Mechanisms of Tumor-derived Exosome TEX-mediated Immunosuppression

Mechanisms of Tumor-derived Exosome (TEX)-Mediated Immunosuppression

Tumor cells, like most other cells, secrete exosomes, which are small (30-100 nm), membrane-bound extracellular vesicles that are involved in intercellular communication. These vesicles contain both membrane-bound and soluble proteins, lipids, double-stranded DNA, mRNA, and microRNAs (miRNAs) derived from their parent cell. The contents of exosomes may vary, however, depending on the cell type that the exosome originates from, selective sorting of molecules into the exosome, and stress-induced changes in the cellular environment. Following release from its parent cell, an exosome can transfer information to a recipient cell through three possible mechanisms: 1) fusion with the plasma membrane and release of its components into the cytoplasm of the recipient cell, 2) exosome endocytosis through phagocytosis, or 3) receptor-ligand interactions that occur between the exosome and the recipient cell. Once inside the recipient cell, the information obtained from the exosome is functional. Tumor-derived exosomes (TEXs) are abundantly produced in the tumor microenvironment and they carry a variety of immunosuppressive molecules. Interactions between tumor-derived exosomes and immune cells can both directly and indirectly inhibit immune cell functions and the anti-tumor immune response. Due to these properties and the observation that high numbers of tumor-derived exosomes in cancer patients correlate with tumor burden and disease progression, tumor-derived exosomes are now considered to be potential targets for cancer immunotherapy. The mechanisms by which TEXs have been described to mediate immunosuppression are detailed below.

Induction of Antigen-Specific Tolerance by Tumor-derived Exosomes

Tumor-derived exosomes (TEX) carry MHC components, tumor-associated antigens, and other proteins that indicate that they are capable of either immune activation or immunosuppression, but multiple studies suggest TEX promote immunosuppression in the tumor microenvironment. Most studies have demonstrated that the immunosuppressive properties of TEX are primarily antigen-independent, but a few have also suggested that they may suppress anti-tumor immune responses in an antigen-dependent manner as well. In a mouse DTH model, tumor-derived exosomes were shown to be taken up by dendritic cells, inhibit DC maturation and function, and confer antigen-specific immunosuppression. This was associated with increased levels of TGF-beta 1 and IL-4 in the lymph nodes, suggesting that TEX may inhibit tumor antigen-specific immune responses by affecting the functions of antigen-presenting cells (APCs). The presence of TEX has also been shown to be associated with low level expression of co-stimulatory molecules by dendritic cells and the production of inhibitory cytokines, indicating that TEX may condition DCs toward a suppressive or tolerogenic phenotype. An additional study using a subcutaneous tumor model suggested that APCs take up TEX, or tumor-associated antigens from apoptotic tumor cells and subsequently release tolerogenic, MHC class II+ exosomes that suppress tumor-specific immune responses. TEX have also been reported to inhibit CD3 zeta chain expression and it has been further suggested that TEX may impair T cell activation by forming TCR-MHC-peptide complexes with T cells that are unable to properly signal through the TCR.

MicroRNA (miRNA)-Induced Inhibition of Immune Cell Functions and Metabolic Reprogramming of Stromal Cells by Tumor-derived Exosomes

A second mechanism by which tumor-derived exosomes (TEX) inhibit the anti-tumor immune response is through the transfer of microRNAs (miRNAs) to different immune cell types. These miRNAs bind to mRNAs at specific sites and inhibit their translation, which can have negative effects on immune cell functions, depending on the affected mRNAs. Specific miRNAs found in tumor-derived exosomes from pancreatic cancer cells were shown to reduce the expression of MHC class II molecules, TLR4, TNF-alpha, and IL-12 in TEX-pulsed dendritic cells (DCs), which would likely negatively affect the antigen-presentation capabilities of DCs and their abilities to prime T cell activation. Overexpressed miRNAs in TEX from nasopharyngeal cancer were also shown to promote a shift in the T cell-mediated immune response from a Th1/Th17 immune cell phenotype towards a Th2/Treg phenotype. This change in the tumor microenvironment would also suppress the anti-tumor immune response. TEX in some other types of cancers have been shown to carry miRNAs that inhibit the activities of natural killer (NK) cells and monocytes. Additionally, TEX have been demonstrated to metabolically reprogram normal stromal cells to produce metabolites with immunosuppressive properties that support the metabolic needs of the tumor and promote tumor growth.

Interference with TCR and IL-2 Signaling by Tumor-derived Exosomes

Tumor-derived exosomes (TEX) interfere with both T cell receptor (TCR) and IL-2 receptor signaling. TEX disrupt TCR signaling by down-regulating CD3 zeta chain expression, thereby inhibiting T cell activation. TEX have also been shown to reduce the expression and phosphorylation of Janus kinase (Jak) in T cells and natural killer (NK) cells. This is significant as Jak is a critical intracellular kinase involved in mediating signaling by the common cytokine receptor gamma-chain family cytokines including IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. IL-2, IL-7, and IL-15 in particular, play a critical role in promoting the survival and proliferation of activated T cells and NK cells, as well as NK cell cytotoxicity. As a result, TEX can inhibit both T cell and NK cell proliferation and functions by targeting Jak.

Direct Engagement of T Cell Inhibitory and Apoptotic Receptors by Tumor-derived Exosomes and TEX Incorporation of PTEN

Tumor-derived exosomes (TEX) may also suppress T cell functions through the direct engagement of T cell inhibitory and apoptotic receptors. TEX may express Fas ligand (FasL), TNF-alpha, TRAIL, and Galectin-9, which bind to their respective T cell-expressed receptors, Fas/CD95, TNF RI, TRAIL R2, or Tim-3 and promote T cell apoptosis. Like many tumor cells themselves, TEX may also express high levels of PD-L1. PD-L1 binds to the PD-1 receptor expressed on CD4⁺ and CD8⁺ T cells and can inhibit TCR signaling and induce T cell arrest, anergy, and apoptosis. TEX may also express the phosphatase, PTEN, which has been shown to be capable of regulating PI 3-K/Akt activity in CD8⁺ T cells. Down-regulation of Akt phosphorylation in these cells results in a decrease in the expression of anti-apoptotic Bcl proteins and an up-regulation of pro-apoptotic Bax expression.

Expression of Decoy Natural Killer (NK) Cell/CD8⁺ T Cell Ligands by Tumor-derived Exosomes

Tumor-derived exosomes (TEX) may directly inhibit natural killer (NK) cells and CD8⁺ T cells through their expression of membrane-bound NKG2D ligands such as MICA, MICB, or ULBP1-6. These ligands interact with the NKG2D activating receptor expressed on the surface of NK and CD8⁺ T cells, resulting in prolonged stimulation of these cells. This over-stimulation leads to a down-regulation of NKG2D expression or responsiveness and a decrease in NK cell or CD8⁺ T cell tumor cell recognition and NK cell cytotoxicity.

Release of Inhibitory Cytokines by Tumor-derived Exosomes

Tumor-derived exosomes also load and release IL-10 and TGF-beta 1, which together inhibit the activation, differentiation, proliferation, and functions of effector T cells, and drive the expansion of T cells with suppressive phenotypes. TGF-beta 1 induces the expression of FoxP3 in CD4⁺CD25^- conventional T cells thereby promoting the expansion of regulatory T cells, while IL-10 promotes the conversion of activated conventional T cells to IL-10-, TGF-beta 1-secreting Tr1 cells. In addition, TGF-beta 1 down-regulates NKG2D and NKp30 expression on natural killer (NK) cells and CD8⁺ T cells, reduces NK cell proliferation and cytotoxicity, promotes the polarization of macrophages toward a M2 phenotype, and inhibits the expression of co-stimulatory molecules and IL-12 by dendritic cells (DCs), as well as DC maturation and migration.

Production of Extracellular Adenosine by Tumor-derived Exosomes

Tumor-derived exosomes (TEX) from different cancer types also express the surface ectonucleotidases, CD39 and CD73, which work in concert to generate extracellular adenosine from ATP. Extracellular adenosine is an immunosuppressive metabolite that signals through the A2a and A2b receptors expressed on different immune cell types to regulate the activities of these cells and drive tumor escape. Adenosine inhibits the proliferation and functions of natural killer (NK) cells, natural killer T (NKT) cells, and effector T cells, and stimulates regulatory T cell (Treg) expansion as well as Treg suppressive activity. Adenosine also indirectly affects T cell functions by acting on antigen-presenting cells. Activation of A2a/A2b receptors on dendritic cells (DCs) inhibits their production of IL-12 and TNF-alpha, promotes their expression of immunosuppressive molecules, and suppresses DC maturation.

Production of PGE₂ by Tumor-derived Exosomes

Another mechanism by which tumor-derived exosomes (TEX) may inhibit the anti-tumor immune response is through the production of prostaglandin E2 (PGE₂). PGE₂ in the presence of TGF-beta inhibits monocyte differentiation into antigen-presenting dendritic cells, and instead favors the expansion of myeloid-derived suppressor cells (MDSCs) and an increase in their suppressive activity. Additionally, PGE₂ has been shown to inhibit the cytolytic activity of natural killer (NK) cells and IFN-gamma production, and to suppress IL-2 production and responsiveness in T cells.

Expression of Tumor-Associated Antigens/Antibody Sequestration

Finally, since tumor-derived exosomes (TEX) carry tumor-associated antigens (TAA), they may condition dendritic cells (DCs) toward a suppressive or tolerogenic phenotype. In addition, TEX may bind and sequester tumor-reactive antibodies, significantly reducing antibody-dependent cellular cytotoxicity (ADCC) mediated by immune effector cells. As this is one of the major mechanisms by which immune cells expressing Fc receptors recognize and destroy antibody-coated cells expressing tumor-associated antigens, TEX may directly promote tumor escape by sequestering antibodies away from tumor cells.

To learn more, explore the following related resources:

Exosomes: Contents by Cell Type and Process Poster

MDSC-Mediated Mechanisms of Immunosuppression Pathway

Mechanisms of Regulatory T Cell-Mediated Suppression Pathway

Mechanisms of Tumor-Associated Macrophage (TAM)-Mediated Immunosuppression

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