The recent progress of tumor cell-derived exosomes in the pathogenesis, diagnosis and therapeutic strategies of tumors.

The roles of TEXs in tumor biology. TEXs obtain the characteristics of the tumor cells, which meanwhile transfer the various substances from the tumor cells to regulate the other cells and the microenvironment. TEXs promote tumor progression and metastasis by regulating metabolism, epithelial-mesenchymal transition, angiogenesis, vascular permeability and immunosuppressive TME. Besides, TEXs facilitate chemotherapy resistance and radiotherapy resistance of tumors. In addition, TEXs promote the recurrence of tumors by remodeling the tumor cells and immunosuppressive TME. TEXs also promote the progression of tumors by inhibiting ferroptosis. (Created in BioRender) Rapidly dividing cancer cells demand high levels of energy to sustain their proliferation [ 7 ]. TEXs elevate blood glucose levels by inhibiting insulin secretion, thereby providing adequate glucose for tumor cell proliferation. Exosomal miR-122 from breast cancer cells targets pyruvate kinase M to inhibit insulin secretion, thereby increasing blood glucose concentrations in the body [ 8 ]. Pancreatic cancer (PC)-derived exosomes contain adrenomedullin (AM) and CA19-9, which interact with receptors on β-cells to inhibit insulin secretion [ 9 ]. TEXs also promote the breakdown of adipose cells surrounding tumor cells to provide energy for tumor cell proliferation. TEXs derived from breast cancer cells carry miR-144 and miR-126 to induce beige/brown differentiation and metabolic reprogramming of adipocytes, thus promoting tumor metastasis [ 10 ]. AM in PC-derived exosomes interacts with receptors on adipocytes, which activates p38, extracellular signal-regulated kinase (ERK1/2) and mitogen-activated protein kinases (MAPKs) to promote lipolysis [ 11 ]. Furthermore, TEXs can inhibit glucose uptake by non-tumor cells at the distant site of colonization, thereby supplying energy for tumor cell metastasis. Breast cancer cell-derived exosomes carry miR-199b-5p to suppress the utilization of energy substances of neurons and astrocytes, leading to the extracellular accumulation of glutamate, glutamine and lactate. These substances are then utilized by breast cancer cells to promote their colonization in the brain [ 12 ]. EMT refers to the process in which epithelial cells lose apical-basal polarity and transform into an invasive mesenchymal phenotype. This process enables cells to penetrate the basement membrane. EMT is generally characterized with the downregulation of epithelial markers (E-cadherin and keratin) and the upregulation of mesenchymal markers (N-cadherin and vimentin). This process reduces intercellular adhesion, thereby promoting cells to metastasize. EMT promotes tumor cells to detach from the primary tumor. TEXs promote EMT to facilitate tumor cell migration and invasion. TGF-β1 stimulates cervical cancer to secrete more exosomal miR-663b, which is internalized by adjacent or distant cervical cancer cells. This process inhibits the expression of MGAT3, accelerates EMT and ultimately promotes both local and distant metastasis of cervical cancer [ 13 ]. Exosomes from PC cells can induce EMT in adjacent human PC cells, partly through the action of TGF-β1 [ 14 ]. Under hypoxic conditions, paclitaxel-resistant breast cancer (PR-BC) cells secrete exosomes with overexpressed HSP gp96 and HIF-1α. Exosomes with overexpressed HSP gp96 and hypoxia-inducible factor (HIF-1) are transported to paclitaxel-sensitive breast cancer (PS-BC) cells, thus accelerating the EMT process [ 15 ]. Hypoxic exosomes from nasopharyngeal carcinoma (NPC) cells carry MMP-13 to significantly upregulate the expression of vimentin and reduce the level of E-cadherin in recipient NPC cells, thereby promoting EMT and enhancing their migration and invasion capabilities [ 16 ]. Without angiogenesis, tumor cells can’t grow more than 2 mm in diameter. Blood vessels transport nutrients to tumor cells to power their proliferation. TEXs induce tumor angiogenesis via carrying angiogenic stimulators which include vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF) and transforming growth factor-β (TGF-β) [ 17 ]. Under HOXD3 induction, liver cancer cell-derived exosomes promote CCR6-enhanced metastasis, invasion and angiogenesis of HCCs [ 18 ]. The miR-374b-3p in exosomes from glioblastoma stem cells enhances tumor angiogenesis by inducing M2 polarization of macrophages, thereby promoting the malignant progression of glioblastoma [ 19 ]. The miR-214-3p in gastric cancer cell-derived exosomes migrates to vascular endothelial cells and targets zinc finger protein A20 in vascular endothelial cells. This process negatively regulates ACSL4 which is a key enzyme involved in lipid peroxidation during ferroptosis, thereby inhibiting ferroptosis in vascular endothelial cells and reducing the anti-angiogenic efficiency of apatinib [ 20 ]. Exosomes from prostate cancer cells carry phosphoglycerate mutase 1 (PGAM1) which binds to γ-actin (ACTG1) to stimulate angiogenesis-related gene expression in human umbilical vein endothelial cells, thereby promoting neovascular sprouting [ 21 ]. TEXs can increase vascular permeability, which makes tumor cells cross the endothelial barrier of blood vessels and provides opportunities for tumor cell metastasis. The circ-ZNF609 in exosomes from hypoxic esophageal squamous cell carcinoma (ESCC) cells acts as a sponge for miR-150-5p to activate VEGFA and promote angiogenesis and vascular permeability [ 22 ]. The miR-103 in liver cancer cell-derived exosomes targets endothelial cells to inhibit the expression of ZO-1, thereby increasing vascular permeability [ 23 ]. The miR-30a-5p in exosomes from intrahepatic cholangiocarcinoma (ICCA) increases vascular permeability depending on PDCD10, thus promoting the development of ICCA [ 24 ]. Protease 17 (ADAM17) is highly expressed in exosomes from various types of tumors, especially metastatic tumors. ADAM17 in metastatic CRC-derived exosomes disrupts VE-cadherin-mediated adhesion to vascular endothelial cells, which increases endothelial barrier permeability to enhance tumor cell metastasis. The ADAM17 inhibitors can act as a potential drug to inhibit hematogenous metastasis of CRC [ 25 ]. TEXs allow tumor cells to evade anti-tumor immunity by creating an immunosuppressive TME. TEXs recruit immunosuppressive cells while suppress the functions of immune effector cells (Table  1 ). Table 1 Tumor cell-derived exosomes regulate immune cells Cells of origin Functional molecules Immune cells Function on the recipient cells Refs. Prostate cancer cells PGE2, TGF-β DCs Inhibit the differentiation and maturation of DCs [ 25 ] Pancreatic cancer cells miR-203 DCs Inhibit the differentiation and maturation of DCs [ 27 ] Renal cell carcinoma cells HLA-G DCs Inhibit the differentiation and maturation of DCs [ 26 ] Melanoma cells S100A9 DCs Inhibit the differentiation and maturation of DCs [ 28 ] Glioblastoma multiforme cells LGALS9 DCs Inhibit the function of DCs [ 30 ] Hepatocellular carcinoma cells circUHRF1 NK cells Inhibit the proliferation while promote the apoptosis of NK cells [ 32 ] Cervical squamous cell carcinoma cells miR-142-5p T-cell Inhibit T-cell proliferation while promote T-cell apoptosis [ 38 ] Triple-negative breast cancer cells ICAM1 T-cell Inhibit T-cell proliferation while promote T-cell apoptosis [ 39 ] Melanoma cells hsa-miR-3187-3p, hsa-miR-498 hsa-miR-122 hsa-miR149 hsa-miR-181a/b T-cell Suppress T-cell activation [ 41 ] Hepatocellular carcinoma cells 14–3-3ζ T-cell Induce the conversion of T-cells into regulatory T cells [ 43 ] Hepatocellular carcinoma cells miR-21-5p Macrophages Induce the M2 polarization of macrophages [ 44 ] Esophageal squamous cell carcinoma cells hsa-circ-0048117 Macrophages Induce the M2 polarization of macrophages [ 45 ] Mouse breast cancer cell line 4T1 miR-183-5p Macrophages Regulate the cytokine secretion of macrophages [ 46 ] Pancreatic cancer cells MIF MDSCs Promote the proliferation and activation of MDSCs [ 47 ] Murine gastric cancer cell lines MFCs PD-L1 MDSCs Promote the proliferation and activation of MDSCs [ 50 ] Mouse BC cell line 4T1 miR-200b-3p MDSCs Promote MDSC recruitment [ 52 ] Tumor cell-derived exosomes regulate immune cells hsa-miR-3187-3p, hsa-miR-498 hsa-miR-122 hsa-miR149 hsa-miR-181a/b TEXs can inhibit the differentiation and maturation of DCs. DCs are the important immune cells which activate the anti-tumor immune responses. DCs differentiate from both myeloid and lymphoid progenitors in the bone marrow or derive from monocytic cells. TEXs suppress the differentiation of monocytes and bone marrow progenitor cells into DCs through their biomolecules (COX-2, PGE2, IL-6, TGF-β, HSP70, and HLA-G). Prostate cancer-derived exosomes carry PGE2 and TGF-β to promote the conversion of monocytic cells into myeloid-derived suppressor cells (MDSCs) rather than DCs. MDSCs play a key role in the suppression of anti-tumor immunity. TEXs derived from 4T1 breast cancer cells or Lewis lung cancer not only inhibit the differentiation of myeloid precursor cells into CD11c + DCs but also induce apoptosis of CD11c + DC cells [ 26 ]. Renal cell carcinoma-derived exosomes carry HLA-G to impair the differentiation of monocytes into DCs [ 27 ]. Pancreatic cancer-derived exosomes deliver miR-203 to downregulate TLR4 and downstream cytokines in DCs, thus inhibiting DC maturation [ 28 ]. Melanoma-derived exosomes carry S100A9 to reduce the expression of four costimulatory molecules (CD83/86, IL-12/15) in DCs, thus disrupting DC maturation [ 29 ]. In addition, TEXs also inhibit the function of DCs. DCs are antigen-presenting cells (APCs) which recognize, process and present antigens on the cell surface to T cells. DCs activate anti-tumor specific T cells through major histocompatibility complex (MHC) molecules, costimulatory molecules and cytokines. TEXs from non-small cell lung cancer (NSCLC) can inhibit antigen presentation from DCs to T cells, thus interrupting antitumor immunity of cytotoxic T cells [ 30 ]. Glioblastoma multiforme (GBM) cell-derived exosomes carry LGALS9 to inhibit DCs antigen presentation, thus inhibiting cytotoxic T cell activation in the cerebrospinal fluid [ 31 ]. TEXs inhibit the proliferation while promote the apoptosis of NK cells. Exosomes derived from MDA231 (human breast cancer), A2058 (human melanoma) and 4T1 (mouse breast cancer) cell lines suppress IL-2-induced NK cell proliferation [ 32 ]. Additionally, circUHRF1 in hepatocellular carcinoma (HCC)-derived exosomes downregulates miR-449c-5p while upregulates the expression of TIM-3 in NK cells, thus inducing the apoptosis of NK cells [ 33 ]. TEXs also inhibit NK cell activity. Exosomes from colorectal cancer cells activate the TGF-β signaling pathway in NK cells, which subsequently inhibits the viability and cytotoxicity of NK cells thereby leads to immune evasion [ 34 ]. Similarly, TEXs derived from pancreatic ductal adenocarcinoma (PDAC) induce the phosphorylation of Smad2/3 in NK cells, thereby suppressing their cytotoxicity [ 35 ]. Urinary bladder cancer-derived exosomes inhibit the expression of the important functional receptors (NKG2D, NKp30 and CD226) on NK cells as well as the secretion of perforin and granzyme-B from NK cells, which inhibits their viability and cytotoxicity [ 36 ]. TEXs inhibit T-cell proliferation while promote T-cell apoptosis. TGF-β in TEXs inhibits CD4+ T cell proliferation and Th1 cytokines production [ 37 ]. The immunoinhibitory molecules, CD39 and CD73, are expressed on the surface of TEXs and can convert ATP into adenosine, which significantly suppresses CD4+ T cell proliferation [ 38 ]. Exosomes from cervical squamous cell carcinoma cells deliver miR-142-5p to lymphatic endothelial cells (LECs). The miR-142-5p upregulates IDO expression of LECs through the ARID2-DNMT1-IFN-γ signaling pathway, thus inducing CD8+ T cell apoptosis [ 39 ]. Exosomes from triple-negative breast cancer carry intercellular adhesion molecule 1 (ICAM1) to induce CD8+ T cell apoptosis thereby initiating and promoting the occurrence and progression of bone metastasis in TNBC. ICAM1 expression is higher in strong invasive phenotype cells than in weak invasive phenotype cells [ 40 ]. In addition, TEXs also suppress T-cell activation. Prostate cancer-derived exosomes induce CD73 expression on DCs to inhibit antigen presentation to T-cells by APCs, thereby suppressing T-cell activation [ 41 ]. Melanoma-derived exosomes contain specific miRNAs (hsa-miR-3187-3p, hsa-miR-498, hsa-miR-122, hsa-miR149 and hsa-miR-181a/b). These exosomes decrease T-cell receptor (TCR) signaling and diminish TNFα secretion by T cells, thus inhibiting T-cell responses and cytotoxic activity [ 42 ]. Gastric cancer cell-derived exosomes can be effectively uptaken by Vγ9Vδ2 T cells, suppressing their activation and inducing apoptosis [ 43 ]. Moreover, TEXs induce the conversion of T-cells into regulatory T cells (Treg). Treg cells primarily promote immune tolerance to suppress immune responses while Th cells secrete various cytokines to stimulate immune responses. Exosomes from nasopharyngeal carcinoma inhibit the proliferation of T-lymphocytes. These exosomes also inhibit the differentiation of T cells into Th1 and Th17 cells while promote the formation of Tregs, thus leading to immunosuppression40. Exosomes from HCC cells carry 14-3-3ζ, which can be phagocytosed by T-cells to induce the conversion of T cells into Tregs [ 44 ]. Both M1 macrophages and M2 macrophages play important roles in immune responses. M1 macrophages mainly inhibit while M2 macrophages mainly promote the growth and progression of tumors. TEXs induce the M2 polarization of macrophages to promote tumor progression. Exosomes from small cell lung cancer (SCLC) induce the M2 polarization of macrophage via the NLRP6/NF-κB pathway, which promotes immunosuppression to facilitate SCLC metastasis [ 45 ]. HCC-derived exosomes carry miR-21-5p to regulate macrophage polarization by targeting RhoB, thus leading to M2 polarization of macrophages and HCC progression [ 46 ]. Exosomes from the esophageal squamous cell carcinoma (ESCC) cells in a hypoxic state are riched in hsa-circ-0048117. Exosomal hsa-circ-0048117 promotes M2 polarization of macrophage to enhance the invasion and migration ability of ESCC cells [ 47 ]. In addition, TEXs regulate the cytokine secretion of macrophages. Exosomes from the mouse breast cancer cell line 4T1 deliver miR-183-5p to macrophages, thus promoting the secretion of proinflammatory cytokines by inhibiting PPP2CA expression. Proinflammatory cytokines can remain the proinflammatory TME and promote the growth and metastasis of BC [ 48 ]. MDSCs can induce the expansion of Treg and inhibit the function of immune cells (B cells, NK cells and DC cells) to enhance immunosuppression. TEXs promote the proliferation and activation of MDSCs to create an immunosuppressive TME. Pancreatic cancer cell-derived exosomes carry macrophage migration inhibitory factor (MIF) to induce the differentiation and activation of MDSCs, which promote pancreatic cancer progression. MIF tautomerase inhibitor IPG1576 inhibits MDSC differentiation in vitro and reduces tumor growth [ 49 ]. Exosomes from the murine melanoma cell line B16-F10 promote the differentiation and activation of MDSCs in vitro, which induce immunosuppression [ 50 ]. Exosomes from murine melanoma cell line B16-F10 enhance the proliferation and function of MDSCs via the JAK2 pathway, thus inhibiting T cell proliferation and exhibiting a significant immunomodulatory effect. JAK2 inhibitors can suppress the activation of TEXs and decrease the number of MDSCs [ 51 ]. Exosomes from the murine gastric cancer (GC) cell lines MFCs carry PD-L1 to stimulate MDSC proliferation by triggering the IL-6/STAT3 signaling pathway in vitro, which promotes the progression of GC [ 52 ]. TEXs promote MDSC recruitment to create an immunosuppressive TME. Exosomes from the murine melanoma cell line B16-F10 activate S100A10 (a plasminogen receptor) in lung fibroblasts to induce the expression of CXCL1 and CXCL8 chemokines, thus promoting MDSC recruitment. MDSCs regulate the lung immune microenvironment and promote the metastasis of melanoma to the lung [ 53 ]. Exosomes from the mouse BC cell line 4T1 carry miR-200b-3p to promote CCL2 expression by targeting PTEN. The high expression of CCL2 promotes the metastasis of BC to the lung by recruiting MDSCs in the lung [ 54 ]. Furthermore, TEXs always act as a substitute for tumor cells to receive immune attacks, assisting tumor cells in the evasion of immune recognition. TEXs express tumor-specific antigens. These tumor antigens deplete antibodies that specifically target the tumor cells, thereby reducing the binding and attack of antibodies to tumor target cells. In aggressive B-cell lymphoma, exosomes that carry tumor-specific antigens are secreted into the plasma. These exosomes can release antigens that induce antibody binding. Thus these exosomes function as decoys against complement-mediated cytotoxicity, protecting tumor cells from immune attacks and complement-dependent cell lysis [ 55 ]. Pancreatic ductal adenocarcinoma (PDAC)-derived exosomes bind circulating antibodies in PDAC patients’ sera, thereby inhibiting complement-dependent cytotoxicity and antibody-dependent cell-mediated cytotoxicity toward PDAC tumor cells [ 55 , 56 ]. Meanwhile, TEXs trigger inflammation and infiltrate stromal cells to create a favorable microenvironment for tumor growth and metastasis [ 7 ]. Rectal cancer cell-derived exosomes carry miR-21 to bind to TLR7, thus inducing the polarization of liver macrophages. Inflammatory cytokines (IL-6 and members of the S100A family) secreted by activated macrophages alter the pre-metastatic niches (PMN) to support liver tumor cell metastasis [ 57 ]. Additionally, TEXs can induce immune tolerance of tumor cells, increasing their resistance to immunity. Exosomes from HER2+ breast cancer (BC) cells can bind to trastuzumab and interfere with trastuzumab’s antiproliferative effect on breast cancer cell proliferation. They can also induce tumor cells to express HER2 to enhance immune tolerance and facilitate metastasis [ 58 ]. Currently, many anti-tumor drugs exhibit poor effects. A main reason for that is tumor drug resistance. TEXs play an important role in drug resistance of tumors. TEXs facilitate drug efflux from tumor cells to promote tumor drug resistance. Exosomes from breast cancer cells treated with doxorubicin (DOX) accumulate a high level of DOX and are subsequently released from breast cancer cells, leading to the efflux of DOX [ 59 ]. Drug exporter pumps including P-glycoprotein (MDR1/P-gp), multidrug-resistant protein-1 (MRP-1) and breast cancer resistance protein (BCRP/ABCG2) participate in drug efflux from the tumor cells. TEXs can deliver these pumps to drug-sensitive cells, thus resulting in drug resistance. For example, exosomes from docetaxel-resistant prostate tumor cells deliver MDR-1/P-gp to drug-sensitive prostate tumor cells, which induces docetaxel resistance [ 60 ]. In addition, TEXs also regulate anti-tumor immune responses to promote tumor drug resistance. The lnc-TALC in GBMs-derived exosomes can bind to ENO1 in microglia, which activates the p38 MAPK pathway and induces the release of C5/C5a. Then, C5/C5a regulates the M2 polarization of microglia to promote TMZ resistance of GBM cells [ 61 ]. Moreover, TEXs increase the autophagy activity of tumor cells after chemotherapy to induce drug resistance of tumor cells. Resistant osteosarcoma exosomes deliver miR-331-3p to induce autophagy and confer resistance to sensitive cells. The inhibition of miR-331-3p decreased the drug resistance of osteosarcoma cells [ 62 ]. Radiation exposure can damage the DNA of tumor cells as well as stimulate them to secrete more TEXs. These TEXs transmit radiation resistance among tumor cells through DNA repair mechanisms, ultimately weakening the effectiveness of radiotherapy. For example, HNSCC cells subjected to ionizing radiation (IR) secrete more exosomes. These exosomes not only enhance DNA repair in irradiated cells but also signal to unirradiated HNSCC cells, which reduces apoptosis and endows both irradiated and unirradiated cells with radiation-resistant phenotypes [ 63 , 64 ]. Similarly, Du et al. discovered that IR induces high expression of HMGB1 in exosomes derived from esophageal squamous cell carcinoma, thus activating the PI3K/AKT/FOXO3A signaling pathway to induce IR resistance. Exosomal HMGB1 can enhance DNA repair in recipient cells, thus leading to stronger radiation resistance of tumor cells [ 65 ]. Bladder cancer-derived exosomes activate the PI3K/Akt and MAPK/Erk pathways, which promotes the anti-apoptotic effect of tumor cells [ 66 ]. After exposure to 4 Gy radiation, both MDA-MB-231 (human breast cancer cells) and HeLa (cervical cancer cells) secrete more exosomes, which leads to enhanced activation of the PI3K/Akt and MAPK/Erk pathways for the inhibition of tumor cell apoptosis [ 67 ]. Furthermore, hypoxia is a key factor in radiation resistance. Hypoxia exosomes decrease the tumor cell apoptosis induced by IR while increase the repair of DNA damage. miR-340-5p is highly expressed in hypoxia exosomes. Hypoxia exosomes transfer miR-340-5p to normoxic OSCC cells, thereby inducing radiation resistance [ 68 ]. Tumor recurrence is associated with tumor cell remodeling, immunosuppression, drug resistance and activation of cancer stem cells. The epithelial-mesenchymal transition (EMT) is an important way of the tumor cell remodeling to promote tumor recurrence. TEXs induce EMT of surrounding cells to promote tumor recurrence. For example, exosomes from highly metastatic MHCC97H cells trigger EMT in low-metastatic HCC cells through the MAPK/ERK signaling pathway, thus endowing the latter with invasiveness [ 69 ]. GC-derived exosomal miR-552-5p inhibits NK cell activity to promote EMT in surrounding GC via the PD-1/PD-L1 axis [ 70 ]. On the other hand, TEXs can modulate the immune system to promote tumor recurrence. For example, in patients with recurrent HNSCC, exosomes significantly enhance the production of 5'-AMP and adenosine in CD4+CD39+ Treg, thus leading to immunosuppression and tumor recurrence [ 71 – 73 ]. SERPINE1 is overexpressed in GC cells. SERPINE1 facilitates the transfer of exosomal let-7g-5p to macrophages and then induces M2 polarization, which promotes tumor recurrence [ 74 ]. Oral cancer stem cell-derived exosomes promote M2 macrophage polarization and suppress CD4+ T cell activity by transferring UCA1 and targeting LAMC2, which involves in tumor recurrence [ 75 ]. In addition, TEXs transfer drug resistance, thus promoting tumor recurrence. miR-891-5p, enriched in circulating exosomes from recurrent ovarian cancer patients, can transfer the drug-resistant phenotype to recipient cells and reduce chemotherapy sensitivity [ 76 ]. ABCB4 is highly expressed in glioblastoma stem cells (GSCs), which promotes the resistance of GSCs to TMZ. GSCs-derived exosomes transfer ABCB4 to glioblastoma and then induce TMZ resistance, which promotes tumor recurrence [ 77 ]. Moreover, exosomes from cancer stem cells facilitate tumor progression through angiogenesis and PMN formation, which is also the origin of tumor formation, metastasis and recurrence. For example, Wang et al. discovered exosomal miR-26a derived from glioma stem cells promotes angiogenesis of microvascular endothelial cells in gliomas [ 78 ]. The breast cancer stem cell-derived exosomes carry lnc-PDGFD to induce lung fibroblast activation through Y-box binding protein 1 (YBX1)/NF-κB signaling, thereby promoting the formation of the PMN [ 79 ]. Ferroptosis is a mode of cell death. TEXs confer drug resistance to tumor cells by inhibiting ferroptosis, thus protecting the tumor cells [ 80 – 83 ]. For example, cisplatin induces ferroptosis to inhibit tumor proliferation in NSCLC [ 84 ]. However, NSCLC-derived exosomes carry miR-4443 to suppress ferroptosis, thereby leading to cisplatin resistance [ 84 ]. Conversely, the enhanced ferroptosis can overcome tumor drug resistance. Exosomal miR-214-3p from gastric cancer inhibits ferroptosis in vascular endothelial cells to promote angiogenesis, which induces tumor progression [ 20 ]. Zhang et al. found that gastric cancer-derived exosomes carry lncFERO to inhibit ferroptosis, which suppresses the apoptosis of GCSC. Targeting the exo-lncFERO/hnRNPA1/SCD1 axis combined with chemotherapy may be an effective treatment strategy for gastric cancer [ 85 ]. Therefore, research on ferroptosis provides directions for tumor therapy.

The roles of TEXs in the diagnosis of tumors. The biomarkers carried in TEXs can help in the diagnosis of tumors (A). In addition, TEXs can be used to differentiate the subtypes and stages of tumors due to their ability to reflect the physiological and pathological state of their cells of origin (B). Furthermore, TEXs can be used to predict, monitor and evaluate drug resistance by detecting the relative biomarkers in exosomes (C). What’s more, TEXs can be used to evaluate the prognosis of tumors (D). (Created in BioRender) Tumor cell-derived exosomes serve as biomarkers TEXs can exhibit the genetic or phenotypic alterations of cancer cells. Thus, TEXs can serve as potential biomarkers for the diagnosis of tumors [ 86 ]. The surface proteins of TEXs can serve as tumor markers. Specifically, proteins such as CLDN4, EPCAM, CD151, LGALS3BP, HIST2H2BE and HIST2H2BF on TEXs have been identified as markers for the diagnosis of pancreatic ductal adenocarcinoma [ 86 , 87 ]. In ovarian cancer, CA125, HE4 and C5a on TEXs are expressed higher in malignant cases compared to benign ones. The combination of exosomal CA125, HE4 and C5a shows enhanced sensitivity and specificity for differential diagnosis of ovarian tumors [ 88 ]. RNAs, the content of TEXs, can also serve as tumor markers. For example, studies have found miR-1281 is highly expressed in the exosomes derived from the cell lines of ovarian clear cell carcinoma (the HAC-2 and OVAS cell lines) as well as the cell line of endometrioid carcinoma (the TOV-112D cell line). Additionally, miR-21, miR-29 and miR-30 are overexpressed in exosomes from ovarian clear cell carcinoma, which symbolizes the development and progression of endometriosis-derived tumors [ 89 , 90 ]. hsa-miR-21-5p is up-regulated in BC cells and their exosomes, which can be used as a biomarker for BC diagnosis [ 91 ]. TEXs carry representative cargos of tumors. Genomic and proteomic scanning and analysis of TEXs enable the determination of subtypes and stages of tumors. The exosome marker CD63 and the tumor marker EpCAM protein are expressed and coexist on the surface of most TEXs. A method utilizes two allosteric aptamers targeting CD63 and EpCAM to create orthogonal labeling barcodes, thus facilitating the selection of tumor cell-derived exosomes. The analysis of the miRNAs (miR-222, miR-1290, miR-182, miR-21, miR-221 and miR-10b) in tumor cell-derived exosomes can differentiate prostate cancer (PCa) and benign prostatic hyperplasia, which can also differentiate metastatic and nonmetastatic PCa [ 92 ]. The miR-375 in exosomes from the MCF-7 breast cancer cell line can be used to differentiate the ER-positive (ER+) luminal subtype, HER-2-positive (HER-2+) subtype and triple-negative subtype based on a newly developed electrochemical system [ 93 ]. Breast tumor cell-derived exosomes are enriched in sphingolipids and glycerophospholipids. Based on these relevant lipid species including PE(14:0_16:0) [M−H]−, PE(O‐18:2_22:6) [M−H]−, PE(O‐16:1/16:1) [M−H]−, PC(O‐20:0_22:2) [M+H]+ and PI(18:0_18:1) [M−H]−, we can discriminate tumors between benign tumors and malignant tumors. Advanced tumor node metastasis (TNM) stage is related to elevated levels of circATP8A1 in exosomes from gastric cancer cells [ 94 ]. We can also differentiate breast cancer subtypes based on the lipid composition of TEXs [ 95 ]. In mouse experiments, glycan analysis of CRC-derived exosomes revealed that the lectin UEA-I can serve as a biomarker for SW480 CRC subtype diagnosis and dynamic monitoring of SW480 tumor progression [ 96 ]. Cao et al. designed biomimetic vesicles using breast cancer cell membranes. They can fuse with breast cancer cell-derived exosomes with phenotypic homology, which enables stage-specific monitoring of breast cancer [ 93 ]. In conclusion, TEXs show potential in the application for the determination of tumor subtypes and stages. Protein and RNA profiles of TEXs can predict tumor response to treatments, reflecting drug sensitivity or resistance of tumors [ 97 , 98 ]. Chemotherapy promotes BC cells to secrete chemotherapy-elicited exosomes enriched in miR-378a-3p and miR-378d. These exosomes are absorbed by chemotherapy-surviving BC cells, resulting in drug resistance. Therefore, exosomal miR-378a-3p and miR-378d are potential biomarkers for chemotherapy resistance in breast cancer patients [ 99 ]. Exosomal miR-99a-5p and miR-125b-5p from chemotherapy-resistant calls of diffuse large B-cell lymphoma (DLBCL) are significantly higher than those from sensitive cells. Thus, these exosomal miRNAs can serve as biomarkers for evaluating chemotherapy resistance [ 100 ]. Similarly, high expression of miR-891-5p in exosomes from ovarian cancer cells is associated with platinum resistance and recurrence, making exosomal miR-891-5p a potential biomarker for platinum resistance [ 76 , 101 , 102 ]. Furthermore, overexpression of exosomal miR-6836 from epithelial ovarian cancer cells promotes cisplatin resistance, indicating exosomal miR-6836 potential as a biomarker for cisplatin resistance [ 103 ]. On the contrary, cervical cancer-derived exosomes carry miR-651 to target ATG3 and then inhibit cisplatin resistance. The downregulation of miR-651 expression indicates cisplatin resistance in cervical cancer [ 104 ]. The downregulation of miR-6836-5p expression in exosomes from NSCLC cells via the MSTRG.292666.16-miR-6836-5p-MAPK8IP3 axis promotes osimertinib resistance [ 105 ]. Additionally, NSCLC cell-derived exosomes carry circRNA_102481 to enhance EGFR-TKI resistance via the microRNA-30a-5p/ROR1 axis [ 106 ]. DDX3 regulates the secretion and the miRNA content of exosomes from HCC cells to affect the drug resistance of HCC [ 107 ]. Exosomes from Taxol-resistant nasopharyngeal carcinoma (NPC) can transfer DDX53 to normal NPC cells. DDX53 upregulates the expression of MDR1 in NPC cells, thus promoting paclitaxel resistance of NPC cells [ 108 ]. miR-25-3p is highly expressed in exosomes derived from glioblastoma (GBM) cells treated with TMZ. miR-25-3p promotes GBM cell proliferation as well as TMZ resistance by targeting FBXW7 [ 109 ]. Tang et al. discovered that high expression of HOTAIR in breast cancer-derived exosomes is related to poor response to neoadjuvant chemotherapy and tamoxifen [ 110 ]. CRC-derived exosomes carry miR-208b to promote Treg expansion by targeting PDCD4, leading to tumor growth and resistance to oxaliplatin. Thus, exosomal miR-208b can serve as an oxaliplatin treatment response indicator [ 111 ]. In conclusion, monitoring the molecular expression levels of TEXs helps to assess chemoresistance. HCC-derived exosomes carry miR-26a to effectively inhibit the progression of HCC both in vitro and in vivo. HCC patients with high expression of miR-26a in exosomes have a favorable prognosis. Therefore, miR-26a can be a non-invasive prognostic marker for HCC patients [ 112 ]. Zhou et al. identified differentially expressed genes (DEGs) in HR+/HER2-breast cancer-derived exosomes. Among these, 315 upregulated tumor-derived exosome genes (UTEGs) are used to classify HR+/HER2-breast cancers into two categories, revealing a difference in survival rates between the groups. They developed a predictive model for breast cancer patients based on the expression of four UTEGs (PDPK1, WSB2, PIR, MTHFD2). The high expression of four UTEGs significantly shortens the distant relapse-free survival (DRFS), which is related to poor prognosis. This model can determine tumor subtypes, predict the efficacy of chemotherapy and endocrine therapy as well as provide prognostic insights [ 113 ]. Additionally, the DEG_FTCD in HCC cell-derived exosomes stimulates macrophage polarization towards the M1 phenotype, thereby inhibiting HCC cell proliferation. Low expression of FTCD is associated with a poor prognosis in HCC [ 114 ]. Real-time quantitative PCR results show the different expression of lncRNA in exosomes from normal ovarian epithelial cells and ovarian cancer (OC) cells. Cui et al. established an exosome-related lncRNA signature (ERLS) model for OC. Patients are stratified into high-risk and low-risk groups based on the model. The combination of the ERLS score with PDL1 or CTLA4 expression can predict prognosis. Patients with low expression of PDL1 or low expression of CTLA4 and a high ERLS score have the poorest survival rates, while those with low expression of PDL1 or high expression of CTLA4 and a low ERLS score have a better prognosis [ 115 ]. Thus, the ERLS model can assist in predicting the prognosis. The migratory potential of TEXs is related to metastasis and recurrence, which leads to poor prognosis. LncRNA-MIR193BHG is highly expressed in exosomes from breast cancer cells. Exosomal lncRNA-MIR193BHG is delivered to osteoclasts, thus promoting bone metastasis and recurrence of breast cancer by targeting the miR-489-3p/DNMT3A signaling axis in osteoclasts. Therefore, lncRNA-MIR193BHG is a promising biomarker for metastasis and recurrence of breast cancer [ 116 ]. Perineural invasion (PNI), a mode of metastasis for various malignant tumors, is related to pancreatic cancer recurrence. LncXIST is highly expressed in exosomes derived from pancreatic cancer cells. Exosomal lncXIST is delivered to neural cells and promotes their secretion of GDNF (glial cell line-derived neurotrophic factor). GDNF can promote PNI, thereby inducing metastasis and recurrence of pancreatic cancer. Therefore, lncXIST is a promising biomarker for metastasis and recurrence of pancreatic cancer [ 117 ]. miR-203a-3p is highly expressed in exosomes derived from pancreatic cancer cells to potentiate muscle wasting by inducing muscle ferroptosis, which leads to poor prognosis. Therefore the miR-203a-3p can be a biomarker to evaluate the prognosis of pancreatic cancer [ 118 ]. The TEX/total sEV ratio in plasma is proved to be the most effective biomarker for distinguishing between patients with recurrence and those who remained disease-free. In a phase I clinical trial ( NCT01935921 ), the molecular profiling of sEV subsets is continuously monitored in a small group of patients with recurrent/metastatic HNSCC who are treated with cetuximab, IMRT and ipilimumab. The study reveals that patients with early recurrence have higher total sEV levels during and after immunoradiotherapy as well as a higher TEX/total sEV ratio [ 119 , 120 ]. Additionally, TEXs can be label-free characterized through FTIR spectroscopy to unveil changes in the cellular phenotype. This research unveils a set of effective spectral biomarkers that can monitor tumor metastasis [ 121 ]. TEXs can serve as predictors of targeted/immune-therapy outcomes in cancer patients. A key mechanism involves the detection of specific biomarkers carried by TEXs, which correlate with treatment efficacy. TEXs carry PD-L1 on their surface, which has immunosuppressive properties by inhibiting T-cell activation, thus we can predict clinical outcomes by tracking the secretion levels of exosomal PD-L1. Immune checkpoint inhibitor (ICI) therapies designed to block checkpoint proteins (PD-L1) are an important immunotherapy. The high level of PD-L1-TEXs at initial diagnosis is associated with poor overall survival of osteosarcoma patients [ 122 ]. In the therapy of signet ring cell carcinoma with the ICI plus XELOX (oxaliplatin and capecitabine), the patients expressing low exosomal PD-L1 or lactate in peripheral blood plasma before treatment initiation demonstrated a significantly increased objective response rate (ORR) and prolonged progression-free survival (PFS) compared to that with high exosomal PD-L1 or lactate. Thus, the levels of exosomal PD-L1 and lactate can predict the clinical outcomes of ICI plus XELOX. In addition, the levels of PD-L1 and lactate in exosomes associate with Treg cells and CD8 T-cells, leading to different clinical outcomes of ICI plus XELOX [ 123 ]. Exosomal PD-L1 from NSCLC cells can be predictors for treatment response and survival in NSCLC patients undergoing treatment with ICIs [ 124 ]. MDA-MB-231 cells (breast cancer cells)-derived exosomes highly express PD-L1. The secretion levels of PD-L1-TEXs are positively correlated with tumor cell viability. While the ICI treatment time increases, PD-L1-TEXs decrease. Thus PD-L1-TEXs can be predictors of ICI treatment effects [ 125 ]. Exosomal PD-L1 from NSCLC cells show high sensitivity and specificity for identifying non-responders to ICIs in NSCLC patients [ 126 ]. In addition, exosomes from JeKo-1 mantle cell lymphoma cells carry DOK3, which can predict the long-term efficacy of gefitinib in lung tumors [ 127 ]. Exosomes from HER2+ BC cells carry HER2. Jia et al. constructed a HER2+ exosome capture detection system (HER2-MEDN) to extract and detect HER2-TEXs, which can predict treatment response [ 128 ].

Application of TEXs in tumor therapy. TEXs can serve as excellent delivery carriers to enhance the efficacy of drugs due to their good biocompatibility, low toxicity, low immunogenicity, high safety and favorable stability. In addition, TEXs and the molecules in the exosomes can be utilized as therapeutic targets to develop new low-toxicity inhibitors. What’s more, TEXs can be developed into tumor vaccines to activate anti-tumor immune efficacy and hinder tumor progression. (Created in BioRender) TEXs can promote tumor metastasis and progression. Therefore, preventing tumor metastasis and progression requires targeting TEXs through reduced biogenesis/release, clearance of circulating vesicles, blockade of cellular uptake, and suppression of drug-resistance molecules [ 129 ]. Currently, numerous therapeutic targets have been identified based on these mechanisms. Some exosome inhibitors based on these targets have been applied in clinical settings. Inhibiting the transfer of TEXs carrying drug resistance molecules can reverse chemotherapy resistance. For example, exosomes from CRC with multi-drug resistance (MDR), specifically exosomes with circ_0001610, fuse with chemotherapy-sensitive cells and confer the drug-resistant phenotype to the chemotherapy-sensitive cells. Inhibiting the transfer of TEXs can prevent the spread of MDR, thereby enhancing chemotherapy sensitivity [ 130 ]. Injecting exo-circ_0001610 inhibitors into mice makes tumors more sensitive to oxaliplatin [ 131 ]. Additionally, experimental evidence suggests that combining chemotherapy drugs with exosome inhibitors yields better therapeutic outcomes. For example, hsa-miR-101-3p, which shows an oncogenic nature, is expressed in exosomes from CRC cell lines. The efficacy of CRC is significantly improved when 5-FU is used synergistically with a hsa-miR-101-3p inhibitor compared to 5-FU alone [ 132 ]. Tian et al. analyzed exosomes from 124 early-stage hepatocellular carcinoma patients who underwent surgical treatment. They found that the levels of miR-21 and miR-10b are closely related to tumor size, cell differentiation and recurrence. Exosomal miR-21 and miR-10b derived from HCC cells promote tumor proliferation and metastasis. They developed nanoparticles (PDCM-miR-21 and PDCM-miR-10b) that target exosomal miR-21 and miR-10b. These nanoparticles significantly inhibit tumor growth and reduce the number of metastatic lung nodules [ 133 ]. Lin et al. found that exosomal miR-4454 promotes HCC cell progression by targeting Vps4A and Rab27A. miR-4454 inhibitor significantly inhibits HCC cell proliferation, migration, invasion and angiogenesis while accelerates cycle arrest, apoptosis and reactive oxygen species (ROS) [ 134 ]. The mortality of BC ranks second among female tumors. The treatment of advanced BC is prone to develop doxorubicin resistance, thus leading to a poor prognosis. miR-181b-5p is highly expressed in exosomes from drug-resistant BC cells. Exosomes with miR-181b-5p fuse with recipient cells and confer the drug-resistant phenotype. Experiments in vivo have demonstrated that inhibitors of miR-181b-5p exhibit significant tumor control effects and reverse doxorubicin resistance [ 135 ]. CirmiR-20a-5p in triple-negative breast cancer cell-derived exosomes can induce CD8+ T cell dysfunction, thereby promoting cancer cell growth and immunosuppression. Inhibitors of cirmiR-20a-5p may overcome anti-PD-1 immunotherapy resistance in triple-negative breast cancer [ 136 ]. Tumor cell-derived exosomes as delivery carriers Lung adenocarcinoma MDA-MB-231 cells Anti-tumor drugs generally have poor stability, low solubility and a short half-life [ 137 ]. Exosomes, which obtain the advantages of low toxicity, high biocompatibility, targeted delivery, high production and small particle size, are potential candidates for drug delivery [ 138 , 139 ]. Additionally, exosomes encapsulating anticancer drugs can effectively prevent cellular drug resistance, thereby enhancing drug efficacy. Specially, compared to exosomes derived from other types of cells, TEXs contain proteins and lipids similar to their parental cells and preferentially fuse with parental cells, resulting in higher targeting specificity and drug accumulation in tumor cells [ 112 , 140 ]. TEXs show significant potential in drug carriers, gene-editing tool carriers and the creation of hybrid exosomes. TEXs solve the problem of poor efficacy caused by the low solubility of tumor drugs. For example, the first-line antineoplastic agent DTX exhibits poor efficacy due to low solubility in NSCLC. Wang et al. encapsulated DTX into TEXs to create EXO-DTX. It is shown that EXO-DTX significantly inhibits the proliferation of A549 cancer cells in vitro while promotes cytotoxicity in vivo. Furthermore, EXO-DTX promotes cell apoptosis, induces G2/M phase cell cycle arrest and increases ROS, thus exhibiting anti-cancer effects in vitro. Experimental results in vivo showed that EXO-DTX releases DTX more slowly than free DTX, leading to higher drug efficacy [ 141 ]. Drug delivery via TEXs can also reduce the toxic side effects of drugs on the body. For example, a drug-targeting delivery system based on exosomes from lung adenocarcinoma MDA-MB-231 cells and doxorubicin (DOX) has been shown to reduce DOX-induced cardiotoxicity [ 142 ]. The Exo-SPIONs are developed by combining superparamagnetic iron oxide nanoparticles (SPIONs) with NSCLC cell-derived exosomes, which is a “dual-targeting” anti-cancer drug delivery platform. The Exo-SPIONs encapsulated with DOX are optimal for drug delivery to tumor tissue, which promotes tumor suppression as well as reduces the toxicity of the DOX to normal tissues [ 143 ]. The R-EXO-T/D for glioma therapy is developed from the GL261 cell line-derived exosomes encapsulated with TMZ and dihydrotanshinone (DHT). The R-EXO-T/D addresses TMZ resistance in gliomas and shows good antitumor efficacy both in vitro and in vivo. Therefore, the R-EXO-T/D is a potential drug delivery system for glioma therapy [ 144 ]. The exosomes from glioblastoma deliver bevacizumab to cross the blood–brain barrier, thereby increasing the apoptosis of tumor cells and significantly prolonging the survival time of the model animals [ 145 ]. Exosomes derived from metastatic breast cancer 4T1 cells are used as drug carriers in animal experiments. In this study, TEXs encapsulated with DDL/DOX can prevent the elimination of therapeutic drugs, enhance the therapeutic efficacy of DDL/DOX, reduce liver injury caused by DDL and inhibit lung metastasis of BC. Furthermore, results showed this exosome drug carrier produces similar outcomes in humans [ 146 ]. CRC cell-derived exosomes deliver siRNAs to silence coiled-coil domain-containing protein 80 (CCDC80) which is a protein associated with drug resistance and metastasis. The results showed that these TEXs significantly reverse chemoresistance, improve the chemotherapy effect and reduce CRC liver metastasis [ 147 ]. Human colorectal cancer HCT116 cell-derived exosomes loaded with DOX and the photodynamic therapy agent 5-aminolevulinic acid (ALA) can pre-target PET/CT imaging to monitor drug delivery [ 148 ]. Research on hybrid exosomes is ongoing in recent years. Hybrid exosomes are developed from the fusion of exosomes and liposomes, which deliver drugs in tumor therapy. They obtain the advantages of both liposomes and exosomes, including low toxicity, high targeting specificity, evasion from the clearance of mononuclear phagocytic system (MPS) and a long circulation half-life. Therefore, hybrid exosomes can overcome chemotherapy resistance and enhance drug delivery efficiency [ 149 , 150 ]. Hybrid TEXs enhance the anti-tumor activity of drugs. For example, hybrid TEXs are developed from the fusion of PDAC-derived exosomes with dasatinib-loaded liposomes. The study indicated a rank of tumor cell toxicity: dasatinib hybrid exosomes > dasatinib liposomes > dasatinib alone. Furthermore, hybrid exosomes loaded with dasatinib also exhibit higher drug uptake-rates to parental PDAC cells [ 151 ]. Hybrid TEXs can overcome chemotherapy resistance. Cisplatin resistance is a big challenge for ovarian cancer (OC) therapy. The overexpression of miR497 in OC cell-derived exosomes may overcome OC chemotherapy resistance. The hybrid exosomes miR497/TP-HENPs are generated from the fusion of OC cell-derived exosomes with TP-loaded liposomes, which can co-deliver miR497 and TP. These hybrid TEXs inhibit cisplatin resistance and eliminate tumor cells by blocking the PI3K/AKT/mTOR signaling pathway in OC cells, reducing intracellular glutathione levels, increasing ROS and upregulating the M1 macrophage polarization pathway [ 149 ]. Additionally, hybrid TEXs can reduce the toxic side effects of drugs and tumor metastasis. Compared to free DOX, TEXs fused with long-circulating and pH-sensitive liposomes that are loaded with DOX are less toxic in animal experiments, which also reduces the number of lung metastasis lesions [ 152 ]. Hybrid TEXs also significantly improve drug solubility. For example, TEXs loaded with thalidomide (THD) fuse with liposomes to deliver THD. The hybrid exosomes show improved drug solubility, optimized drug targeting efficiency and inhibition of the Treg proliferation [ 153 ]. TEXs can deliver nucleic acid molecules (siRNA, miRNA, mRNA, DNA and lncRNA) for tumor therapy. VPS28G and RASGRP1 genes are related to the promotion of TMZ resistance via RAS-GDP to RAS-GTP transformation and TMZ efflux. Glioblastoma cell-derived exosomes deliver TMZ and siRNAs which target VPS28G and RASGRP1 genes to reverse TMZ resistance in the mesenchymal-glioblastoma subtype [ 154 ]. miR-34a is a tumor suppressor gene. TEXs from the mouse colon cancer cell line CT-26 in starved condition can deliver miR-34a to CRC cells, which decreases the viability and migration while increases the apoptosis of tumor cells [ 155 ]. Exosomes from the mouse breast cancer cell line EMT6 are developed as the exosome platform, which can co-deliver IL-2 mRNA, the photothermal agent IR806 and antilymphocyte activation gene-3 inhibitory antibody for the treatment of triple-negative breast cancer [ 156 ]. Exosomes from the topotecan-treated breast cancer cells functionally deliver DNA fragments (< 2 kbp) into the cytoplasm of GM-CSF-induced DCs to activate a STING-dependent pathway, thus reinforcing anti-tumor immunity [ 157 ]. Long non-coding RNA maternally expressed gene 3 (lncRNA MEG3) obtains anti-tumor properties. Osteosarcoma cell-derived exosomes deliver lncRNA MEG3 to osteosarcoma cells, which inhibits proliferation and migration while promotes the apoptosis of osteosarcoma cells [ 158 ]. Additionally, TEXs can deliver CRISPR/Cas9 for targeted gene therapy. The CRISPR/Cas9 system is a gene-editing tool. TEXs can bypass tumor defense, which thus can be extremely effective carriers to be applied in CRISPR/Cas9-based tumor therapy. The tumor-intrinsic gene YTHDF1 promotes tumor immune evasion and resistance to immune checkpoint inhibitors. Murine melanoma B16-F10 cell-derived exosomes deliver CRISPR/Cas9 to specifically deplete YTHDF1 in vivo, thus restoring tumor immune surveillance [ 159 ]. Poly(ADP-ribose) polymerase-1 (PARP-1) is one of the targets for tumor therapy. Ovarian cancer cell-derived exosomes deliver CRISPR/Cas9 to inhibit the expression of human locus PARP-1, which induces the apoptosis of ovarian cancer cells as well as enhances the chemical sensitivity and synergistic cytotoxicity of cisplatin [ 160 ]. Tumor cell-derived exosomes as tumor vaccines Currently, tumor vaccines often show poor clinical benefits. One of the main reasons for that is the poor immunogenicity of tumor antigens [ 161 ]. Nevertheless, TEXs carry a variety of tumor antigens and possess strong immunogenicity [ 162 ]. TEXs present tumor antigens from malignant cells to DCs as well as facilitate DCs to mature, ultimately stimulating a tumor-specific CTL response [ 163 , 164 ]. Moreover, TEXs present a more complete and enriched collection of tumor antigens than tumor lysate, thus contributing to the stimulation of a larger number of T cell clones [ 165 ]. TEXs serve as an ideal replacement repository of tumor antigens, which are promising to be developed highly effective and personalized tumor vaccines [ 166 , 167 ]. Breast cancer cell-derived exosomes loaded with the ELANE (an immunogenic cell death inducer) and Hiltonol (TLR3 agonist) can serve as an in situ DC-primed vaccine, which enhances the antigen cross-presentation activity of cDC1s and induce potent CD8+ T cell responses in vitro to increase anti-tumor immunity in breast cancer [ 168 ]. Breast cancer 4T1/Her2 cell-derived exosomes loaded with two potent immune adjuvants such as CpG ODN and p(I:C) can serve as tumor vaccines, which can induce stronger humoral and cellular anti-tumor specific immune responses to regress tumor growth in tumor-bearing mice [ 169 ]. Exosomes from the mouse prostate cancer cell line RM-1 are modified by interferon-γ to formulate a novel exosomal vaccine. These exosomal vaccines promote M1 macrophage differentiation to engulf RM-1 cell-derived exosomes and induce the production of antibodies against RM-1 cell-derived exosomes, thus inhibiting tumor growth [ 170 ]. Exosomes from renal cell carcinoma cells act as tumor vaccines to promote the proliferation and activation of CD8+ T cells, which enhances anti-tumor immunity [ 171 ]. The Lipo@HEV is generated from the fusion of lipids and exosomes from murine melanoma cell line B16-F10. The Lipo@HEV loaded with a natural adjuvant (outer membrane vesicle from Akkermansia muciniphila, Akk-OMV) acts as a tumor vaccine to promote synergistic tumor immunotherapy by inducing memory T cells, promoting DC maturation and activating CTL response [ 172 ]. A hybrid nanovaccine is developed from the fusion of TEXs and DC membrane vesicles, which can elicit a robust T‐cell response for HNSCC therapy [ 173 ]. Exosomes from the mouse breast cancer cell 4T1 or colon cancer cell CT26 encapsulated GM-CSF inside and incorporated Ce6 in the surface. These exosomes can be developed as hydrogel vaccines which can elicit tumor‐specific immune responses to kill cancer cells [ 174 ]. miRNA-124 promotes the polarization of naive CD4+ T cells into T helper 1 (Th1) cells, which also mediates the immunosuppressive state to the immunostimulatory state by regulating cytokine balance. Exosomes from the murine colorectal carcinoma cell line CT-26 are loaded with miR-124-3p, which act as tumor vaccines to inhibit tumor growth and increase median survival time in tumor-bearing mice [ 165 ]. Exosomes from the human hepatoma cell lines HepG2 and 97H carry tumor antigens and overexpress Rab27a. TEXs overexpressing Rab27a are loaded onto DCs to form tumor vaccines, which can promote the differentiation and maturation of DCs [ 175 ]. Additionally, TEXs can be modified for the codelivery of tumor antigen and adjuvant. For example, exosomes from the streptavidin (SAV)-transfected mouse melanoma B16BL6 cells carry SAV, which are combined with biotinylated CpG DNA to form CpG DNA-modified exosomes (CpG-SAV-exo). CpG-SAV-exo acts as a tumor vaccine to exhibit a stronger anti-tumor effect in tumor-bearing mice [ 176 ]. Although TEXs can act as tumor vaccines to promote immune activation, they also possibly induce immunosuppression. Therefore, the safety of TEX vaccines deserves careful consideration as well as confirmation by comprehensive and further research.

TEXs carry various cargos including proteins, RNAs, DNAs and lipids. TEXs with these cargos are involved in regulating tumor cells and the TME by exchanging cargos among cells, which thus play important roles in the pathogenesis, development and metastasis of tumors. TEXs prevent the apoptosis of tumor cells by inhibiting ferroptosis. TEXs promote the spread of tumor cells by regulating metabolism, angiogenesis, vascular permeability, epithelial-mesenchymal transition and immunosuppressive TME. TEXs also facilitate chemotherapy resistance and radiotherapy resistance of tumors. TEXs have been studied and recognized as potential biomarkers for the diagnosis of tumors as well as the differentiation of the subtypes and stages of tumors due to their abilities to reflect the physiological and pathological state of their cells of origin. TEXs also can be used in the evaluation of the prognosis and chemotherapy/radiotherapy resistance of tumors based on components which are aberrantly expressed in TEXs. For tumor therapy, TEXs and the molecules in the exosomes can be utilized as therapeutic targets to develop new low-toxicity inhibitors. TEXs can serve as excellent delivery carriers due to their good biocompatibility, low toxicity, low immunogenicity, high safety and favorable stability. TEXs can be developed into tumor vaccines to activate anti-tumor immune efficacy and hinder tumor progression. In recent years, tumor diagnosis and therapy based on TEXs are promising due to their specific advantages. Therapeutic TEXs can maintain anti-tumor efficacy even after undergoing frozen storage for 5 months. TEXs can cross the blood-brain barrier to treat brain diseases. TEXs obtain low immunogenicity, good tolerability and favorable safety. TEXs also can prolongate the half-life of drugs in the body. However, some important questions regarding the application of TEXs need to be addressed. The isolation and purification of TEXs are complex and time-consuming, which leads to high production costs of TEXs and limits their clinical application as biomarkers for tumor diagnosis as well as drug carriers and vaccines for tumor therapy. The specificity and efficiency of exosome delivery remain to be further improved. The safety of clinical trials based on therapeutic TEXs is necessary to evaluate rigorously. Moreover, most current research on TEXs focuses on cellular and animal studies, which remain in the preclinical stage. The clinical translation trials are awaiting further exploration to confirm the efficacy and safety of TEXs-based strategies.

Exosomes are a class of extracellular vesicles originating from cells with a diameter of 30–150 nm, which are the smallest extracellular vesicles [ 1 ]. After the formation of the endosomal system, then the formation of intraluminal vesicles (ILVs) and finally the fusion of multivesicular bodies (MVBs) with the plasma membrane, the exosomes are released from the cells to the extracellular environment [ 2 , 3 ]. Exosomes mainly contain proteins, lipids, DNAs and RNAs. The types of RNAs carried in exosomes include microRNAs, message RNAs, long non-coding RNAs, and circular RNAs. The microRNAs can be transported to the recipient cells and regulate the expression of the target genes. The message RNAs are transcribed into proteins, thus involving in protein synthesis. The lncRNAs can exert regulatory effects on the gene expression of the recipient cells. The circRNAs can act as miRNA sponges to adsorb miRNAs thereby relieving their inhibition on target genes. Most of the RNAs carried in exosomes are microRNAs (65%–90%). Meanwhile, RNAs carried in exosomes also include message RNAs (5%–15%), long non-coding RNAs (1%–10%), and very little circular RNAs [ 4 ]. Exosomal microRNAs play a very important role in regulating the pathophysiological process of cells. Exosomes with these cargos are involved in the communication between tumor cells and the tumor microenvironment (TME), which thus participate in the pathogenesis and development of tumors [ 5 ]. Some of the proteins, RNAs and lipid molecules in TEXs are tumor-specific, which can be used as biomarkers for disease diagnosis, monitoring and prognosis. In addition, TEXs derive from the tumor cells of the patients themselves, so they obtain good biocompatibility and meanwhile may carry tumor antigens. Therefore, they can be used as carriers for drug delivery as well as tumor vaccines for the therapy of tumors. Moreover, TEXs can deliver genetic material such as RNAs, DNAs and gene editing tools, which realize gene therapy through gene editing [ 6 ]. The application and research on TEXs are promising and ongoing currently, which will be systematically reviewed in the following parts.