**1. Introduction**

Over the past several months, the world has had to endure another global outbreak, the likes of which have not been seen since the Spanish flu pandemic of 1918 [1]. Coronavirus disease 2019 (COVID-19) caused from the virus now known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is understood to undergo human-to-human transmission by respiratory droplets and known to cause a broad range of symptoms contributing to its rapid spread [2]. As of February 14, 2021, there have been over 109 million reported cases and over 2.39 million deaths worldwide, with the United States having over 27.6 million reported cases along with over 484,000 deaths [3]. As cases continue to accumulate and cause significant strain on medical resources and society, the need for an urgent, effective and safe treatment is paramount. Current measures to curb the COVID-19 pandemic revolve around a broad range of pharmaceutical remedies and the distribution of a vaccine [4]. With vaccines being a prophylactic measure, current treatment options being unproven, non-definitive and suboptimal, and the emergence of new viral strains, attention needs to be placed on alternatives.

Investigations have identified that the majority of Intensive Care Unit (ICU) patients with COVID-19 have high plasma levels of granulocyte colony-stimulating factor (GCSF), tumor necrosis factor-alpha (TNF-α), interferon gamma inducible protein-10 (IP-10), monocyte chemoattractant protein-1 (MCP1) and macrophage inflammatory protein 1-alpha (MIP1A) [5]. These factors have been shown to be interconnected with the recruitment of proinflammatory cells and the production of a cytokine storm. A cytokine storm is a large and abrupt increase in proinflammatory cytokines that is suggested to be the main cause of acute respiratory distress syndrome (ARDS) and other severe pathophysiological effects seen in COVID-19 patients [6]. The cytokine storm induces a vast signaling cascade that recruits immune cells such as humoral B-cells, T-cells, and macrophages (MOs) as well as shifts most of these cells into a proinflammatory state [7]. Interestingly, clinicians have found that through the attenuation of the cytokine storm with mesenchymal stem cells (MSCs) patients have been able to recover even in severe cases [6].

MSCs have been successfully and safely used to treat pneumonia, acute lung injury (ALI), and ARDS in the past [8, 9]. Their effectiveness has been attributed to their ability to be directly antiviral, immunomodulate, induce tissue regeneration, inhibit apoptosis/fibrosis and clear alveolar fluid [10]. MSCs have also been shown to aggregate within the lung microvasculature when intravenously (IV) administered, affecting the local environment in an efficient manner [11]. MSCs are able to be so affective by inhibiting the function, recruitment and activation of MOs, dendritic cells (DCs), T-cells and B-cells, subsequently reducing proinflammatory cytokines such as interleukin-6 (IL-6) and TNF-α among others [12]. MSCs have also been shown to differentiate into a multitude of tissues, and secrete cytokine (CKs), growth factors (GFs) and extracellular vesicles (EVs), all of which play an integral part in their mechanism of action [13, 14]. MSCs can be derived from various types of tissues from both allogenic and autogenic sources. These tissues include: adipose, bone marrow, placenta, amniotic fluid, umbilical cord, and umbilical cord-derived Wharton jelly [15–18].

Extracellular vesicles (EVs) are composed of hypoimmunogenic properties that resemble amphipathic structures such as the lipid bilayer that allow the vesicles to migrate rapidly as well as harmlessly towards the target organs, without the occurrence of blood flow coagulations [19]. EVs can be obtained from any MSC source and act in a paracrine manner delivering enclosed biological molecules such as DNA, RNA, proteins, and lipids [20]. These EVs include microvesicles (MVs) and exosomes and provide microenvironment that further decreases inflammation, promotes tissue regeneration, and overall enhance the effects of MSCs [21].

In the face of the COVID-19 pandemic, scientists rush to generate and successfully distribute viable therapeutics and vaccines. Due to the urgent need and limitations with the current options, MSCs and their EVs may be a viable option. The cooperative mechanism of actions of MSCs and their EVs that include their ability to be directly antiviral, immunomodulate, induce tissue regeneration, inhibit apoptosis/fibrosis and clear alveolar fluid as well as sequester into the lung microvasculature make them an exciting alternative therapy.

#### **2. Current treatments and therapeutic status**

The scientific and medical community have been quick to adapt and have explored a plethora of therapeutic approaches. Treatments originally known for their efficacy against prior viral infections such as corticosteroids, and convalescent plasma (CP) have been repurposed for SARS-CoV-2 [22–24]. Recent novel

treatments and vaccines have emerged such as the monoclonal antibodies casirivimab and imdevimab (REGN-COV2) and the Pfizer and Moderna mRNA vaccines [4]. Unfortunately, these current treatments have limitations and potentially dangerous adverse effects and although vaccinations are an effective preventive measure, they do not treat COVID-19 [4]. Considering these limitations and emergence of new viral strains there is an urgent need for a safe and effective therapeutic option [25].

Corticosteroids have long been used due to their immunomodulation and they have been a therapeutic option in many autoimmune diseases and in conditions such as ARDS [26]. Although they have a long history, their use in COVID-19 still remains controversial. Data from their prior use in viral infections indicated that they were associated with increased mortality, longer hospitalizations and increased tendency for mechanical ventilation [27–29]. In addition, observational studies in patients with SARS and MERS suggested that the use of corticosteroids delayed viral clearance, increased rates of secondary infections and had somewhat severe adverse effects of psychosis, hyperglycemia, and avascular necrosis [27, 30, 31]. Thus, similar adverse effects and outcomes can be expected in pateitns with COVID-19. Passive immunity with convalescent plasma has also been used and has been shown to improve the survival rate of patients with prior viral epidemics [32]. CP is a therapy that utilizes artificial passive immunity from pooled plasma of patients with resolved SARS-CoV-2 infections [32, 33]. Although the science is sound, there are several limitations associated with CP [34]. The efficacy of CP is highly reliant on the time of its administration, as it seems to only be beneficial to patients a week after infection when viremia is at its highest [35]. Additionally, the effect of CP on SARS-CoV-2 is highly dependent on the neutralizing antibody titer which has to be >1:160, seen 12 weeks after onset of disease [34].

CP infusions can also have severe adverse effects such as anaphylaxis, transfusionrelated ALI and cardiac overload. Additionally, there are several limitations to the collection of CP such as age, weight, state of health, and informed consent all of which make CP a limited treatment option to the current pandemic [34].

Recently attention have been geared towards novel treatments such as REGN-COV-2, which is a cocktail of two human antibodies (casirivimab and imdevimab) using both transgenic mice and B cells from recovered COVID-19 patients [36]. REGN-COV-2, although approved by the FDA, has specific criteria that have to be met before a patient can receive it. REGN-COV-2 is authorized for use in mild to moderate COVID-19 in adults, pediatric patients (12 years or older) with a weight of 40 kg and who have had a positive SARS-CoV-2 test with a high risk of progressing to severe COVID-19. Patients are not indicated for treatment if they are hospitalized, require supplemental O2, and/or currently using chronic supplemental O2 due to another underlying condition [37].These criteria are limitations and important obstacles associated with REGN-COV-2.

Currently, the emergence of new vaccines against SARS-CoV-2 have drawn much excitement. There are several vaccine candidates that are subdivided into five general categories: protein subunit, virally vectored, nucleic acid (mRNA), inactivated and live attenuated [23]. The Pfizer and Moderna vaccines utilize nucleic acids (mRNA) and are composed of a lipid particle with nucleosidemodified RNA, encoding for the S protein [38]. These two vaccines have the most data and have been the most widely used [4]. Although the data suggest that these vaccines are 95% effective at preventing SARS-CoV-2 infections they fail to actively treat disease once patients develop symptoms leaving a substantial amount of the population without a safe and efficacious treatment [38, 39].

Considering the limitations and adverse effects associated with current treatments as well as vaccines being only a preventative measure the need to develop a safer and more efficacious therapy is vital. MSCs and their EVs lack severe adverse effects and studies suggest they have high efficacy making them a potential candidate for COVID-19 treatment. MSCs and their EVs immunomodulatory effects and regenerative capabilities make them an exciting new option combating COVID-19 [12].
