The Magical World of Circulating Vesicles.

Abstract

Cells use vesicles to efficiently compartmentalize bimolecular processes and without them, higher eukaryotic life would be unthinkable. Over 20 years ago, researchers realized that many cells also shed vesicles and that a fraction of these find their way into circulation. Initially believed to be “detritus” and “purification artifacts”, it became clear that these extracellular vesicles (EV) had important biological functions and consequences. Next, different types of vesicles were discovered, and the race to fundamentally understand their biogenesis, composition and biological functions was on. Today EV are used for therapeutic and diagnostic (liquid biopsy) purposes. Their role in diverse immunologic, proliferative and repair, processes continues to be elucidated. Extracellular vesicles are abundant in plasma and other biofluids, stable in circulation, and produced by all cell types. In the normal human organism, the majority of EV are produced by endothelial cells and red blood cells. Tumor cells can produce large quantities of EV (∼4,000x more than normal host cells). These tumor cell derived EV (tEV) partly resemble parental tumor cells in their molecular make-up, reason why they are so attractive for diagnostics. The main challenges with EV diagnostics have largely centered around the following questions: which type of circulating vesicle (exosome, microvesicles, others) is diagnostically most relevant; which molecular biomarker (or combination) is best suited to detect tEV; how does one best separate tumor and host cell derived EV; given the heterogeneity of EV, will future analyses have to rely on single EV diagnostic tools; and what is the best technology to analyze human plasma samples at high throughput and reasonable cost so that patients can benefit from new discoveries? Needless to say, these questions are also relevant for therapeutic applications of EV. This issue of Advanced Biosystems focuses on diagnostic aspects of EV analysis. The compilation of 12 articles have been prepared by current or past members of a large exosome consortium in Boston dedicated to basic and translational biology. Article number 1900305 deals with the pharmacokinetics and biological distribution of EV. Using mathematical modeling, this manuscript lays the groundwork to estimate detectable tumor burden by EV analysis. One take-home message is that tumors as small as 1 mm3 are theoretically detectable in patients when single EV analytical techniques are used for diagnosis. A second take-home message is that single EV analysis will ultimately enable cancer detection multiple years before mass lesions become detectable by imaging. Article 2000069 reviews the different proteomic analysis of EV. One of the current challenges in EV research is the selective isolation of EV subtypes at quantities sufficient for efficient downstream analyses. The review focuses on the diverse techniques and protocols used to isolate and purify EVs with a special emphasis on their adequacy for proteomics applications. Articles 1900310, 2000003, 2000007, and 2000203 describe new EV analytical techniques. Article 1900307 showcases a new way to study EV in vivo by genetically engineering them to display a membrane-bound form of Sortase A, a bacterial transpeptidase that can catalyze the transfer of reporter molecules on the much bigger surface of EV-binding cells. One of the most anticipated clinical role of EV diagnostics is for CNS diseases and glioblastoma where biopsies are not always feasible, carry considerable morbidity and can not be performed serially. Articles 2000035, 2000029, and 1900312, focus on various aspects of EV biology in glioblastomas. Article 2000017 focuses on the recently discovered paradigm in which PD-L1 containing tumor cell-derived EVs cause immune suppression by the direct engagement of PD1 on T cells, decreasing their activation and providing a further barrier to protect tumor cells from T cell killing. Article 1900309 is dedicated to neuroinflammation and degenerative CNS diseases. We hope the articles in this issue of Advanced Biosystems will inspire readers to develop new methods, applications, therapeutic uses or simply understand human EV biology using the newly developed tools. Ralph Weissleder is the Thrall Professor of Radiology at Harvard Medical School, Director of the Center for Systems Biology at Massachusetts General Hospital (MGH), and Attending Clinician (Interventional Radiology) at MGH. Dr. Weissleder is also Professor of Systems Biology and a faculty a member of the Department of Systems Biology at HMS and the Dana Farber Harvard Cancer Center. The focus of his research lab is to obtain a deeper understanding of human biology in health and disease, to translate new biological understanding into clinically useful diagnostics, and to identify new therapeutic approaches and drug targets. His research has been translational and several of his developments have been licensed to companies and led to advanced clinical trials.