Exosomes carry a variety of substances, including proteins, lipids, nucleic acids (DNA, mRNAs, and ncRNAs) and metabolites (Fig.
1b) [
18]. There are thousands of exosome cargos, such as proteins, mRNAs, and miRNAs, according to the ExoCarta database (
http://exocarta.org/) [
44]. These cargos are critically involved in regulating diverse biological processes in both physiological and pathological conditions, and can be employed as biomarkers and effective therapeutic targets for human diseases. Over 1600 proteins participating in different biological functions, such as signal transduction, structural dynamics and metabolism modulation, are carried in exosomes [
23]. Multiple exosomal proteins have been found to be associated with membrane transport and fusion, such as RAB GTPases, ESCRTs, heat shock proteins (HSPs) such as Hsp60, Hsp70, and Hsp90 and the accessory factors Alix and VPS4 [
23,
45,
46]. Tetraspanins including CD9, CD63, CD81, CD82, and Tspan8, are the most common membrane proteins in exosomes, and have been found to facilitate the packaging of specific cargos into exosomes [
47]. Notably, several exosomal proteins that are nonspecific molecules are usually regarded as markers for the identification of exosomes, such as CD9, CD63, CD81, HSP70 and Alix [
14,
48]. Lipids are also a key component in exosomes, and play an indispensable role in exosome biogenesis, shape maintenance and homeostasis regulation in recipient cells [
14]. For example, lysobisphosphatidic acid and ceramide are crucial mediators during exosome release from the cytoplasm [
49,
50]. In addition, growing evidence has shown that ceramide participates in the regulation of autophagy, which may influence MVB homeostasis indirectly [
51]. Moreover, the lipid composition of target cells has been found to change after fusing with exosomes, which subsequently affects the homeostasis of exosomes in recipient cells [
14]. Notably, nucleic acids, especially RNAs make up the most significant fraction of functional components in exosomes and significantly facilitate the roles of exosomes as critical modulators of intracellular communication and diverse signalling pathways [
52]. Exosomal RNA packaging is a specific process, as indicated by the preferential accumulation of certain exosomal RNAs that can be transferred to recipient cells to exert their effects [
43]. RNAs in exosomes can modulate the biological behaviours of recipient cells in various ways, and exosomal RNAs in different body fluids can function as diagnostic and prognostic biomarkers in various human diseases [
53]. In addition to proteins, lipids and RNAs, exosomes also carry several types of DNA, including single-stranded DNA (ssDNA), double stranded DNA (dsDNA) and mitochondrial DNA (mtDNA). KRAS and p53 mutations can be identified in genomic DNA from exosomal DNA for pancreatic cancer prediction [
54]. The transfer of exosomal DNA into recipient cells can also endow recipient cells with diverse biological functions. For example, topotecan-treated cancer cell-derived exosomal DNA can trigger the activation of dendritic cells and subsequent CD8+ T-cell activation through the CGAS-STING signalling pathway [
55].