**6. The mesenchymal stem cell treatment in COVID-19: what to prove**

The therapy using MSCs usually covers the processes of isolation, culture, subculture, proliferation, and differentiation of exogenously obtained stem cells, which are then transplanted into patients for immune regulation and microenvironment repair. The therapy success determinants are the safety and efficacy for any treatment. Hence the safety and effectiveness of MSCs are important as shown in a number of clinical trials besides fundamental studies.

MSCs have been widely used in the treatment of inflammatory diseases, such as in graft vs. host disease [128] and lupus erythematosus [129]. Some studies have shown that MSCs have definite efficacy in improvement in cardiovascular, kidney, liver, and other diseases [130, 131].

MSCs are able to regulate the immune response by controlling the function and proliferation of various immune cells. They alsocan inhibit monocyte differentiation into dendritic cells (DCs) which results in upregulation of regulatory cytokines and downregulation of inflammatory cytokines [132]. It was suggested that systemic administration of MSC resulted in reduction of H5N1 influenza virus-induced mortality in older patients with severe pulmonary illness [133]. Also, in patients with H7N9 induced ARDS, a significant improvement in survival rate was observed [134]. So far, MSC transplantation in human subjects with diverse disease conditions has not showed any severe adverse events [135]. Therefore, it is plausible that MSC-therapy can be used to treat COVID-19 patients.

MSCs are evaluated as one of the most promising candidates for SARS-CoV-2 infection treatment. Since the key target for the treatment of SARS-CoV-2 infection resides in the cytokine storm management in lungs, MSCs are well-suited considering their main mechanism of action is through their immunomodulatory and anti-inflammatory properties [129].

MSCs have immunomodulatory effects and they:

1.prevent uncontrolled cytokine or inflammatory factors production,

2.inhibit excessive immune responses, and.

3. reduce immune damage to tissues and organs.

Having the immunemodulatory properties, MSCs not only take part in suppressing immune injury, but also replace and repair damaged tissue and inhibit lung fibrosis. Treating COVID-19 with MSCs has presented considerably good results [136]. Stem cell therapy can suppress the storm of cytokine release, promote endogenous repair by improving the microenvironment, slow the progression of acute lung inflammation down and relieve the symptoms of respiratory distress [137]. The reports suggested that the potentially COVID-19 can be successfully treated with MSCs therapy by the MSC regulation mechanism of the immune system. Studies revealed that when the MSCs are exposed to an inflammatory microenvironment, they can regulate immune cells and inflammatory factors, such as cytokines, leading the alterations in the specific or nonspecific immune responses in vivo. The said modulation is shown to be related to exosomes or the cytokines secreted by MSCs, such as transforming growth factor (TGF)-b, prostaglandin (PG)E-2, and interleukin (IL)-10 [138, 139].

The regulation of the T and B lymphocytes' functions is of special interest as it has been done in several ways. One of these is the T cell proliferation, which is controlled by inflammatory stimulation. A study on cell cycle analysis revealed that T cell subsets can be blocked at the G0/G1 phase. Another way of modulation is that the MSCs can control T cell function via cytokines, by releasing TGF-β, inhibiting the immune activity of Th17 cells, inducing their altering to form T regulatory cell Treg cells, or secreting hepatocyte growth factor to regulate the Th17/Treg cell balance [140]. The modulation of the B cells' proliferation, differentiation, and antibody secretion by the MSCs is also important, since MSCs can affect the G0/G1 phase transition of B cells and regulate the antibody secretion ability of B cells through various transcription pathways [141]. MSCs help to regulatory B cells to multiply; these B cells express IL-10. MSCs activate T cells to release interferons, as well. Suppression of activated B cells regulates the immune function of B cells, and MSCs can also affect innate immune cells, including macrophages and dendritic cells, to realize immune regulation. Under inflammatory conditions, MSCs regulate macrophage function, as well [142]. Once the proinflammatory macrophages (M1) secrete the inflammatory agents, activated MSCs can up-regulate the cyclooxygenase (COX)-2 signal and increase PGE2 secretion. This thereby promotes the transformation of macrophages from activated proinflammatory type to selectively activated anti-inflammatory type (M2).

The MSCs releasing the anti-inflammatory factor TSG-6 and the CD44 macrophages act collectively to destroy the interaction between CD44 and toll-like receptor-2, inhibit the nuclear factor-jB signal downstream, and reduce the inflammatory response [143]. On the other hand, the MSCs can secrete HGF under endotoxin stimulation to induce differentiation into regulatory dendritic cells and alleviate acute lung injury [144].

As explored by research, COVID-19 patients' blood have large numbers of inflammatory factors including interferon-c, interferon-inducible protein-10, and monocyte chemoattractantprotein-1. Additionally, when the patients staying in ICU is compared with the patients in the inpatient clinics, the concentration of granulocyte colony-stimulating factor (G-CSF), MCP-1, tumor necrosis factor (TNF)-a, and other inflammatory factors were shown to be dramaticaly higher in the ICU patients, hence there is a positive correlation between the severity of the cytokine storm and the clinical manifestations of COVID-19 [145]. As discussed previously (see section 2.2.2) COVID-19 have a variety of clinical manifestations changing from a mild disease to a severe disease. This change in severity results both from complications of the viral infection and the cytokine storm. The cytokine storm damaging effects are well-known. Cytokine storm in patients with severe COVID-19 can lead to the release of nitricoxide, which affects the normal systolic and diastolic function of blood vessels, thereby causing hypotension and multi-organ hypoxia [146]. Severe patients have IL-6 levels ten-times higher than those in non-severe patients. In addition, the IL-6 levesl are closely related to the serum SARS-CoV-2 virus load and vital signs of patients. Some study reports have now shown that tozumab (anti-IL-6 receptor) use can prevent worsening of the disease [147]. The MSCs of umblical cord origin, can also inhibit monocyte activation and IL-6 production to inhibit the development of cytokine storm, these result in the improved patient's prognosis. It has been reported that the microenvironment having high IL-6 levels, lead the MSCs to produce cytokines and exosomes enriched with mirR-455-3p, thus calming cytokine storm down and treating acute inflammatory injury. However, the effect of MSCs on cytokine storm in patients with COVID-19 still needs further confirmation [148].

MSCs may suppress ARDS exacerbation and pulmonary fibrosis. Studies have revealed that, once infused or transplanted intravenously, MSCs can reside in the lungs and help improving the microenvironment of the lungs, protecting alveolar epithelial cells, promoting neovascularization, and preventing pulmonary fibrosis

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

[149, 150]. So it seems that one of the most important outcome of the MSC treatment is its reparative action. The reperative function of the MSCs is managed over a variety of the cytokines, particularly keratinocyte growth factor (KGF) [151]. KGF functions through promoting alveolar fluid clearance and alleviating the acute lung injury induced by endotoxin by up-regulating ACE-2 [152]. Another up-regulation managed by KGF is that the activity of sodium potassium ATP enzyme in alveolar cells, resulting in the improvement in alveolar fluid transport, and this play a therapeutic role in ARDS and lung injury [153].

MSCs may have bacteriostatic role. There was a controversy in whether the virus could cause MSCs to lose their function when the MSCs are invaded by bacteria. Although conducted in limited number of patient size, the clinical trial reported from Beijing, showed that the COVID-19 virus could not infect umbilical cord MSCs that were infused intravenously [136]. MSCs can exert their anti-COVID-19 virus effect through direct and indirect mechanisms, according to the recent research. Direct function of the MSCs can be lined up as the direct anti-viral effect by secreting antibacterial peptides and proteins, indoleamine 2,3-dioxygenase, IL-17, and other molecules. MSCs can activate a large number of anti-virus genes independent of interferon, such as the IFITM gene, which can encode protein structures that prevent viruses from invading cells [154]. When it comes to the indirect function of the MSCs combating against COVID-19, they also exhibit an indirect antiviral effect through regulating the coordination of pro-inflammatory and anti-inflammatory actors of the patient's immune system and inducing the macrophages' functions [155–157].

The in vitro sepsis model, ARDS model, and alveolar epithelial fibrosis model use in the research activities demonstrated the immunoregulation and antibacterial and antiviral values of MSCs [156, 157]. Studies show that MSCs secrete at least four AMPs including, antibacterial peptide LL-37, human defensin 2, hepcidin, and lipocalin-2. The function of these AMPs includes killing cells, inhibiting the synthesis of essential proteins, DNA, and RNA of infected cells, interacting with certain targets in infected cells, and playing an active regulatory role in the infection and inflammatory progress of COVID-19 patients [158].

The therapeutic properties of the MSCs against SARS-CoV-2 infection include:

