**6.1 MSC clinical studies**

It has been observed that most of the clinical trials for COVID-19 treatment have used allogeneic stem cell source. The curative effect of MSCs in the treatment of COVID-19 has been shown by the two recent clinical trials. In one of them, human umbilical cord derived MSCs were used in three consecutive intravenous infusions administered to patients with COVID-19; it was reported from this trial that subject demonstrated the neutrophil levels decreased significantly, lymphocytes increased, CD4+ T and CD8+ T cells returned to normal level, and vital signs were improved, after the second intravenous infusion [161]. The other trial recruited

seven patients with COVID-19 (two mild cases, four severe cases, and one critical case) to receive one intravenous MSC transplantation each. According to the published results, The patient's regulatory dendritic cell population increased, the level of the pro-inflammatory factor TNFa decreased, and the level of antiinflammatory factor IL-10 increased, after 2–4 days after MSC transplantation [136]. This was a pilot study Clinical grade MSCs were injected intravenously (1 × 106 cells/ kg body weight) and the patients were followed-up for 14 days. From clinical point of view, a significant reduction in clinical symptoms and pneumonia infiltration was observed in chest CT of critically ill COVID-19 patient within 2–4 days of MSC-therapy. An increase in peripheral lymphocyte levels, decrease in C-reactive protein (CRP), drastic disappearance of activated cytokine-secreting immune cells (CXCR3+ CD4+ T-cells, CXCR3+ CD8 + T-cells and CXCR3 + NK-cells) and restoration of regulatory DC cell population to normal levels was observed after day 6 of MSC transplantation. From cytokines point of view, the level of anti-inflammatory cytokine IL-10 was increased and the levels of serum pro-inflammatory cytokine TNF-α was significantly decreased. These were considered as the indicators of the efficient regulation of cytokine storm in COVID-19 patients on MSC transplantation. On the other hand, the absence of ACE-2 receptor and TMPRSS2 on the transfused MSCs affirmed that they cannot get infected with SARS-Cov-2, suggesting the beneficial effects of the MSC-therapy in COVID-19 infection. The authors suggested that this clinical trial showed that transplantation of MSCs can improve the prognosis of patients with COVID-19 [145]. In a case report of one critically all COVI-19 case who is 65-year-old woman with underlying with type-II diabetes and hypertension, it was reported that after receiving MSC-based treatments her health improved and she left the ICU. The authors of this case report proposed that the possible effects of hUCMSCs might be anti-inflammation and tissue repair to COVID-19 patient. They also suggested that MSCs could down regulate proinflammatory cytokines and chemokines and increase IL-10 and VEGF which could promote the lung repair [161]. The patient didn't respond to any anti-viral drug and the disease progressed to multiple organ injury. During this critical stage when the patient is ventilated, hUC-MSC was infused in 3 consecutive administrations in 50 × 106 cells/dose. After second MSC administration, ventilator was removed as the vital signs had improved with gradual decrease in serum albumin and CRP levels. CT images showed no infiltration patches of pneumonia by the end of MSC infusions. These results suggest that hUC-MSC can be beneficial for patient who showed resistantance to anti-viral drugs. The therapeutic potential of MSCs in viral infections and immunomodulation capabilities to alleviate the cytokine storm, are being tested in clinical studies that have been initiated to further evaluate their efficiency for COVID-19 treatment.

The evidence pf the published results of the clinical trials in which the MSC transplantation is used for curative purposes, shows the beneficial effect of MSCs on the treatment of severe patients. However, more clinical data are still needed to confirm its effectiveness [162].

Several anti-viral drugs such as remdesivir, favipiravir, ribavirin functioning as RNA dependent RNA polymerase inhibitors, lopinavir, ritonavir which are protease inhibitors and drugs suc as hydroxychloroquine targeting endocytic pathway are being evaluated for COVID-19 but standard therapeutics yet not available. To fight against the cytokine storm, immune-therapy targeting TNFα, IL-1, IL-2, and IL-6 and are evaluated. One of the promising immune-modulators is the MSCs administered as add-on therapy can surmount the severity of COVID-19 infections. Recent studies have shown that MSC-therapy significantly dampens the cytokine storm in critically ill COVID-19 patients [163].

The published results of MSC add-on therapy for ARDS, with focused clinical outcome measures' analysis on safety, efficacy, and related immunologic and

#### *Cellular Therapy as Promising Choice of Treatment for COVID-19 DOI: http://dx.doi.org/10.5772/intechopen.96900*

pulmonary responses [164]. The clinical studies have demonstrated that MSC therapy is safe and has the potential to mitigate inflammatory and physiologic damage for a variety of conditions involving the central nervous, [165] cardiac, [166] renal, [167] gastrointestinal, [168] and respiratory [169, 170] systems. The data in the literature suggests similar results for MSC therapy for treating ARDS in COVID-19.

As expected, safety is the most important matter for all new therapies, especially in patients at high risk for death from the condition being treated and was carefully evaluated for MSC-treated patients in the clinical trials published. According to the literature review, out of the 200 ARDS patients were treated with intravenously or intratracheally administered MSCs or placebo, 30patients died in the active treatment group. None of these 30 deaths were found to be related to MSC therapy. Also, no other SAEs attributed to the MSC therapy. Some transient adverse effects reported, but all of them resolved on its own in short term. This safety profile is consistent with the experience of other human clinical trials involving MSC therapy [165, 171].

The clinical trials of cell-based therapy using MSCs and their safety has been reported in several clinical trials related to GVHD and SLE [127, 172–174]. The approach of MSC transplantation has been used to treat H7N9-induced ARDS patients and the outcome showed significant reduction in mortality rates [134]. Similarly, the study of MSC-based treatment for SARS-CoV-2 suggested that MSCs lack SARS-CoV-2 infection-vital receptors (ACE2- and TMPRSS2-); so MSCs are SARS-CoV-2 infectionfree. Also, the these cells' infusion in SARSCoV- 2-infected patients improved the outcomes because of their extraordinary immunosuppressant potential [136].

The potential efficacy of MSC therapy for ARDS in COVID- 19-infected patients is reported from a phase 1 trial. There were 9 patients enrolled. In-hospital mortality was reported as 33.3% (3/9), including two with septic shock and one with ventilator-induced severe pneumomediastinum and subcutaneous emphysema. No serious prespecified cell infusion-associated or treatment-related adverse events was identified in any patient. The circulating inflammatory (CD14CD33/ CD11b+CD16+/CD16+MPO+/CD11b+MPO+/CD14CD33+) and MSC markers (CD26+CD45-/CD29+CD45-/CD34+CD45-/CD44+CD45-/CD73+CD45−/ CD90+CD45-/CD105+CD45-/CD26+CD45-) were reported as progressively reduced and the immune cell markers such as Helper-T-cell/Cytotoxity-T-cell/ Regulatory-T-cell were notably increased after cell infusion. As a result, this phase I clinical trial showed that a single-dose intravenous infusion of hUC-MSCs was safe with favourable outcome in nine ARDS patients [175]. According to the available evidence, SARS-CoV-2 affects not only the lung, but also the heart and kidney with reported cardiomyopathy and kidney injury [171, 176]. It has been reported that the improved resolution of multiple organ failure or increased organ failure-free days with MSC treatment, which further supports their consideration for clinical use.

The safety and efficacy profile of MSCs is well-constituted based on the results from several completed clinical studies conducted on the therapeutic potential of these therapies in lung diseases such as ARDS [134, 177] as well as bronchopulmonary dysplasia cardiovascular diseases), diabetes [178, 179] and also spine injuries [180]. Although it has been still in experimental phase, the stem cell types investigated for possible cure of SARS-CoV-2 infections include human induced pluripotent stem cells. Recently, it has been reported that when iPSCs were exposed to SARS-CoV-2, it was presented a deleterious effect on the cells in vitro where the pluripotency of iPSCs was lost leading to fibroblast-like phenotype [181, 182]. Therefore, evidence-based selection of stem cell type for the treatment of COVID-19 is critical for safety and efficacy.

Wraping up, it seems the MSC-therapy, when applied as add-on treatment, suppresses the over activated immune system through its immuno-modulatory properties and promotes the tissue repair of alveolar cells in lung microenvironment of SARS-CoV-2 infected patients. Clearly, the data of the recent studies are encouraging, however they have major limitations such as the small-sized patient recruitment. Hence, the need for larger randomized control trials to establish the effectiveness and safety of MSC-therapy in SARS-Cov-2 infection is obvious.

The immense knowledge available with reference to the mechanism of action of MSCs and their effective potencies at a specific disease stage makes MSCs as an promising and effective therapeutic candidate.

#### **6.2 Mesenchymal stem cell treatment action of mechanism**

It has been demonstrated that MSCs have broad immunomodulatory, antiinflammatory capacity [183, 184], as well as regenerative properties [185]. MSCs can induce the repair of damaged tissue, and eventually prevent long-term lung damage resulting from COVID- 19. The stabilization of the endothelial fluid leakage and maintenance of the alveolar-capillary barrier function are also characteristics demonstrated by MSCs; obviously, these features are irrevocable to decrease lung permeability and attenuating the development of interstitial lung oedema [186]. These are the main grounds for the MSC based cellular therapy as potentially effective treatment for COVID-19 infection.

Severe cases of COVID-19 infection is characteristic with high levels of cytokines in the plasma, particularly IL-6 which is a biomarker of inflammation and immune response. From this perspective, clinical trials using the medications such as Sarilumab and Tocilizumab, the antibodies anti-IL6 receptors, has been testing such therapeutic strategy in hospitalized COVID-19 infected patients.

Azithromycin is an antibiotic with immunomodulatory effects and invasion inhibitory activity. That is why this drug has been also administrated for the therapy of chronic inflammatory conditions, such as bronchiolitis and rosacea. Although the exact mechanisms of this anti-inflammatory effect are still not fully known, some studies presented a reduction of IL-6 levels after azithromycin treatment [187, 188]. What is more, another study has demonstrated that azithromycin increases rhinovirus-induced interferons and interferon-stimulated mRNA and protein expression as well as decreases rhinovirus replication and release, resulting in induced anti-viral responses in epithelial cells of the human brochiols [189].

After administered systemically, the majority of MSCs reside in the vascular bed of lungs through the interactions with the capillary endothelial cells. When labelled MSCs are traced, it was seen that most are cleared within 24–48 h, and there can be persistence in injured or inflamed lungs for a longer period [190]. It has been suggested that the apoptosis and subsequent efferocytosis and phagocytosis by resident inflammatory and immune cells could be amongst the clearance process [191]. MSCs can secrete various soluble mediators including anti-inflammatory cytokines [192], antimicrobial peptides [193], angiogenic growth factors, as well as extracellular vesicles [194] in their vicinity.

There are evidence for cell–cell transmission of mitochondria from MSCs to respiratory epithelial and immune cells [195]. This reveals the release of antiinflammatory mediators is specific for the inflammatory lung environment and is mediated through differential activation of damage- and pathogen-associated molecular pathogen receptors expressed on MSC surfaces [196, 197]. Amongst these receptors, Toll-like receptors are crucial; since these are activated by viral RNA in COVID-19 and viral unmethylated CpG-DNA (e.g. TLR9). This leads to modulate the pathways of cell signalling resulting in MSC activation [198]. MSCs derived angiopoietin-1 and keratinocyte growth factor (KGF) contribute to the reparation or restoration of alveolar–capillary barriers disrupted as part of ARDS pathogenesis [199]. On the other hand, the specific inhibitory microRNAs in extracellular vesicles are also described as mediating the protective effects of MSCs in pre-clinical models of infectious or non-infectious acute lung injuries [200].
