Research Article Natural Immunity Enhances the Activity of a DR5 Agonistic Antibody and Carboplatin in the Treatment of Ovarian Cancer Ahmed El-Gazzar 1 , Paul Perco 5 , Eva Eckelhart 2 , Mariam Anees 1 , Veronika Sexl 2 , Bernd Mayer 5 , Yanxin Liu 6 , Wolfgang Mikulits 3 , Reinhard Horvat 4 , Thomas Pangerl 1 , Dexian Zheng 6 , and Michael Krainer 1 Abstract The tumor necrosis factor–related apoptosis-inducing ligand (TRAIL) induces apoptosis specifically in can- cer cells with little effect on normal cells. We have previously shown that TRAIL signaling is altered in most ovarian cancer patients and that resistance to TRAIL contributes to ovarian cancer progression. In this study, we investigated whether resistance to TRAIL may be overcome by a monoclonal TRAILR2 (DR5) agonistic antibody (AD5-10). We found that the joint presence of AD5-10 with TRAIL and natural killer (NK) cells expressing TRAIL resensitizes ovarian cancer cells to apoptosis in vitro and in vivo, respectively. The combi- nation of AD5-10 with carboplatin exerts a more than additive effect in vitro, which may at least partially be explained by the fact that carboplatin triggers DR5 expression on ovarian cancer cells. Moreover, AD5-10 restores the sensitivity of platin-resistant ovarian cancer to carboplatin in vivo. In addition, we found that TRAIL expression and NK cells are abundant in the tumor microenvironment and that depletion of NK cells abolishes the antitumor activity of AD5-10. This indicates that NK-mediated immunosurveillance against ovarian cancer might be mediated by TRAIL and that apoptosis induced by AD5-10 requires the presence of NK cells. In conclusion, this study indicates a key role and strong antitumorigenic effect of DR5 and high- lights a novel link between NK-mediated immunosurveillance and activation of DR5-mediated apoptosis in ovarian cancer. Mol Cancer Ther; 9(4); 1007–18. ©2010 AACR. Introduction Ovarian cancer is the most lethal gynecologic cancer and the fifth leading cause of cancer-related deaths among women in western industrialized countries (1, 2). Ovarian cancer originating from the ovarian surface epi- thelium is the most common form and displays a range of histologic subtypes (3). Whereas most ovarian cancers are sensitive to platin-based chemotherapy at the time of di- agnosis, recurrence of the disease is frequent, and ulti- mately, platin-resistant disease develops in all patients. Apoptosis is important for maintaining cellular ho- meostasis in normal tissues by eliminating disordered cells, and defects in the apoptosis pathway may lead to cancer (4). The apoptotic cascade can be stimulated by death receptors, resulting in activation of caspases (5). Trimerization of tumor necrosis factor–related apoptosis- inducing ligand (TRAIL) functional receptors TRAILR1 (DR4) or TRAILR2 (DR5) by their ligands leads to the assembly of death-inducing signaling complex, which initiates apoptotic cascade (6). DR4 and DR5 are charac- terized by an extracellular cystein-rich domain and an intracellular death domain, giving them the ability to trigger the assembly of the death-inducing signaling complex. We have previously found that TRAIL is highly expressed in the human ovarian cancer microenviron- ment, but that tumor tissues display a reduced number of TRAIL functional receptors (7). One major physiologic role of TRAIL is the mediation of natural immunity and the elimination of developing tu- mors (8, 9). Previous studies have shown that soluble TRAIL or agonistic monoclonal antibodies specific for functional TRAIL receptors exhibit tumoricidal activities, a phenomenon that has been tested in clinical trials (10). The agonistic human DR5-specific monoclonal antibody AD5-10 used in this study was reported to mediate antitu- mor effects in various tumor cells and, due to its unique binding site, does not compete with TRAIL for binding to DR5 in contrast to other agonistic DR5 antibodies (11). In the current study, we identified the functional role of DR5 in ovarian cancer progression and shed light on a novel strategy to eliminate ovarian cancer in a preclinical mouse model. Moreover, we show for the first time that the function of natural killer (NK) cells is necessary for Authors' Affiliations: 1 Division of Oncology, Department of Medicine I, 2 Institute of Pharmacology, 3 Institute of Cancer Research, Department of Medicine I, and 4 Department of Pathophysiology, Medical University of Vienna; 5 Institute for Theoretical Chemistry, University of Vienna, Vienna, Austria; and 6 National Laboratory of Medical Molecular Biology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China Note: Supplementary material for this article is available at Molecular Cancer Therapeutics Online (http://mct.aacrjournals.org/). Corresponding Author: Michael Krainer, Division of Oncology, Depart- ment of Medicine I, Medical University of Vienna, Waehringer Guertel 18-20, 1090 Vienna, Austria. Phone: 43-1-40400-7572; Fax: 43-1-40400- 1685. E-mail: michael.krainer@meduniwien.ac.at doi: 10.1158/1535-7163.MCT-09-0933 ©2010 American Association for Cancer Research. Molecular Cancer Therapeutics www.aacrjournals.org 1007 on February 3, 2016. © 2010 American Association for Cancer Research. mct.aacrjournals.org Downloaded from Published OnlineFirst April 6, 2010; DOI: 10.1158/1535-7163.MCT-09-0933