Mahika Rawat, Chiao Ng, Emaad Khan, Rayyan A. Shah, Suha Ashfaq, Ghaith A. Bahader, Zahoor A. Shah

Abstract Coronavirus disease 2019 (COVID-19) has affected a broad demographics, eliciting a more significant effect on specific groups such as males, African Americans, and Hispanic minorities. Treatment of COVID-19 often requires antiviral drugs or monoclonal antibodies. However, immunotherapies such as mesenchymal stem cells and mesenchymal stem cells-derived exosomal vesicles should be evaluated as treatment options for COVID-19. Mesenchymal stem cell therapy offers regenerative, anti-inflammatory, and immunomodulatory properties that can speed up the recovery from COVID-19. Mesenchymal stem cell therapy can also benefit COVID-19 patients who suffer from strokes, as COVID-19 increases the risk of strokes due to increased cytokines and clotting factors. Most stroke cases that occur in COVID-19 patients are ischemic strokes. Therefore, with the help of mesenchymal stem cell therapy and mesenchymal stem cells-derived exosomes, COVID-19-induced stroke patients might benefit from dual-ended treatment. The objective of this review was to discuss COVID-19 and stroke incidence and the available treatment options.

Key Words: COVID-19; cytokines; immunotherapy; interleukin-7; mesenchymal stem cells; neuroinflammation; sex differences; stroke

Introduction

Coronavirus disease 2019 (COVID-19) results in a pandemic mutating as it spread across continents, killing millions of people worldwide (Vilar and Isom, 2021). Along with respiratory symptoms such as coughing, shortness of breath, and wheezing, the virus also causes long-term damage to other organs. For example, emerging studies have revealed damage to the heart muscles even months after individuals recover from COVID-19, potentially increasing the likelihood of heart complications (Puntmann et al., 2020). Furthermore, the virus also impacts the brain, as revealed by clinical studies and observations (Misra et al., 2021). One intriguing effect of COVID-19 is its ability to increase the incidence of strokes (Puntmann et al., 2020; Elkhider et al., 2020).

In a meta-analysis of 67,845 COVID-19 patients, 1.3% experienced a stroke, with 1.1% ischemic and 0.2% hemorrhagic type. COVID-19 causes a predisposition to thrombotic embolism, as per the data collected from 28 sites in 16 countries; researchers observed that ischemic stroke associated with COVID-19 had higher mortality and severe outcomes than non-COVID-19 ischemic strokes. The likely reasons are viral infections accelerating thrombosis through immune-mediated platelet activation, infection-induced cardiac arrhythmias, and dehydration (Ntaios et al., 2020). A meta-summary of five observational cohort studies revealed that the incidence of acute ischemic stroke was between 0.9% and 2.7% for COVID-19 patients (Tan et al., 2020).

Another study suggested that the depletion of angiotensin-converting enzyme 2 (ACE2) leads to organ damage and stroke in COVID-19 patients. COVID-19 binds to ACE2, a dipeptidyl carboxypeptidase, reducing its concentration in the renin-angiotensin system. ACE2 neutralizes angiotensin II, an enzyme if increased, can produce pro-inflammatory, procoagulant effects and elevate blood pressure, ultimately advancing to organ damage and also increasing the risk of stroke. Similarly, venous and arterial thromboembolism caused by a surge in cytokine levels led-increase in tissue factors is known to activate coagulation and generate thrombin (Hess et al., 2020; Ni et al., 2020; Pathangey et al., 2021). Therefore, in this review, we discussed COVID-19 and stroke incidence, sex differences in COVID-19, COVID-19 in children, and the available immunotherpies as treatment options.

Search Strategy

An electronic search of the PubMed database for literature describing COVID-19 and stroke from 2019 to 2022 was performed using the following conditions: COVID-19 (MeSH Terms) AND Mesenchymal Stem Cell Therapy OR Stroke. The results were further screened by title and abstract to present humans, rats, and mice. In addition, an electronic search of the PubMed database for sex differences and children was completed. This included publications 2019 onwards, with the following search criteria: sex differences in COVID-19, stroke and COVID, COVID-19 and children. Subsequent searches were completed that were specifically relevant to immunotherapies and narrowed down to mesenchymal stem cells (MSCs) (Figure 1).

Figure 1｜Schematic overview of the proposed mechanism of action of MSCs in treating COVID-19 and related strokes.

Sex Differences in COVID-19

Studies have found males at higher risk of suffering from the severest form of COVID-19 and dying from it than females (Cai et al. 2020; Pradhan and Olsson, 2020; Danielsen et al., 2022). The female innate immune system allows for better survival against COVID-19 but worsens the disease progression of COVID-19 in males. So, males having a noticeably weaker immune system than females is the likely reason that males have higher COVID-19 mortality rates, particularly in aged males (over 60 years of age) (Ciarambino et al., 2021). A female robust immune system is rooted in the sex-linked chromosome. The sex chromosome XX in females increases stress endurance to diseases and is more potent than XY chromosomes in males. The XX chromosomes and estrogen in females also boost their immune system’s stress endurance, whereas testosterone lowers male stress endurance to diseases. In males, higher cytokines are found in the plasma, compared to females, with higher activation of T cells (Takahashi et al., 2020). This may contribute to the speed of the immune system’s reactivity and specificity to COVID-19, as innate immunity occurs faster than adaptive immunity, in which T cells are activated. Adaptive immunity, however, offers a more tailored response to the virus. The lowered T cell response in men and aged individuals may contribute to the progression of COVID-19, suggesting sex and age differences in responding to COVID-19 (Samson et al., 2022). T cell activation is essential for viral clearance as it releases interferon-gamma that induces the production of other cytokines (tumor necrosis factor-α, interleukin-2, and type I interferons) to control virus progression or clearance from the host (Rosendahl Huber et al., 2014). Thus, immunotherapy may help target improved T cell activation in males to slow disease progression and lower mortality.

Children and COVID-19

Initial studies showed that children with COVID-19 generally have minor symptoms than adults. But recent evidence suggests that children with COVID-19 have a surprisingly higher rate of neurological symptoms (Pierce et al., 2022). A study conducted at the Medical University of South Carolina discovered that 22% of children with acute COVID-19 suffered from temporary symptoms of seizures, altered awareness, crawling, and difficulty walking. Some had more severe and long-term symptoms, despite most being temporary. Forty-three patients developed life-threatening neurological disorders out of 1695 patients nationwide. Some conditions included brain damage or stroke, with about 11 children deaths (Son et al., 2021).

Immunotherapy for COVID-19

Although no specific treatment is approved for COVID-19, efforts are made to subdue symptoms through supportive therapies, including ventilator support, oxygen therapy, and antibiotics for secondary bacterial infections. Another form of supportive treatment is immunotherapy. Type II pneumocytes of the lung and the gut enterocytes are the primary host cells for COVID-19 (Lamers et al., 2020). The virus enters these cells through ACE-2 receptors. Although the immune system’s role is to defend the body against COVID-19, many factors inhibit or confound a proper antiviral immune response. Especially in immunocompromised patients, the immune system’s delay in producing a sufficient antiviral reaction leads to the virus rapidly infecting the lower respiratory tract and causing pneumonia, severe inflammation, and acute respiratory distress syndrome. Another mechanism by which COVID-19 dodges a sufficient antiviral response is its ability to elude identification by the immune system through disturbance in RNA-sensing and type I interferonproducing pathways (Esmaeilzadeh and Elahi, 2021). Thus, immunotherapies may be significantly beneficial and applicable to COVID-19 patients

MSCs therapy, an immunotherapeutic that has gained attention recently, could be the next step in COVID-19 treatment. As mentioned previously, the immune system of a patient infected with COVID-19 releases inflammatory factors such as cytokines, producing a “cytokine storm”. MSCs therapy aims to inhibit the cytokine storm and utilize stem cells to restore healthy cells. The multipotent MSCs can reach the lungs and protect the alveolar epithelial cells that line the alveolar compartment, preventing pulmonary fibrosis (Samarelli et al., 2021). MSCs therapy suppresses lung inflammation and treats pneumonia and lung dysfunction caused by COVID-19 (Esquivel et al., 2021). MSCs therapy is advantageous over other forms of immunotherapy because the MSCs can proliferate rapidly within the lungs. In addition, MSCs are easily extracted from different tissues, such as bone marrow and dental pulp, through a minimally invasive procedure and do not have ethical issues due to their safety profile (Shimabukuro-Vornhagen et al., 2018).

MSCs therapy has been tested in preclinical and clinical studies regarding cerebrovascular diseases like stroke, and the results are quite encouraging. MSCs effectiveness is due to their immunoregulatory function and high regenerative capacity. Neurotrophic factors released by MSCs are shown to recover spinal cord injuries and help regain neuromotor activity. In an experimental mouse model of stroke, MSCs therapy resulted in an enhanced peripheral immune response (Paris et al., 2021). Another study reported that after MSCs therapy, rats displayed higher levels of neurovascular recovery and plasticity compared to control or non-treated rats (Zhang et al., 2015). Besides, MSCs secrete extracellular vesicles that carry important biomolecules required to maintain homeostasis near the injury sites (Zhang et al., 2015) and can be used as a therapeutic agent. These MSC-derived exosomes are formed during the inward pudding of late-stage endosomes, creating multi-vesicular bodies, which are then released as exosomes (Yu et al., 2014). Exosomes illicit their effect by releasing their contents (nucleic acids, cytokines, enzymes, and other proteins) into the recipient cell cytosol. The group further demonstrated that administrating MSC-derived exosomes in rats with traumatic brain injury resulted in better functional recovery, an increase in angiogenesis and neurogenesis, and a decline in neuroinflammation. The benefits were seen after stroke in rats as well. The study explains how the results of using MSCs and the MSC-derived exosomes are similar in animal models of stroke and traumatic brain injury. Essentially, the exosomes produced identical effects as their parent cells, illustrating their cell origin-dependent manner. Exosomes have a crucial role in cell communication due to their ability to partake in small functional molecule delivery and are better suited than MSCs to be stored and delivered (Zhang et al., 2015). Thus, the vesicles secreted by MSCs may work as therapeutics for stroke. This cell-free therapy may produce better results and have inherently low self-immunogenic side effects than standalone MSCs therapy.

The limitations of MSCs therapy are concerns about immunogenicity, tumor formations, and integration in the target area. Furthermore, the efficacy of such treatments depends heavily on comorbidities, dietary habits, and the lifestyle of the person being treated. Therefore, epigenetics must be considered when deciding on MSCs therapies for COVID-19-related strokes.

Other Immunotherapies for COVID-19

The other immunotherapies that can be utilized to treat COVID-19 are hostdirected immunotherapies which can reduce inflammation and inflammationassociated lung damage, and cytokine release syndrome and prevent ICU hospitalization (Moore and June, 2020). It is observed that COVID-19 causes an increase in interleukin-6, causing lung tissue damage (McGonagle et al., 2020). Patients with cytokine release syndrome that receive T-cellengaging immunotherapies show elevated serum levels of interleukin-6. Immunotherapies such as the utilization of the chimeric monoclonal antibody, siltuximab (used to target interleukin-6), and the recombinant humanized monoclonal antibody, tocilizumab (used to target interleukin-6 receptor), led to rapid diagnosis of cytokine release syndromes in those patients. A study involving 21 severely ill COVID-19 patients showed that tocilizumab quickly reduced their fever and inflammation and normalized peripheral blood T cell counts in most patients (Bonam et al., 2020).

Additionally, a recently published randomized trial including 129 patients by French investigators proposed that tocilizumab improves the outcome of patients with COVID pneumonia. Immunomodulators, such as intravenous immunoglobulin, have extensive anti-inflammatory effects (Bonam et al., 2020). Obtained from the pooled plasma of several thousand healthy donors, intravenous immunoglobulin therapy is standard immunotherapy for many autoimmune and inflammatory diseases. Furthermore, a recent open-label trial involving three patients reported the benefits of intravenous immunoglobulin therapy in severe COVID-19-induced pneumonia, providing another option for managing COVID-19 patients. A meta-analysis reported a significantly reduced mortality rate in COVID-19-infected patients following infusion of convalescent plasma. As a result, the patients reported a decline in their virus load.

Furthermore, the pluripotent cytokine interleukin-7 has been effective in multiple other viral infections (Francois et al., 2018). Interleukin-7 raises lymphocyte counts in septic patients with low absolute lymphocyte counts and restores protective immunity in human polyomavirus 2 (JC) virus-induced progressive multifocal leukoencephalopathy. Its effectiveness and the use of other immune stimulants have only begun to be explored in sepsis conditions and should also be considered in COVID-19 infections. Although immune stimulants such as interleukin-7 or nivolumab could theoretically feed the cytokine storm, both are given to patients with sepsis and interleukin-6 concentrations like that of COVID-19 patients without exacerbating inflammatory responses (Remy et al., 2020). Like sepsis, antimicrobials (antivirals in this case) and supportive therapies are likely to remain the bedrock of therapeutic interventions for COVID-19 infection. Nevertheless, the pathophysiology and mechanisms of COVID-19 are still under investigation; besides vaccinations, various other therapeutic approaches are being tested in clinical trials.

Conclusion and Future Perspectives

The COVID-19 pandemic has affected millions worldwide and continues mutating into strains that pose a more significant threat. As a result, scientists and clinicians have been striving hard to find therapies to combat the COVID-19 symptoms and long-term effects on brain physiology. MSC-derived exosome therapy holds promise as it can treat COVID-19 symptoms and has the neuromodulatory capacity to alleviate various neurological complications by enhancing immune response and reducing immunosuppression. In addition, there is a minimal self-immunogenicity factor as compared to standalone MSCs therapy. Therefore, clinical studies are needed to apply these therapies to COVID-19 patients as promising therapies.

Author contributions:MR wrote the first draft. EK, RAS, SA contributed to different topics of the review. CN overviewed the article preparation and contributed to writing and summarizing the article. GAB prepared the figure, and ZAS reviewed and finalized the final draft of the review. MR, EK, RAS, and SA are high school students. All authors approved the final version of the manuscript.

Conflicts of interest:The authors declare that there is no conflict of interest regarding the publication of this manuscript.

Data availability statement:The data are available from the corresponding author on reasonable request.

Open access statement:This is an open access journal, and articles are distributed under the terms of the Creative Commons AttributionNonCommercial-ShareAlike 4.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as appropriate credit is given and the new creations are licensed under the identical terms.

Open peer reviewer:Aurel Popa-Wagner, University Medicine Rostock, Germany.

Additional file:Open peer review report 1.