Tumor-Derived Extracellular Vesicles and the Immune System-Lessons From Immune-Competent Mouse-Tumor Models

Abstract

Tumor-derived extracellular vesicles (TEVs) are important regulators of the immune response in cancer; however, most research so far has been carried out using cell culture systems. Immune-competent murine tumor models currently provide the best platform to assess proposed roles of TEVs using in vivo animal models and therefore are important for examining interactions between TEVs and the immune system. In this review, we present the current knowledge on TEVs using in vivo tumor-bearing animal models, with a focus on the role of TEVs in mediating crosstalk between tumor cells and both adaptive and innate immune cells. In particular, we address the question how animal models can clarify the reported heterogeneity of TEV effects in both anti-tumor responses and evasion of immune surveillance. The potential of TEVs in mediating direct antigen-presenting functions supports their potential as cancer vaccine therapeutics, therefore, we provide an overview of key findings of TEV trials that have the potential as novel immunotherapies, and shed light on challenges in the path toward the first in-human trials. We also highlight the important updates on the methods that continue to enhance the rigor and reproducibility of EV studies, particularly in functional animal models.

Keywords: adaptive immunity; cancer; extracellular vesicle heterogeneity; extracellular vesicles; immune system; immunotherapy; mouse tumor models; tumor exosomes.

Figure 1

Overview of selected immune-modulatory tumor-derived extracellular vesicle (TEV) cargo. Both immunosuppressive and immune-stimulating roles have been proposed for TEVs. These effects can be mediated by receptors bound to the surface of TEVs, such as PD-L1, or by nucleic acid or protein contents (e.g., arginin-1, ARG1) encapsulated in the interior of the vesicles (21, 22, 25). However, if uptake occurs, interaction of vesicular and cellular receptors is probably needed (75). Tetraspanins (Tspans) such as CD9, CD63, and CD81 are frequently used for characterization of EVs and are likely involved in fusion of TEVs and recipient cells, similar to what has been described for some viruses (76, 77). While also promoting TEV uptake, integrins and other adhesion molecules are responsible for tissue-specific binding of TEVs (78). Thus, immune responses can be modified by TEVs not only globally, but also influence local local responses. This may promote pre-metastatic niche formation (51). Recently, a role in immune cell targeting has been also revealed for N glycans (47). (Figure created using BioRender.com).

Figure 2

Experimental design influences the heterogeneity of reported tumor-derived extracellular vesicle (TEV) effects. TEVs are regularly harvested from the supernatants of tumor cell lines (1). They can be either used for in vitro pulsing of immune cells (e.g., differentiated dendritic cells, 2a) or for direct injection (2b). In pulsing experiments, TEV-associated antigens are presented to dendritic cells, which are then injected into an animal and induce strong T cell responses (3a) (41). This promotes tumor growth restriction and increases survival of tumor-bearing mice (4a). On the other hand, when TEVs are injected separately, they interact with a plenitude of other immune cells, such as regulatory T cells, CD34⁺ dendritic cell precursors and myeloid derived suppressor cells (3b), which are commonly activated by TEVs to enable immune evasion (18, 36). This, in turn, reduces the tumor-specific T cell response and enhances tumor progression (4b). (Figure created using BioRender.com).