Microenvironment and Immunology Proin fl ammatory Characteristics of SMAC / DIABLO-Induced Cell Death in Antitumor Therapy

Molecular mimetics of the caspase activator second mitochondria-derived activator of caspase (SMAC) are being investigated for use in cancer therapy, but an understanding of in vivo effects remains incomplete. In this study, we offer evidence that SMAC mimetics elicit a proinflammatory cell death in cancer cells that engages an adaptive antitumor immune response. Cancer cells of different histologic origin underwent apoptosis when transduced with lentiviral vectors encoding a cytosolic form of the SMAC mimetic LVtSMAC. Strikingly, treatment of tumor-bearing mice with LV-tSMAC resulted in the induction of apoptosis, activation of antitumor immunity, and enhanced survival. Antitumor immunity was accompanied by an increase of tumor-infiltrating lymphocytes displaying low PD-1 expression, high lytic capacity, and high levels of IFN-g when stimulated. We also noted in vivo a decrease in regulatory T cells along with in vitro activation of tumor-specific CD8þ T cells by dendritic cells (DC) isolated from tumor draining lymph nodes. Last, tumorspecific cytotoxic T cells were also found to be activated in vivo. Mechanistic analyses showed that transduction of cancer cells with LV-tSMAC resulted in exposure of calreticulin but not release of HMGB1 or ATP. Nevertheless, DCs were activated upon engulfment of dying cancer cells. Further validation of these findings was obtained by their extension in a model of human melanoma using transcriptionally targeted LV-tSMAC. Together, our findings suggest that SMAC mimetics can elicit a proinflammatory cell death that is sufficient to activate adaptive antitumor immune responses in cancer. Cancer Res; 72(6); 1– 11. 2012 AACR. Introduction Deregulated apoptosis is a hallmark of many cancers (1). Several abnormalities have been described, including overexpression of inhibitor of apoptosis proteins (IAP; refs. 1–3) and failure of IAP antagonists to translocate from the mitochondria to the cytosol (4). Second mitochondria-derived activator of caspases (SMAC), also known as direct inhibitor of apoptosis-binding protein with low pI (DIABLO) is an IAP antagonist that is a potentially interesting therapeutic target. Cleavage of the mitochondrial targeting signal of SMAC is required for its translocation to the cytosol, in which SMAC binds to the baculovirus IAP repeat domain of IAPs (5). As such SMAC competes with caspase-3 and -9 for binding to IAPs, resulting in their release, cleavage of their substrates, and induction of apoptosis. This knowledge has led to the development of small molecules that mimic SMAC functions (6–9) and to the development of gene vectors encoding full-length SMAC (10), pro-SMAC (4, 11), or processed SMAC (tSMAC; ref. 12), illustrating that SMAC plays a pivotal role in the onset of cancer cell apoptosis. Dendritic cells (DC) in the tumor environment are mainly immature (13), as such ideally equipped to engulf and process dying cells (14). However, tumor-derived factors keep DCs immature (13), trap them within the tumor (15) hence impair effective presentation of ingested antigens to T cells. Moreover, these immature DCs have immune suppressive rather than stimulating properties (16). It has been suggested that this blockade on antigen-presenting cells can be overcome by proinflammatory signals such as pathogen-derived factors (13). Because we showed that lentiviral vectors (LV) activate DCs through Toll-like receptors (TLR; refs. 17, 18), we evaluated whether lentiviral delivery of tSMAC (LV-tSMAC) to tumor cells induces apoptosis and boosts antitumor immunity. We report on the antitumor efficacy of this strategy, together with the immunologic mechanisms responsible for its outcome. Authors' Affiliations: Department of Immunology-Physiology, Laboratory of Molecular and Cellular Therapy; Laboratory of HematologyImmunology, Vrije Universiteit Brussel, Jette; VIB Department for Molecular Biomedical Research, Research Unit for Molecular Pathophysiology and Experimental Therapy, University of Ghent, Ghent, Belgium; and Centre for Molecular Oncology and Imaging, Barts Cancer Institute, Queen Mary University of London, United Kingdom Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Corresponding Author: Karine Breckpot, Laboratory of Molecular and Cellular Therapy, Department of Immunology-Physiology, Vrije Universiteit Brussel, Laarbeeklaan 103/E, B-1090 Jette, Belgium. Phone: 0032-2-4774565; Fax: 0032-2-477-4568; E-mail: kbreckpo@vub.ac. be doi: 10.1158/0008-5472.CAN-11-240