Mesenchymal stem cells and their derived exosomes for the treatment of COVID-19

Table 4 Comparison of different sources of mesenchymal stem cells for treatment of coronavirus disease 2019

Types	Advantages	Disadvantages
UC-MSCs	Prevent fibrosis and restore the oxygenation index and down-regulated CS in critically ill COVID-19 hospitalized patients; readily available and rapidly expanded to clinically required numbers without raising ethical issues and with minimal allograft rejection[147,148]	More extensive randomized trials and phase III clinical trials of UC-MSCs are still needed to investigate the exact molecular mechanisms of UC-MSCs in treating COVID-19 patients
BM-MSCs	Inhibit CS[149]	Adverse events such as low cryopreservation survival, cell product heterogeneity, immunogenicity, and thrombus generation, which have been observed with BM-MSCs products, as well as the low number of MSCs in bone marrow aspirates and the invasive nature of the process of obtaining MSCs have also prevented the generalization of BM-MSCs[150-152]
PL-MSCs	Higher amounts of CD106 are expressed because surface markers such as CD106 and CD54 are important for immunizing MSCs through cell-to-cell contact[153]	Differences in autologous or allogeneic preparation protocols and ethical concerns about PL-MSCs[154]
ADSCs	Rich tissue sources and tissue collection methods are simple[155]	Some severe side effects have been shown, such as three cases of vision loss after patients with AMD received bilateral intravitreal injections of autologous adipose tissue-derived stem cells at a stem-cell clinic[156]

ADSCs: Adipose-derived mesenchymal stem cells; AMD: Age-related macular degeneration; BM-MSCs: Bone marrow-derived mesenchymal stem cells; PL-MSCs: Placenta-derived mesenchymal stem cells; UC-MSCs: Umbilical cord mesenchymal stem cells; MSC: Mesenchymal stem cell; COVID-19: Coronavirus disease 2019; CS: Cytokine storm.