**Abstract**

The extracellular environment exhibits a potent effect on cellular growth and development. Exosomes secreted into this milieu carry functional proteins and nucleic acids from the cell of origin to recipient cells, facilitating intercellular communication. This interaction is particularly influential in the tumor microenvironment, transporting oncogenes and oncoproteins within a tumor and to distant sites. The mechanisms by which cells internalize exosomes vary greatly and the factors dictating this process are still unknown. Most cancers show evidence of exosomal transfer of material, but differences in cell type can dictate the effectiveness and extent of the process. Improving therapeutics requires addressing specific cellular functions, illustrating the need to better understand the forces involved in exosome-cell interactions. This review summarizes what is known about the different types of cells that play a role in exosome internalization.

**Keywords:** exosome, endocytosis, receptors, internalization, uptake

### **1. Introduction**

Intercellular communication is essential to homeostasis and is largely dependent on the cellular secretome [1]. An emerging awareness of the role that the extracellular environment plays is evident in the field of secreted vesicles. The vesicular contribution to the tumor microenvironment (TME) has furthered our understanding of the communication between cells and the surrounding stroma [2]. This relationship has also elucidated many potential therapeutic targets and possible transporters of chemotherapeutics [3, 4]. There are multiple extracellular vesicle types, characterized by biogenesis, size, and common protein markers [5, 6]. Of these, exosomes are the smallest, with sizes ranging from 30 to 150 nm [6]. These vesicles have the most complex synthesis, emerging from the endocytic pathway. They arise from intraluminal invaginations into a multivesicular body (MVB) and are released from the cell when the MVB fuses with the plasma membrane. Exosomes consist of intracellular material surrounded by a lipid membrane that reflects the cellular membrane of the host cell [7]. These specific vesicles have demonstrated promise in several fields of research, including rheumatoid arthritis [8, 9] and neurodegenerative disease [10], but primarily in cancer [11, 12]. Tumor-derived exosomes (TEX) contain oncoproteins and oncogenes from the cell of origin and thus are

very influential in intercellular communication. Numerous studies have used these luminal proteins and genes to better understand tumor growth and metastasis, as well as for improving diagnostic, prognostic, and therapeutic methods [13, 14].

While there has been an exponential growth in research focused on exosome biology, clarification on the mechanisms of transport between the cell of origin and the recipient cell is essential to maximizing on exosome potential in treating and diagnosing disease. The methods by which exosomes influence the cells with which they interact are still under review. Some exosomes have been shown to fuse to the recipient cell [15, 16], while others are internalized by specific receptor-ligand interactions [17, 18] or by stimulating an indirect uptake by macropinocytosis [19]. Exosome binding to cells has been seen both as a mechanism of transferring luminal contents [15, 16] and as an initial step in the endocytosis process [17, 20]. The significance of the effects of cell-exosome binding in comparison to internalization is still unknown. Most types of endocytosis have been described in the process of exosome uptake [21], but which factors determine the specific mechanism used, are still unclear. Previous reviews have clearly identified a number of ligands and receptors involved in exosome trafficking [21–23], but little is known about the dependence of uptake mechanism on cell-type. This review presents the current understanding of the endocytosis process utilized by specific cells involved in exosomal internalization.
