Extracellular Vesicles Present in Human Ovarian Tumor Microenvironments Induce a Phosphatidylserine-Dependent Arrest in the T-cell Signaling Cascade

Abstract

The identification of immunosuppressive factors within human tumor microenvironments, and the ability to block these factors, would be expected to enhance patients’ anti-tumor immune responses. We previously established that an unidentified factor, or factors, present in ovarian tumor ascites fluids reversibly inhibited the activation of T cells by arresting the T cell signaling cascade. Ultracentrifugation of the tumor ascites fluid has now revealed a pellet that contains small extracellular vesicles (EV) with an average diameter of 80nm. The T cell arrest was determined to be causally linked to phosphatidylserine (PS) that is present on the outer leaflet of the vesicle bilayer, as a depletion of PS expressing EV or a blockade of PS with anti-PS antibody significantly inhibits the vesicle induced signaling arrest. The inhibitory EV were also isolated from solid tumor tissues. The presence of immune suppressive vesicles in the microenvironments of ovarian tumors and our ability to block their inhibition of T cell function represent a potential therapeutic target for patients with ovarian cancer.

Exosomes Associated with Human Ovarian Tumors Harbor a Reversible Checkpoint of T-cell Responses

“…The absence of a positive spot for GM130 indicated that our exosome preparations were not contaminated with cellular material. As we and others have previously reported, tumor-associated exosomes also express a negatively charged glycerophospholipid, phosphatidylserine (PS) (31), representing a lipid marker expressed on the surface of exosomes.…”

“…Extracellular vesicles derived from cancer patients’ sera/plasma (32) or from patients’ ovarian tumor ascites fluids (31) have been reported to inhibit the activation of T cells. However, those studies used a method to active the T cells that depended on antibodies to CD3 and CD28 immobilized on antibody-coated beads (32).…”

Apoptotic Bodies Elicit Gas6-Mediated Migration of AXL-Expressing Tumor Cells

“…However, tumor derived exosomes have been reported to express PS in ovarian cancer [20,38]. Since exosomes fall within the size range of the SUV liposomes used in this study (Fig.…”

“…However, even in non-apoptotic tumor cells, PS can appear on the outer leaflet, which is accompanied by increased shedding of microvesicles [20,21]. In addition, higher rates of apoptosis within the tumor environment, as well as therapy-induced apoptosis, contribute to bursts of PS exposure and release of PS-expressing apoptotic bodies.…”

Antibody-Mediated Phosphatidylserine Blockade Enhances the Antitumor Responses to CTLA-4 and PD-1 Antibodies in Melanoma

“…When intratumoral dendritic cells bind and ingest PS-expressing cells, they maintain an immature phenotype that prevents the expression of costimulatory molecules required for functional antigen presentation (26,38). Moreover, PS externalized on tumor-derived microvesicles suppresses activation of T-cell responses (39), and administration of PS-expressing liposomes containing insulin restores tolerance in a murine model of autoimmune diabetes (40). Lastly, although chemotherapy induces tumor cell apoptosis, the concomitant expression of PS on these cells (41,42) further suppresses potential immune-associated antitumor responses (43).…”