INTRODUCTION
The world has been facing a dreadful situation due to the spread of the Severe Acute Respiratory Syndrome–Coronavirus-2 (SARS-CoV-2) (1). However, neither confirmed effective antiviral medications nor vaccines are available to deal with this emer-gency (2). Many reports have suggested that it is the cytokine storm in COVID-19 that leads to acute respiratory distress syndrome (ARDS) (3). The cytokine storm in COVID-19 refers to the fact that a variety of cytokines are rapidly produced after viral infections (4). In addition, such a cytokine storm induces hypoxia, and direct viral infection can cause cellular damage. Multiorgan damage and injury have been concomitant with COVID-19, and can be observed more in patients with a more severe form of the disease (5).
Stem cells are specialized cells that can renew themselves by means of cell division and can differentiate into multilineage cells. Mesenchymal stem cell (MSCs) have immunomodulatory features and secrete cytokines and immune receptors that regulate the microenvironment in the host tissue (6). In addition, it has been observed that the crucial role of MSCs in therapy has been mediated by exosomes released by the MSCs. These exo-somes have exhibited immunomodulatory, antiviral, anti-fibrotic, and tissue-repair-related functions in vivo; similar effects have been observed in vitro (6).