Glioma-Derived Extracellular Vesicles - Far More Than Local Mediators

Abstract

Extracellular vesicle (EV) secretion is a ubiquitous cellular process with both physiologic and pathologic consequences. EVs are small lipid bilayer vesicles that encompass both microvesicles and exosomes and which are secreted by virtually all cells including cancer cells. In this review, we will focus on the roles of EVs in mediating the crosstalk between glioblastoma (GBM) cells and innate and adaptive immune cells and the potential impact on glioma progression. Glioma-derived EVs contain many bioactive cargoes that can broaden and amplify glioma cell mediated immunosuppressive functions and thereby contribute to shaping the tumor microenvironment. We will discuss evidence demonstrating that the low oxygen (hypoxia) in the GBM microenvironment, in addition to cell-intrinsic effects, can affect intercellular communication through EV release, raising the possibility that properties of the tumor core can more widely impact the tumor microenvironment. Recent advances in glioma-derived EV research have shown their importance not only as message carriers, but also as mediators of immune escape, with the capacity to reprogram tumor infiltrating immune cells. Exploring EV function in cancer-immune crosstalk is therefore becoming an important research area, opening up opportunities to develop EV monitoring for mechanistic studies as well as novel diagnostic glioma biomarker applications. However, robust and reproducible EV analysis is not always routinely established, whether in research or in clinical settings. Taking into account the current state of the art in EV studies, we will discuss the challenges and opportunities for extending the many exciting findings in basic research to a better interpretation of glioma and its response to current and future immunotherapies.

Keywords: biomarkers; extracellular vesicles; glioma; hypoxia; immunosuppression; tumor microenvironment.

Figure 1

Extracellular vesicle biogenesis and types. Microvesicles are formed by direct budding of the plasma membrane and release into the extracellular space. Exosome biogenesis begins with the formation of early endosomes. Early endosomes accumulate intraluminal vesicles (ILVs) through membrane shedding, leading to the formation of multivesicular bodies (MVBs). Subsequently, late endosomes/MVBs either fuse with lysosomes, in which case the ILVs will be destroyed, or else they fuse directly with the cell membrane, releasing exosomes into the extracellular space. Apoptotic bodies are shed directly into the extracellular environment by apoptotic cells. Each of these types of EVs can carry different cargos that are loaded on their membrane (proteins, glycoproteins) or packed in their lumen (proteins, DNA and RNAs). EV cargo composition can reflect donor cells (apoptotic bodies) or harbor more specifically sorted biomolecules (exosomes).

Figure 2

GBM EVs can trigger various processes in the cells present in the tumor microenvironment. They can have an effect on neighboring cancer cells, resident brain cells (astrocytes and microglia), and on infiltrating immune cells (T cells and macrophages). However, the process is bidirectional and many of the EV-recipient tumor-suppressive cells can also secrete EVs that further suppress the antitumor functions of immune infiltrating cells and support GBM cell proliferation.

Figure 3

Potential of EVs as a non-invasive clinical biomarker in GBM. Diagnosis of GBM usually comprises magnetic resonance imaging (MRI) and histopathological analysis of a tumor biopsy. Most patients receive treatment composed of surgery, followed by radiation and chemotherapy with temozolomide. Despite this treatment, GBM recurrence generally occurs. However, radiological distinction between tumor progression and pseudoprogression is often difficult, particularly after experimental treatments such as immunotherapy (immunomodulatory antibodies, vaccines, chimeric antigen receptor T cells). Methodologically, EVs can be rapidly characterized by nanoparticle tracking analysis (NTA) that can detect size and concentration, as well as surface expression of EV and tumor associated antigens with the use of fluorochrome-conjugated antibodies. EVs and their cargo detected in patients’ plasma or CSF offer great potential as a biomarker that can improve diagnosis and treatment decisions.