TEM8 specific T cells target the tumor cells and tumor-associated vasculature in triple negative breast cancer

Abstract

Background: Tumor endothelium marker 8 (TEM8) was discovered by St Croix, et. al. as one of nine gene products preferentially upregulated in the tumor-associated vs. normal endothelium [1]. Interestingly, TEM8 has also been identified as a tumor restricted antigen in triple negative breast cancers (TNBC) [2,3]; a clinical entity associated with a particularly poor prognosis. Being null for HER2, estrogen and progesterone receptors, targeted therapies for TNBC are quite limited. Purpose: To use T cells expressing TEM8-specific chimeric antigen receptors (CAR) as a novel approach to target both TNBC cells and their tumor-associated vasculature. Methods/ results: We used in silico design to construct a novel TEM8- specific CAR molecule. The antigen recognition exodomain consisted of a single chain variable fragment based on the TEM8-specific monoclonal antibody, SB5. The signaling endodomain consisted of the costimulatory molecule CD28 and CD3-zeta chain. The encoding DNA was codon optimized, synthesized and then sequence verified. We used a retroviral transduction system to express the TEM8 CAR transgene on HEK 293T, then on primary T cells. Approximately 70% of primary human T cells expressed the TEM8 CAR, as detected by flow cytometry. The expression of TEM8 was characterized using flow cytometry and western blot on a battery of TNBC lines, TEM8 transduced (modest and high expressers) cell lines as well as TEM8 negative cell lines. TEM8 CAR T cells recognized and killed TEM8 positive target cells in an antigen-dependent fashion in 51Cr release assays and secreted immunostimulatory cytokines upon encounter of TEM8 positive cells. There was no reactivity against TEM8 negative cell lines. No cytotoxicity or cytokine release was exhibited by T cells expressing an irrelevant (CD19 specific) CAR or non-transduced T cells from the same blood donor. We are currently testing this strategy in a vascularized orthotopic breast cancer murine model. Conclusion: TEM8 specific CAR T cells could serve as a tumor and vascular-targeted immunotherapeutic modality for triple-negative breast cancer.