Abstract 3241: Targeting the tumor stroma using CAR T-cells

Abstract

Abstract The majority of breast cancer deaths are due to metastatic disease, thus developing new strategies to target the metastatic cascade is clinically relevant and of scientific importance. Breast cancer metastasis is a multi-step process influenced by many factors, including the tumor microenvironment (TME). Key interactions between the TME and tumor cells have been identified as promoting metastasis, thus presenting novel clinical targets. One such target is endosialin (CD248), a transmembrane glycoprotein which has limited expression in the healthy adult but is strongly upregulated on tumor-associated pericytes and fibroblasts in most solid cancers, including breast cancer. Using endosialin knockout mice, our laboratory has demonstrated that expression of endosialin in the tumor stroma promotes tumor cell intravasation, thereby promoting metastatic dissemination. Endosialin-expressing cells present an interesting target for CAR T-cell therapy, an emerging approach within cancer immunotherapy. Thus far, targeting tumor cells with CAR T-cells has proven challenging in solid neoplasms owing to the harsh, immunosuppressive environment that CAR T-cells encounter. Here, we describe an alternative approach of targeting cells of the microenvironment, which are more accessible to the infiltrating CAR T-cells, to limit breast cancer metastasis. Experiments comparing the effects of endosialin-directed CAR T-cells (Endo3 CARs) and mock- transduced T-cells (mock T-cells) in a spontaneously metastatic BALB/c breast cancer model reveal that Endo3 CAR treatment significantly reduces the number of metastatic deposits in the lungs. Endo3 CARs expand by at least 2-fold in tumor-bearing mice and endosialin staining confirms that Endo3 CAR treatment results in the depletion of endosialin-positive cells in the tumor stroma, without affecting normal tissue. Strikingly, Endo3 CAR treatment also results in significant growth inhibition and necrosis of the primary tumor. However, some evidence of Endo3 CAR T-cell associated toxicity is observed in treated mice, and we are currently testing therapeutic options to minimize these side effects. No off- target activity is observed in parallel experiments with tumor-bearing endosialin knockout mice. In conclusion, these promising results demonstrate that Endo3 CAR T-cell therapy presents a novel approach to limiting breast cancer metastasis via the targeting of activated stromal cells in the tumor microenvironment. Citation Format: Sarah Ash, Frances Turrell, Clare Isacke, Steven Lee. Targeting the tumor stroma using CAR T-cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3241.