Allergic rhinitis (AR) is a chronic inflammatory disease of the nasal mucosa mediated by immunoglobulin E (IgE) (1). Dendritic cells (DCs), which are a major subtype of antigen-presenting cells (APCs), serve a key role in the immunopathogenesis of AR (1,2). When allergens enter the body, they are presented by DCs to CD4 T cells to trigger allergic inflammatory responses, leading to the activation and maturation of DCs (2,3). Exosomes are secreted during the maturation and differentiation of DCs in AR (4,5). However, almost all cells and not only DCs can produce exosomes (5-8). Exosomes are cell-derived, nm-sized extracellular vesicles (EVs) that are formed through the endocytosis and inward budding of the endosomal membrane mediated by extracellular components and cell surface proteins. They are distributed in almost all bodily fluids and have been previously associated with the occurrence and progression of several diseases (9,10). Exosomes can carry important signaling molecules for intercellular communication and material transfer (11). DC-derived exosomes (Dex) are nano-scale lipid-membrane vesicles formed within DCs by the inward budding of the endosomal membrane after DCs receive immune signals (12,13). The composition and function of DCs and Dex are strikingly similar. Dex, which mimics the biology of donor DCs, can transfer functional major histocompatibility complexes (MHC) to DCs, leading to the activation of CD8 and CD4 T cells (14-16). In addition, Dex carry MHC and T-cell costimulatory molecules to present allergens to induce the production of Th2 cytokines in allergic donors, which are important immunostimulatory factors of anaphylactic immune responses (17,18). Therefore, allergen-carrying Dex may be important targets for AR immunotherapy (17,18). Since the diverse and complex mode of information transfer between Dex and various cells may serve an integral role in the occurrence and progression of AR, Dex engineered to carry anti-allergic drugs may have the potential to interrupt the allergic and immune processes underlying AR on a novel level (5,19).

DCs are a class of bone marrow-derived cells that are typically found in blood, tissues, and lymphoid organs. They primarily initiate immune responses by presenting antigens to naive T cells in lymphoid tissues (20,21). Once activated, DCs increase the expression levels of the MHC peptide complex and costimulatory molecules, allowing them to efficiently activate T cells (22). As the most efficient type of APCs, DCs serve a central role in the immune system. They are typically classified according to their location, function, and cell surface marker profile, namely plasmacytoid DCs (pDCs) and conventional DCs (cDCs; Fig. 1) (20,23-25). DCs can develop from different hematopoietic or myelopoietic progenitors, where to the best of our knowledge, no interconversion from one type to another has been found to date (26).

pDCs are also known as 'lymphoid DCs' and form a subset of DCs with antigen-presenting potential, accounting for <0.3% of all blood mononuclear cells (27,28). Although they share a similar origin with cDCs, pDCs have a different life cycle, since they primarily accumulate in the blood and lymphoid tissues, entering lymph nodes through the blood circulation (29). pDCs can develop in situ in the bone marrow or from common lymphoid progenitors (CLPs; Fig. 1) (24,30). They acquire the functions of APCs after activation, where their expression combination of costimulatory molecules CD40, CD80, and/or CD86 can dictate which specific T cell function is activated (24,31). After recognizing foreign nucleic acids, pDCs will produce large quantities of IFN-I and acquire the ability to present foreign antigens, which serve an important role in antiviral immunity (29,30). In addition, pDCs can directly inhibit allergic immune responses in the airway in addition to indirectly promoting the induction of regulatory T (Treg) cells in mice (32,33).

The majority of cDCs are short-lived hematopoietic cells that are constantly replaced by blood-derived precursors (29). cDCs account for a much larger proportion of DCs compared with pDCs and are typically distributed in most lymphoid tissues and non-lymphoid tissues. With highly efficient antigen-presenting ability, they can capture relevant antigens and present them to T lymphocytes after intracellular processing (29). Serving the role of 'sentinels', cDCs can respond to environmental stimuli and alert the immune system to the presence of foreign antigens, including allergens. cDCs have been reported to be required for the initiation of Th2 immune responses (24,32). Allergens can either signal directly through specific receptors on cDCs or indirectly by inducing cytokine production in surrounding tissues or inflammatory cells, which can then compel cDCs into promoting Th2 responses (32).

DCs in different types of tissues appear to serve different cellular functions, where various stimuli can induce the maturation of specific and distinct DC phenotypes that mediate different functions (34). However, during the resting state, when the DCs are immature (imDCs), they can acquire self-antigens but do not activate T cells. After being stimulated by injury, pathogens, or inflammatory cytokines, imDCs are then transformed into mature DCs (mDCs), which then migrate to secondary lymphoid tissues, where they prime naive T cells into initiating adaptive immune responses (35,36). In particular, only viable, mature, and fully functional DCs migrating into lymph nodes can stimulate T-cell responses (35). The maturation of DCs is accompanied by the enhanced expression of MHC II, costimulatory molecules, and chemokine receptors (37). It is mainly during the maturation of DCs that exosomes are produced.

Dex are nm-sized vesicles formed within the cell by the inward budding of the endosomal membrane (12). There are a variety of proteins in Dex, such as integrin α and β chains (αMβ2), immunoglobulin family member intercellular adhesion molecule (ICAM), and milk fat globule epidermal growth factor 8 (MFG-E8), cytoskeleton proteins and anti-apoptosis-related proteins, which dock their membranes onto those of host cells. CD9, CD63, and CD81 are also components that are frequently found on the Dex surface membrane (83-85). The composition of Dex membranes differs from those of DCs in that they are richer in sphingolipid content but poorer in phosphatidylcholine content, in addition to being deprived of cholesterol (13). The lipid composition of Dex membranes can have an impact on their function (86). HSP70, HSP90, and heat shock cognate protein 73 have also been found in Dex, which can increase the immunogenicity of Dex (87). There is also a variety of different types of RNAs in Dex, which can transfer onto other cells. In particular, Dex has been documented to contain several immunomodulatory molecules with different structures and biochemical properties, depending on the intracellular origin of Dex (88). The composition and features of Dex are shown in Table III.

DCs can secrete different types of exosomes to regulate the adaptive immune response. In addition, exosomes from different sources can modulate the differentiation, maturation, and function of DCs (77). Dex, in addition to the known immunostimulatory capabilities of DCs, has been reported to regulate a variety of immune processes, including antigen presentation, immunomodulation, and signal transduction (12,92). With the ability to activate naive T-cells and facilitate the transfer of MHC complexes between DCs, Dex can be produced in large quantities and efficiently diffuse into tissues, rendering them potentially more potent compared with DCs in activating T lymphocytes and natural killer cells (93-95). The immunomodulatory effect of Dex is closely associated with the maturation status of DCs (96). imDCs and mDCs secrete exosomes with similar morphology, where the potency of exosomes secreted by mDCs is substantially higher compared with that of exosomes from imDCs (97). In general, exosomes from mDCs (mDex) have higher levels of immune-related molecules and superior antigen presentation compared with those from imDCs, which are prone to exosome production but do not effectively stimulate T-cell responses (84,93,94,97).

Dex can transfer MHC I and II complexes to DCs to amplify the immune response (84,98). LPS has been found to promote Dex production from DCs, which contain high concentrations of MHC molecules (86). In addition, Dex has been reported to stimulate T-cells and enhance their activity, thereby potentiating the immune response (86). The shuttling of miRNAs between DCs using Dex can serve as a means of communication and post-transcriptional modification, which may regulate the overall function of DCs (87). Rao et al (99) previously found that Dex can selectively enter DCs, since they have a high affinity for DCs and can act on them to alter the distribution and differentiation of T-cells by encapsulating triptolide. In another previous study, Zhang et al (100) found that Dex contains cargoes secreted by DCs that can activate Treg cells, which leads to the improvement of inflammation. However, it is important to note that Dex from different subsets of DCs may mediate different functions, ultimately leading to different downstream outcomes (14).

DCs have been extensively reported to be involved in the pathogenesis of AR. Therefore, it is highly likely that Dex will play a potentially important role in AR. Dex can be recaptured by DCs and remain on the cell surface, where they can present allergens and induce Th2 cytokine production in allergic donors to elicit allergic immune responses (18,101). Dex share similarities with DCs in promoting allergic immune responses (102). Dex can present antigens directly to T-cells or transport MHC complexes back to the surfaces of DCs for presentation to T-cells after docking onto APCs (103). CD40 on Dex has been found to induce T-cell responses to promote IgE production (104-106). In addition, CD63 and CD81 in Dex have been shown to inhibit FcεRI-induced degranulation by mast cells (MCs), which further affects signaling that normally mediates allergic inflammation (107). Costimulatory molecules, such as CD80 and CD86, on the Dex surface can also contribute to the maturation of DCs and promote Th2-type inflammation, leading to an imbalance in the differentiation of naive T-cells towards to Th2 subtype (108). In addition, CD80 and CD86 can activate allergen-specific Th2 cells to potentiate antigen-specific immune responses (109,110). By contrast, miRNAs in Dex can regulate serum IgE levels and the severity of allergy symptoms through blood, nasal mucosa, and nasal secretions in AR (111). Therefore, miRNA cargoes in Dex can be used to determine the extent of allergic inflammation and immune response (106). Changes in the expression levels of ICAM and MFG-E8 can also regulate the immune response (112). SM can drive allergic inflammation and promote airway hyperresponsiveness by serving as important signaling molecules for mediating inflammatory and immune responses (113-115). This suggests that a wide variety of cargoes carried by Dex can mediate an impact on AR, though different cargoes are likely to exert different effects on AR.

The immunostimulatory or suppressive function of Dex is dependent on the type or maturity stage of DCs that secrete them (116). Therefore, Dex in different states is also likely to have different effects on AR. Exosomes from imDCs (imDex), which primarily reduce T-cell-dependent immune activation, contribute to inhibiting the Th17 response whilst enhancing the population of Treg cells (15,117). By contrast, mDex can directly act on T-cells to exert specific immune responses (118). By functioning as an antigen-presenting molecule, Dex can modulate immunity and inflammation by perpetuating the response of Th2 cells to DCs (119). Dex carries leukotriene synthase, which stimulates granulocyte translocation to promote the recruitment and migration of immune cells to sites of inflammation (77,110,120). Choi et al (121) previously showed that DCs can excrete allergen-bound Dex, which can trigger the degranulation of adjacent MCs, leading to anaphylaxis. Furthermore, DCs have been reported to secrete TNF-α and other proinflammatory cytokines in response to Dex stimulation, leading to increased inflammation (122). In another study, Huang et al (123) activated DCs using thymic stromal lymphopoietin (TSLP), which induced Dex release and in turn promoted the proliferation and differentiation of CD4⁺ T-cells into the Th2 subtype through the OX40 ligand. These studies suggest that Dex can serve a significant role in the pathogenesis of AR through different types of cargo.

It is noteworthy that Dex can not only aggravate allergic reactions but also prevent them. Exosome-mediated transfer of allergens can promote allergic inflammation whereas regulatory and/or tolerogenic exosomes can suppress allergic and hypersensitivity reactions (19). Immunotherapy using Dex typically involves loading antigens directly into Dex or by modifying them (122). DCs can be modified to produce immunosuppressive Dex for the treatment of allergic inflammation (19). Dex can also be modified to carry anti-allergic drugs that can reduce allergic airway inflammation. In addition, lipids and proteins carried by Dex can enhance the permeability of biological membranes, which facilitates the efficiency of the delivery of the anti-allergic drugs they carry (19,124,125). In particular, mDex can promote the activation of T and B cells, leading to Th1-type immune responses and increased IgG titers (101). Since high IgG titers inhibit IgE-mediated effector function, it also suppresses the allergic inflammation that causes AR (101). In addition, imDCs treated with IL-10 and IL-4 have been reported to produce tolerogenic Dex to attenuate Th2 cell responses, thereby inhibiting inflammatory responses, in a mice model of delayed-type hypersensitivity (DTH) (84,126). The inhibitory capacity of IL-10-treated imDCs depends on the presence of CD80 and CD86 (127). Bianco et al (128) previously revealed that Dex overexpressing indoleamine 2,3-dioxygenase has anti-inflammatory effects in a mouse model of DTH, which are dependent on the costimulatory molecule B7. Furthermore, Jia et al (129) showed that DCs-derived forkhead box p3-exosomes inhibited the proliferation of CD4⁺T cells, which in turn reduced the population of Th1 and Th17 cells whilst increasing that of Treg cells without affecting the level of Th2 cells. In another previous study, Yu et al (130) found that Dex modified with IL-2 can upregulate Treg differentiation to suppress allergic inflammation. Kim et al (131) also showed that genetically modified Dex derived from FasL-expressing DCs can exert anti-inflammatory and immunosuppressive effects by suppressing DTH in an antigen-specific and MHC-II-dependent manner, though this process was independent of MHC I. These aforementioned findings suggest that Dex can either be processed or modified to inhibit allergic inflammation and thereby treat AR. Since Dex as a candidate for the treatment of AR is less susceptible to the effects of the surrounding environment (132), these findings could inspire further exploration into novel methods for treating AR with Dex.

Dex, with superior biocompatibility, biodegradability, and safety, can activate various immune cells and hold significant advantages in terms of delivery efficiency (85). Dex is more stable, can be stored for longer, and more immunogenic than DCs, which are highly susceptible to external factors that induce their maturation under pro-inflammatory conditions and promote immune responses, in addition to inducing their tolerance and moderate immune responses in response to IL-10 and TGF-β stimulation (85,133). These advantages of Dex suggest their viability for the treatment of AR. Treating AR with Dex not only eliminates the need for direct contact with natural allergens but is also less prone to triggering IgE/MC reactions. Therefore, they tend to be safer and more effective for the treatment of AR (19). The feasibility and safety of Dex therapy has the potential to be one of the alternatives to conventional treatments of AR (110). It is encouraging that Dex-based therapies are already in clinical trials (101). However, the scope of clinical trials for Dex is limited compared to DC vaccines, where their potential for application has not been fully evaluated (122). The role of exosomal vaccines is dependent on the environment, antigen, and cell type (122), which will require more in-depth research in the future.

From the aforementioned studies, it is likely that Dex is involved in the pathogenesis and can be exploited for the diagnosis and treatment of AR. However, their roles can vary during the different stages of AR. The role of Dex in AR opens another door to understanding the pathogenesis of AR, furthering the potential to design interventions and/or sensitizations to the immunotherapy of AR on a novel level. Although rapid progress has been made in the understanding of Dex (12), the clinical exploitation of Dex remains hindered by a series of problems, such as low efficiency, poor yields, difficulty associated with expression, and low purity (10,122). In addition, research on Dex biomarkers or targeted therapies for AR remains in the early stages and requires further development (77). The complexity of Dex requires thorough understanding. In addition, it remains necessary to carefully monitor the potential adverse events associated with Dex in future trials (12). Therefore, the role of Dex in AR will need to be enhanced further with a specific focus on the problems currently obstructing progress to adequately refine and improve the application of Dex in AR therapy. It is hoped that Dex can be used as an important marker for the diagnosis, treatment, and prognosis of patients with AR in the future, which requires more in-depth research on the isolation and purification, sensitivity, and cost-effectiveness of Dex. In addition, a more thorough exploration into the composition and mechanism of action mediated by the various cargoes contained within Dex needs to be performed.

Is it possible to adjust or change the composition and function of Dex as needed to diagnose or treat AR? It is possible to design Dex to stimulate or inhibit immune responses as needed? Can Dex be a candidate for the treatment of AR? How can Dex be produced for the personalized clinical diagnosis and treatment of AR according to the situation? These questions need to be addressed in future studies. A more thorough understanding of the role of Dex in AR could help prevent AR or develop more effective treatment strategies. This provides a novel insight for exploration into the pathogenesis of AR and offers a new direction for the efficient treatment of AR.

Overall, the application of Dex in the diagnosis and treatment of AR is a promising approach, where novel insights in this field will drive the development of new therapeutic or preventive measures. However, the role of Dex in AR warrants further investigation in the future.