INTRODUCTION
Since decades of extensive research, hematopoietic stem cells (HSCs) have been shown to exhibit tissue-specific stem cell characteristics and have been continuously used in many clinical procedures. Upon clinical application, they
provide a deep understanding of the fundamentals of stem cell sciences. The different fields of HSCs being studied and their clinical applications are believed to benefit significantly. The properties of self-renewal and differentiation have put stem cells at the center of regenerative medicine. The combination of these features is leading to improve ways to treat many medical conditions [1]. The undifferentiated stem cells have their origin in the different stages of life. They can be found in the embryonic, fetal, and adult stages of life. They give rise to different lineages of cells that are necessary for building up various organs and tissues. Many studies have shown the use of stem cells in cellular therapy. In the regeneration of organs and the replacement of damaged cells, these cells are highly valued. Some recent research has shown that stem cell-associated specific cells in some diseases can be transformed to develop drugs [2]. In the jewels of medicine, cellular therapy is considered a favorable and encouraging approach that, instead of generating a new organ, focuses on the regeneration and restoration of various injured tissues [3]. Genetically modified stem cells and the production of engrafts with bio cultures scaffolds have enhanced therapeutic outcomes in animal models.
However, several vigorous pre-clinical studies are required to move forward in the field of stem cell therapy [4]. There are a lot of researches that have shown assuring effects of stem cell therapy. The scope of stem cell therapy can be seen during the developing phase of the embryo in various cardiovascular diseases, including cardiomyopathies, degenerative nervous disorders, metabolic diseases like diabetes, and osteoarthritis [5]. MSCs induce immunomodulatory and anti-inflammatory effects and have ascertained a dynamic role in the cure of COVID-19. The MSCs therapy has been a cost-effective treatment in COVID-19 [6]. This chapter will discuss the fundamentals of stem cells, their origin, and their types. Also, we will discuss the use of stem cells in novel coronavirus (COVID-19), the immune reactions, and the future perspective of HSCs for COVID-19.
Overview of Stem Cells
The term stem cell denotes a type of cell that holds the property to differentiate into several other types of cells while maintaining the self-renewal feature. Stem cells have the following main characteristics: a). The capability of expansive proliferation, i.e., the process of renewing itself b). Potentiality to form different types of cells and c). The ability to grow from a single cell [2]. Going afar and crossing the limitations of cell lineages, hematopoietic stem cells possess a versatile nature to proliferate and give rise to abundant cell types. This process is also called transdifferentiation [7]. Different components in the blood are a reason for the differentiation of the multipotency feature of hematopoietic stem cells. HSCs are solely in charge of forming blood and its elements in all the spans of life. They develop, maintain, and regenerate a major portion of blood which itself is connective tissue. Multipotent stem cells act as the origin to give rise to the lineage of myeloid and lymphoid cells. Myeloid lineage then differentiates to common myeloid progenitors or CMP, while lymphoid lineage gives rise to common lymphoid progenitors or CLP. CMP and CLL themselves are left with no or negligible transdifferentiating characteristics reaching the stage of these progenitors. They also lose self-renewal ability and show very little division. From the CLP arise the cells that are essential components of cellular immunity. The cells that arise from CLP include T cells, B cells, natural killer cells (NK), lymphocyte subset forming lineages, and antigen-presenting cells like dendritic cells.
CMP differentiates into granulocyte monocyte progenitors (GMP), which can differentiate into granulocytes and monocytes. CMP also gives rise to megakaryocyte erythrocyte progenitors (MEP), which further develops a change to megakaryocytes and erythrocytes [8]. Outbreaking research has been conducted on HSC circulation under different conditions. HSCs don’t follow a haphazard path circulating in the blood, but they distribute under proper physiologic rhythmic regulatory signals. The central nervous system has a major part in regulating the circadian clock, which draws the HSCs to the bone marrow [9]. High resistance to viral infections has been found in different stem cells. It has been noticed that stem cells can protect themselves from viruses by inducing interferon stimulating genes (ISGs). Interferon reaction is the first main mechanism of many cells against viral invasion. It has been reported that when stem cells differentiate into other types of cells, as they cannot differentiate any further, the ISGs become lower. Different types of stem cells have a particular way of expressing the ISGs [10]. The stem cells are generated when the embryo cells migrate and specify in multiple sites during its developmental phase. In upper-class vertebrates, various locations can be seen to undergo hematopoiesis, the process of blood formation. Hematopoiesis can be consecutively seen in the yolk sac, aorta gonad mesonephros region that encircles the dorsal aorta, the fetus's liver, and lastly, the bone marrow. Placental tissue has been recently recognized as another spot for the development of HSCs [11]. HSCs can be transplanted in two different ways, i.e., autologous and allogenic. Therapeutically autologous transplantation of stem cells is related to radiotherapy and chemotherapy. This has been regulated to kill the tumor cells and recover the patient later on by Stem cell therapy. Allogenic stem cell transplantation is different from autologous. It uses stem cells of hematopoietic origin from a matched donor to revive the patient without using chemotherapy. The allogeneic transfer can lead to a graft-versus-tumor effect [12]. The hematopoietic stem cell lineages have been illustrated in Fig. (1) .
Fig. (1))
LT-HSC: Long Term Hematopoietic Stem Cells, ST-HSC: Short Term Hematopoietic Stem Cells, CMP: Common Myeloid Progenitor, CLP: Common Lymphoid Progenitor, GMP: Granulocyte-Monocyte Progenitor, MEP: Megakaryocyte-Erythroid Progenitor, RBC: Red Blood Cells, NK: Natural Killer Cell, PDCs: Plasmacytoid Dendritic Cells, MDC: Monocyte Dendritic Cell.
Different Stem Cells
Although Stem cells can be divided into different types, two main types have been broadly categorized, namely the Pluripotent cells and Multipotent cells. The cells that have the property to differentiate into cells forming the endoderm, ectoderm, and mesoderm are known as pluripotent cells. These cells include embryonic stem cells, embryonic germ cells and also share some properties with induced pluripotent stem cells (iPSCs). The other types of stem cells based on potency are multipotent stem cells. These cells are different from embryonic or pluripotent cells in the way that they can be isolated from many fetal and animal tissues. HSCs, hepatic stem cells, neuronal stem cells are all lineages of multipotent cells [12]. HSCs have the characteristic to proliferate into both myeloid and lymphoid cell types. Multipotent stem cells can differentiate into a single layer of germ cells. Mesenchymal stem cells (MSCs) also belong to the cell types of multipotent stem cells. Several tissues are a source of multipotent stem cells, including peripheral blood, adipose tissue, bone marrow, bone, Wharton’s jelly, and umbilical cord blood [2]. A cluster of stem cells has been recorded and hierarchized into different types based on potency and origin.
The different types of stem cells based on the potency to differentiate are as follows:
- Totipotent or omnipotent cells are differentiable into embryonic as well as extra-embryonic tissues. Examples include an oocyte that has undergone fertilization. Embryonic and extra-embryonic tissues with development form into the embryo and the placenta.
- Pluripotent stem cells including embryonic stem cells.
- Multipotent stem cells include Mesenchymal stem cells.
- Oligopotent stem cells can form two or more cell lineages belonging to a particular tissue. Hematopoietic stem cells can be included in this group.
- Unipotent stem cells for example muscle stem cells which differentiate into only one type of cell.
Based on the origin of stem cells, they can be classified as:
- Embryonic stem cells: The source of these cells is a 5-6-day stage of embryo known as the blastocyst. The blastocyst is formed of two cell types, i.e.., the inner cell mass developing inti embryo and the outer cell mass, also known as trophoblasts, give rise to the placenta.
- Fetal stem cells: Whose source is terminated pregnancy fetus. Fetal stem cells lack many features ascribed to embryonic stem cells. They cannot differentiate indeterminately unless forced to do so under certain conditions in biological cultures. Neural stem cells can identify lineages that can be increased using fetal stem cells.
- Perinatal stem cells: The origin of these cells may be the umbilical cord, placenta, and amniotic fluid. The blood of the placenta is a rich source of stem cells. Fetal epithelial cells are commonly found in amniotic fluid.
- Adult stem cells: They are also known as resident stem cells or tissue-restricted stem cells. They can be found in the tissues and organs of an adult. Adult stem cells can be sourced to originate from bone marrow, menstrual blood, and adipose tissue, as shown in Table 1.
- Induced Pluripotent stem cells (iPS): Which shares the features of embryonic stem cells and adheres to their in vitro indefinite division [13]. In Table 1 below most important types of stem cells with their sources and/or examples are shown.
| Type of Stem Cell | Examples |
|---|---|
| 1. Totipotent 2. Pluripotent 3. Multipotent 4. Oligopotent 5. Nullipotent |
Zygote Embryonic Stem Cells (ESCs) and induced Pluripotent Stem Cells (iPSCs) Mesenchymal Stem Cells (MSCs) Lymphoid or Myeloid Stem Cells Osteoblasts Osteocytes |
Stem Cells and Immune System Responses During COVID-19
Among the different stem cells, mainly mesenchymal stem cells have shown beneficial effects in COVID-19. MSCs inhibit many lymphocytes like T lymphocytes, monocytes, and macrophages. MSCs stimulate the differentiation of these cells into regulatory T cells (Treg). They also induce anti-inflammatory effects on macrophages. Mesenchymal stem cell therapy impedes the secretion of different cytokines, e.g., IL-1, TNF-a, IL-6, IL-12, and IFN-g. Inhibition of such cytokines decreases the occurrence of cytokine storms in COVID-19 patients. MSCs protect the damaged respiratory tissue and pulmonary fibrosis; also relieve the COVID-19 patient from acute respiratory distress syndrome (ARDS). This effect has been reported based on the secretion of some factors by MSCs such as IL-10, vascular endothelial growth factor (VEGF), hepatocyte growth factor, and keratinocyte growth factor [14]. MSCs act through paracrine secretions, interacting with different cells of the immune system and regulating immunomodulatory effects. In various disease settings, the condition was improved after MSC transplantation. In such clinical settings, immune cells that secrete multiple cytokines have shown a decline in the blood. These cells include CXCR3+ NK cells, CXCR3+ CD4+ T cells, and CXCR3+ CD8+ T cells. MSCs also help in the proliferation of peripheral lymphocytes. In addition to the mentioned immune responses, CD14+ CD11c+ CD11bmid dendritic cells (DCs) have surged after the infusion of MSC therapy. Other immune responses found in many trials showed a reduction in TNF-a and an increment in IL-1 [15]. It should be noted that whenever the virus gets entry into the cells, it first presents its antigen to antigen-presenting cells (APCs). The peptides of antigen are presented by human leukocyte antigen (HLA) or major histocompatibility complex (MHC) in humans. Cytotoxic T lymphocytes (CTLs) then recognize these antigens. CD8+ and CD4+ T cells are reduced in COVID-19 [16]. It has been noted that there is a strong affinity of HLA molecules to bind to COVID-19 viral peptides [17]. Embryonic stem (ES) cells can be used in transplantation against MHC barriers without provoking an immune response [18]. Some studies reveal that mesenchymal stem cells inhibit the proliferation and differentiation of dendritic cells from CD34+ cells in humans while stimulating the regulatory dendritic cell differentiation [19]. It is known that lymphocytes and their subsets have a major role in regulating cellular immunity, and these cells work by restricting and regulating their common cascade. In novel coronavirus pneumonia, CD4+ T cells show a serene decrement in mild symptomatic COVID-19 patients, whereas in severely ill patients, this number is further decreased. Similarly, the CD8+ T cells are shown a gentle reduction in mild patients while a high drop is found in severe reported cases. This shows that immune system suppression was more noted in severe patients [20]. In the earlier course of the disease, activated partial thromboplastin time (aPTT) was considerable of shorter duration in more severe forms of COVID-19 affected patients. The quantity of D-dimer has also been reported to be elevated in novel coronavirus. Some other monitored findings are reduced number of B cells and higher than normal levels of NK cells, respectively more in severe cases than milder ones. Reduction in the levels of Treg subsets such as CD4+, CD25+, and CD127+ has also been reported in mild and severe forms of the disease. CD45RA+ Treg is decreased rapidly in severe cases as compared to mild reported cases [21]. Naïve Th cell count is increased, and memory Th cell count is reduced in COVID-19 patients who are severely infected. Cytotoxic suppressor T cells, which are CD28+, are also decreased below the normal levels. There is no big difference in the count of activated T cells, which are CD3+, HLA-DR+, and activated suppressor T cells which are CD8+, HLA-DR+, and CD3+. It was observed in seriously ill patients that regulatory T cells are decreased. No major change was seen in Treg cells and naïve T cells [22]. Much clinical analysis reported in certain cases of COVID 19 was explored for immune responses. These reports revealed high levels of immune factors in the serum of COVID-19 infected patients. One sample has shown high M-CSF, IL-10, IL-2, IL-4, IL-7, IL-12, IL-13, IL-14, IL-1a, IL-1ra, IL-1B, IP-10, IFN-a2, IFN-g, G-CSF, and HGF. Further immune-stimulating factors including cytokines and chemokines such as TRAIL, IL-16, IL-18, TNF-B, IL-2Ra, CTACK, LIF, GRO-a, MIF, MCP-3, b-NGF, SCF, SDF-1a, and SCGF-B have also been noticed. One study reveals the excess number of CD14+ and CD16+ monocytes responsible for the secretion of IL-6, which has also been accounted for in COVID-19 [23]. SARS-CoV2/COVID-19 suppresses and inhibits the TNF receptor-associated factors (TRAF), such as TRAF3 and TRAF6. Both of them have a crucial role in inducing TLR3/7 with the help of stimulating IRF-3/7. TRAF indirectly activates the RIG-I, MDA-5, and NFKB pathways. COVID-19 hinders the signaling pathway of T1IFN and activates phosphorylation reactions to inhibit the STAT family factors [24]. An early phase of the disease in hospitalized patients revealed low levels of IgM and IgG. Later on, with the course of the disease, IgM, and IgG, both immunoglobulin levels were reported to be positive, i.e., there was an increment in the levels of IgG and IgM [25].
Mesenchymal Stem Cells in COVID-19
In various clinical studies, it has been put forth that MSCs can be used therapeutically as these cells treat via immunomodulation. Several clinical trials have shown an increasing amount of interest in MSCs. A study reveals that post mesenchymal stem cell transplantation, the lungs' alveolar lining is protected. It has also shown that pulmonary fibrosis can also be prevented, and the dysfunction of lungs can also be treated. As the immune system releases a storm of cytokines in COVID-19, mesenchymal stem cell therapy can prevent this phenomenon by promoting endogenous repair, which is a characteristic of stem cells. MSCs are a group of multipotent stem cells. They can be stored and then used repetitively for therapeutic purposes. MSCs are safe and effective with no nasty effects [26].
MSCs have been known to drift towards injured and inflammatory tissues and upregulate receptors like CCR2, CCR3, CCR4, and CXCR4 for RANTES and MDC. They also express PDGF and IGF receptor tyrosine kinase [27]. Bone marrow mesenchymal stem cells secrete FAS-regulated monocyte chemotactic protein 1 (MCP-1) and cause FASL-mediated T cell apoptosis. These T cells that undergo apoptosis recruit macrophages. Macrophages, in turn, secrete TGFB, which expresses CD4+ CD25+ and FOXP3 Treg cells. Treg cells mediate immune tolerance paving a way to affect the functioning of the immune system [28]. Mesenchymal Stem cells, which have been recently known as mesenchymal stromal cells, have been found to adhere to their immunomodulatory properties. They affect the T cells and show an interaction with B cells. This mechanism of mesenchymal stem cells shows that they can reduce inflammation in COVID-19 patients [29]. MSCs increase the activity of phagocytosis and secrete antimicrobial peptides and proteins (AMPs). In this way, they employ antimicrobial activity by coordinating pro and anti-inflammatory components of the immune system. While circulating in the bloodstream, mesenchymal stem cells invade the parenchyma of the lung and start refining the alveolar structure of the lungs. They also improve lung compliance and avoid further fibrosis of the respiratory tissue. They have inhibitory properties against the overstated reaction of the immune system, which helps to promote, revitalize, and regenerate the microenvironment. The mesenchymal stem cells derived from the umbilical cord are allogenic and are available in sparse quantity.
In contrast, the mesenchymal stem cells from bone marrow and adipose tissue can be accessed easily with ample quantity possessing autologous nature. The cells can regenerate epithelium and meshwork of alveoli in the lungs, thus bringing down mortality and morbidity and helping to boost the quality of life in patients with COVID-19 [30]. Among many sources of MSCs, umbilical cord blood and placenta are two excellent references from where MSCs can be derived. MSCs have been studied in a range of studies and clinical applications and have shown positive outcomes in treating the ARDS, acute lung injury and enhancing the repair of tissues [31]. Intravenous administration of human umbilical cord mesenchymal stem cells (hUCMSCs) helps to return the levels of CD3+ T cell, CD4+ T cell, and CD8+ T cell to normal. Thymosin a1, in combination with hUCMSCs, can significantly induce anti-inflammatory effects and regenerate the damaged organs and help in regaining the activity of antiviral cells of the immune system. These cells also have the potency to inhibit the inflammatory cytokines as G-CSF and IL-6 by activating homing receptors to heal the tissues injured due to abnormal immune responses in COVID-19 [32].
Recently, the infusion of mesenchymal stem cells of haploid nature represses graft versus host disease (GVHD). Hematopoietic mesenchymal stem cells interact with the APCs. They change the APCs to deletional APCs, which display immunomodulatory features. Stem cells of mesenchymal origin with the expression of HLA-DR17 and CD80-10 stimulate T cells [33]. MSCs express integrin-A and B-subunits, which interact with extracellular matrix receptors of collagen, laminin, fibronectin, and vitronectin. In this way, they can manage the extracellular matrix. MSCs can upregulate the ligands for interaction with hematopoietic stem cells. They upregulate cell-binding factors such as ICAM-1, ICAM-2, vascular cell adhesion molecule 1, lymphocyte-associated antigen-3, CD72, and leukocyte adhesion molecule. All these features help to generate a scaffold for hematopoietic stem cells. MSCs can evade the immune system as the major histocompatibility complex is expressed at low levels or is not expressed. The communication via the FAS ligand cannot be seen in MSCs. Costimulatory elements like B7-1, B7-1, CD40, or CD40L are not expressed as well. Thus mesenchymal stem cells can form a backbone to develop a stem cell niche ex vivo and can be infused in COVID-19 patients to lessen their complications [34].
Synthetic Stem Cells for COVID-19
Stem cells are available throughout life. Their function is to sustain and form functional tissues with their self-renewal mechanism. Stem cells are used to get a particular and vastly applicable synthetic tissue. Bioengineered stem cells are cultivated in humanized materials mimicking the bodily compartments of the extracellular matrix in which stem cells proliferate quickly. By providing such an environment, stem cells can be easily allowed to proliferate and differentiate. Some of the materials that are known to be compatible to synthesize stem cells such as peptide hydrogels, polylactic acids (LGA and PLA), and hydroxyapatite (HA) like bioceramics [35]. 3D hydrogel scaffold has been utilized to preserve the characteristics of different stem cells. This 3D culture system mimics the microenvironment and extracellular matrix of stem cells in vivo. This scaffold can be used to generate stem cells that proliferate and differentiate easily in this 3D architecture hydrogel [36]. Silk-derived fibroin proteins can be used in vitro in biomaterials for synthesizing stem cells. Silk fibroin provides adhesive support to stem cells; regulates the molecular mechanism and structure of these cells. Silk-based microenvironments drive the proliferation, differentiation of stem cells which can then be infused into damaged tissues to repair them [37].
Nanotechnology's use of bioengineered stem cells known as “LIFNano” has been made available. With mesenchymal stem cell therapy, advantages of LIFNano have been registered in COVID-19 subjects suffering from pneumonia. LIFNano has shown useful properties in revitalizing the damaged tissues and reducing the cytokine release in pneumonia [38]. Secretome is a Mesenchymal Stem Cell derivative consisting of biological products that primarily include lipid derivatives, nucleic acids, soluble proteins, and extracellular vesicles. Secretome-based MSC therapy has been noted as a novel way to treat degenerative and inflammatory diseases. Several research groups have registered the major effects of MSC-derived secretome in suppressing the myofibroblastic differentiation, reducing the production of cytokines, and initiating immunosuppression. In the COVID-19 patients, secretome-based MSC therapy can be of great use. MSC sourced secretome reduces the influx of monocyte, neutrophil, and eosinophil infiltration. They also decrease and overturn the activation of T cells and Dendritic cells, thus helping to rejuvenate the microenvironment in the lungs [39]. MSC-derived secretome is becoming a new research tool for cell-free treatment therapy in acute and chronic pulmonary disorders. Anti-protease, promoting angiogenesis, modulating immune responses, and stopping inflammation are some features of mesenchymal stem cells that can be seen in the secretome as well. Many cytokines can be acted upon at the same time by the secretome. It also possesses some synergistic activity. Many have reported secretomes to be safer than cells. Another feature that can be related to secretome is that they lack the power of self-replication, thus cannot generate tumors. Secretomes yield low immune responses and, on intravenous infusion, has a low possibility of embolism [40].
Synthetic stem cells can be used in multiple therapeutic possibilities for treating a disease. A biosynthesized stem cell niche can be used to generate a unique type of cell population from pluripotent stem cells (PSCs) in vitro [41]. Thus, making a stable stem cell niche for the therapeutic purpose in COVID-19 adhering to the complex nature and interactions between HSCs can provide a vital foundation of microenvironment ex vivo.
Stem Cells in Clinical and Preclinical Research Stage
Stem cells are known for their peculiar biological characteristics. They are the only cells needed for engrafting while considering tissue transplantations. Pre-clinical and clinical studies have to lead to detrimental observations with experimental employing of hematopoietic stem cells. Hematopoietic stem cell transplantation (HSCT) has been beneficial therapeutically in graft versus tumor (GVT) in common hematology-related malignancies. HSC infusion restores graft versus tumor without GVHD. Some of the pre-clinical and clinical case studies indicate if HSCs can improve autoimmune diseases keeping in view the donors are regular. In the case of solid organ transplantation, grafts of HDCs permit a long duration for survival. This has been presented in heart grafts of neonates who are matched with the donor [8]. Hematopoietic and mesenchymal stem cells have also been reported to be the progenitor for many cells, which can give rise and differentiate to form cardiac muscle cells. Injured myocardium can be refurbished with bone marrow cells. These bone marrow cells are attributed to plasticity and differentiate into endothelial cells, smooth muscle cells, and myocytes [42]. Many pre-clinical studies have been carried out on the use of umbilical stem cells for multiple sclerosis, ataxia, motor nervous system disorders, atrophy, and paraplegia without eliciting an acute immune response. Stem cells originating from the placenta are in Phase 3 of clinical-stage research to treat ischemia of limbs. In multiple sclerosis, CD34+ HSCs have shown immunosuppression and are considered to inhibit the immune responses of autoreactive T lymphocytes. Umbilical cord cells are indicated to generate in vitro neurons according to some pre-clinical data. Clinical research on HSC therapy is going on for genetic disorders blood like b-thalassemia and sickle cell disease. Adipose derives stromal stem cells are found in abundant quantities and can be accessed with ease. They have the property of soft tissue reconstruction and regeneration. The fetus, neonate, or brain of the adult can be a source of neural stem cells (NSCs). These cells have been reported in pre-clinical studies to divide into cells like astrocytes, oligodendrocytes, and neurons. They undergo clinical trials for lysosomal storage disease, Parkinson’s disease, glioma, and other disorders. Pluripotent human embryonic stem cells (ESCs) are also undergoing clinical research and are considered safe in thoracic spinal cord injury diseases [43]. Testicular or spermatogonial stem cells possess a significant part in constituting germ cell lineage. These cells are diploid and are thought to have eternal nature. Testicular stem cell grafting has proven beneficial in the generation of sperm. This innovative approach can help retain fertility in young patients who are at potential risk of developing fertility disorders due to undergoing treatment for cancers or being exposed to many toxins that affect gonads [44]. A combination of cell-based therapy using different sourced stem cells is a new clinical and pre-clinical trial experimented with by different researchers. Animal-based studies using such therapies with a combination of multipotent MSCs, adipose-derived stem cells, bone marrow mononuclear stem cells, and induced pluripotent stem cells have been used together with angiogenic and/or growth factors. This approach in animals has resulted in intensified vasculogenesis, decreased automatic cell death, and improved heart function [45]. Pre-clinical results and clinical study of stem cell-based therapy infused in Myocardial infarction and ischemic cardiac diseases have proven beneficial with improved left ventricular ejection fraction and has been marked safe [46]. Stem cells in combination with biomaterials can provide relief in terms of available bone engrafts. Bone marrow has been found to inherit an abundant amount of osteoprogenitors and growth factors in addition to stromal stem cells. MSCs can be sourced in the bone marrow and have been recognized as the most persuading cells with osteogenic activity. Stem cells of mesenchymal or adipose origin have the vital capacity to regenerate the bone. Pre-clinical trials have shown that these cells play a vital role in bone regeneration via secreting factors needed for angiogenesis while maintaining the environment of hypoxia and inflammation. Stem cells can be stimulated by inducing bioceramic materials and osteoinduction to develop bone [47].
Serum Cytokine/Chemokine/Growth factors Up- and Down-Regulation During Stem Cell Therapy
As with the previous strains of coronavirus like SARS and MERS, we have seen dysregulation of immune responses. The same is true for the novel COVID-19 or SARS-CoV-2. Various studies across the globe have noted the irregularities of different immune responses in novel coronavirus patients. There has been upregulation of many cytokines and chemokines in COVID-19. It includes IL6, CXCL1, CXCL5, CXCL10, and CCl2. IL-10 and TNFa were higher in Intensive Care Unit patients than those who had moderate clinical progress. The term “Cytokine storm” has been commonly associated with recent studies regarding novel coronavirus [48]. These immune responses stimulated by COVID-19 occur in two phases, first during the incubation period second during the non-severe phase of the disease when the patient has been discharged from the hospital. Higher levels of pro-inflammatory cytokines TNF and IL-1 lead to the induction of EpCAM+ in the alveolar tissue of the lungs. Also are potent inducers of fibroblasts and hyaluronan (HA-synthase-2) in CD31+ endothelial cells [17]. Acute phase reactants like fibrinogen, serum amyloid A, CRP (C-reactive protein), hepcidin, and inhibiting the synthesis of albumin have been associated with the elevations of IL-6. Thus IL-6 serves as an essential biomarker for inflammation and can conduct us to early severity of COVID-19 [49]. Laboratory data reveals an abnormally high level of CRP and hyperferritinemia in many COVID-19 reported cases. Elevated CRP can lead to macrophage activation syndrome (MAS) or haemophagocytic lymphohistiocytosis (HLH) [50]. High plasma levels TNFa, IL2, IL6, IL7, IL10, IP10, GCSF, MAP1a, and MCP1, have been reported in COVID-19 patients. The different growth factors are also highlighted to be involved in novel coronavirus infection having viral protein interactions between cytokines and cytokine receptor pathways. According to one study 15 chemokines include CCL2, CCL4L1, CCL4L2, CCL5, CCL10, CCL12, CCL16, CCL17, CCL27, CCL28, CCL32, CXC3CL1, and BAFF. They are involved with 12 cytokines, including IL3, IL6, IL12, IL13, IL17A, IL17B, IL17E, IL19, IL20, IL21, IL32, and IFNG. Growth factors involved in COVID-19 infection have been found as TGFB1 and NGF. In the onset of the acute phase of COVID-19 immunoregulatory pathways like MAPK, ERK1/ERK2, JAK-STAT, and PI3K are also activated. These studies reveal a more precise understanding of inflammation and immune signaling pathways in COVID-19 [51]. Dysfunctional T cells express PD-1, which inherits inhibitory functions. This leads to the immune dysregulation commonly seen in COVID-19. The responses of the immune system are directed from immunoregulatory T cells towards immunosuppressive T cells. It has been clear from many types of research immune dysfunction is the main target in COVID-19 and still needs to be observed precisely to understand the mechanism behind the cytokine storm [52].
Role of ACE2-Mesenchymal Stem Cells for COVID-19
Angiotensin-converting enzyme (ACE2) is a host protein in which COVID-19 uses a co-receptor to enter the human body tissues. Its path is intracellular and usually targets the respiratory system and brain. The expression of ACE2 can be found in almost all body tissues. High expression of ACE2 can be seen in the kidney and ileum. In comparison, the moderate expression has been reported in nasal and oral mucosa, liver, stomach, vasculature, adipose tissue, brain stem, heart, and lungs. SPIKE or S protein is expressed on the coating of COVID-19, which has a high affinity to the ACE2. ACE2 also binds to peptides in maternal and fetal circulation [53]. Many researchers have received the interaction of COVID-19 and ACE2.
ACE2 helps COVID-19 to enter the cells. ACE2 also interacts with the kallikrein-kinin system. It has been proposed that the downregulation of ACE2 by COVID-19 leads to the failure of inactivating the B1 bradykinin receptor, a receptor attributed to endothelial cells. The loss to downregulate the B1 receptor is mutually related to the downregulation of ACE2. This mechanism of COVID 19, via downregulation of ACE2 and failure in the inactivation of B1 ligand, has been related to a severe form of infections [54]. A study conducted on the genetic profiling of mesenchymal stem cells indicated that MSCs are ACE-, proving the virus-free nature of mesenchymal stem cells. Henceforth mesenchymal stem cells are efficient in treating COVID-19 induced pneumonia [26]. Mesenchymal stem cells have the trait of not showing the ACE2 [30]. Many benefits have been registered for ACE2- the nature of mesenchymal stem cells. Some of which we will discuss in this section. After the infusion of MSCs, COVID 19 patients with acute onset have shown improvement in pulmonary function, protection of respiratory epithelium, and healing of fibrosis. MSCs refurbish the immune cell compartments and their functions. ACE2(-) mesenchymal stem cells prevent the cytokine storm by inhibiting T and B lymphocyte proliferation and secreting anti-inflammatory factors. They also normalize the immunomodulatory functions of different cytokines [55]. Some research on mice reveals that the ACE2 gene is overexpressed with bone marrow mesenchymal stem cells offering protection of endothelium and anti-inflammatory effects in response to lung injury caused by endotoxins [56]. ACE2 binds to the Fc domain of IgG in COVID-19 patients. MSCs with neutralizing antibodies can target ACE2-Fc and prove to help treat the COVID-19 by inhibiting the mentioned binding domains [57]. Responding to the beneficial effects of immune cells and their immunoregulatory and immunomodulatory effects in antiviral activities, ACE2- MSCs may provide a breakthrough to combat the COVID-19 [58].
Clinical Outcomes of Stem Cells for COVID-19
Many cases can be illustrated based on the overall outcome of stem cell transplantation for COVID-19 in different hospitals worldwide. A 65-year-old female COVID-19 patient with a critical condition was hospitalized in China. hUCMSCs were infused intravenously in three phases for the mentioned case. At the end of the 2nd phase, autoimmune responses were greatly changed. SGOT/SGPT, CRP, and bilirubin levels in serum were notably decreased. T lymphocyte subsets like CD3+, CD4+, and CD*+ T cells were also noted to return to normal levels. After the treatment, pneumonia in the patient was relieved. WBC count was normal. But the neutrophil count was increased. There was a decrease in lymphocyte numbers.
The patient was taken out of ICU with normal vital signs, and the swab sample taken from the throat was also reported negative. hUCMSCs have displayed potent capabilities to regulate immune system responses and heal tissue damage without any side effects [32]. Several clinical trials have been carried out in China on the safety of stem cells in COVID-19 pneumonia patients. A trial conducted on 7 patients who were infected with novel coronavirus revealed some beneficial effects in treating COVID-19 complications. All 7 patients have infused stem cells and are observed for 14 days. These patients recovered after receiving stem cell therapy without any notable complications. Another pre-clinical trial was conducted in Wuhan on MSCs in mice, which revealed a significant reduction of viral load, reduced pulmonary injury, anti-inflammatory characteristics, and decreased the mortality rate [59]. RNA sequencing performed on MSCs therapy of COVID-19 patients showed the safe and infectious-free nature of MSCs. In many patients after MSCs therapy, the levels of pro-inflammatory cytokines like TNF-a were decreased, and anti-inflammatory cytokines like IL-10 were increased. Biomarkers of dendritic cells, i.e., CD14+CD11bmid, were also alleviated after the infusion of MSCs [60]. Investigations of 7 COVID-19 patients after MSC therapy showed improvement in pulmonary functions. All the pulmonary complications have been reported to have diminished in 2-4 days after MSC introduction [61]. In a clinical trial of 48 COVID-19 patients with severe pneumonia in Wuhan Union Hospital, 24 patients received conventional treatment while 24 others were infused with MSCs. The investigation was based on the fact that MSCs induce anti-inflammatory and immunoregulatory effects. MSCs helped to promote tissue repair and protect from apoptosis and fibrosis of lung tissue. The patients who received MSC therapy showed better outcomes than those getting conventional treatment. Another trial has been carried out in Puren Hospital to demonstrate the safety of MSCs in COVID-19 [30].
FDA/WHO Approved Stem Cells Therapy for COVID-19
Owing to the growing clinical research and safety of stem cells, Food and Drugs Association (FDA) has given a green signal to many projects to be carried out on stem cell therapy for their use in COVID-19. 4 projects have been approved in China, and another one has also registered their project of interest to FDA [62]. Many changes have been made in FDA regulations in response to stem cell providers. Under the current FDA guidelines, autologous bone marrow transplantation of stem cells has taken over stem therapy in the US [59]. Hope Biosciences’ has been given a green signal by the US Food and Drug Administration (FDA) to evaluate the mesenchymal stem cells as a treatment type in COVID-19 patients. Based on the trial, patients with severe signs and symptoms of COVID-19 will be infused with adipose-originated mesenchymal stem cells [63]. The FDA has also approved the Global Institute of Stem Cell Therapy and Research (GIOSTAR) to carry on the clinical trials of mesenchymal mesenchymal-based stem cell therapy for their utilization in critical cases COVID-19 the new guidelines keeping in view some important outcomes of mesenchymal stem cells and “expanded access for compassionate use” project [64]. Authority to test the protective characteristics and effectiveness of umbilical cord mesenchymal stem cells has been granted to a team of scientists by the US FDA. They will explore the benefits of these cells by intravenous administration in COVID-19 patients suffering from inflammatory responses in the lungs [65]. RESTEM company has received authentication from the FDA to conduct a trial of umbilical cord stem cells on 60 patient studies in response to the treatment of COVID-19. This study will assess the success of systemized umbilical cord mesenchymal stem cells in COVID-19 patients with acute respiratory distress syndrome (ARDS) [66]. A biotech-linked company, mesoblast, has received acceptance from the FDA for the clinical application of their stem cell remestemcel-L. Remestemcel-L has its origin from allogeneic mesenchymal stem cells.
Several hospitals and COVID-19 patients with ARDS are going to be part of this study [67]. FDA has also approved Caladrius Biosciences, which proposed CLBS119 CD34+ cell therapy for lung injury in COVID-19. Organicell™ Flow, a stem cell product of Organicell Regenerative Medicine, has been agreed upon by the FDA to undergo clinical trials in moderate and severely suffering COVID-19 patients. The aforementioned trial will be one of its kind to involve randomized, placebo-controlled, and multicenter investigations to prove the usefulness of amniotic fluid sourced stem cells for novel coronavirus [68]. After the outbreak of novel coronavirus worldwide, the US FDA changed their policies and guidelines and came up with proper issuances to help many benefactors carry on clinical and pre-clinical trials on participants while maintaining passivity during the pandemic trials. FDA formed a unique platform and database for potential cell therapies named the Coronavirus Treatment Acceleration Program (CTAP) [69]. World Health Organization (WHO) has mentioned the use of umbilical cord mesenchymal stem cells, human dental pulp stem cells, and bone marrow-derived mesenchymal stem cells as experimental non-drug treatment under advanced therapy medicinal products (ATMPs) [70].
FUTURE PERSPECTIVE OF HSC FOR COVID-19
Hematopoietic stem cell therapy is a novel approach to treat many diseases. The research on stem cells to be used in COVID-19 has been geared up and is still undergoing many clinical and pre-clinical trials. UCMSCs have been successful in repairing lung tissue and renovating the airway permeability in endothelial cells. MSC therapy has also successfully treated the alveolar damage in various strains of influenza-induced viral pneumonia such as ARDS induced by H7N9. This can serve as a platform to induce MSCs in vitro for clinical trials in COVID-19 infected patients [56]. Stem cells from a patient may prove to be a conceivable cornerstone in cellular therapies and a novel approach of treatment in molecular science [71]. Different sourced stem cells can suppress the immune system, regulate the levels of serum cytokines and chemokines. Stem cells have also been shown to reduce the number of macrophages in the damaged pulmonary tissue. They increase the number of regulatory dendritic cells in the tissue undergoing inflammation. MSCs can help enhance the recovery in COVID-19 patients in obligation to regenerative, renewal, and rejuvenate features of these cells [55]. Stem cells can have promising effects to treat acute lung injury in COVID-19. MSCs reduce the pro-inflammatory cytokines. This can serve as an integral approach to treat the cytokine storm in novel coronavirus [31]. ACE2 is expressed in multiple tissues of the body like the heart, lungs, and gastrointestinal tract et al. In COVID-19, ACE2 has been observed to express itself on lymphocytes. Thrombocytopenia and venous thromboembolism have been reported in COVID-19 patients with the expression of ACE2. MSCs can suppress the expression of ACE2 and thus prevent many complications in COVID-19 patients. There is the necessity of stem cell donors, which over time, will increase the need for blood donors to prevent severe complications and disorders in seriously infected patients with COVID-19 [72].
MSCs act via paracrine routes and can disperse in the lungs. These factors help to reduce and save the mugs from further fibrosis. The respiratory tract microenvironment in COVID-19, especially in elderly patients, is well preserved with the intravenous infusion of stem cells. The elderly population is at high risk of respiratory suppression, and morbidity is also high in these individuals. A thorough understanding of stem cells will prove essential to save the COVID-19 induced respiratory disorders [15]. Clinical use of recombinant stem cells that can be viewed in future use for COVID-19 is adipose tissue-derived stromal stem cells, embryonic stem cells derived from amnion, and umbilical cord stem cells. Umbilical cord mesenchymal stem cells have been studied thoroughly and have yielded positive outcomes in various patients.
Further studies to use cell therapy in combination with other drugs to alleviate the condition of the COVID-19 infected patients will bring a new perspective and challenging techniques ahead [59]. Although different stem cells have shown combating promises to regulate and rejuvenate the immune system in COVID-19, these cell therapies also have some side effects noted by many researchers. To have a clear understanding of the mechanism of hematopoietic stem cell therapy in COVID-19 patients, a vigorous qualitative and quantitative analysis is the requirement of the time. Extra evaluation of stem cells on the clinical applications will uncover many mysteries that cover up the treatment plans of COVID-19 [73].
CONCLUSION
Stem cell therapy is valid cell therapy in many diseases. Stem cell therapy has yielded surprising outcomes in some clinical trials conducted on COVID-19 patients. It has been noted that COVID-19 is more of a respiratory viral infection causing damage to the lungs. COVID-19 causes inflammation, fibrosis, and pneumonia. Stem cell infusion suppresses the cytokine storm, a process that causes leakage of cytokines and chemokines responsible for the severity of disease in the pulmonary system. This can help to reduce the complications of COVID-19. Different stem cells exert a particular mechanism to deal with COVID-19. A more prosperous outlook of stem cells in clinical trials can be of great benefit in decreasing mortality and morbidity in COVID-19. Mesenchymal stem cells derived from the umbilical cord can be a breakthrough to modulate the immune responses in COVID-19. WHO and FDA should allow researchers and clinicians to evaluate clinical trials and the potent effects of stem cells in COVID-19. Timely interventions of stem cell therapies can prevent rising cases of COVID-19, thus provide a sigh of relief to the millions of people worldwide.