and with multiple pharmacological effects and thera- Barcelona (UAB), Barcelona, death, even when not of advanced age or predisposed peutic efficacies have been included in the potential Catalunya, Spain 4 Cardiology Service, Germans by pre-­existing comorbidities or chronic diseases.5 armamentarium against COVID-19.9 10 For instance, Trias i Pujol University Hospital, In these patients, infection by SARS-­CoV-2 activates among the antivirals preliminary tested, remde- Badalona, Catalunya, Spain both the innate and adaptive immune responses, sivir has showing the best efficacy, with significant 5 Servei de Pneumologia, provoking the production of large amounts of pro-­ improvements in shortening the time to recovery Germans Trias i Pujol University Hospital, Badalona, Catalunya, inflammatory cytokines and chemokines, resulting and evidence of lower respiratory tract infection.11 Spain in a clinical presentation (ie, unremitting high It was issued an emergency use authorisation on 1 6 CIBERES, Instituto de Salud fever, lymphadenopathy, hepatosplenomegaly, cyto- May 2020 for the treatment of COVID-19 patients Carlos III, Madrid, Spain penia, hyperferritinaemia, central nervous system hospitalised with severe disease by the Food and Correspondence to Dr Santiago Roura, ICREC, Fundació Institut d’Investigació en Ciències de la Salut Germans Trias i Pujol, Badalona 08916, Catalunya, Spain; ​sroura@​gmail.​com Received 4 July 2020 Accepted 28 August 2020 Published Online First 15 December 2020 © Author(s) (or their employer(s)) 2021. Re-­use permitted under CC BY-­NC. No commercial re-­use. See rights and permissions. Published by BMJ. To cite: Monguió- Figure 1 COVID-19 pathophysiology, potential disease mechanisms and treatment options. ACS, acute coronary Tortajada M, Bayes-­Genis A, syndrome; ARDS, acute respiratory distress syndrome; CRP, C reactive protein; DAMPs, danger-­associated molecular Rosell A, et al. Thorax patterns; IL, interleukin; LDH, lactate dehydrogenase; MSC, mesenchymal stem cell; MSC-­EV, MSC-­derived extracellular 2021;76:196–200. vesicles; PAMPs, pathogen-­associated molecular patterns; TNFα: tumour necrosis factor alpha. 196   Monguió-Tortajada M, et al. Thorax 2021;76:196–200. doi:10.1136/thoraxjnl-2020-215717 State of the art review Drug Administration (FDA), and on 3 July 2020, it became the capabilities once implanted in vivo.26 For example, MSCs induce first treatment against COVID-19 with a conditional marketing a marked shift in the pro-­inflammatory pulmonary microenviron- authorisation for patients over 12 years old with pneumonia who ment induced by paraquat poisoning, ventilator-­ induced injury require supplemental oxygen by the European Medicines Agency. or infectious models of lung injury.27 28 Particularly, the beneficial Moreover, some researchers have promoted the antimalarial effects of MSCs are shown to be associated with the reduction in 4-­aminoquinolines, chloroquine and hydroxychloroquine (HCQ), TNF-α, IL-1 and IL-6 through the release of hepatocyte growth for the blocking of the progression to the dangerous ARDS phase factor (HGF), prostaglandin-­E2 (PGE2), lipoxin A4 (LXA4) and of the disease due to their immunomodulatory effects.9 However, TNF-­stimulated gene 6 protein (TSG-6), suppression of inflamma- both drugs, even the less toxic HCQ, have been removed as ther- tory T-­cell proliferation by indoleamine 2,3-­dioxygenase expres- apeutic options for treating COVID-19 patients as they do not sion, switch from Th1 and Th17 responses to Th2, and inhibition lead to clinical improvement. Indeed, the FDA recently revoked of monocytes and myeloid dendritic cells maturation (figure 2). emergency use ruling for HCQ. Since large amounts of secreted MSCs induce M2 polarisation by contact-­dependent and paracrine interleukin 6 (IL-6) has been correlated to disease severity, admin- factors such as HGF, PGE2 and TSG-6, resulting in a monocyte-­ istration of the humanised anti-­IL-6 receptor antibody inhibiting aided increase in anti-­inflammatory IL-10, which synergistically IL-6 tocilizumab is being tested in patients with severe SARS-­CoV-2 induces regulatory T cells and switches on tissue repair mech- infection complicated with CRS and ARDS. Published data from anisms. Furthermore, MSCs interfere in different pathways that some of these clinical trials have revealed that response to tocili- promote lung fibrosis resulting in protective effects, as demon- zumab is rapid, sustained and associated with significant clinical strated in several lung injury models.27 Indeed, MSC delivery also improvements, including reduction of lethality rate at 30 days.12–14 reduces collagen-­ rich scarring and pulmonary fibrosis Ashcroft On the other hand, CRS also comprises massive IL-1 secretion score by inhibiting the expression of TGF-β, IFN-γ and TNF-α, and other cytokines. There are a number of ongoing studies balancing extracellular matrix remodelling enzymes (matrix metal- using the recombinant human IL-1 receptor antagonist anakinra loproteinase (MMPs) and tissue inhibitor of MMPs), collagen to clinically block the proinflammatory effects of IL-1α/β against deposition and impairing myofibroblasts differentiation through COVID-19.15 Preliminary findings by Huet et al and Cavalli et al HGF and TSG-6 in rodent models of bleomycin-­induced acute are promising due treatment with anakinra reduces both the need lung injury.29–31 Moreover, MSCs have the potential to release for invasive mechanical ventilation and mortality in patients with antimicrobial peptides such as LL37, also stimulating the phago- severe forms of COVID-19, without serious life-­threatening side-­ cytic capacity of macrophages together with the anti-­apoptotic effects.16 17 Based on its ability to inhibit IL-1 production, colchi- HGF secretion.32 MSCs can protect against infection-­associated cine is also an anti-­inflammatory and immunomodulatory agent alveolar damage such as influenza A (H5N1) virus infection, and which is currently being tested in the COLCORONA (Colchicine prevent or reduce acute lung injury in vivo.33 Specifically, MSCs Coronavirus SARS-­ CoV-2) trial (NCT04322682). Moreover, reduce the injury-­related alveolar oedema and endothelial perme- there is no sufficient evidence supporting tumour necrosis factor ability by the release of keratinocyte growth factor (KGF) to (TNF)-α or Janus kinase inhibition (etanercept and baricitinib, enhance the sodium-­dependent alveolar fluid clearance through respectively, used for rheumatoid arthritis), or non-­steroidal anti-­ epithelial sodium channel (ENaC), and have anti-­apoptotic and inflammatory agents, especially ibuprofen, as potential treatment anti-­ oxidative mechanisms to restore cytokine-­ damaged alve- options for COVID-19.18–21 Of note, the corticoid dexametha- olar type II cells and epithelial and endothelial injury through sone, which is used to treat common conditions such as arthritis, the release of angiopoietin-1, lipoxin A4 (LXA4) and TSG-6. In blood, hormone and immune system disorders, allergic reactions, fact, the successful use of allogeneic MSC therapy in influenza certain skin and eye conditions, is the first anti-­ inflammatory (H7N9)-­induced ARDS has been recently documented with an drug to exert very significant clinical and mortality improvement open-­label controlled trial showing significantly lower mortality in hospitalised patients with severe respiratory complications of in the treatment group (17.6%) to the control group (54.5%),34 COVID-19, as reported by the RECOVERY (Randomised Evalua- and proving safe in a phase IIa clinical trial for ARDS.35 In light tion of COVID-19 thERapY) investigators.22 The study concluded of these findings, although there is no clinical evidence published that treatment with dexamethasone at a dose of 6 mg once daily on ARDS-­associated lung fibrosis reduction after MSC therapy per for up to 10 days reduced 28-­day mortality in patients who were se, MSCs modulate apoptosis and oxidative injury, avoid inflam- receiving respiratory support, but induced no benefit or even possi- mation, have antifibrotic properties, reduce fibroblasts activation bility of harm among those who did not require oxygen. and, thus, enhance lung repair. This could have a positive impact Alternatively, treatment with mesenchymal stromal or stem cells in COVID-19 patients if MSCs are used to treat ARDS in the early (MSCs) has been proposed as a promising therapeutic strategy to stages of SARS-­CoV-2 infection. halt the uncontrolled inflammatory cascade and, at the same time, For that, numerous clinical studies have begun to probe whether reduce lung fibrosis and abnormal pulmonary function occurring treatment with human MSCs from distinct origins improves the post-­COVID-194 23 24 (figure 1). In particular, an incidence of lung course and prognosis of COVID-19 patients with no adverse fibrosis in approximately 7% to 10% of survivors, similar to other effects.36 37 In this context, Zhang and colleagues have reported ARDS conditions and to the pathogenesis found in MERS and data from the intravenous infusion of MSCs from human umbilical SARS, could be estimated from compelling data.4 24 MSCs repre- cord Wharton’s jelly (WJ-­MSCs) in a single-­patient study.38 Cell-­ sent a heterogeneous population of non-­ haematopoietic multi- treated patient, who already received antiviral, anti-­inflammatory potent stromal cells with a well-­established, specific cell surface and antipyretic therapy and displayed a C reactive protein of expression pattern, low alloreactivity (ie, low expression of major <60 2 weeks before cell infusion, experienced disappearance of both histocompatibility complex (MHC)-­ I and lack of MHC-­ II and fever and shortness of breath on the second day. Chest CT imaging co-­stimulatory molecules) and the ability to differentiate tissues of also showed that the ground-­glass opacity and pneumonia infiltra- mesodermal lineages.25 Despite the remarkable heterogeneity of tion had extensively diminished on the sixth day after transplan- MSC sources and the lack of specific biomarkers predicting the tation of WJ-­MSCs. Additional analyses revealed that COVID-19 success of prospective MSC-­ based therapeutic products, MSCs lymphopenia improved, with increased counts of T lymphocytes show powerful immunomodulatory, anti-­oxidant and angiogenic (both CD4+ and CD8+), and the level of pro-­inflammatory IL-6 Monguió-Tortajada M, et al. Thorax 2021;76:196–200. doi:10.1136/thoraxjnl-2020-215717 197 State of the art review Figure 2 Potential mechanisms of action of MSCs and derived EVs to reduce lung injury in acute respiratory distress syndrome following SARS-­ CoV-2 infection. (A) SARS-­CoV-2 infection causes cell and tissue injury, release of DAMPs, PAMPs and inflammatory mediators that are sensed by neighbouring cells and alveolar macrophages that amplify an inflammatory response in the alveolus, promoting immune cell infiltration and accumulation of protein-­rich alveolar oedema fluid. Infused MSCs and derived EVs can reduce inflammation and trigger tissue repair by (B) increasing alveolar fluid clearance and have anti-­apoptotic and anti-­oxidative effects to restore injured alveolar, epithelial and endothelial cells; (C) reducing neutrophil activation, which also decreases inflammatory cell infiltration; (D) enhancing phagocytosis and viability of alveolar macrophages; (E) reducing inflammation and inducing M2 polarisation of monocytes that results in IL-10 production to induce (F) regulatory T cells and Th2 switch from Th1 and Th17 responses, together with a modulation of T-­cell proliferation; (G) protecting from lung fibrosis. ANG-1, angiopoietin-1; COX2, cyclooxygenase-2; DAMPs, danger-­associated molecular patterns; ECM, extracellular matrix; ENaC, epithelial sodium channel; HGF, hepatocyte growth factor; IDO, indoleamine 2,3-­dioxygenase; IFNγ, interferon gamma; IL, interleukin; KGF, keratinocyte growth factor; LL37, antimicrobial peptide β-cathelicidin; LXA4, lipoxin A4; MMP, matrix metalloproteinase; MSC, mesenchymal stem cell; MSC-­EV, MSC-­derived extracellular vesicles; PAMPs, pathogen-­associated molecular patterns; PGE2, prostaglandin E2; TGFβ, transforming growth factor beta; TIMP, tissue inhibitor of MMP; TNF-α, tumour necrosis factor alpha; Treg, regulatory T-­cell; TSG-6, TNF‐stimulated gene 6 protein. and TNF-α cytokines, and C reactive protein was significantly as extracellular vesicles (EVs). This foundation arises from growing decreased after cell treatment. Leng et al reported a clinical trial research showing that MSC-­EVs are harmless and trigger, at least, pilot study treating seven COVID-19 patients (two common, four similar effects as their parent cells in acute and chronic lung injury, severe and one critically severe case) with MSCs. Post-­treatment, sepsis and ARDS models.43 44 Thus, MSC-­EVs could be more valu- pulmonary function improved within 2 days, the levels of TNF-α, able than the MSC themselves to halt COVID-19 inflammatory C reactive protein and overactivated lymphocytes decreased cascade. Specifically, a better and safer refining of the paracrine significantly, while levels of IL-10, regulatory dentritic cell popu- function of therapeutic MSCs could enhance therapy outcomes for lations and peripheral lymphocytes increased. They concluded COVID-19 and, in turn, avoid some potential side-­effects from that the intravenous infusion of MSCs improved the functional MSC administration. But what are the scientific fundamentals outcome of patients with COVID-19 pneumonia, especially underlying this alternative therapy? First, the fact that a minimum patients whose condition was critically severe, without any adverse amount of retained cells exert beneficial effects points out that the effects.39 More recently, 13 COVID-19 adult patients under inva- contribution of MSCs is mainly driven by paracrine signalling, sive mechanical ventilation who had received previous antiviral rather than a direct cell-­to-­cell effect in vivo. In line, increasing data and/or anti-­inflammatory treatments were infused with allogeneic suggests that paracrine activity of MSCs is accompanied by their adipose tissue-­derived MSCs.40 MSC treatment was followed by a well-­determined production and secretion of EVs, which promote reduction in C reactive protein, IL-6, ferritin, LDH and D-­dimer), similar functional properties as the parental cells.45 EVs, which especially in those patients with clinical improvement (70%). Also, constitute stable, lipid bilayer-­ encapsulated nanostructures and no adverse events related to cell therapy were observed. Remark- specifically transfer intercellular proteins, lipids and non-­coding ably, ongoing industry trials such as those conducted by Mesoblast nucleic acids through the circulation to tissues and/or cells at distal and Athersys are greatly expected to confirm initial encouraging sites, act as great versatile immune regulators. In terms of advan- results from the evaluation of novel efficient MSC-­based products tages, non-­replicative EVs are theoretically unaltered by micro- for ARDS treatment.41 42 Collectively, the preliminary published environmental factors (eg, the inflammatory milieu) since their reports support that administration of MSCs is safe, alleviates double-­leaflet lipid membranes protect the inner molecular cargo the aggressive inflammatory status and is followed by clinical and from degradation and guarantee entry into surrounding or distant biological improvement in most COVID-19 patients. targeted cells nor teratogenic, with low alloreactivity, besides easy In addition, a number of researchers devoted to the develop- handling and storage. Second, EVs may arrive easily at sites of ment of MSC-­based therapies for years have turned their attention the lungs getting damaged and scarred by pulmonary fibrosis for to derivatives prepared from MSC-­conditioned media, referred to a targeted modulation of inflammation due to their characteristic 198 Monguió-Tortajada M, et al. Thorax 2021;76:196–200. doi:10.1136/thoraxjnl-2020-215717 State of the art review nanosize which, in turn, omits the possibility of pulmonary throm- declare the COVID-19 pandemic as a real global health emer- bosis after intravascular MSC delivery as the majority of admin- gency for the WHO. Although COVID-19 infection may be istered cells are initially trapped in the lungs.46 Several studies asymptomatic or cause only mild symptoms in the majority of comprehensively described the thromboembolism effect of MSC the cases, it provokes interstitial pneumonia, CRS and ARDS infusion and potential thrombolytic regimens for safer applications in nearly 15% of the cases, especially in people who are old and to maximise clinical benefit to the patients.47 48 In this regard, and have co-­ morbidities. In general, treatments with the SARS-­CoV-2 infection has been reported to cause damage to other focus on modulating this exacerbated inflammation may thus parts of the body beyond the respiratory tract, such as the cardio- improve clinical outcomes, and ultimately improve disease vascular system, causing severe forms of myocarditis and endothe- death rate. For that, a battery of anti-­inflammatory drugs has liitis in addition to the respiratory dysfunction.49 The blood vessels been tested to limit exacerbated immune response in severe present high expression of ACE2 receptors and, thus, excessive patients with yet modest results. Alternatively, more compat- blood clotting has been observed in COVID-19 patients as a direct ible, biological therapies, such as those based on MSCs, are consequence of the viral infection on the circulatory system.50 gaining interest to mitigate ARDS. In this context, meanwhile, Particularly, a large amount of D-­dimer, which is a product that the numerous ongoing clinical trials of MSCs for COVID-19 indicates the presence of blood clots, is detected and consequently will determine whether adoptive MSC transfer therapy is an the vast majority of COVID-19 patients require further anticoag- ideal choice for treatment; their secreted EVs are also envi- ulant treatments. Furthermore, the infection by SARS-­CoV-2 can sioned as promising in terms of lack of risk of thrombosis, cause a marked constriction of blood vessels that causes reduction inalterability over time, low alloreactivity and better usage in blood circulation and damage to peripheral tissues such as subcu- and biodistribution. Indeed, EV administration should be taneous inflammatory affections, mainly in the trunk and extrem- appropriately manufactured, and pre-­clinically and clinically ities.51 Indeed, these circulatory alterations could be reinforced by tested following rational clinical trial design for rigorous and MSC-­based therapies due to some relevant issues that have been meaningful safety and efficacy data. raised regarding the safety of MSC infusions and the thrombogenic risk they could provoke. The size of MSCs could be a significant Contributors MM-­T, AB-­G, AR and SR approved the final manuscript as submitted cause of blood vessel obstructions favouring lethal pulmonary and are accountable for all aspects of the work, including conception and design, thromboembolism, as well as MSCs have been shown to exhibit and writing of the manuscript. In specific, MM-­T also conceived and created a marked procoagulant activity linked to the expression of tissue schematic images. factor that, in turn, can potentially initiate unwanted coagulation Funding This work was supported in part by the PERIS (SLT002/16/00234) and SGR processes when in contact with blood.52 Some authors have also programmes (2017-­SGR-483) from the Generalitat de Catalunya, ’La Caixa’ Banking described a dual activation of both the coagulation and the comple- Foundation, grants from the Spanish Ministry of Economy and Competitiveness-­ MINECO (SAF2017-84324-­C2-1-­R and PID-2019-­11137RB-­I00), Instituto de Salud ment pathway referred to as instant blood-­mediated inflammatory Carlos III (PIC18/00014) funds, Red de Terapia Celular-­TerCel (RD16/00111/0006), reaction.53 Consequently, approaches to detect and prevent these CIBER Cardiovascular (CB16/11/00403) CIBER Enfermedades Respiratorias unreported yet harmful MSCs’ effects are under investigation, (CB06/06/1089) projects, as part of the Plan Nacional de I+D+I, and co-­funded by which could in turn give an explanation to the limited benefits and ISCIII-­Subdirección General de Evaluación y el Fondo Europeo de Desarrollo Regional cell engraftment described in many clinical trials. On the contrary, (FEDER), and by the Call for Expression of Interest (EoI) for Collaborative Projects on Regenerative Medicine 2019 of Centre for Regenerative Medicine of Barcelona among their multifaceted role in the modulation of biological (CMR(B)). responses, the involvement of MSC-­EVs in angiogenesis-­related Competing interests None declared. diseases is gaining interest.54 Other important concerns should be taken into account in order to keep treating COVID-19 patients Patient consent for publication Not required. with MSCs or to inspire alternative clinical practice such as those Provenance and peer review Not commissioned; externally peer-­reviewed. based in EVs. As mentioned above, they include the pulmonary Open access This is an open access article distributed in accordance with the retention of intravenously administered MSCs,55 56 from where Creative Commons Attribution Non Commercial (CC BY-­NC 4.0) license, which they secrete anti-­ inflammatory factors or agents with systemic permits others to distribute, remix, adapt, build upon this work non-­commercially, effects (including EVs), and cell viability, which is crucial for the and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use success of cell therapies,35 a drawback that EVs could greatly is non-­commercial. See: http://​creativecommons.​org/​licenses/​by-​nc/​4.​0/. circumvent. Of concern, together with the ongoing clinical trials using MSCs in COVID-19 that need to counteract limitations related REFERENCES to patient coagulopathy and thromboprophylaxis while probing 1 Grasselli G, Zangrillo A, Zanella A, et al. Baseline characteristics and outcomes of cell product quality and delivery route to guarantee treatment 1591 patients infected with SARS-­CoV-2 admitted to ICUs of the Lombardy region, Italy. JAMA 2020;323:1574–81. safety and efficacy, the latest research news highlight a number 2 Marini JJ, Gattinoni L. Management of COVID-19 respiratory distress. JAMA of registered clinical trials evaluating the potential benefit of 2020;323:2329. MSC-­EVs.57 However, leading voices for advancing clinical 3 Ackermann M, Verleden SE, Kuehnel M, et al. Pulmonary vascular Endothelialitis, translation of cell and gene therapies worldwide such as Interna- thrombosis, and angiogenesis in Covid-19. N Engl J Med Overseas Ed tional Society for Cell and Gene Therapy and the International 2020;383:120–8. 4 Sun P, Qie S, Liu Z, et al. Clinical characteristics of hospitalized patients with SARS‐ Society for Extracellular Vesicles have rapidly launched their CoV‐2 infection: a single arm meta‐analysis. J Med Virol 2020;92:612–7. recommendations on providing ‘appropriate manufacturing and 5 Moore JB, June CH. Cytokine release syndrome in severe COVID-19. Science quality control provisions, pre-­clinical safety and efficacy data, 2020;368:473–4. rationale clinical trial design, and proper regulatory oversight’ 6 Gao Y-­M, Xu G, Wang B, et al. Cytokine storm syndrome in coronavirus disease 2019: of EV-­based therapeutics for COVID-19 before administration a narrative review. J Intern Med 2020. doi:10.1111/joim.13144. [Epub ahead of print: 22 Jul 2020]. to patients.58 7 England JT, Abdulla A, Biggs CM, et al. Weathering the COVID-19 storm: lessons from In conclusion, since the first cases reported in Wuhan, hematologic cytokine syndromes. Blood Rev 2020;100707:100707. China, COVID-19 accounts for more than 68 000 000 infec- 8 Stebbing J, Phelan A, Griffin I, et al. COVID-19: combining antiviral and anti-­ tions and over 1 500 000 deaths worldwide. This has led to inflammatory treatments. Lancet Infect Dis 2020;20:400–2. Monguió-Tortajada M, et al. Thorax 2021;76:196–200. doi:10.1136/thoraxjnl-2020-215717 199 State of the art review 9 Soy M, Keser G, Atagündüz P, et al. Cytokine storm in COVID-19: pathogenesis 35 Matthay MA, Calfee CS, Zhuo H, et al. Treatment with allogeneic mesenchymal and overview of anti-­inflammatory agents used in treatment. Clin Rheumatol stromal cells for moderate to severe acute respiratory distress syndrome (start study): 2020;39:2085–94. a randomised phase 2A safety trial. Lancet Respir Med 2019;7:154–62. 10 Choudhary S, Malik YS, Tomar S. Identification of SARS-­CoV-2 Cell Entry Inhibitors by 36 Golchin A, Seyedjafari E, Ardeshirylajimi A. Mesenchymal stem cell therapy for Drug Repurposing Using in silico Structure-­Based Virtual Screening Approach. Front COVID-19: present or future. Stem Cell Rev Rep 2020:1–7. Immunol 2020;11:1664. 37 Khoury M, Cuenca J, Cruz FF, et al. Current status of cell-­based therapies for 11 Beigel JH, Tomashek KM, Dodd LE, et al. Remdesivir for the treatment of Covid-19 — respiratory virus infections: applicability to COVID-19. Eur Respir J 2020;55:2000858. final report. N Engl J Med Overseas Ed 2020. 38 Zhang Y, Ding J, Ren S, et al. Intravenous infusion of human umbilical cord Wharton’s 12 Xu X, Han M, Li T, et al. Effective treatment of severe COVID-19 patients with jelly-­derived mesenchymal stem cells as a potential treatment for patients with tocilizumab. Proc Natl Acad Sci U S A 2020;117:10970–5. COVID-19 pneumonia. Stem Cell Res Ther 2020;11:207. 13 Toniati P, Piva S, Cattalini M, et al. Tocilizumab for the treatment of severe COVID-19 39 Leng Z, Zhu R, Hou W, et al. Transplantation of ACE2- Mesenchymal Stem pneumonia with hyperinflammatory syndrome and acute respiratory failure: a single Cells Improves the Outcome of Patients with COVID-19 Pneumonia. Aging Dis center study of 100 patients in Brescia, Italy. Autoimmun Rev 2020;19:102568. 2020;11:216–28. 14 Perrone F, Piccirillo MC, Ascierto PA, et al. Tocilizumab for patients with COVID-19 40 Sánchez-­Guijo F, García-­Arranz M, López-­Parra M, et al. Adipose-­Derived pneumonia. The TOCIVID-19 prospective phase 2 trial. MedRxiv 2020. mesenchymal stromal cells for the treatment of patients with severe SARS-­ 15 U.S. National library of medicine shares information about clinical trials using CoV-2 pneumonia requiring mechanical ventilation. A proof of concept study. Anakinra against COVID-19 either in recruitment or pre-­recruitment stage. Available: EClinicalMedicine 2020;25:100454. https://​clinicaltrials.​gov/​ct2/​results?​cond=​Covid19&​term=​anakinra&​cntry=&​state=&​ 41 Mesoblast shares information about its clinical trials, including that devoted to city=&​dist= [Accessed 19 Aug 2020]. mesenchymal stem cells for the treatment of moderate to severe COVID-19 acute 16 Huet T, Beaussier H, Voisin O, et al. Anakinra for severe forms of COVID-19: a cohort respiratory distress syndrome (NCT04371393). Available: https://www.​mesoblast.​ study. Lancet Rheumatol 2020;2:e393–400. com/​product-​candidates/​clinical-​trials" https://www.​mesoblast.​com/​product-​ 17 Cavalli G, De Luca G, Campochiaro C, et al. Interleukin-1 blockade with high-­ candidates/​clinical-​trials [Accessed 18 Aug 2020]. dose anakinra in patients with COVID-19, acute respiratory distress syndrome, 42 Athersys Inc. Shares information about the MACOVIA study (MultiStem administration and hyperinflammation: a retrospective cohort study. Lancet Rheumatol for COVID-19 induced ARDS). Available: https://www.​athersys.​com/​clinical-​trials/​ 2020;2:e325–31. ards/" https://www.​athersys.​com/​clinical-​trials/​ards/ [Accessed 18 Aug 2020]. 18 Mehta P, McAuley DF, Brown M, et al. COVID-19: consider cytokine storm syndromes 43 Worthington EN, Hagood JS. Therapeutic use of extracellular vesicles for acute and and immunosuppression. Lancet 2020;395:1033–4. chronic lung disease. Int J Mol Sci 2020;21:2318. 19 Stebbing J, Phelan A, Griffin I, et al. COVID-19: combining antiviral and anti-­ 44 Mahida RY, Matsumoto S, Matthay MA. Extracellular vesicles: a new frontier inflammatory treatments. Lancet Infect Dis 2020;20:400–2. for research in acute respiratory distress syndrome. Am J Respir Cell Mol Biol 20 Wang M, Zhou Y, Zong Z, et al. A precision medicine approach to managing 2019 2020;63:15–24. novel coronavirus pneumonia. Precis Clin Med 2020;3:14–21. 45 Monguió-Tortajada M, Roura S, Gálvez-­Montón C, et al. Nanosized UCMSC-­derived 21 Day M. Covid-19: ibuprofen should not be used for managing symptoms, say doctors extracellular vesicles but not conditioned medium exclusively inhibit the inflammatory and scientists. BMJ 2020;368:m1086. response of stimulated T cells: implications for nanomedicine. Theranostics 22 RECOVERY Collaborative Group, Horby P, Lim WS, et al. Dexamethasone in 2017;7:270–84. Hospitalized Patients with Covid-19 - Preliminary Report. N Engl J Med 2020. 46 Coppin L, Sokal E, Stéphenne X. Thrombogenic risk induced by intravascular doi:10.1056/NEJMoa2021436. [Epub ahead of print: 17 Jul 2020]. mesenchymal stem cell therapy: current status and future perspectives. Cells 23 Mo X, Jian W, Su Z, et al. Abnormal pulmonary function in COVID-19 patients at time 2019;8:1160. of hospital discharge. Eur Respir J 2020;55:2001217. 47 Tatsumi K, Ohashi K, Matsubara Y, et al. Tissue factor triggers procoagulation in 24 Thille AW, Esteban A, Fernández-­Segoviano P, et al. Chronology of histological lesions transplanted mesenchymal stem cells leading to thromboembolism. Biochem Biophys in acute respiratory distress syndrome with diffuse alveolar damage: a prospective Res Commun 2013;431:203–9. cohort study of clinical autopsies. Lancet Respir Med 2013;1:395–401. 48 Wu Z, Zhang S, Zhou L, et al. Thromboembolism induced by umbilical cord 25 Dominici M, Le Blanc K, Mueller I, et al. Minimal criteria for defining multipotent mesenchymal stem cell infusion: a report of two cases and literature review. mesenchymal stromal cells. The International Society for cellular therapy position Transplant Proc 2017;49:1656–8. statement. Cytotherapy 2006;8:315–7. 49 Guzik TJ, Mohiddin SA, Dimarco A, et al. COVID-19 and the cardiovascular system: 26 Carreras-­Planella L, Monguió-Tortajada M, Palma E. Stem cells: Immunotherapy in implications for risk assessment, diagnosis, and treatment options. Cardiovasc Res solid organ transplantation. Encyclopedia of Tissue Engineering and Regenerative 2020;116:1666–87. Medicine, Elsevier, 2019: 81–90. 50 Varga Z, Flammer AJ, Steiger P, et al. Endothelial cell infection and endotheliitis in 27 McIntyre LA, Moher D, Fergusson DA, et al. Efficacy of mesenchymal stromal cell COVID-19. Lancet 2020;395:1417–8. therapy for acute lung injury in preclinical animal models: a systematic review. PLoS 51 Klok FA, Kruip MJHA, van der Meer NJM, et al. Incidence of thrombotic complications One 2016;11:e0147170. in critically ill ICU patients with COVID-19. Thromb Res 2020;191:145–7. 28 Laffey JG, Matthay MA. Fifty years of research in ARDS. cell-­based therapy for acute 52 Oeller M, Laner-­Plamberger S, Hochmann S, et al. Selection of tissue factor-­deficient respiratory distress syndrome. biology and potential therapeutic value. Am J Respir cell transplants as a novel strategy for improving Hemocompatibility of human bone Crit Care Med 2017;196:266–73. marrow stromal cells. Theranostics 2018;8:1421–34. 29 Moodley Y, Atienza D, Manuelpillai U, et al. Human umbilical cord mesenchymal stem 53 Hoogduijn MJ, Roemeling-­van Rhijn M, Engela AU, et al. Mesenchymal stem cells reduce fibrosis of bleomycin-­induced lung injury. Am J Pathol 2009;175:303–13. cells induce an inflammatory response after intravenous infusion. Stem Cells Dev 30 Ortiz LA, Dutreil M, Fattman C, et al. Interleukin 1 receptor antagonist mediates the 2013;22:2825–35. antiinflammatory and antifibrotic effect of mesenchymal stem cells during lung injury. 54 Todorova D, Simoncini S, Lacroix R, et al. Extracellular vesicles in angiogenesis. Proc Natl Acad Sci U S A 2007;104:11002–7. Circ Res 2017;120:1658–73. 31 Srour N, Thébaud B. Mesenchymal stromal cells in animal bleomycin pulmonary 55 Fischer UM, Harting MT, Jimenez F, et al. Pulmonary passage is a major obstacle fibrosis models: a systematic review. Stem Cells Transl Med 2015;4:1500–10. for intravenous stem cell delivery: the pulmonary first-­pass effect. Stem Cells Dev 32 Gupta N, Krasnodembskaya A, Kapetanaki M, et al. Mesenchymal stem cells enhance 2009;18:683–92. survival and bacterial clearance in murine Escherichia coli pneumonia. Thorax 56 Eggenhofer E, Benseler V, Kroemer A, et al. Mesenchymal stem cells are short-­lived 2012;67:533–9. and do not migrate beyond the lungs after intravenous infusion. Front Immunol 33 Chan MCW, Kuok DIT, Leung CYH, et al. Human mesenchymal stromal cells reduce 2012;3:297. influenza A H5N1-­associated acute lung injury in vitro and in vivo. Proc Natl Acad Sci 57 Khoury M, Rocco PRM, Phinney DG, et al. Cell-­Based therapies for coronavirus U S A 2016;113:3621–6. disease 2019: proper clinical investigations are essential. Cytotherapy 2020. 34 Chen J, Hu C, Chen L, et al. Clinical study of mesenchymal stem cell treating acute doi:10.1016/j.jcyt.2020.04.089. [Epub ahead of print: 17 Apr 2020]. respiratory distress syndrome induced by epidemic influenza A (H7N9) infection, a hint 58 Börger V, Weiss DJ, Anderson JD, et al. ISEV and ISCT statement on EVs from for COVID-19 treatment. Engineering 2020. doi:10.1016/j.eng.2020.02.006. [Epub MscS and other cells: considerations for potential therapeutic agents to ahead of print: 28 Feb 2020]. suppress COVID-19. Cytotherapy 2020;22. 200 Monguió-Tortajada M, et al. Thorax 2021;76:196–200. doi:10.1136/thoraxjnl-2020-215717