Glioma is a heterogeneous, complex brain tumor often with a poor prognosis, originating in the central nervous system. The clinical management of malignant gliomas remains a major challenge owing to the highly infiltrative growth and chemoresistance of the tumor and the presence of the blood-brain barrier (BBB) (79). The ability of exosomes to cross the BBB and be easily produced in almost all types of human biological fluids make them potential biomarkers for gliomas. Exosomal non-coding RNAs (ncRNAs) in gliomas include microRNAs, lncRNAs, and circRNAs, which can be selectively encapsulated, released, and delivered in exosomal cells to regulate many features of gliomas, such as multiplication, infiltration, vascular growth, immune evasion, and therapeutic tolerance (80). Myeloid-derived suppressor cells (MDSCs) play a key role in mediating the formation of an immunosuppressive environment and assisting tumors to evade host immune responses. However, Guo et al. (43) reported that glioma-derived exosomes (GEXs) can stimulate the differentiation of functional MDSCs by transferring miR-29a and miR-92a to MDSCs both in vitro and in vivo, and that hypoxia-induced GEXs have a stronger ability to induce MDSCs than normoxia-induced GEXs. However, the question of whether miR-29a and miR-92 can be essential biomarkers useful in the diagnosis and prognosis of glioma remains unanswered. Xu et al. found that the removal of the oncogene miR-375 from glioma cells by exosomes leads to the continuous activation of the CTGF-EGFR oncogenic pathway, which promoted the multiplication, metastasis, and aggression of glioma (44), indicating that exosomal miR-375 may be a promising glioma biomarker. Furthermore, taking advantage of the inherent inflammatory chemotaxis of neutrophils and their excellent BBB crossing ability, Wang et al. (81) proposed a neutrophil-exosome (NEs-Exos) drug delivery system containing adriamycin (DOX), which could rapidly permeate the BBB and metastasis into the brain to target aggressive tumor cells in inflammatory brain tumors, effectively suppressing tumor growth and prolonging survival. PD-L1 on exosomes may be another mechanism by which glioblastoma suppresses antitumor immunity. Ricklefs et al. (82) first identified glioblastoma exosomes containing PD-L1 and blocking TCR-induced T-cell activation. Glioblastoma can upregulate PD-L1 expression upon stimulation with IFNγ, which in turn increases PD-L1 levels on exosomes, thereby inhibiting T-cell activation.

In recent years, there have been several research advances in the application of exosomal immunotherapy in breast cancer. Ham et al. (45) showed that breast cancer-derived exosomal gp130 activates the IL-6/STAT3 pathway in macrophages and causes the expression of protumor cytokines, thus potentially tilting macrophages toward a procancer phenotype, suggesting that breast cancer-derived exosomal proteins may play a key role in cancer progression. Moreover, Nabet et al. (83), found that triggering stromal NOTCH-MYC in breast cancer cells led to an increase in POL3-driven RN7SL1 expression, which resulted in the production of RN7SL1 in stromal exosomes that were not shielded by the RNA-binding protein SRP9/14. Upon metastasis to breast cancer cells, activation of PRR RIG-I by unscreened RN7SL1 could enhance tumor growth, metastasis, and treatment resistance. Recently, it was found that tumor exosome-derived miR-9 and miR-181a promote the formation of IL-6-high-expressing early myeloid-derived suppressor cells in breast cancer by suppressing key regulators, including SOCS3 and PIAS3, in the negative feedback circuit of the JAK/STAT signaling pathway (46). This may provide a potential therapeutic target for breast cancers with high IL-6 expression. Yang et al. (47) also found that chemotherapy activated the EZH2/STAT3 axis in breast cancer cells, which in turn secreted miR-378a-3p- and miR-378d-rich exosomes that were taken up by surviving breast cancer cells and then activated the WNT and NOTCH stem cell pathways by targeting DKK3 and NAMB, ultimately leading to drug resistance (47). Therefore, blocking the secretion of such exosomes during chemotherapy may reduce the occurrence of drug resistance and, thus, maximize the therapeutic effect of breast cancer therapies.

Ludwig et al. (23) showed that plasma exosomes from patients with head and neck cancer (HNC) carry immunosuppressive molecules that interfere with immune cell function and correlate with disease activity in HNC. They also showed that the presence, number, and molecular content of isolated plasma-derived exosomes could be used to distinguish between patients with HNC and patients with active disease (AD) and those with no apparent disease (NED) after tumor treatment, showing that plasma exosomes can be potential biomarkers of HNC progression (23). Ghosh et al. (84) used Nanosight/TEM/immunoblotting and anti-Alix/anti-GRP78/anti-Asgr2 to validate the presence of HBV/HCV-infected low methemoglobin non-hepatocellular carcinoma (non-HCC) exosomes. They found that in exosomes with miR-21-5p and miR-10b-5p/miR-221-3p/miR-223-3p expression was significantly higher than that of non-HCC patients (84); thus, exosome-encapsulated microRNAs could be used as circulating diagnostic markers for low methemoglobin HCC. Yin et al. (48) demonstrated that in pancreatic ductal adenocarcinoma tissue, M2 macrophage-derived exosomal miR-501-3p inhibited the expression of TGFBR3, a tumor suppressor gene, by activating the transforming growth factor-β signaling pathway, thereby promoting tumor development (48), and possibly providing a new target for the molecular therapy of pancreatic ductal adenocarcinoma. Zhou et al. (49) found that miR-1468-5p encapsulated in exosomes secreted from cervical cancer promoted lymph angiogenesis, upregulated PD-L1 expression, impaired T-cell immunity, directly targeted the SOCS1 promoter in HMBOX1, activated the JAK2/STAT3 pathway in tumor-associated lymphatic endothelial cells (LECs), and enabled cancer cells to evade anticancer immunity. This suggests that exosomal miR-1468-5p can induce an intact immunosuppressive tumor microenvironment in LECs and may be a prognostic biomarker and therapeutic target for cervical cancer.

Based on the role of exosomes in disease suppression, Rayamajhi et al. (85) designed hybrid exosomes (HE) by hybridizing exosomes from mouse macrophages with synthetic liposomes. With higher colloidal stability, drug loading, and pH-sensitive drug release, they had different targeting and cytotoxicity capacities to normal and cancer cells and could mimic immune cells to target tumors (85), suggesting that HEs could be a powerful platform for tumor-targeting drug delivery. Additionally, Shi et al. (86) developed genetically engineered exosomes with anti-CD3 and anti-HER2 antibodies to produce SMART- Exos, which showed highly effective and selective induction of tumor-specific immunity expressing HER2 both in vitro and in vivo, thus creating an innovative class of immunotherapy against HER2-positive breast cancer (86). Interestingly, tumor vaccines are emerging immunotherapies, but their clinical application is hampered by difficulties in neoantigen recognition, immune cell activation, and tumor infiltration. Liu et al. (87) developed a tumor vaccine (hEX@BP) that encapsulates black phosphorus quantum dots with exosomes (HE). In comparison with black phosphorus quantum dots alone, hEX@BP showed better long-term photothermal therapy in vivo performance and higher tumor temperature and tumor targeting and promoted the infiltration of T lymphocytes into tumor tissue. In addition, Kim et al. (88) used fusion exosomes to introduce fusion viral antigen (VSV-G) to the surface of xenograft tumor cells, making them more easily recognized and phagocytosed by DCs, thus effectively presenting tumor antigens to T lymphocytes and inhibiting tumor growth (88).

SLE is a common type of systemic autoimmune inflammatory connective tissue disease involving almost every organ, mostly occurring in young women, with significant morbidity and mortality rates. It is characterized by the production of multiple antigen-driven autoantibodies against intracellular and extracellular molecules. Lee et al. (89) showed that circulating exosomes from patients with SLE induced a higher production of IFN-α, TNF-α, IL-1β, and IL-6 than exosomes from healthy individuals and that exosome levels correlated with disease activity in SLE patients, whereas depleted SLE serum and mechanically fragmented SLE exosomes both did not induce the production of any significant cytokines (89). Therefore, circulating exosomes of SLE patients have immunological activity and may be used as a new biomarker of SLE disease activity. It is worth mentioning that the overdose of type I interferon is a biomarker for and main pathogenic mechanism of many autoimmune diseases such as SLE. In addition, Salvi et al. (31) reported that microRNA delivered by SLE exosomes activates human plasmacytoid DCs to secrete IFN-α through TLR7, indicating that microRNA can be used as a potential target for the treatment of SLE. Recently, López et al. (90) found that T cell-derived exosomes (CD3-Ex) are related to CD4+CD28- cells. The activation status of the senescent subgroup is related to the interferon score of patients with SLE. Type I interferon signals may promote the excessive activation of leukocytes and inflammatory mediators in patients with SLE (90). In patients with SLE, T cell-derived exosomes are expected to become a promising therapeutic tool for SLE. In addition, Tan et al. (91) showed that the downregulation of serum exosomal miR-451a expression is associated with disease activity and SLE kidney damage related to the role of exosomes in cell-to-cell communication in SLE (91), which also provides a potential strategy for the treatment of SLE.

Rheumatoid arthritis (RA) is a chronic systemic inflammatory disease of unknown etiology. It is an autoimmune disease that mainly affects joints throughout the body, mostly the small joints of the hands, wrists, and feet, resulting in joint deformities of varying degrees, leading to lifelong disabilities and increased mortality. The levels of amyloid A (AA) and lymphatic vessel endothelial cell hyaluronan receptor-1 (LYVE-1) exosomes differed between the clinical remission (CR group) and non-clinical remission groups (non-CR group) in patients with RA, and both serum/exosomal AA and LYVE-1 levels were significantly and positively correlated with C-reactive protein (CRP) levels in the non-CR group (92). Therefore, exosomal AA and LYVE-1 are expected to be effective biomarkers for assessing disease activity in patients with RA. In addition, exosome-encapsulated microRNAs have been recognized as promising biomarkers of autoimmune diseases. Xu et al. (50) found that exosome-encapsulated miR-6089 from patients with RA inhibited lipopolysaccharide (LPS)-induced cell proliferation and activation of macrophage-like THP-1 cells and regulated IL-6, IL-29, and TNF-α production by targeting the TLR4 signaling pathway. It is evident that exosome-encapsulated miR-6089 can regulate LPS/TLR4-mediated inflammatory responses and potentially act as a novel and promising biomarker for RA. Prospective studies of these biomarkers could help develop new diagnostic tools to assess disease activity and prognosis of RA and other autoimmune diseases. Besides, Wang et al. (93) found that miRNAs in exosomes of patients with RA were more abundant than those in exosomes of healthy individuals and that upregulated miR-17 in patients with RA may inhibit Treg induction by suppressing transforming growth factor β receptor II (TGF-βRII) expression. Thus, altered miRNA expression in exosomes of patients with RA may be involved in the pathogenesis of RA by disrupting the homeostasis of Treg cells. It has also been shown that miR-424 expression of exosomes derived from synovial fibroblasts (SFS-EXO) is increased in patients with RA and can significantly induce T cell differentiation by targeting the transcription factor FoxP3, resulting in an increase in the number of Th17 cells and a decrease in the number of Treg cells (51). Therefore, miR-424 may also be a potential therapeutic target for RA. It is worth acknowledging that glucocorticoids (GCs) have a strong therapeutic effect on RA, but the serious adverse effects after long-term administration limit their clinical application. Recently, Yan et al. (94) established a bionic exosome of dexamethasone sodium phosphate nanoparticles (Exo/Dex), resulting in an active targeted drug delivery system that significantly reduced joint inflammation by inhibiting the expression of proinflammatory cytokines and increasing the expression of anti-inflammatory cytokines, hence providing a promising strategy for improving the therapeutic effects of GCs on RA.

Inflammatory bowel disease (IBD) is a chronic autoimmune disease with intestinal immune dysfunction, primarily including Crohn’s disease (CD) and ulcerative colitis (UC). Eichenberger et al. (24) reported that the interaction of hookworm secreted exosomes with host cells prevented inducible colitis in mice. This suggests that the proteins and miRNAs contained in worm exosomes have great potential applications in the development of drugs for the treatment of worm infections and chronic non-infectious diseases caused by immune system dysregulation, such as inflammatory bowel disease. Additionally, human breast milk is rich in exosomes that may also prevent necrotizing small intestinal colitis (21). Bulek et al. (95) found that epithelial-derived gastrin D-mediated release of IL-1β exosomes was associated with the development of intestinal inflammation. In fact, Lian et al. (96) previously found that “auto-DNA” released by exosomal secretion enters the cytoplasm of innate immune cells and activates AIM2 inflammatory vesicles, leading to the secretion of mature IL-1β and IL-18, intestinal mucositis, and delayed diarrhea. Recent studies have revealed that mast cell-derived exosome-mediated transfer of functional miRNAs from human mast cells-1 (HMCs-1) to intestinal epithelial cells inhibited the expression of tight junction-related proteins (including TJP1, OCLN, CLDN8), thereby significantly increasing intestinal epithelial permeability, disrupting intestinal barrier function, and promoting the development of inflammatory bowel disease (52). These studies provide new insights into the mechanisms of innate immune responses that cause intestinal toxicity and new options for the use of exosomes in the prevention and treatment of disease.

Multiple sclerosis (MS) is the most common autoimmune disease characterized by inflammatory demyelinating lesions in the white matter of the central nervous system, often resulting in severe physical disability in patients. Selmaj et al. (97) used next-generation sequencing (NGS) technology to analyze the overall RNA profile of serum exosomes comparing patients with relapsing-remitting multiple sclerosis (RRMS) and healthy controls and found that in patients with RRMS, four circulating miRNA classes in serum had significantly lower exosome expression during relapse. The possible targets of these miRNAs include STAT3 as well as the cell cycle regulator aryl hydrocarbon receptor, suggesting that intercellular communication is disrupted in MS (97). This suggests that exosomal miRNAs may be useful biomarkers for differentiating MS relapses. Galazka et al. (53) found that exosomes expressed three major myelin basic proteins, proteolipid proteins, and myelin oligodendrocyte glycoprotein (MOG), which promoted the expansion of MOG-T cell receptor transgenic T cells. This demonstrates that serum exosomes maintain the immunogenicity of MOG and may enhance and/or prolong the antimyelin immune response in MS (53), suggesting that exosomes are involved in the development and progression of MS. Furthermore, exosomes from bone marrow mesenchymal stem cells were found to reduce inflammation and demyelination in the rat central nervous system by modulating the polarization of microglia in an experimental autoimmune encephalomyelitis model (54). Consequently, the application of bone marrow MSC-derived exosomes may be a potential therapeutic approach for the treatment of MS or other inflammatory diseases.

Asthma is a complex multifactorial disease influenced by multiple factors and is thought to be a heterogeneous disease characterized by chronic inflammation of the airways involving multiple cells and cellular components. Esser et al. (53) found that exosomes from human macrophages and DCs produce leukotriene biosynthetic enzymes that stimulate granulocyte transfer, suggesting that exosomes can contribute to inflammation by participating in leukotriene biosynthesis and granulocyte recruitment (55). Following this, Paredes et al. (98) found that bronchoalveolar lavage fluid (BALF) exosomes contribute to cytokine and leukotriene production in allergic asthma and may promote subclinical inflammation by increasing cytokine and leukotriene C4 (LTC₄) production in the airway epithelium of patients with asthma. According to findings by Kulshreshtha et al. (56), under the influence of IL-13, epithelial cell-derived exosomes induced enhanced proliferation and chemotaxis of undifferentiated macrophages in the lungs under inflammatory conditions in asthma. However, GW4869 inhibited the production of exosomes, decreased the proliferation rate of monocytes, and alleviated asthma symptoms. Apparently, airway epithelial cell-derived exosomes can induce the generation and development of asthma inflammation. Du et al. (57) found that exosomes derived from bone marrow mesenchymal stem cells in patients with asthma upregulated the expression of IL-10 and transforming growth factor-β-1 (TGF-β1) in PBMCs, thereby promoting the proliferation and immunomodulation of Tregs (57). Simultaneously, it has been found that the overexpression of transforming growth factor-β-2 (TGF-β2) induces enhanced release of cellular exosomes of TGF-β2, resulting in reduced epithelial cell proliferation. In contrast, exosomes from fibroblasts of patients with severe asthma showing lower TGF-β2 levels contribute to epithelial cell proliferation, thereby promoting airway remodeling (58). This suggests that exosomes play a key role in the immune regulation of bone marrow mesenchymal stem cells and asthma airway remodeling by bronchial fibroblasts, which have the potential for the treatment of asthma.

Recent studies suggest a potential pathophysiological mechanism for exosomes in complex regional pain syndrome (CRPS). CRPS is a term describing debilitating pleiotropic pain and dysfunction characterized by chronic inflammation. McDonald et al. (99) showed that miRNAs, altered in the circulation of patients with CRPS, are transported by exosomes. Through their systemic signaling capacity, exosomes can trigger the multifactorial pathological effects of chronic pain. Therapeutic plasma exchange (PE) or plasma replacement is an in vitro procedure used to treat immune disorders. Ramanathan et al. (59) identified 17 miRNAs from PE-induced exosomes of patients with CRPS that were significantly different before and after treatment and found that exosomal miR-338-5p levels were higher in PE responders. Overexpression of miR-338-5p can downregulate IL-6 mRNA and protein levels in vitro, thereby suppressing inflammation (59). In the same year, they also found, from comparative data of primitive immune cell-derived exosomes from patients with CRPS and controls, that while the relative expression of miR-939 was higher in B, T, and NK cell-derived exosomes than in monocyte-derived exosomes in controls, only B cell-derived exosomes showed significantly higher levels of miR-939 in patients with CRPS (100). These studies suggest that differential miRNA sorting of exosomes and their effects on receptor cell function may contribute to the study of the underlying pathophysiological mechanisms of CRPS. Meanwhile, the strategy of studying exosomal miRNAs to stratify patients with CRPS and, thus, maximize the effectiveness of PE for CRPS has some feasibility.

Periodontitis is a chronic inflammation of periodontal supporting tissues (e.g., gingiva, periodontal membrane, alveolar bone, and dental bone) caused mainly by local factors, such as plaque bacterial infection, which often leads to alveolar bone resorption and progressive tooth loosening. The relationship between periodontitis and circulating exosomal miRNAs is still unclear, but it has been found that the expression levels of serum exosomal miR-207, miR-495, and miR-376b-3p in mice with periodontitis were significantly upregulated after 2 weeks with periodontitis (101). This suggests that these serum exosomal miRNAs could be used as biomarkers for periodontitis. Interestingly, Sundaram (102), found that ginger exosome-like nanoparticles could significantly attenuate the pathogenic mechanism of periodontitis by Pseudomonas gingivalis by interacting with heme-binding protein 35 (HBP35) on the surface of Porphyromonas gingivalis and being selectively taken up by it (102). This suggests that edible plant exosome nanoparticles can be used as potential therapeutic agents to treat chronic periodontitis. Nakao et al. (60) found that the pretreatment of gingival tissue-derived mesenchymal stem cells (GMSCs) with TNF-α not only increased the number of exosomes secreted by GMSCs, whose exosome miR-1260b could target the WNT5a-mediated NF-κB ligand-receptor activator (RANKL) expression pathway and inhibit its osteolytic activity, but also the expression of CD73 exosomes. This resulted in the induction of anti-inflammatory M2 macrophage polarization (60), which showed that GMSC-derived exosomes pretreated with TNF-α have a significant ability to regulate inflammation and osteoclastogenesis, potentially introducing a new level of treatment for periodontitis. In addition, Wang et al. (103) showed that cyclic retraction force can induce periodontal membrane cells to secrete exosomes, which inhibit LPS-induced NLRP3 inflammasome signaling in human macrophages by suppressing the NF-κB signaling pathway in macrophages. This helps maintain the dynamic balance of periodontal inflammation. Interestingly, P2X7 receptor (P2X7R) gene modification can reverse inflammation-mediated damage to periodontal membrane stem cells. It has recently been found that P2X7R gene-transduced cells can also positively affect other coexisting cells in in vivo through an exosome-mediated paracrine mechanism, either by transplanting small amounts of P2X7R gene-modified cells or using their exosomes to act as initiators to alter the local inflammatory microenvironment to aggregate stem cells and promote tissue regeneration for the treatment of periodontitis (104). This not only improves the safety and efficacy of stem cell therapy without causing extra side effects but also provides a viable and promising approach to the clinical treatment of periodontitis.