Extracellular Vesicles: Novel Opportunities to Understand and Detect Neoplastic Diseases

Abstract

With a size range from 30 to 1000 nm, extracellular vesicles (EVs) are one of the smallest cell components able to transport biologically active molecules. They mediate intercellular communications and play a fundamental role in the maintenance of tissue homeostasis and pathogenesis in several types of diseases. In particular, EVs actively contribute to cancer initiation and progression, and there is emerging understanding of their role in creation of the metastatic niche. This fact underlies the recent exponential growth in EV research, which has improved our understanding of their specific roles in disease and their potential applications in diagnosis and therapy. EVs and their biomolecular cargo reflect the state of the diseased donor cells, and can be detected in body fluids and exploited as biomarkers in cancer and other diseases. Relatively few studies have been published on EVs in the veterinary field. This review provides an overview of the features and biology of EVs as well as recent developments in EV research including techniques for isolation and analysis, and will address the way in which the EVs released by diseased tissues can be studied and exploited in the field of veterinary pathology. Uniquely, this review emphasizes the important contribution that pathologists can make to the field of EV research: pathologists can help EV scientists in studying and confirming the role of EVs and their molecular cargo in diseased tissues and as biomarkers in liquid biopsies.

Keywords: biomarker; cancer; exosomes; extracellular vesicles; microvesicles pathogenesis; pathology.

Figure 1.

Mechanisms of extracellular vesicle (EV) biogenesis, release, and uptake. Different types of EVs are produced and released in different ways by donor cells: by formation of multivesicular bodies and fusion to the membrane (exosome), or by direct blebbing of the membrane (microvesicles, large oncosome, apoptotic bodies). Once in the intercellular space, EVs can be uptaken by target cells by receptor-mediated recognition, endocytosis, phagocytosis, or direct fusion with the plasma membrane. MVB, multivesicular body; E, exosome; MV, microvesicle; LO, large oncosome; AB, apoptotic body.

Figure 2.

Roles of extracellular vesicles (EVs) in physiological processes. A variety of cell types in the body communicate via EVs. EV-mediated transfer of bioactive cargo influences processes in early development, the immune system, nervous system, and circulatory system.

Figure 3.

Roles of extracellular vesicles (EVs) during retroviral infection. EVs produced and released by infected cells can both facilitate and suppress viral infection by different mechanisms. EVs can carry viral proteins, receptors, and RNAs. EVs released by inflammatory cells that have been activated by viral infection may also play a key role in the pathogenesis of viral diseases.

Figure 4.

Main roles of cancer-derived extracellular vesicles (EVs) in tumor pathogenesis. Tumor-derived EVs alter the behavior of cancer cells, thereby facilitating cancer progression. Tumor-derived EVs induce alterations in immune cells, endothelial cells, fibroblasts, and mesenchymal stem cells in order to establish a tumor microenvironment that promotes tumor cell survival and dissemination. CAFs, cancer-associated fibroblasts.

Figure 5.

Extracellular vesicle (EV)-based therapy. EVs can be exploited both as therapeutic targets, to inhibit their biogenesis, release, and uptake, and as therapeutic agents, using them as vaccine or nanocarriers to transport therapeutic molecules. E, endothelial cells; L, lymphocytes; DC, dendritic cells; SC, stem cells; CAF, cancer-associated fibroblasts; Tag, tumor antigen; D, drug.

Figure 6.

Most frequently used methods for the isolation of extracellular vesicles (EVs). The isolation of EVs can be based on size, buoyant density, or by detecting an antigen. Differential ultracentrifugation uses centrifugal force to separate EVs based on size, as larger EVs collect earlier and at a lower centrifugation speed compared to small EVs. In size-exclusion chromatography, a column with a porous matrix is used to separate EVs by size. Filtration concentrates EVs in a sample by passing them through a filter. In precipitation, a reagent is added to a sample concentrate EVs in a pellet. In density gradient centrifugation, EVs are separated into specific layers of a density gradient, as they settle in the layer with their equilibrium density. The immunoaffinity isolation method uses antibodies to capture EVs based on their antigenicity.