key: cord-0761218-vylz544n
authors: Dauletova, Meruyert; Hafsan, Hafsan; Mahhengam, Negah; Zekiy, Angelina Olegovna; Ahmadi, Majid; Siahmansouri, Homayoon
title: Mesenchymal stem cell alongside exosomes as a novel cell-based therapy for COVID-19: A review study
date: 2021-03-06
journal: Clin Immunol
DOI: 10.1016/j.clim.2021.108712
sha: 472d6e69b393b2187951aef96aedde25b17e8c6d
doc_id: 761218
cord_uid: vylz544n

In the past year, an emerging disease called Coronavirus disease 2019 (COVID-19), caused by Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been discovered in Wuhan, China, which has become a worrying pandemic and has challenged the world health system and economy. SARS-CoV-2 enters the host cell through a specific receptor (Angiotensin-converting enzyme 2) expressed on epithelial cells of various tissues. The virus, by inducing cell apoptosis and production of pro-inflammatory cytokines, generates as cytokine storm, which is the major cause of mortality in the patients. This type of response, along with responses by other immune cell, such as alveolar macrophages and neutrophils causes extensive damage to infected tissue. Newly, a novel cell-based therapy by Mesenchymal stem cell (MSC) as well as by their exosomes has been developed for treatment of COVID-19 that yielded promising outcomes. In this review study, we discuss the characteristics and benefits of MSCs therapy as well as MSC-secreted exosome therapy in treatment of COVID-19 patients.

Unfortunately, in 20% of cases with COVID-19, disease enter to stage 3, in which the respiratory system is completely damaged by the virus and the disease worsens. Investigations show that the mortality rate is about 2%, which is significantly related to the patient's conditions and risk factors like age (14) . The virus migrates to the gas exchange parts of the lung and affects alveolar type II cells. Both influenza and SARS-CoV are more likely to infect alveolar type II cells than Elderly infected patients are at exceptional risk due to a compromised immune response and diminished ability to repair damaged epithelium. Another noteworthy point is the reduced mucociliary clearance ability in the elderly patients that allows the virus spread and settle in the oxygen exchange parts of the lungs more readily (24).

The SARS-CoV-2 can induce vascular damage. Afterwards, the circulating platelets are exposed to collagen and endothelium, and become the active platelet form. Activated platelets release the essential factors, including adenosine diphosphate, serotonin, thromboxane A2, and prothrombin to further activate platelets. On the other hand, 12 coagulation factors are needed to start the clotting process in the arteries. Briefly, the factor XII activated and lead to prothrombin conversion to thrombin. finally, fibrinogen convert to fibrin which constitute a network of fibrins macrophages, monocytes, and T lymphocytes to the local of infection, increasing subsequent inflammation with high levels of interferon (IFN)-γ released by T cells and beginning a new proinflammatory response. In an incomplete immune reaction, this may assist in the reposition of immune system cells in the lungs, resulting in a mass production of pro-inflammatory cytokines that finally damages the lung tissue. In addition, the resulting cytokine storm spreads to other tissues and can damage several organs (31). Despite the limited immune response, preliminary studies show patients who recovered from COVID-19 developed specific memory T cells against virus, which were traceable within 24 months after recovery (38).

About 7 days after the onset of first symptoms in patients with COVID-19, the response of B cells with the assistance from follicular helper T cells can be assessed. In patients with COVID-19 infection, the B cell response is usually first against the N protein, then antibody responses to the S protein are found within 4-8 days after the onset of symptoms (39, 40). Protective antibodies are produced in the second week of illness, often in response to protein S, but most people have neutralizing antibodies in the third week. Additionally, the principal target of the neutralizing antibodies produced in COVID-19 is a part of the S1 protein subunit. This region is called "receptor-binding-domain (RBD)" and is made up of 193 amino acids (amino acids 318 -J o u r n a l P r e -p r o o f 510). RBD is able to bind to the ACE2 expressed on the host cell and initiate the infection process (41-43).

The binding of the virus-antibody complexes to the Fc receptors on immune cells like alveolar macrophages could impel the production of pro-inflammatory proteins, including MCP1 and IL-8, which enhance the immune stimulatory conditions. These complexes agitate the complement system and result in another undesirable inflammation. Hence, it is substantial to consider designing therapeutic antibodies without pro-inflammatory effects and keep their neutralizing capacity against the virus. For example, changes in the Fc region of antibodies or its glycosylation can modify its binding affinity to the Fc receptor (44-46).

A recent survey suggests that specific antibodies against the SARS-CoV-2 particles may only be preserved for 2 months. This causes a concern that effective immunity to the virus may not persist for a long term. Likewise, rapid reduction in the titer of antibodies have been illustrated in mild cases. However, with a half-life of nearly 21 days for IgG, it is expectable for titer reduction in COVID-19 patients. Laboratory studies on the level of IgM and IgG antibodies in subjects with COVID-19 indicate that these antibodies are also found in the asymptomatic individuals, but it is noteworthy that the antibody titer is much lower in such patients (47-49).

Natural killer (NK) cells are part of the innate immune system, which have capability to target virus-infected cells (50). NK cells have the potential to lysis abnormal target cells through cytotoxic mechanisms and ignore self-cells expressing MHC I. Investigations reported that cytotoxicity function of NK cells can controlled through a factor called inhibitory natural killer J o u r n a l P r e -p r o o f receptor (51). Immune cell profile laboratory tests indicated that NK cell counts will reduce during COVID-19 infection because of infiltration into the sites that have been affected by COVID-19, like the lung (52-54). NK group 2A (NKG2A) receptor as an inhibitory signaling transmitter, has some functional effects on T cells and NK cells, such as cytotoxicity reducing and cytokine secretion suppressing. Studies have shown in individuals who infected with SARS-CoV-2 the expression of NKG2A is dramatically high, while the expression level of markers like TNF-ɑ, IL-2, IFN-ɣ, and CD107a is remarkably lower, as activator factors (54). Besides, it is proved that the hyper activation of localized NK cells in the lung can be more damaging than beneficial and cause lung injury (55, 56).

The main mission of neutrophils is the deletion of pathogens via the phagocytosis process. They also can release Neutrophil Extracellular Traps (NETs) for virus inactivation and cytokine to stop virus replication (57) (58) (59) . NETs contain chromatin fibers affiliated with some enzymes, including myeloperoxidase, cathepsin G, and neutrophil elastase (60) . NETs function is like to a double edge of sword; primary role of this traps has recorded in anti-inflammatory response and in opposite, they can develop tissue damage too (61, 62) . The data analysis of a study reported that neutrophil degranulation and activation are highly activated processes in SARS disease (63) .

Neutrophil penetration and localization in lung capillaries with migration to alveolar space was illustrated in lung autopsies sampled from patients who passed away from COVID-19, demonstrating inflammation in the lower overall part of the respiratory system (64, 65) . On other hand, not only immature neutrophils but also inefficient mature neutrophils have been reported in SARS-CoV-2 patients (66) . Broncho alveolar fluid testing from COVID-19 patients recorded great levels of CXCL-8 and CXCL-2, chemokines that simplify the neutrophils recruitment to the infection zone (67, 68) , and extended activation of these phagocytic cells may lead to J o u r n a l P r e -p r o o f harmful effects in the respiratory system and cause ARDS (69) . In addition, neutrophils secrete toxic factors that might contribute to ARDS too (70) . Some of ROSs like H2O2 and superoxide radicals can produced follow a respiratory burst by neutrophil cells. Finally, this mechanism develop oxidative stress that associates with blood clots and cytokine storm in COVID-19 patients (71) .

Monocytes are a kind of leukocytes that are deriving from myeloid progenitors and circulate in 

J o u r n a l P r e -p r o o f Today, incredible advances have been achieved in the treatment of diseases through cell therapy, particularly stem cells, which have provided a bright horizon for incurable patients (77) .

Mesenchymal stem cells (MSCs) have several unique properties including many resources for cell purification, self-renewal, high proliferation, non-invasive procedure to obtain, immunosuppression, and multidirectional differentiation. These cells can transform into adipocytes, chondrocytes, and osteocytes in the induction medium (78) . There are many advantages to apply MSC therapy as a new method in analogy with other available treatments.

They are more accessible and can be separated and purified from several tissues, such as the umbilical cord blood, menstrual blood, bone marrow, adipose tissues, buccal fat pad, dental pulp, 

The International Society of Cell Therapy has set three criteria for MSCs that it must meet in order to be utilizable in the clinics. First, MSCs must have the property of adhering to plastic when maintained under suitable culture protocols. Second, they must express some specific markers, such as CD73, CD90, and CD105, and not express CD11b, CD14, CD45, CD79a, CD34, CD19, and human leukocyte antigen (HLA)-DR molecules. Third, MSCs must be able to differentiate into adipocytes, chondroblasts, and osteoblasts in vitro (85) . MSCs ought to express modest ratio of MHC class I molecules. However, they should not express MHC class II and other stimulant molecules, including CD80, CD86, and CD40, leading to low immunogenicity of MSCs. Given these features, it can be assumed that these cells do not elicit an immune response in the recipient and have already been used in clinical trials (86, 87) . inquired about their potential in finding out the mechanistic and regenerative facets in treating diseases. The utilization of MSC-derived exosomes as a cell-free therapy has acceptable advantages over cell therapy, such as low immunogenicity, high stability, ability to cross the blood-brain barrier, and easy storage approaches (97) . Pulmonary edema is inevitable following impairment of the endothelial and epithelial barrier of the lungs and increased permeability of the alveolus during infection. This unexpected physiological mechanism disrupts the air exchange action of the lung. One animal study revealed that MSCs-derived exosomes detract extravascular lung fluid by 43% with a decline in pulmonary edema and permeability of lung (106) . This potential of exosomes was partly mediated by CD44-dependent pathway of exosomes for localization inside the injured host cells (107) .

Therefore, it appears that with the assistance of high potential that exosomes have in accelerating the healing of damaged lung tissue, promising insights have been raised in the treatment of COVID-19 patients and can be prescribed as a nano-medicine.

Today, among the most suitable drug carriers for drug delivery are liposomes or vesicles with phospholipid membranes (108) . Exosomes have lots of positive characteristics that are J o u r n a l P r e -p r o o f impressive in their role as drug delivery vehicles. They help implement a defensive barrier against premature transformation and omission. Liposomes can easily constitute hydrophobic and/or hydrophilic drugs in their hydrophobic membranes or within an aqueous core, respectively. They can also break the plasma membrane to release their loaded medication.

Liposomal membranes have potential to insert modified antibody segments or ligands on their surface, facilitating the interaction with specific target cells, conferring specific drug delivery.

They may also be developed with an inert polymeric structure like polyethylene glycol to decrease liposome identification by opsonins and elimination of the liposomes. Amid the several secreted membrane carriers, exosomes are the most appropriate for expansion as a drug delivery carrier (109) . Firstly, the attendance of nucleic acids and protein in MSCs-derived exosomes annotates that such biological molecules could be placed into exosomes. Secondly, exosomes are sustained in the body ideally, as demonstrated by their immense dispensation in biological fluids (110) . Thirdly, exosome vesicles are able to pass through the plasma membrane to transfer their loaded pharmaceutical cargo into the target cells. For instance, DCs-derived exosomes can transmit peptide-packed MHC class II and I complexes to another DC to regulate the immune response (111) . Fourthly, exosomes have an inherent potency to target tissues. Exosomes have special homing target sites depending on the sources from which they are derived (112) . Finally, exosomes are flexible to membrane rectification that augment targeting of a specific cell (113).

Clinical achievements of the medications targeting the COVID-19 requires the recognition of a 

Nasal delivery of drug-carrying theranostic nanoparticles is presented as a successful treatment strategy to control the COVID-19. These types of drug carriers can be classified into three different categories, including inorganic, organic as well as virus-like structures or self-assembly tiny proteins. The vehicle system impressively dominates the drug delivery difficulties related to the mucosal track and retain a highly effective dose of the medicine at the local of infection, while expressing little side effects to the healthy tissues and cells (114).

The medication particles are bonded to the antibodies by a chemical reaction and release their cargo at the target site. Preclinical data have documented that SARS-CoV-2 antibody STI-1499, J o u r n a l P r e -p r o o f that entirely inhibited the viral contamination. The antibody reportedly stopped the viral dispersion by blocking the interaction of ACE2 with the S1 subunit of the virus, a necessary incident for the entry of the COVID-19 into the host cells (115).

The vesicular drug delivery method gives an interesting innovatory approach as theranostics in J o u r n a l P r e -p r o o f

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