Microenvironment and Immunology Vaccine-Elicited CD 8 þ T Cells Cure Mesothelioma by Overcoming Tumor-Induced Immunosuppressive Environment

Eradicating malignant tumors by vaccine-elicited host immunity remains a major medical challenge. To date, correlates of immune protection remain unknown for malignant mesothelioma. In this study, we demonstrated that antigen-specific CD8þ T-cell immune response correlates with the elimination of malignant mesothelioma by a model PD-1–based DNA vaccine. Unlike the nonprotective tumor antigen WT1-based DNA vaccines, the model vaccine showed complete and long-lasting protection against lethal mesothelioma challenge in immunocompetent BALB/c mice. Furthermore, it remained highly immunogenic in tumor-bearing animals and led to therapeutic cure of preexisting mesothelioma. T-cell depletion and adoptive transfer experiments revealed that vaccine-elicited CD8þ T cells conferred to the protective efficacy in a dose-dependent way. Also, these CD8þ T cells functioned by releasing inflammatory IFNg and TNFa in the vicinity of target cells as well as by initiating TRAIL-directed tumor cell apoptosis. Importantly, repeated DNA vaccinations, a major advantage over livevectored vaccines with issues of preexisting immunity, achieve an active functional state, not only preventing the rise of exhausted PD-1þ and Tim-3þ CD8þ T cells but also suppressing tumor-induced myeloid-derived suppressive cells and Treg cells, with the frequency of antigen-specific CD8þ T cells inversely correlating with tumor mass. Our results provide new insights into quantitative and qualitative requirements of vaccine-elicited functional CD8þ T cells in cancer prevention and immunotherapy. Cancer Res; 74(21); 6010–21. 2014 AACR. Introduction Mesothelioma is a life-threatening malignant tumor in humans (1). Significant progresses have been made with vaccines that can prevent oncovirus-associated liver and cervical cancers (2, 3). The passive administration of tumorspecific antibodies or immune cells such as cytotoxic T lymphocytes (CTL), lymphokine-activated killer cell, and dendritic cells (DC) has been clinically beneficial to patients (4–6). Despite these progresses, preventing and eradicating malignant tumor such asmesotheliomaby vaccine-induced immune surveillance remains difficult even in animal model systems. Immune surveillance prevents and eliminates transformed tumor cells (7). The activation of oncogenes induces malignant tumor while rendering them vulnerable to antitumor immune effector cells such as CTLs and CD4þ Th1 cells (8, 9). When the host immune system fails to eliminate tumor cells in time, they may hijack ineffective immune responses to foster the growth and progression of cancer (10). On the one hand, tumor cells express inhibitory ligands on their surface to dysregulate T-cell function through interaction with T-cell receptors such as programmed cell death protein 1 (PD-1), CTL-associated antigen 4 (CTLA-4), and immune regulator T cell immunoglobulin mucin 3 (Tim-3; refs. 8, 11). On the other hand, the growing tumor evades immune surveillance by sustaining immunosuppressive cells such asmyeloid-derived suppressive cells (MDSC) and CD4þCD25þFoxp3þ regulatory T lymphocytes (Treg) systematically as well as in the tumor microenvironment (12). We hypothesized that an effective cancer vaccine should harness the immune system and reinstate immune surveillance by overcoming tumor-associated immune suppression. To test this hypothesis, we sought to determine the efficacy and immune correlates of protection of conventional tumor antigenWT1DNAvaccines and a soluble PD-1 (sPD1)-based fusion DNA vaccine, namely sPD1-p24fc, via in vivo electroporation (EP) delivery against malignant mesothelioma in immunocompetent BALB/c mice. The model xenoantigen p24 is derived from HIV capsid protein GAG (13). sPD1-p24fc/EP DNA vaccination induces uniquely high frequency of AIDS Institute and Department of Microbiology, Li Ka Shing (LKS) Faculty of Medicine, The University of Hong Kong, Hong Kong SAR, P.R. China. Department of Surgery and Centre for Cancer Research, The University of Hong Kong, Hong Kong SAR, P.R. China. Research Center for Infection and Immunity, LKS Faculty of Medicine, The University of Hong Kong, Hong Kong SAR, P.R. China. Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Z. Tan and J. Zhou contributed equally to this article. Corresponding Author: Zhiwei Chen, AIDS Institute and Department of Microbiology, Li Ka Shing Faculty of Medicine, The University of Hong Kong, L5-45, 21 SassoonRoad, Pokfulam, HongKongSAR, China. Phone: 852-3917-9825; Fax: 852-3917-9095; E-mail: zchenai@hku.hk doi: 10.1158/0008-5472.CAN-14-0473 2014 American Association for Cancer Research. Cancer Research Cancer Res; 74(21) November 1, 2014 6010 on July 25, 2017. © 2014 American Association for Cancer Research. cancerres.aacrjournals. org Downloaded from Published OnlineFirst August 14, 2014; DOI: 10.1158/0008-5472.CAN-14-0473