IMMUNOMODULATORY POTENTIAL OF MSC-EXOS
Exosomes are nanoparticles with a diameter of 40-150 nm. To generate and isolate the exosomes, MSCs can be conditioned to increase the release of exosomes by treatment with cyto-kines or by serum starvation or hypoxia (29). The exosomes are then purified and can be subsequently introduced into the body. MSC-Exos can inhibit CD4+ and CD8+ T cells and NK cells (30). They inhibited T cells expressing IL-17 and induced IL-10-expressing regulatory cells that are involved with suppression of inflammation. MSC-Exos also aid in suppressing the differentiation of CD4+ and CD8+ T cells by releasing mole-cules like TGFβ and prevent inflammation in vivo (31). Similarly, treatment with MSC-Exos reduced the proliferation and activa-tion of NK cells (32). MSC-Exos could shift macrophages from the M1 to the M2 phenotype, further suppressing pro-inflam-matory states (33). Moreover, sepsis is an important lethal factor in COVID-19 patients, and treatments with MSC-Exos have increased the rate of survival in mice with sepsis (34). Concomitantly, MSC-Exos also suppressed release of the pro-inflammatory factors TNF-α, IFN-γ, IL-6, IL-17, and IL-1β (35) and promoted release of anti-inflammatory factors, such as IL-4, IL-10 and TGF-β (36). Additionally, MSC-Exos also reduced the number of chemokines in the serum when injected (37). These immunomodulatory effects of MSC-Exos have also been attributed to their anti-inflammatory cargo, such as IDO, HLA-G, PD-L1 and galectin-1 (38, 39). These mechanisms are illustrated in Fig. 3.