**3. Up-take of EVs and reprogramming of recipient cells**

Once exosomes are released from MVBs into the extracellular space, they are disseminated throughout all tissues, enter the circulation and freely cross the blood brain barrier (BBB) and all tissue barriers [10]. Exosomes are taken up and internalized by recipient cells in the circulation and tissues by a variety of mechanisms ranging from endocytosis, phagocytosis or membrane fusion to receptor-ligand mediated entry and signaling as reviewed elsewhere [11] and deliver their cargos to recipient cells, which may be either near or distantly located. Through this mechanism, "*exosome-releasing cells*" can profoundly alter the phenotype and function of "*exosome-receiving cells*". For example, immune cells found in patients with cancer (tumor-infiltrating as well as circulating cells) become polarized or reprogrammed through interactions with TEX, so that from effector cells, they turn into cells facilitating tumor growth [12]. Further, these reprogrammed immune cells now release exosomes that are equipped with immunosuppressive molecules and effectively suppress tumor antigen-specific immune responses. This process of the immune cell "corruption" or "subversion" by TEX is orchestrated by the tumor which utilizes TEX to initiate a "cascade" of secondary EVs, thereby changing the TME into one promoting tumor growth and suppressing anti-tumor functions of immune cells [13].

Body fluids are a diverse mixture of various EV subsets, and in cancer patients, TEX constitute a substantial and functionally important EV subset that is engaged in immune regulation. Mechanistically, TEX-mediated immune suppression involves activation in recipient immune cells of numerous inhibitory pathways, leading to a loss of anti-tumor functions [9, 13]. The result of TEX-driven reprogramming of immune cells is that not only TEX but also immune cell-derived exosomes in the plasma of cancer patients are enriched in immunosuppressive proteins and upon co-incubation with primary normal immune cells or upon injection into experimental animals, these exosomes mediate immune suppression [14]. Although, TEX carry tumor-associated antigens (TAAs) and thus could be immunogenic, TEX interactions

with reprogrammed antigen-presenting cells (APCs) in the TME do not support antigen processing/presentation which normally culminates in T cell responses [15]. Instead, T cells cross talking with TEX are suppressed or induced to acquire a suppressive phenotype (i.e., develop into Treg or myeloid-derived suppressor cells). Suppressive activities of TEX appear to be the major mechanism underlying negative regulation that prevails in the TME.
