**1. Introduction**

Mesenchymal stem cells (MSCs) are multipotent stem cells that can be isolated from human tissues or organs, such as the bone marrow, adipose tissue, umbilical cord, lung, spleen, liver or kidney [1]. Despite being derived from multiple sources, MSCs display similar biological phenotypes and functions [2,3]. Because of their autocrine and paracrine actions, MSCs have been shown to possess potency in repairing tissue damage [4]. Critically, delivery of only a small population of MSCs can result in accelerated damage repair in the host [5–7]. In addition, exosomes are crucial components that account for the paracrine action of MSCs [8–10]. For example, they exchange genetic material across cells by transferring bioactive molecules [11]. Similar to other cellular exosomes, MSCexosomes are extracellular vesicles with a lipid bilayer structure and an average diameter of 100 nm [1,12]. They carry bioactive molecules, including miRNAs, lncRNAs, lipids and cytokines [1], thus providing a context for researching the biological functions of MSC-exosomes.

Treating diseases with MSC-exosomes has shown promise in the field of regenerative medicine, and numerous studies exploring the therapeutic effects of MSC-exosomes on neurological, immunological and cardiovascular diseases have been published [13]. In summary, the benefits of delivering MSC-exosomes in disease models mainly include the attenuation of inflammation, promotion of angiogenesis and improvement in the survival and proliferation of stem or progenitor cells within injured tissues or organs [14]. In fact, such benefits can be achieved with MSCs as well. Although it has also been shown

**Citation:** Pu, X.; Ma, S.; Gao, Y.; Xu, T.; Chang, P.; Dong, L. Mesenchymal Stem Cell-Derived Exosomes: Biological Function and Their Therapeutic Potential in Radiation Damage. *Cells* **2021**, *10*, 42. https://doi.org/10.3390/cells10010042

Received: 31 October 2020 Accepted: 24 December 2020 Published: 30 December 2020

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that MSCs can exert therapeutic effects on radiation damage, the therapeutic potential of MSC-exosomes has not been widely explored in this field. Nevertheless, in a previous study, irradiated cells exhibited enhanced uptake of exosomes because of an increase in the formation of the integrin and tetraspanin complex CD29/CD81 on the cell surface [15], thus indicating the specific role of exosomes in mediating biological processes in injured cells. Moreover, MSC-exosomes were found to protect against acute or chronic radiation damage via their miRNA cargo, suggesting that irradiated cells might utilize MSC-exosomes to increase their resistance to ionizing irradiation [16–18]. For example, a study showed that exosomal miRNA-210 could elicit efficient DNA damage repair by controlling the transcriptional activity of HIF-1, thus enhancing cellular radio-resistance [17,19]. In this review, we explore the pro-regenerative properties of MSC-exosomes in the field of radiation damage and aim to provide new insight into the management of radiation damage by using MSC-exosomes.
