Table 1.

EVs have differences in comparison to their producer cells.

Open issues		Significant differences
Interaction with the immune system	Stability	- More stable in the circulation (39).
	Allogeneic Administration	- Carry immunogenic proteins with questionable ability for transferring the proteins to the recipient cells (29, 177, 180). - The immune rejection rate of EVs is considerably low compared to the cells (181).
	Heterogeneity	- Heterogeneous temporal, spatial, or disease-associated nature. - Have more common features in comparison to the cells.
	Mechanism of Tolerance Induction	- The way EVs communicate with the effector cells of the immune cells is incompletely described (182). - EVs can penetrate deeper into the damaged tissues, so they may have a higher chance of regulating the immune cells (183). - EVs have less cell-dependent and independent mechanisms to exert their immunomodulatory properties (183–187). - EVs immunoregulatory properties leading to reduced immune reactions, increased tolerance, and homeostasis (188, 189). - EVs have been shown to carry, mediate, and regulate cytokine transport from cells to members of the immune system (190). - Allogeneic EVs do not stimulate T cells during in vitro experiments unless in the presence of APCs. - MSC-EVs cannot impact the production of CD3+ T cells. They do not change the ratio of B cells and plasma cells in vitro (182, 191–193). - MSC-EVs inhibit the complement activation in a CD59-mediated manner (194). - EVs are less functional against DCs in comparison to the cells (185, 195). - Allogeneic EVs would activate T cells in vivo only if delivered into an inflammatory microenvironment (196).
Homing capabilities	Systemic Administration	- Proteins and glycoproteins on the surface of EVs and the recipient cells mediate EV-cell interactions. - EVs interact with recipient cells through various targeting mechanisms to transmit selective biological information. - Superficial cytokines on EVs could act as barcodes recognized by the recipient cell cytokine receptors (197). - On the contrary, some studies suggest that EV uptake is not cell-specific (198).
	Uptake Mechanism	- The interactions between EVs and the neighboring or distant acceptor cells occur through diverse mechanisms (115). - It is proposed that MSC-EVs have the same receptors of MSCs on their membrane, and that could be why MSC-EVs could find the injury site, perhaps through a similar mechanism (199). - EV size and surface components affect their recognition and capture by acceptor cells (115). - EVs content could randomly be released into the cytoplasm upon fusing vesicles with the cell membrane. - Acceptor cells may internalize vesicles to intracellular specific molecular targets.
	Communication with the Environment	- Although different from their producer cells, EVs also alter the microenvironment. - It is possible for EVs to acquire soluble proteins from the surrounding extracellular milieu (200–202). - EVs harbor their cargo from enzymatic degradation while trafficking through the extracellular milieu (203). - EVs preserve normal tissues, support tumorigenesis, provide nutrition, and facilitate immune escape (204).
Application	Clinically Relevant Superiorities	- EVs mediate a significant part of the paracrine action of stem cells and most of their functional properties (139). - Unlike cells, there is no concern regarding the possibility of necrosis or abnormal differentiation (181). - They do not show self-replicative and tumor-formation properties (139). - They are not seriously affected by the surrounding inflammatory microenvironment (186). - They reach deeper into injured tissue layers (183). - EVs are highly bio-stable, hence their contents are protected from macrophage-based phagocytosis (205). - EVs have a more straightforward pre-banking capacity and are more stable during freeze-thaw cycles. - EVs can be combined with existing compositions or drug delivery methods (66).