Emerging Role and Therapeutic Application of Mesenchymal Stem Cell (MSC) and MSC-Derived Exosome in Coronavirus Disease-2019 (COVID-19) Infection

Abstract

Over the past few years, the coronavirus disease-2019 (COVID-19) pandemic has infected billions of people worldwide. Most patients infected with COVID-19 present with fever, cough, headache, fatigue, and other clinical manifestations. For elderly patients or people with low immunity and underlying diseases, it is frequent to develop into severe or critical illness, which may even lead to multiple organ failure and death. Symptomatic treatment remains the most common treatment for patients with severe COVID-19 infection, whereas the effectiveness is limited. A large number of studies have shown that mesenchymal stem cells (MSCs) can inhibit viral growth, enhance tissue repair, and reduce inflammation, infection-induced cytokine storm, and multi-organ failure by secreting a variety of paracrine factors. In this paper, we summarized current relevant research, describe the mechanism of action and therapeutic effect of MSCs in patients with severe COVID-19 infection-related diseases, and discuss the therapeutic potential of MSCs and their exosome derivatives in patients with critical infections.

1. Introduction

Expanding of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) caused a novel epidemic of coronavirus disease worldwide, which is named with coronavirus disease 2019 (COVID-19) on account that it was firstly reported in 2019. SARS-CoV-2 belongs to the order Nidovirales of the β-coronavirus genus and is a class of enveloped single plus-stranded RNA viruses [1]. Humans of all ages are susceptible to SARS-CoV-2, while the virus can be transmitted via air, contact through nose, mouth, eye mucosa of infected patients, or inhalation of droplets of infected patients [2, 3]. The incubation period of SARS-CoV-2 virus is 3–14 days, then patients may present with typical symptoms, including fever, dry cough, dyspnea, fatigue, along with decreased white blood cell count, and obvious lesions in lung [4].

Invasion of SARS-CoV-2 is initiated with combination between viral S protein with the major receptor and angiotensin-converting enzyme 2 (ACE2) [5]. ACE2 is normally enriched in respiratory epithelial cells (e.g., type II alveolar epithelial cells) and capillary cells (e.g., endothelial cells), endowing these sites as major targets of the SARS-CoV-2. After initial recognition, activation of the fibrillin and decreased pH of endosomes trigger fusion of viral envelope to endosomal membrane, followed with entry of SARS-CoV-2 genetic material into cytosol to launch transcription and replication [6]. The newly synthesized RNA is subsequently transported to endoplasmic reticulum and Golgi apparatus, to be assembled with structural proteins before release as vesicles [7].

During the progress of SARS-CoV-2 infection, the abnormal activation and recruitment of immune cells promote release of a large number of cytokines, resulting in pulmonary inflammation, fibrosis, cell apoptosis, and alveolar fluid accumulation, eventually leading to respiratory failure and even multiple organ failure in severe patients [8]. Compared to healthy individuals, serum levels of inflammatory factors, such as interleukin (IL)-2, IL-7, granulocyte colony-stimulating factor (GCSF), monocyte chemoattractant protein-1 (MCP-1), and tumor necrosis factor (TNF)-α in patients with COVID-19 admitted to ICU, were significantly elevated [9]. A stereotyped bronchiolar-alveolar pattern of lung remodeling was observed in lung tissue of patients with COVID-19, presented with symptoms including basal epithelial cell hyperplasia, mucinous differentiation, and immune activation [10]. Since the pathogenesis of severe COVID-19 is complicated and rapidly progressing, it is urgent to explore more effective therapeutic options in addition to active symptomatic treatment.

2. Introduction of mesenchymal stem cells (MSCs)

In 1966, Friedenstein et al. found that fibroblast-like cells obtained from mouse bone marrow would differentiate into bone cells when transplanted subcutaneously, which are named with mesenchymal stem cells (MSCs) [11]. In addition to be derived from bone marrow, MSCs can also be derived from other tissues such as adipose tissue, placenta, and umbilical cord [12]. MSCs are marked with self-renew property, manifested with regeneration, and multidirectional differentiation, as well as immune modulatory and anti-inflammatory function. Previous studies have shown that MSCs can exert anti-inflammatory effects by increasing the number of lymphocytes, enhancing antigen presentation of dendritic cells (DCs), and reducing the levels of pro-inflammatory cytokines (e.g., IL-6, IL-8, TNF-α, etc.). Additionally, MSCs can affect both innate and adaptive immune cells to play an immunomodulatory role [13]. Cytokines secreted by bone marrow MSCs, including IL-10, transforming growth factor (TGF)-β, and tryptophan catabolase indoleethylamine 2, 3-dioxygenase (IDO) can inhibit the overgrowth of T cells and alter the cytokine expression profile of T cells [14]. Proliferation, differentiation, and chemotactic properties of B cells are also reported to be affected by MSCs [15]. MSCs can also affect immune homeostasis, regulate inflammatory processes, repair damaged cells by binding to cytokines, chemokines, and cell surface molecules, and repair vascular barrier [16].

Based on the function of repairment, MSCs have been widely explored for organ regeneration and tissue repair. In the past few years, safety and efficacy of MSC-based treatment have been preliminarily confirmed in neurogenesis and a variety of diseases such as traumatic injuries, neurogenesis and traumatic injury [17], osteosarcoma [18], type 1 diabetes [19], rheumatoid arthritis [20], acute liver failure [21], and acute kidney injury [22]. The therapeutic potential of stem cells and their derivatives in severe cases and sequelae of COVID-19 have been preliminarily demonstrated. Literature and clinical trials of MSC and its derivatives-based treatment in COVID-19 infection and sequelae were comprehensively summarized, as well as the molecular mechanisms involved.

3. Molecular mechanism of MSCs-based treatment of COVID-19

3.1 Antiviral effects of MSCs

During viral infection, innate immune response plays a central defensive role. Interferon (IFN) can be autogenously synthesized, inducing a batch of intracellular interferon-stimulated genes (ISGs) [23]. Products of those ISGs include non-constitutive ISG-expressed proteins such as metallothionein (MT)1X, MT1G, serpin family G member (SERPING)1, spermidine/spermine N1-acetyltransferase (SAT)1, IFNA receptor (IFNAR) [24, 25], and constitutive ISG express proteins such as IFNA inducible protein (IFI)6, IFN stimulated gene (ISG)15, and the C-C motif chemokine ligand (CCL)2 [26]. Those ISG-encoding proteins play a targeted inhibitory role in multiple stages of the viral infection cycle such as preventing SARS-CoV-2 cell membrane invagination process, mRNA transcription, genome amplification, protein translation, virus assembly, and release [25, 27, 28]. However, previous studies have revealed that coronaviruses (such as SARS-CoV-2 and MERS-CoV) can blunt IFN-mediated antiviral responses. For example, the nonstructural protein 2 (NSP2) of SARS-CoV-2 can directly interact with GIGYF2 protein. This interaction enhances the binding of GIGYF2 to 4EHP, which is the mRNA cap-binding protein, thereby repressing the translation of the IFN-β1 mRNA [29]. Meanwhile, cells in the target site of SARS-CoV-2 initial infection, including nasal epithelial cells, induced pluripotent stem cell-derived alveolar type 2 cells (iAT2), and cardiomyocytes (iCM), manifest with weak expression of IFN or OAS-RNase L [30]. In addition, MSCs stimulated with IFN-γ express IDO, which reduces the content of TRP in cells and is involved in the inhibition of virus synthesis [31].

3.2 Anti-inflammatory and immunomodulatory effects of MSCs

Cytokine storm syndrome (CSS) is considered as the major pathogenic mechanisms of respiratory failure and multi-organ damage in patients with COVID-19 [32]. A variety of plasma cytokines and numbers of immune cells are abnormally augmented in patients with severe COVID-19, which are involved in generation of cytokine storm. Along with the persistent infiltration of monocytes and macrophages, intensive inflammation leads to atrophy of the spleen, lymph nodes, and lymphopenia, which is also accompanied by thrombosis and multi-organ dysfunction [33]. Under this condition, MSCs can be recruited to inflammatory sites by chemokines to play a systemic immunomodulatory function through direct contact and paracrine effects. The anti-inflammatory mediators released by MSCs are specific to different pathogens, mediated by corresponding pathogen-related receptors on the surface of MSCs [34]. Under viral infection, unmethylated viral DNA or viral RNA can activate the toll-like receptor-9 (TLR-9) and TLR-3 signaling pathways, respectively, inducing synthesis and secretion of numerous inflammatory factors [35]. In the early phase of infection, the production of these factors, such as type-I IFNs limits virus propagation; whereas uncontrolled increase of them leads to aberrant inflammation in the late phase of the infection, which is associated with poor clinical outcome [36].

Correspondingly, a variety of paracrine factors secreted by MSCs show mutual influence on immune cells and improvement of COVID-19 patients’ condition. For example, indoleamine 2,3-dioxygenase, transforming growth factor-β (TGF-β), human leukocyte antigen (HLA), and prostaglandin E2 (PGE2) have been identified as the main effectors [37]. Growth factors, such as keratinocyte growth factor (KGF) and angiopoietin-1 (Ang1), can promote the recovery of the disrupted alveolar-capillary barrier during COVID-19 [38]. Beside, activated TLR-4 signaling is also reported to improve regeneration in alveolar epithelial cells during fibrotic status [39].

3.3 Repair effect of MSCs on damage cell/tissue

Aside from being effective in controlling viral replication, MSCs also show recover impact on injured organs during COVID-19. Generation of inflammatory factors induced by SARS-CoV-2, such as ILs and TNF as mentioned above, contributes to severe destruction of alveolar epithelial cells and cardiac tissue damage [40]. Patients suffer from long-term cardiopulmonary damage, presenting with continuous breathlessness, coughing, fatigue, and limited exercise ability [41]. The unique homing properties of MCSs enable them to migrate those damaged tissues through blood flow [42]. On the one hand, MCSs can repair both the histology and function of those damaged tissues. In a bleomycin-induced lung injury and fibrosis mice model, MSCs home to injury area and initiate epithelioid trans-differentiation, which significantly alleviated inflammation and collagen deposition in lung tissue [42]. Through scratch experiment and co-culture experiment, MSCs are firmed to ameliorate wound healing and protect primary small airway epithelial cells, by promoting the migration and proliferation of epithelial cells [43]. In addition, MSCs are found to transfer endothelial mitochondria to damaged alveolar epithelial cells, increase alveolar adenosine triphosphate (ATP) concentration, and reduce endotoxin-induced alveolar damage, thereby facilitate lung fluid clearance [44].

On the other hand, differentiation of MSCs is found to stimulate regeneration of those damaged areas and reconstruct the microenvironment in the case of pulmonary infection [45]. This is attributed to their tolerance to cytotoxic agents and the inhibition of signaling cascades in response to lung injury, as well as the release of growth factors, anti-inflammatory cytokines, extracellular vesicles, etc. [46]. Type-2 alveolar epithelial (AT-II) cells are generated by human induced pluripotent stem cells (hiPSCs), which are applied for SARS-CoV-2 infection and drug testing [47]. iPSCs-induced endothelial cells and pneumocytes can engraft in lungs of emphysematous mice and form functional lung units to ameliorate emphysema [48].

4. Clinical progress of treatment with MSCs in COVID-19 and severe complications

During the past years, MSCs and their released products have been proved to be effective in COVID-19 therapy, based on their immunomodulatory and repair capacities [49, 50, 51]. In a systematic review about stem cell-based therapy in ARDS of COVID-19 patients, CRP levels were found to be decreased in 14 of the 17 trials (82.3%). In addition, IL-6 levels were diminished in nine (52.9%) of seventeen studies [52]. Indicators involved in severe cases, including oxygen saturation and the PaO2/FIO2 ratio, are increased in 12 out of 14 (85.7%), while the lung picture on chest CT or radiography improved in 15 out of 17 studies (88.2%) [52]. MSCs based therapy showed obvious effect in reducing cytokine storm, recovering injured alveolar epithelial cells, and facilitating tissue repair by secreting anti-inflammatory cytokines and antifibrotic growth factors [53], suppressing excessive immune responses to protect the alveolar epithelial lining during acute respiratory distress syndrome [54, 55].

4.1 Cytokine release/storm syndrome (CRS/CSS)

CRS, known as CSS, is an abnormal systemic inflammatory response triggered by a variety of factors (infection, drugs, and other factors) [56]. In many patients infected with SARS-CoV-2, types of inflammatory cytokines are sharply stimulated and released into both the pulmonary and circulatory system, including IL-2, IL-7, IL-10, G-SCF, MCP-1, MIP-1α, and TNF-α [5, 9, 57, 58]. Clinical studies have found that CRS is much more common in critical patients with COVID-19, which is corresponded to poorer outcome or deterioration, such as ARDS and multiple organ failure [59, 60]. Patients with CRS presented with massive alveolar damage, interstitial inflammation, intra-alveolar edema, fibrin and collagen deposition, bronchiolitis, and leukocyte infiltration, contributing to progressive respiratory failure [61]. Glucocorticoids and immunosuppressive agents are recommended in the current treatment of CRS as they might increase the risk of side effects, such as osteonecrosis [56].

The immune modulatory and repair function of MSCs makes them as an ideal option toward rapidly developing CRS [59]. In a single-arm pilot study that was conducted in critical patients with COVID-19, transplantation of UC-MSCs increased oxyhemoglobin saturation, and improved cytokine storm without adverse reactions, demonstrating the safety and feasibility of UC-MSCs in the treatment of COVID-19-associated CRS [62]. The study performed by Zhinian Guo et al. also showed that injection of MSCs could restore oxygenation and down-regulate cytokine storm in COVID-19 patients without risk of side reactions. The average pressure ratio of arterial partial pressure of oxygen to the fraction of inspired oxygen (PaO2/FiO2) and lymphocyte count increased, and serum C-reactive protein, procalcitonin, D-dimer and IL-6 decreased after UC-MSCs infusion in 31 COVID-19 positive patients [63]. In another study, 210 severe/critical patients with COVID-19 were transplanted with 1–2 × 10⁶/kg UC-MSCs, showing recovery of oxygenated SaO2 and remission of CRS [64]. The results of these clinical studies showed that MSCs treatment could effectively reduce the levels of IL-6 and other cytokines, which proved the preliminary potential of the treatment of COVID-19-related CRS.

4.2 Acute respiratory distress syndrome (ARDS)

ARDS is an acute respiratory disease with sustained serious lung injury, characterized with increased bilateral lung texture, blurred edges, and severe hypoxemia [65]. There are about 15–30% of hospitalized patients with COVID-19 will develop into COVID-19-related acute respiratory distress syndrome (CARDS), which is closely related to occurrence of CRS [66]. Development of ARDS is rapid, while ARDS patients have of lower survival rate and quality of life compared to COVID-19 patients with mild symptoms. Due to the persistent inflammation of the lung, the permeability of alveolar endothelial cells and epithelial cells increases, resulting in accumulation of pulmonary edema fluid [67, 68]. At present, the major therapeutic option is mechanical ventilation to improve oxygenation, which shows limited influence on patient mortality in related studies [68]. Current approved therapies, such as intravenous remdesivir and dexamethasone, have a modest effect on moderate to severe COVID-19 [69].

As evidenced by clinical results, MSCs-based therapy is effective in regulating the inflammatory process, repairing epithelial and endothelial cell damage, enhancing alveolar fluid clearance, and delaying the process of ARDS [70, 71]. Injection of MSCs derived from umbilical cord (UC-MSCs), placental (PL-MSCs), and bone marrow (BM-MSCs) have been proved to be safe and reliable in patients with COVID-19 induced ARDS, without showing any severe adverse reactions in relevant clinical trials. In the experiment conducted by Antoine Monsel et al., infusion with BM-MSCs significantly increased the survival rate of ARDS patients at both 28 days and 60 days (Day28, 100%:79.2%, p = 0.025; Day60, 100%:70.8%, p = 0.0082) [72, 73]. Compared with the patients in control group, the level of D-dimer and pulmonary microcirculation thrombosis was also reduced in patients treated with BM-MSCs, indicating alleviated ventilation disorder and restored coagulation [74]. Similar therapeutic efficacy was observed in patients under UC-MSCs treatment, performed as remodeled anti-inflammatory immunity and improved patient outcome. After being injected with UC-MSCs, levels of proinflammatory cytokines, such as IL-6, IFN-γ, TNF-α, and IL-17, were diminished, while levels of anti-inflammatory cytokines such as TGF-β, IL-1β, and IL-10 were increased [75, 76]. In the trial led by Seyed-Mohammad Reza Hashemian et al., lung opacity was significantly reduced after PL-MSCs treatment as determined by computed tomography (CT), and the recovery degree was better than that of the conventional treatment group [77].

4.3 Idiopathic pulmonary fibrosis (IPF)

IPF is a chronic, progressive pulmonary fibrosis disease, attributed to abnormal ECM formation in the alveolar epithelium and disrupted lung function. Accompanied with pandemic of COVID-19, the incidence and prevalence of IPF are increasing globally. IPF is characterized by interstitial fibrosis accompanied by reduced lung volume and hypoxemic respiratory failure [78]. IPF patients manifest with dry cough, fatigue, and dyspnea [79]. In patients with COVID-19-related ARDS, fibrosis can become one of the major long-term complications, with the incidence of persistent lung injury exceeding 30% within one year, and one-third of patients have fibrotic lung injury [80], even in asymptomatic patients with COVID-19 [81, 82]. The mechanism of pulmonary fibrosis caused by COVID-19 is that chronic inflammation leads to epithelial cell damage and fibroblast activation and proliferation, excessive deposition of collagen, and other extracellular matrix (ECM) components, resulting in destruction of normal lung structure and preventing the reconstruction of damaged alveolar epithelium [83]. The progression of pulmonary fibrosis compresses normal lung parenchyma and damages capillaries, leading to respiratory failure [84]. Current treatment of COVID-19-associated IPF mainly contains Pirfenidone and Nintedanib. Pirfenidone can inhibit production of fibrosis-related proteins or ECM to reduce the aggregation of inflammatory cells, while Nintedanib targets on a variety of tyrosine kinases to block fibroblast proliferation. However, patients’ tolerance of them is not satisfactory [85].

The testing of MSCs on IPF patients is positive. Animal experiments have shown that intravenously transplanted MSCs can accumulate in lung tissue, protecting alveolar epithelial cells and restoring the microenvironment in lung [77]. In bleomycin-induced pulmonary fibrotic mice, MSCs reduced the accumulation of collagen and matrix metalloproteinase through augmenting IL-1RA production [86]. IL-1RA is a cytokine that competitively binds with IL-1b, while IL-1b serves as the major inflammatory cytokines in pulmonary edema fluid of patients with ARDS [87]. Several clinical studies have proved that fibrosis-related proteins in the lung tissue are markedly lower in the MSCs treatment group than that of the control group (all P < 0.05) [88]. Forced vital capacity (FVC), carbon monoxide diffusing capacity (DLCO), and other indicators of pulmonary function are reduced in IPF patients [89]. In the clinical trial performed by Daniel C. Chambers et al., treatment with placenta-derived MSC prevents fibrotic progress in patients with COVID-19-associated IPF. Compared to baseline, there is no deterioration of FVC, DLCO, and radiological scoring at 6 months posttreatment [90, 91]. However, due to the limited number of clinical trials, the safety of MSCs in the treatment of IPF needs to be further verified.

5. MSCs-derived exosomes for COVID treatment

A large number of studies have shown that the therapeutic effect of MSCs in repair is mainly attributed to paracrine signals, including secreting extracellular vesicles (EVs), especially exosomes [92]. Exosomes are lipid bilayer vesicles with a typical diameter of 30 to 200 nm, packaged with biomolecules such as cytoskeletal proteins and signal transduction proteins, as well as a variety of nucleic acid components such as messenger RNA (mRNA), ribosomal RNA (rRNA), and microRNA (miRNA) [93]. As evidenced previously exosomes have the advantages in safety and expense over MSCs [94]. Beside, based on its carriage property and nano-size, exosomes can be used as an ideal loading vehicle for deliver functional components in COVID-19 patients.

A number of preclinical studies have shown that exosomes have good therapeutic effects in animal models of inflammatory diseases such as acute lung injury (ALI), ARDS [95], and IPF [93]. MSC-derived exosomes can improve indices of ALI and reduce total proteins in extravascular lung water and bronchoalveolar lavage in a KGF-mediated mechanism [96]. In a clinical study, 24 patients with severe COVID-19 were treated with exosomes derived from allogeneic BM-MSCs. At 72 hours after injection, their clinical condition was remised with restored oxygenation and downregulated CRS [97]. In the clinical trial performed by Meiping Chu et al., patients with COVID-19-associated pneumonia who received aerosolized MSCs-derived exosomes showed reduced CRP levels, improved lung lesion absorption [98]. In another clinical trial, seven patients with severe COVID-19-related pneumonia received nebulized human adipose (haMSC)-derived exosomes. All patients showed well tolerance to haMSC-Exos and with different degrees of resolution of pulmonary lesions [99]. Therefore, exosomes derived from MSCs may become a superior therapeutic tool for COVID-19 than MSCs [100]. Although exosomes have shown a good prospect in clinical experiments, the standard of isolating and verifying exosomes, especially obtained from different origins, must be established for quality ensurance.

6. The bottleneck of MSCs application in the treatment of severe COVID-19 patients

Despite the advantages described above for MSCs and exosomal products in COVID-19, many questions remain to be addressed before further application. First of all, the characterization of MSCs is a common problem of MSCs for clinical use. Lack of unified molecular markers makes it difficult to ensure the consistency and stability of MSCs [101]. Secondly, whether MSCs can be infected or hijacked by virus is also controversial. Although sequencing result demonstrated extremed low expression of ACE2 on MSCs, suggesting that MSCs are not suitable targets of SARS-CoV-2 virus [38]. However, there are also studies reported that in the inflammatory microenvironment of COVID-19, expression of ACE2 in MSCs can be abnormally stimulated by IFN expression, leading to an increased risk of viral infection and a detrimental effect on MSCs [102]. In addition, the survival rate of MSCs in damaged tissue areas and the low transplantation potential limit the effectiveness of MSCS in tissue repair [103]. Safety (potential tumorigenicity), cost in isolation, and storage restrict its application in patients infected with SARS-CoV-2 [104, 105].

7. Summary and prospect

Based on preclinical investigation and ongoing clinical trials (as summarized in Table 1), MSCs have shown great immune-modulatory ability in reforming unbalanced immune system, recovering lung tissue from immune damage or CRS, and improving the physical condition of patients with severe COVID-19. However, there are still limitations of MSCs or MSCs-derived exosome-based treatment in terms of lack of homogeneity markers and transplantation efficacy. Whether MSCs can be infected and damaged by viruses is still suspending. MSCs treatment could be considered as a potential candidate for patients with COVID-19, especially for cases suffering from severe symptoms, such as CARDS.

Trial ID	Source of MSCs	Phase	Numbers of patients	Doses and administration routes	Primary outcomes	Reference
NCT04313322	WJ-MSCs	1	5	Three doses of 1 X 10^6 cells/kg; IV	Improved clinical symptoms including fever, cough, and respiratory distress.	NA
NCT04252118	UC-MSCs	1	20	Three doses of 3 X 10^7 cells; IV	Safe and well tolerated.	[106]
NCT04355728	UC-MSCs	2	24	Two doses of 1 10^7 cells; IV	Safe and well tolerated. Improved oxygenation index and PEEP.	NA
NCT04400032	UC-MSCs	1&2	15	Panel 1: Three doses of 25 X 10^6 cells; IV; Panel 2: Three doses of 50 X 10^6 cells; IV; Panel 3: Three doses of 90 X 10^6cells; IV	NA	NA
NCT04898088	MSCs	NA	30	Three doses; IV	NA	NA
NCT04625738	WJ-MSCs	2	30	day 0: 1 X 10^6 cells/kg; IV; day 3: 0.5 X 10^6 cells/kg; IV; day 5: 0.5 X 10^6 cells/kg; IV	NA	NA
NCT04399889	hCT-MSCs	1&2	12	Phase 1: Three doses of 1 X 10^6 cells/kg; IV; Phase 2: One dose of 1 X 10^8 cells/kg; IV	Safe and well tolerated.	NA
NCT04333368	UC-MSCs	1&2	47	Three doses of 1 X 10^6 cells/kg; IV	NA	[107]
NCT04457609	UC-MSCs	1	40	One dose of 1 X 10^6 cells/kg; IV	NA	NA
NCT04269525	UC-MSCs	2	16	Four doses of 1 X 10^8 cells; IV	No adverse effects.; Improved oxygenation index and lymphocyte count.	[108]
NCT04753476	Secretome-MSCs	2	48	1 cc every 12 hours for three days(Dosage not mentioned)	NA	NA
NCT04352803	AD-MSCs	1	20	5 X 10^6 cells/kg; IV	NA	NA
ChiCTR2000029606	MB-MSCs	1	44	Three doses of 3 x10^7 cells; IV	Safe and well tolerated.; Improved SpO2 and chest imaging.	[63]
NCT04416139	UC-MSCs	1	5	One dose of 1 x 10° cells /kg; IV	Improved PaO2/FiO2.	[63]
NCT04392778	UC-MSCs	1&2	30	Three doses of 3 X 10^6 cells/kg; IV	NA	NA
NCT04339660	UC-MSCs	2	58	One dose of 1 × 10^6 cells /kg; IV	Safe and well tolerated.; Improved clinical symptoms, values of inflammatory parameters, and CT scan.	[63]
ChiCTR2000029990	MSCs	1	10	1 X 10^6 cells/kg; IV	Improved pulmonary function.	[106]
ChiCTR2000031494	UC-MSCs	1	41	2 X 10^6 cells/kg; IV	Clinical improvement.	[109]
NCT04288102	UC-MSCs	2	100	Three doses of 4 x10^7 cells; IV	Safe and well tolerated; Increased distance in 6MWD.	[110]
NA	UC-MSCs	1	9	A single dose of 1 × 10^6 cells or 5 × 10^6 cells or 1 × 10^7 cells;IV	Well tolerated; without serious adverse events.	[111]
NCT03042143	UC-MSCs	1	9	A single dose of 1 or 2 or 4 × 108; IV	Well tolerated. Adverse events included; apyrexia, non-sustained ventricular; tachycardia, and deranged liver function.	[112]
NA	BM-MSCs	Case report	2	A single dose of 2 × 106 cells/ kg; IV	Improved oxygenation and pulmonary compliance; reduced inflammation markers.	[113]
NA	UC-MSCs	1	31	1 × 10^6 cells/ kg; IV	No adverse events.; Improved laboratory parameters.	[63]
NCT04276987	MSCs-derived exosomes	1	24	5 doses of 2 × 10^8 nanovesicles	Safe and well tolerated.	[114]
NCT04491240	MSCs-derived exosomes	1&2		20 doses of 0.5–2 × 10^10 exosomes	No adverse effects.	NA

Table 1.

Summarization of clinical trials on COVID-19 patients treated with MSCs.

Abbreviations: MSCs: Mesenchymal stem cells, WJ-MSCs: Wharton’s Jelly mesenchymal stem cells, UC-MSCs: Umbilical cord blood-derived mesenchymal stem cells, AD-MSCs: Adipose-derived mesenchymal stem cells, MB-MSCs: Menstrual blood-derived mesenchymal stem cells; hCT-MSCs: Human cord tissue mesenchymal stromal cells, BM-MSCs: Bone marrow-derived mesenchymal stem cells, MSCs-derived exosomes: Mesenchymal stem cells-derived exosomes. NA: Not available.

Fundings

This study was supported by the National Natural Science Foundation of China (No. 82203539) to SY; Medico-Engineering Cooperation Funds from the University of Electronic Science and Technology of China (No. ZYGX2021YGCX018) to SY; the Fundamental Research Funds from the Central Universities (No. ZYGX2020KYQD002) to SY; Chengdu Medical Research Project (No. 2022427) to YC.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.