Table 4.

Mechanism of action with MSCs and EVs on NK cell.

Cell type	Immune cell	Mechanism	Effect	Reference
MSC	NK cell	regulating indoleamine-2, 3-dioxygenase (IDO) and prostaglandin E2 (PGE2)	inhibit the activity of NK cells	(103, 104)
BM-MSCs	NK cell	inhibit IL-12 and IL-21	Suppression NK cell proliferation but increase IFN-γ and IFN-α production	(105, 106)
TLR4+MSC	NK cell	–	inhibitory effect on NK cells and the receptor NKG2D	(107)
FL-MSC-EXO	NK cell	regulating TGF-β	inhibit the proliferation, activation and cytotoxicity of NK cells	(108)

MSC, Mesenchymal stem cells; FL-MSC, Fetal liver-derived Mesenchymal stem cells; BM-MSC, bone marrow Mesenchymal stem cells; UC-MSC, umbilical cord Mesenchymal stem cells; IDO, Indoleamine 2,3-dioxygenase; PGE2, Prostaglandin E2; TLR4, Toll-like Receptor 4; NK, Natural killer; TLR4⁺MSC, TLR4-positive MSC; IL-12, interleukin 12; IL-21, interleukin 21; NKG2D, type II integral membrane protein; IFN-γ, interferon-γ ; IFN-α, interferon-α.