Exosomes‐mediated crosstalk between glioma and immune cells in the tumor microenvironment

Abstract Gliomas are the most common primary malignant tumors in the central nervous system. However, conventional treatments, such as surgical resection and postoperative combined chemo‐ and radio‐therapy, are ineffective in improving patients' long‐term survival. The tumor microenvironment (TME) consists of stromal cells, tumor components, and innate and acquired immune cells, and these cells, along with the extracellular matrix, regulate and communicate intercellularly to promote TME formation. The immune microenvironment plays a vital role in the development of glioma. Exosomes, which are extracellular vesicles (EVs), facilitate intercellular communication and regulation within the TME. Tumor cells can release exosomes to transmit messages, induce macrophage polarization, and inhibit immune cell activity, ultimately promoting metastasis and immune evasion. Moreover, immune cells can regulate tumorigenesis and progression through exosomes. This review summarized the biological properties of exosomes and their effects on the tumor microenvironment and provides an overview of the interactions between glioma cells and immune cells.

pathogenesis of glioma remains unclear, with tumorigenesis resulting from a combination of environmental and genetic factors.
The tumor microenvironment (TME) consists of tumor cells and surrounding components. 13,14 It includes innate and adaptive immune cells (T lymphocytes and B lymphocytes), mesenchymal fibroblasts, and vascular and lymphatic vessel networks. Various chemokines secreted through autocrine or paracrine make up the TME. [15][16][17] Alterations in the microenvironment affect tumorigenesis and progression. The immune cell is fundamental in determining the fate of cancer and its invasiveness and metastatic capacity. 18,19 The clinical outcome of cancer patients is interrelated to the composition of immune cells that infiltrate tumors. 20,21 2 | OVERVIE WOFE XOSOME S

| Mechanismofexosomeformation
Exosomes are generated in normal physiological conditions or response to external environmental stimuli. They originate from intracellular invaginations, where the cell membrane forms multiple vesicles. These vesicles fuse to form early intracellular vesicles, which further mature and bud inward to form luminal vesicles or intraluminal vesicles (ILVs). [44][45][46] ILVs-rich intracellular bodies are called multivesicular bodies (MVBs). Upon fusion with lysosomes intracellularly, MVBs are degraded, while the other part is secreted outward to form exosomes under the regulation of Rab enzymes of the GTPase family. [47][48][49] Exosomes are extracellular vesicles with a diameter of 30-150 nm generated by almost all cell types. 50,51 Exosomes consist of a lipid bilayer containing transmembrane proteins and wrap around cytoplasmic proteins, lipids, or nucleic acids. 52,53 Exosomes can regulate cellular communication by transporting specific exosomal contents. 54,55 Exosomes are closely associated with normal physiological homeostasis and various diseases, including cancer.
Exosomes are essential mediators of tumorigenesis, proliferation, angiogenesis, and distant metastasis. 56,57 Currently, the extraction of exosomes has become commercially available. The advantages and disadvantages of different methods are presented in Table 1.

| Exosomesfunctions
Exosomes are widely distributed in the body fluid of tumor patients after being released. 58,59 They can enter and deliver biologically active substances to target cells, affecting gene expression, protein synthesis, and other processes that ultimately regulate the target cells' function. 60,61 Exosomal surface proteins bind directly to target cell receptors and stimulate signaling pathways, and they can also fuse with target cell membranes and deliver functional proteins, miRNA, and other biomolecules. 22,62 The mRNA of exosomes can translate corresponding proteins, and miRNA and siRNA regulate target cells by affecting the expression of related genes. 63,64 Phagocytose exosomes can either be re-released in target cells or degraded by the lysosomal pathway. 65,66 The exosomal function is closely related to their cellular origin and the protein and RNA they contain. Exosomes from different sources have different purposes at different physiological and pathological stages. 67,68 Exosomes can regulate physiological activities and maintain intracellular homeostasis, support the body's immune tolerance, and participate in normal physiological processes, and pathological processes. 69,70 Tumor cell-derived exosomes contain antigens, genetics, and other biologically active substances that are vital in tumorigenesis, proliferation, invasion, and metastasis. 71 Proteins, miRNAs, and even DNA in tumor-derived exosomes are potential markers for non-invasive diagnosis. 72,73 As natural carriers, the phospholipid bilayer structure of exosomes protects the stability of proteins, miRNA, and other biologically active substances. Exosomes have similar biological activities as parental cells, are widely distributed, exist for a long time in the body, and have the advantages of a long half-life and natural nontoxicity. Due to their nanoscale structure, exosomes can evade phagocytosis while freely shuttling between cells and matrix, with solid penetration ability and low immunogenicity. 74 Exosomes can also serve as a means of delivering drugs and miRNAs for tumor therapy. Tumor cell-derived exosomes are also potential for tumor vaccine development. 75

| Immunesystemcells-derivedexosomes(IEXs)
IEXs have a comprehensive spectrum of functions in the immune system. IEXs regulate multiple immune signaling pathways, including modulation, antigen presentation, antitumor immunity, and immune system suppression. Dendritic cell-derived exosomes (DEX) can enhance antitumor immunity and activate specific T cells in the direct pathway by expressing the MHCII-peptide complexes, costimulatory molecules, and binding to T-cell receptors. In the in-  Figure 1). Immunotherapeutic strategies targeted at the TME can stimulate or restore the innate tumor-suppressive capacity of the immune system, create a favorable immune microenvironment, and produce a comprehensive response. [85][86][87] Tumor metastasis is the primary cause of mortality. The pre-  88,94 In addition to immunosuppressive factors, the TME accumulates metabolites from tumor proliferation, such as adenosine and lactate. 95,96 Hypoxic TME increases the release of ATP and AMP. The nucleotidases CD39 and CD73 catalyze the conversion of ATP to AMP and AMP to adenosine extracellularly, resulting in a significant increase in adenosine levels in immune regulation. 97,98 TA B L E 1 The separation techniques for exosomes.

Ultracentrifugation
The obtained exosomes are not contaminated by the separation reagent, and the number of separations is large, and the processed sample is small The instrument is expensive, the sample volume is large, the time consumption is long, and protein contamination still exists when exosomes are observed by electron microscopy

| Interactionbetweengliomaandimmunecells intheTME
Glioma is a primary malignant tumor in the central nervous system  (Table 2). Similarly, immune cells can influence glioma progression by delivering substances to glioma cells through exosomes (Table 3).

| Dendriticcells(DCs)
Studies have shown that dendritic cell-derived exosomes (DEX) can enhance antitumor immunity and activate specific T cells to combat tumor cells. Using methods to upregulate and downregulate exosome production by immune cells is a novel way to regulate immunity against tumors and infected cells, as well as immune reactions in some autoimmune and allergic diseases. DCs are the cells in the T cell-mediated immune response to cancer within the organism.
DCs and their precursors in tumors can be recruited and respond to several molecular signals, including cell death, inactivation, and successful maturation in the TME. 100 Immature DCs cannot initiate F I G U R E 1 The primary cells involved in the tumor immune microenvironment.

F I G U R E 2
The primary process of tumor metastasis. Tumor metastasis mainly includes invasion, intravasation, circulation, extravasation, and colonization. It also provides for the involvement of macrophages, neutrophils, and T cells.
T-cell responses to tumors and may induce immune tolerance, while mature DCs can migrate to tumor-draining lymph nodes to create T-cell responses, recruit T cells into the TME, and produce immunostimulatory cytokines to regulate the TME. 101 Research suggests that chaperone-rich cell lysates (CRCLs) may

| Neutrophils
Neutrophils are the most abundant type of leukocytes and play a crucial role in the inflammatory response's progression. 125  formation of immunosuppressive TME. 86,148 Various immunotherapeutic agents targeting TME have been developed in this stage, however, the combination of diverse

ACK N OWLED G M ENTS
We thank the generous support of Liaoning Cancer Hospital & Institute (Shenyang) and Dalian University of Technology(Dalian).

FU N D I N GI N FO R M ATI O N
This Work is supported by the Fundamental Research Funds for the Central University(2021-YGJC-17).

CO N FLI C TO FI NTE R E S TS TATE M E NT
The authors declare no competing interests.

DATAAVA I L A B I L I T YS TAT E M E N T
The data in the current study are available from the corresponding authors upon reasonable request.