Abstract 3795: Novel mechanism underlying tumor immune evasion through tumor endothelial cells
Authors: Taguchi Kazuhiro , Onoe Takashi , Ohdan Hideki 
Published: August 2018

Abstract
Background: There has been a recent interest in how tumor cells escape from immune surveillance while establishing tumor microenvironments. Tumor endothelial cells (TECs), which constitute the lining of the tumor vessels, develop in the tumor microenvironment and come in contact with circulating immune cells. As such, TECs may control the trafficking or the anti-tumor reaction of an immune cell. We hypothesized that TECs may have an immune-suppressive potential in tumor microenvironments; thus, TECs may contribute to tumor immune-evasion.
Objective: We aimed to investigate the immune-suppressive functions of TECs.
Materials and Methods: We established a mouse cancer-model using the B16 mouse melanoma cell line and isolated TECs from the tumors. The immunological character of TECs was assessed by performing flow cytometric phenotyping of TECs and in vitro antigen endocytosis assay using exogenous antigen ovalbumin (OVA). Next, to identify the antigen presentation and immune-suppressive functions of TECs, B16 cells transfected with ovalbumin (B16-OVA) were inoculated in mice and the TECs isolated from the B16-OVA tumor were analyzed. An antibody reacting specifically to complex of OVA-derived antigen presented with mouse MHC class I was used to test antigen presentation. Further, to verify antigen-specific suppressive functions, TECs isolated from B16-OVA tumors were co-cultured with OVA-specific CD8+ T cells (OT-I cells) stimulated by OVA-pulsed bone marrow derived dendritic cells (BMDCs) in vitro. In this suppression assay, we performed a blocking assay using an anti-programmed death-ligand 1 (PD-L1) antibody. We also performed an antigen-specific cytotoxicity assay using B16-OVA cells as targets and OVA-specific CD8+ T cells which had been co-cultured with TECs as effectors.
Results: TEC expressed MHC class I/II and co-stimulating molecules and PD-L1, suggesting that TECs had antigen-presenting and immune-suppressive properties. We found that TECs endocytosed B16-OVA derived protein and presented tumor-derived OVA peptides with their MHC class I molecule. TECs from B16-OVA tumor showed significantly greater suppression of OT-I CD8+ T cell proliferation compared to those from non-transfected B16 tumor. Similarly, in a cytotoxicity assay, TECs from B16-OVA tumor suppressed cytotoxicity of CD8+ T cells in an antigen specific manner. Strikingly, this suppressive function of TECs for the proliferation and cytotoxicity of OVA-specific T cells was abrogated by blocking PD-L1 using an anti-PD-L1 antibody, suggesting that the PD-1/PD-L1 axis contributes towards the immune-evasion mechanism.
Conclusions: TECs have the characteristics of antigen presenting cells and regulate the immune response of tumor antigen specific T cells via the PD-1/PD-L1 pathway. Our data may suggest the novel mechanism of tumor immune-evasion through TECs.
Citation Format: Kazuhiro Taguchi, Takashi Onoe, Hideki Ohdan. Novel mechanism underlying tumor immune evasion through tumor endothelial cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3795.