Tumor–Host Cell Interactions in Ovarian Cancer: Pathways to Therapy Failure

Cytokines, EVs, and miRNAs modulate PI3K–AKT–mTOR, STAT3, and NFκB signaling pathways that mediate chemoresistance. Drugs for these pathways are available and at various stages of clinical development. EV-encapsulated miRNA released by omental CAAs and CAFs blocks chemotherapy-mediated tumor apoptosis by targeting APAF-1, a central component of the apoptosome. CAFs limit the intracellular availability of chemotherapy drugs in cancer cells by restricting the extracellular concentrations of GSH and its precursor cysteine, which is counteracted by IFNγ released by T cells. Tumor-infiltrating T cells reactivated by PD-1 checkpoint blockade improve the cytotoxic effect of chemotherapy in animal models of ovarian cancer and are clinically tested. Ovarian cancers have different types of resistance. The underlying mechanistic diversity requires individualized therapies that combine chemotherapy, signaling inhibition, and T cell checkpoint blockade. Although most ovarian cancer patients are highly responsive to chemotherapy, they frequently present with recurrent metastatic lesions that result in poor overall survival, a situation that has not changed in the last 20 years. This review discusses new insights into the regulation of ovarian cancer chemoresistance with a focus on the emerging role of immune and other host cells. Here, we summarize the complex molecular pathways that regulate the interaction between tumor and host cells, discuss the limitations of current in vitro and in vivo models for translational studies, and present perspectives for the development of innovative therapies. Although most ovarian cancer patients are highly responsive to chemotherapy, they frequently present with recurrent metastatic lesions that result in poor overall survival, a situation that has not changed in the last 20 years. This review discusses new insights into the regulation of ovarian cancer chemoresistance with a focus on the emerging role of immune and other host cells. Here, we summarize the complex molecular pathways that regulate the interaction between tumor and host cells, discuss the limitations of current in vitro and in vivo models for translational studies, and present perspectives for the development of innovative therapies. High-grade serous carcinoma is the most common ovarian malignancy and is usually diagnosed at an advanced stage. Patients presenting with advanced disease have a dire prognosis with an overall 5-year survival rate of less than 40% due to the recurrence of peritoneal metastases after first-line therapy [1Lengyel E. Ovarian cancer development and metastasis.Am. J. Pathol. 2010; 177: 1053-1064Abstract Full Text Full Text PDF PubMed Scopus (1070) Google Scholar, 2Vaughan S. et al.Rethinking ovarian cancer: recommendations for improving outcomes.Nat. Rev. Cancer. 2011; 11: 719-725Crossref PubMed Scopus (929) Google Scholar]. A feature that distinguishes ovarian cancer from other human tumors is the specific tumor microenvironment (see Glossary) [3Ahmed N. Stenvers K.L. Getting to know ovarian cancer ascites: opportunities for targeted therapy-based translational research.Front. Oncol. 2013; 3: 256Crossref PubMed Scopus (276) Google Scholar, 4Bowtell D.D. et al.Rethinking ovarian cancer II: reducing mortality from high-grade serous ovarian cancer.Nat. Rev. Cancer. 2015; 15: 668-679Crossref PubMed Scopus (650) Google Scholar]. Ovarian cancer is a peritoneal disease, where the dissemination of tumor cells is facilitated by the peritoneal fluid as a carrier [3Ahmed N. Stenvers K.L. Getting to know ovarian cancer ascites: opportunities for targeted therapy-based translational research.Front. Oncol. 2013; 3: 256Crossref PubMed Scopus (276) Google Scholar, 5Kipps E. et al.Meeting the challenge of ascites in ovarian cancer: new avenues for therapy and research.Nat. Rev. Cancer. 2013; 13: 273-282Crossref PubMed Scopus (355) Google Scholar]. This fluid, frequently occurring as large volumes of ascites, comprises detached tumor cells, tumor cell spheroids, and numerous types of host cells, including different kinds of innate and adaptive immune cells and activated mesothelial cells, which produce, and are targeted by a plethora of tumor-promoting soluble factors and extracellular vesicles (EVs). Tumor-associated macrophages (TAMs) play a prominent role in this context as major producers of tumor protumorigenic and immunosuppressive factors [6Reinartz S. et al.A transcriptome-based global map of signaling pathways in the ovarian cancer microenvironment associated with clinical outcome.Genome Biol. 2016; 17: 108Crossref PubMed Scopus (73) Google Scholar]. Another feature characteristic of ovarian cancer is the particular relevance of the omentum (Box 1), a structure composed of fatty and connective tissue that covers the ventral surface of the intestines. The omentum is the preferred site of ovarian cancer metastases and represents a key player in tumor progression [1Lengyel E. Ovarian cancer development and metastasis.Am. J. Pathol. 2010; 177: 1053-1064Abstract Full Text Full Text PDF PubMed Scopus (1070) Google Scholar].Box 1Role of the Omentum in Ovarian Cancer ProgressionThe omentum is a large apronlike structure in the abdomen that serves to protect the visceral organs and is part of the peritoneal immune defense system. Its major constituent is a double layer of fatty tissue, but harbors a variety of other cell types, including mesothelial cells, fibroblasts, lymphocytes, and macrophages. Tissue-resident adipocytes and fibroblasts of the omentum are converted to carcinoma-associated adipocytes (CAAs) and CAFs by tumor-derived mediators that induce growth and a metastasis promoting microenvironment. One pathway specific in ovarian cancer is the transfer of fatty acids from CAAs to adjacent tumor cells via the fatty acid-binding protein 4 (FABP4), thereby supporting energy production for metastatic growth [67Nieman K.M. et al.Adipocytes promote ovarian cancer metastasis and provide energy for rapid tumor growth.Nat. Med. 2011; 17: 1498-1503Crossref PubMed Scopus (1427) Google Scholar]. Other examples are the promotion of tumor cell homing and invasion into the omentum by IL-8 secreted by CAAs and CAFs [67Nieman K.M. et al.Adipocytes promote ovarian cancer metastasis and provide energy for rapid tumor growth.Nat. Med. 2011; 17: 1498-1503Crossref PubMed Scopus (1427) Google Scholar] and the activation of the growth-promoting receptor ERBB3 by its ligand neuregulin 1 (NRG1) also released from CAAs [28Pradeep S. et al.Hematogenous metastasis of ovarian cancer: rethinking mode of spread.Cancer Cell. 2014; 26: 77-91Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar]. The omentum is a large apronlike structure in the abdomen that serves to protect the visceral organs and is part of the peritoneal immune defense system. Its major constituent is a double layer of fatty tissue, but harbors a variety of other cell types, including mesothelial cells, fibroblasts, lymphocytes, and macrophages. Tissue-resident adipocytes and fibroblasts of the omentum are converted to carcinoma-associated adipocytes (CAAs) and CAFs by tumor-derived mediators that induce growth and a metastasis promoting microenvironment. One pathway specific in ovarian cancer is the transfer of fatty acids from CAAs to adjacent tumor cells via the fatty acid-binding protein 4 (FABP4), thereby supporting energy production for metastatic growth [67Nieman K.M. et al.Adipocytes promote ovarian cancer metastasis and provide energy for rapid tumor growth.Nat. Med. 2011; 17: 1498-1503Crossref PubMed Scopus (1427) Google Scholar]. Other examples are the promotion of tumor cell homing and invasion into the omentum by IL-8 secreted by CAAs and CAFs [67Nieman K.M. et al.Adipocytes promote ovarian cancer metastasis and provide energy for rapid tumor growth.Nat. Med. 2011; 17: 1498-1503Crossref PubMed Scopus (1427) Google Scholar] and the activation of the growth-promoting receptor ERBB3 by its ligand neuregulin 1 (NRG1) also released from CAAs [28Pradeep S. et al.Hematogenous metastasis of ovarian cancer: rethinking mode of spread.Cancer Cell. 2014; 26: 77-91Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar]. The standard first-line therapy for high-grade serous ovarian carcinoma is a combination of surgery and carboplatin/paclitaxel-based chemotherapy. Although most patients are initially highly responsive to this regimen, the vast majority of patients present with recurrent disease. Apart from inherent and acquired chemoresistance, a poorly understood mechanism of transient or conditional resistance is likely to be responsible for the failure of chemotherapy (Box 2). Prime resistant candidates are detached cancer cells and spheroids in the malignant ascites that express markers characteristic of stem cells [7Foster R. et al.Ovarian cancer stem cells: working towards the root of stemness.Cancer Lett. 2013; 338: 147-157Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar]. However, in contrast to some other human malignancies, a clearly defined 'cancer stem cell' has not been identified in ovarian cancer and may not exist. Nevertheless, ovarian cancer cells expressing stemness markers possess a high tumor-initiating potential, exhibit increased drug resistance, and are enriched in the ascites of patients with relapsed ovarian cancer [8Rizzo S. et al.Ovarian cancer stem cell-like side populations are enriched following chemotherapy and overexpress EZH2.Mol. Cancer Ther. 2011; 10: 325-335Crossref PubMed Scopus (164) Google Scholar]. These findings clearly support the conclusion that ascites-associated tumor cells play a pivotal role in ovarian cancer spread and resistance to therapy. There is also evidence to suggest that the properties of these cells are strongly influenced by factors of both the local and distal tumor microenvironment, the invaded host tissues, and the malignancy-associated ascites [9Kwon M.J. Shin Y.K. Regulation of ovarian cancer stem cells or tumor-initiating cells.Int. J. Mol. Sci. 2013; 14: 6624-6648Crossref PubMed Scopus (57) Google Scholar].Box 2Different Mechanisms of Chemoresistance in Ovarian CarcinomasThe vast majority of ovarian carcinomas are highly responsive to chemotherapy (Figure IA) with only a small fraction of patients showing disease progression during or shortly after chemotherapy due to a presumably genetic mechanism of inherent chemoresistance (Figure IB). Although disease control can initially be achieved, most of these patients present with recurrent tumors within 3 years (Figure IC). In a subset of patients, this relapse results from the selection of genetically altered tumor cells leading to a state of acquired chemoresistance (Figure ID), resulting from diverse genetic mechanisms as suggested by the extensive clonal heterogeneity [68Patch A.M. et al.Whole-genome characterization of chemoresistant ovarian cancer.Nature. 2015; 521: 489-494Crossref PubMed Scopus (930) Google Scholar]. By contrast, many of the relapsed cancers are still highly sensitive to treatment with a combination of carboplatin and paclitaxel, even after multiple cycles of relapse and chemotherapy (Figure IE). Since these cells survived and were refractory to first-line therapy, their state of chemoresistance is likely transient. This feature distinguishes ovarian carcinoma from most other human cancers, where recurrence is usually associated with genetically acquired resistance.Transient chemoresistance may be induced, for example, by changes in the tumor microenvironment and/or stress-induced autophagy triggered, for instance, by a perturbed energy metabolism in individual cancer cells [44Sodek K.L. et al.Cell-cell and cell-matrix dynamics in intraperitoneal cancer metastasis.Cancer Metastasis Rev. 2012; 31: 397-414Crossref PubMed Scopus (111) Google Scholar]. Increased autophagy has indeed been observed in dormant metastatic lesions on serous membranes in relapsed disease compared to primary ovarian cancers [69Lu Z. et al.DIRAS3 regulates the autophagosome initiation complex in dormant ovarian cancer cells.Autophagy. 2014; 10: 1071-1092Crossref PubMed Scopus (53) Google Scholar]. Tumor cell spheroids floating in the peritoneal fluid most likely play a pivotal role in causing this state of transient chemoresistance. On the one hand, these multicellular aggregates are endowed with a high tumor-initiating potential, which has been proposed to be linked to the expression of stemness-associated genes [3Ahmed N. Stenvers K.L. Getting to know ovarian cancer ascites: opportunities for targeted therapy-based translational research.Front. Oncol. 2013; 3: 256Crossref PubMed Scopus (276) Google Scholar, 70Liao J. et al.Ovarian cancer spheroid cells with stem cell-like properties contribute to tumor generation, metastasis and chemotherapy resistance through hypoxia-resistant metabolism.PLoS One. 2014; 9: e84941Crossref PubMed Scopus (263) Google Scholar] and is increased by fluid shear stress to which cells are exposed in malignant ascites [71Ip C.K. et al.Stemness and chemoresistance in epithelial ovarian carcinoma cells under shear stress.Sci. Rep. 2016; 6: 26788Crossref PubMed Scopus (79) Google Scholar]. On the other hand, these cells are protected from anoikis and drug-induced programmed cell death [44Sodek K.L. et al.Cell-cell and cell-matrix dynamics in intraperitoneal cancer metastasis.Cancer Metastasis Rev. 2012; 31: 397-414Crossref PubMed Scopus (111) Google Scholar], which is presumably related to their quiescence [72Correa R.J. et al.Modulation of AKT activity is associated with reversible dormancy in ascites-derived epithelial ovarian cancer spheroids.Carcinogenesis. 2012; 33: 49-58Crossref PubMed Scopus (53) Google Scholar] and the skewing of metabolism toward anaerobic glycolysis [70Liao J. et al.Ovarian cancer spheroid cells with stem cell-like properties contribute to tumor generation, metastasis and chemotherapy resistance through hypoxia-resistant metabolism.PLoS One. 2014; 9: e84941Crossref PubMed Scopus (263) Google Scholar]. It is therefore conceivable that a small number of tumor cell spheroids survive chemotherapy and due to their high tumorigenic potential initiate new metastatic lesions upon attachment to visceral organs. In view of the transient or conditional nature of the chemoresistant state of these cells, their interaction with the microenvironment, including host cells, EVs, soluble mediators, and the extracellular matrix, deserves particular attention. The vast majority of ovarian carcinomas are highly responsive to chemotherapy (Figure IA) with only a small fraction of patients showing disease progression during or shortly after chemotherapy due to a presumably genetic mechanism of inherent chemoresistance (Figure IB). Although disease control can initially be achieved, most of these patients present with recurrent tumors within 3 years (Figure IC). In a subset of patients, this relapse results from the selection of genetically altered tumor cells leading to a state of acquired chemoresistance (Figure ID), resulting from diverse genetic mechanisms as suggested by the extensive clonal heterogeneity [68Patch A.M. et al.Whole-genome characterization of chemoresistant ovarian cancer.Nature. 2015; 521: 489-494Crossref PubMed Scopus (930) Google Scholar]. By contrast, many of the relapsed cancers are still highly sensitive to treatment with a combination of carboplatin and paclitaxel, even after multiple cycles of relapse and chemotherapy (Figure IE). Since these cells survived and were refractory to first-line therapy, their state of chemoresistance is likely transient. This feature distinguishes ovarian carcinoma from most other human cancers, where recurrence is usually associated with genetically acquired resistance. Transient chemoresistance may be induced, for example, by changes in the tumor microenvironment and/or stress-induced autophagy triggered, for instance, by a perturbed energy metabolism in individual cancer cells [44Sodek K.L. et al.Cell-cell and cell-matrix dynamics in intraperitoneal cancer metastasis.Cancer Metastasis Rev. 2012; 31: 397-414Crossref PubMed Scopus (111) Google Scholar]. Increased autophagy has indeed been observed in dormant metastatic lesions on serous membranes in relapsed disease compared to primary ovarian cancers [69Lu Z. et al.DIRAS3 regulates the autophagosome initiation complex in dormant ovarian cancer cells.Autophagy. 2014; 10: 1071-1092Crossref PubMed Scopus (53) Google Scholar]. Tumor cell spheroids floating in the peritoneal fluid most likely play a pivotal role in causing this state of transient chemoresistance. On the one hand, these multicellular aggregates are endowed with a high tumor-initiating potential, which has been proposed to be linked to the expression of stemness-associated genes [3Ahmed N. Stenvers K.L. Getting to know ovarian cancer ascites: opportunities for targeted therapy-based translational research.Front. Oncol. 2013; 3: 256Crossref PubMed Scopus (276) Google Scholar, 70Liao J. et al.Ovarian cancer spheroid cells with stem cell-like properties contribute to tumor generation, metastasis and chemotherapy resistance through hypoxia-resistant metabolism.PLoS One. 2014; 9: e84941Crossref PubMed Scopus (263) Google Scholar] and is increased by fluid shear stress to which cells are exposed in malignant ascites [71Ip C.K. et al.Stemness and chemoresistance in epithelial ovarian carcinoma cells under shear stress.Sci. Rep. 2016; 6: 26788Crossref PubMed Scopus (79) Google Scholar]. On the other hand, these cells are protected from anoikis and drug-induced programmed cell death [44Sodek K.L. et al.Cell-cell and cell-matrix dynamics in intraperitoneal cancer metastasis.Cancer Metastasis Rev. 2012; 31: 397-414Crossref PubMed Scopus (111) Google Scholar], which is presumably related to their quiescence [72Correa R.J. et al.Modulation of AKT activity is associated with reversible dormancy in ascites-derived epithelial ovarian cancer spheroids.Carcinogenesis. 2012; 33: 49-58Crossref PubMed Scopus (53) Google Scholar] and the skewing of metabolism toward anaerobic glycolysis [70Liao J. et al.Ovarian cancer spheroid cells with stem cell-like properties contribute to tumor generation, metastasis and chemotherapy resistance through hypoxia-resistant metabolism.PLoS One. 2014; 9: e84941Crossref PubMed Scopus (263) Google Scholar]. It is therefore conceivable that a small number of tumor cell spheroids survive chemotherapy and due to their high tumorigenic potential initiate new metastatic lesions upon attachment to visceral organs. In view of the transient or conditional nature of the chemoresistant state of these cells, their interaction with the microenvironment, including host cells, EVs, soluble mediators, and the extracellular matrix, deserves particular attention. The impact of ovarian cancer cells on the immune system has been extensively studied (Box 3) and discussed in recent reviews [3Ahmed N. Stenvers K.L. Getting to know ovarian cancer ascites: opportunities for targeted therapy-based translational research.Front. Oncol. 2013; 3: 256Crossref PubMed Scopus (276) Google Scholar, 10Zhu X. Lang J. The significance and therapeutic potential of PD-1 and its ligands in ovarian cancer: a systematic review.Gynecol. Oncol. 2016; 142: 184-189Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar, 11Santoiemma P.P. Powell Jr., D.J. Tumor infiltrating lymphocytes in ovarian cancer.Cancer Biol. Ther. 2015; 16: 807-820Crossref PubMed Scopus (207) Google Scholar]. Reciprocal interactions are less well studied, but a number of recent studies have shed new light on the question of how subverted host cells within the unique microenvironment of ovarian carcinoma impinge on cancer progression and therapy resistance (Figure 1, Key Figure). Here we discuss these new and partly unexpected biological principles, which provide potential explanations for the failure of current treatment regimens and point to the need for innovative therapeutic approaches.Box 3T Cells Are a Major Determinant of Clinical Outcome in Ovarian Cancer and Are a Promising Therapeutic TargetIn a seminal study, Zhang and colleagues [73Zhang L. et al.Intratumoral T cells, recurrence, and survival in epithelial ovarian cancer.N. Engl. J. Med. 2003; 348: 203-213Crossref PubMed Scopus (2606) Google Scholar] showed that the presence of CD3+ T cells is strongly associated with a favorable clinical outcome. The 5-year overall survival rate for patients with tumor infiltrating T cells was 79% and for patients without T cells was 11.9% [73Zhang L. et al.Intratumoral T cells, recurrence, and survival in epithelial ovarian cancer.N. Engl. J. Med. 2003; 348: 203-213Crossref PubMed Scopus (2606) Google Scholar]. This observation was confirmed by a meta-analysis of ten studies with 1815 patients, which also identified CD8+ tumor-infiltrating lymphocytes as crucial effectors of the antitumor response [74Hwang W.T. et al.Prognostic significance of tumor-infiltrating T cells in ovarian cancer: a meta-analysis.Gynecol. Oncol. 2012; 124: 192-198Abstract Full Text Full Text PDF PubMed Scopus (431) Google Scholar]. By contrast, the presence of regulatory T cells (Tregs) or a low CD8+/Treg ratio is associated with poor survival [75Curiel T.J. et al.Specific recruitment of regulatory T cells in ovarian carcinoma fosters immune privilege and predicts reduced survival.Nat. Med. 2004; 10: 942-949Crossref PubMed Scopus (4077) Google Scholar, 76Sato E. et al.Intraepithelial CD8+ tumor-infiltrating lymphocytes and a high CD8+/regulatory T cell ratio are associated with favorable prognosis in ovarian cancer.Proc. Natl. Acad. Sci. U. S. A. 2005; 102: 18538-18543Crossref PubMed Scopus (1836) Google Scholar].Even though the immune surveillance system can efficiently control the initiation and progression of cancers, immune evasion is a hallmark of tumor development. A major mediator of immune evasion is the PD-1 immunomodulatory receptor on T cells, which is activated upon binding by PD-L1 expressed on tumor cells and TAMs [77Gubin M.M. et al.Checkpoint blockade cancer immunotherapy targets tumour-specific mutant antigens.Nature. 2014; 515: 577-581Crossref PubMed Scopus (1433) Google Scholar, 78Abiko K. et al.PD-L1 on tumor cells is induced in ascites and promotes peritoneal dissemination of ovarian cancer through CTL dysfunction.Clin. Cancer Res. 2013; 19: 1363-1374Crossref PubMed Scopus (167) Google Scholar]. PD-1 checkpoint activation inhibits the proliferation, survival, and function of tumor-infiltrating cytotoxic CD8+ T cells. The additional activation of the checkpoint controlled by CTLA4, another inhibitory receptor expressed on T cells, further increases the ratios of both CD4 and CD8 effector T cells to regulatory T cells (Treg) [79Nirschl C.J. Drake C.G. Molecular pathways: coexpression of immune checkpoint molecules: signaling pathways and implications for cancer immunotherapy.Clin. Cancer Res. 2013; 19: 4917-4924Crossref PubMed Scopus (204) Google Scholar].The PD-1/PD-1L pathway is believed to play a pivotal role in ovarian cancer [10Zhu X. Lang J. The significance and therapeutic potential of PD-1 and its ligands in ovarian cancer: a systematic review.Gynecol. Oncol. 2016; 142: 184-189Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar]. Consistent with this notion, the PD-1 blocking antibody nivolumab showed encouraging results in a subgroup of platinum-resistant patients, including full clinical remissions [80Hamanishi J. et al.Safety and antitumor activity of anti-PD-1 antibody, nivolumab, in patients with platinum-resistant ovarian cancer.J. Clin. Oncol. 2015; 33: 4015-4022Crossref PubMed Scopus (770) Google Scholar]. Current experimental and clinical studies that evaluated immune checkpoint inhibitors against ovarian cancer and clinical issues regarding immune checkpoint inhibitors are discussed in detail in recent reviews [66Chester C. et al.Immunotherapeutic approaches to ovarian cancer treatment.J. Immunother. Cancer. 2015; 3: 7Crossref PubMed Scopus (52) Google Scholar, 81Gaillard S.L. et al.The role of immune checkpoint inhibition in the treatment of ovarian cancer.Gynecol. Oncol. Res. Pract. 2016; 3: 11Crossref PubMed Google Scholar]. In a seminal study, Zhang and colleagues [73Zhang L. et al.Intratumoral T cells, recurrence, and survival in epithelial ovarian cancer.N. Engl. J. Med. 2003; 348: 203-213Crossref PubMed Scopus (2606) Google Scholar] showed that the presence of CD3+ T cells is strongly associated with a favorable clinical outcome. The 5-year overall survival rate for patients with tumor infiltrating T cells was 79% and for patients without T cells was 11.9% [73Zhang L. et al.Intratumoral T cells, recurrence, and survival in epithelial ovarian cancer.N. Engl. J. Med. 2003; 348: 203-213Crossref PubMed Scopus (2606) Google Scholar]. This observation was confirmed by a meta-analysis of ten studies with 1815 patients, which also identified CD8+ tumor-infiltrating lymphocytes as crucial effectors of the antitumor response [74Hwang W.T. et al.Prognostic significance of tumor-infiltrating T cells in ovarian cancer: a meta-analysis.Gynecol. Oncol. 2012; 124: 192-198Abstract Full Text Full Text PDF PubMed Scopus (431) Google Scholar]. By contrast, the presence of regulatory T cells (Tregs) or a low CD8+/Treg ratio is associated with poor survival [75Curiel T.J. et al.Specific recruitment of regulatory T cells in ovarian carcinoma fosters immune privilege and predicts reduced survival.Nat. Med. 2004; 10: 942-949Crossref PubMed Scopus (4077) Google Scholar, 76Sato E. et al.Intraepithelial CD8+ tumor-infiltrating lymphocytes and a high CD8+/regulatory T cell ratio are associated with favorable prognosis in ovarian cancer.Proc. Natl. Acad. Sci. U. S. A. 2005; 102: 18538-18543Crossref PubMed Scopus (1836) Google Scholar]. Even though the immune surveillance system can efficiently control the initiation and progression of cancers, immune evasion is a hallmark of tumor development. A major mediator of immune evasion is the PD-1 immunomodulatory receptor on T cells, which is activated upon binding by PD-L1 expressed on tumor cells and TAMs [77Gubin M.M. et al.Checkpoint blockade cancer immunotherapy targets tumour-specific mutant antigens.Nature. 2014; 515: 577-581Crossref PubMed Scopus (1433) Google Scholar, 78Abiko K. et al.PD-L1 on tumor cells is induced in ascites and promotes peritoneal dissemination of ovarian cancer through CTL dysfunction.Clin. Cancer Res. 2013; 19: 1363-1374Crossref PubMed Scopus (167) Google Scholar]. PD-1 checkpoint activation inhibits the proliferation, survival, and function of tumor-infiltrating cytotoxic CD8+ T cells. The additional activation of the checkpoint controlled by CTLA4, another inhibitory receptor expressed on T cells, further increases the ratios of both CD4 and CD8 effector T cells to regulatory T cells (Treg) [79Nirschl C.J. Drake C.G. Molecular pathways: coexpression of immune checkpoint molecules: signaling pathways and implications for cancer immunotherapy.Clin. Cancer Res. 2013; 19: 4917-4924Crossref PubMed Scopus (204) Google Scholar]. The PD-1/PD-1L pathway is believed to play a pivotal role in ovarian cancer [10Zhu X. Lang J. The significance and therapeutic potential of PD-1 and its ligands in ovarian cancer: a systematic review.Gynecol. Oncol. 2016; 142: 184-189Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar]. Consistent with this notion, the PD-1 blocking antibody nivolumab showed encouraging results in a subgroup of platinum-resistant patients, including full clinical remissions [80Hamanishi J. et al.Safety and antitumor activity of anti-PD-1 antibody, nivolumab, in patients with platinum-resistant ovarian cancer.J. Clin. Oncol. 2015; 33: 4015-4022Crossref PubMed Scopus (770) Google Scholar]. Current experimental and clinical studies that evaluated immune checkpoint inhibitors against ovarian cancer and clinical issues regarding immune checkpoint inhibitors are discussed in detail in recent reviews [66Chester C. et al.Immunotherapeutic approaches to ovarian cancer treatment.J. Immunother. Cancer. 2015; 3: 7Crossref PubMed Scopus (52) Google Scholar, 81Gaillard S.L. et al.The role of immune checkpoint inhibition in the treatment of ovarian cancer.Gynecol. Oncol. Res. Pract. 2016; 3: 11Crossref PubMed Google Scholar]. The communication between tumor cells and the microenvironment depends to large extent on EVs, also referred to as exosomes. EVs are small particles that are composed of lipids, proteins, and nucleic acids, and are released by virtually all cell types and thus are present in all biological fluids, including ascites. These vesicles are either shed as microvesicles from the cell membrane, or released as exosomes when multivesicular bodies fuse with the plasma membrane. EVs can convey biological materials to surrounding cells and thereby interfere with gene expression and signaling pathways [12Wendler F. et al.Extracellular vesicles swarm the cancer microenvironment: from tumor–stroma communication to drug intervention.Oncogene. 2016; (Published online August 22, 2016)https://doi.org/10.1038/onc.2016.253Crossref PubMed Scopus (106) Google Scholar]. There is emerging evidence that EVs transfer miRNAs, which shape the plasticity of the cells within the microenvironment (Figure 1). Of note, EVs have a cell-independent capacity to generate mature miRNAs and contain pre-miRNAs, along with processing enzymes such as Dicer or AGO2 [13Melo S.A. et al.Cancer exosomes perform cell-independent microRNA biogenesis and promote tumorigenesis.Cancer Cell. 2014; 26: 707-721Abstract Full Text Full Text PDF PubMed Scopus (1099) Google Scholar]. Given the crucial role of miRNAs in cancer, an miRNA signature associated with the survival of ovarian cancer was established based on the expression of 35 miRN