Abstract LB121: Endothelial cells mediated paracrine signaling alters immune cell modulators on colorectal cancer cells

Abstract Background: Metastatic colorectal cancer (mCRC) is the second leading cause of cancer related deaths in the US. Various factors in the tumor microenvironment (TME) play significant roles in progression of CRC tumors and their response to various therapies. Previous studies from our laboratory and other groups have demonstrated that blood vessels and their constituent endothelial cells (ECs) exert strong effects on CRC cells by altering stemness, enhancing drug resistance, and increasing cell survival. Although ECs have also been shown to affect immune cell function or alter the tumor immune environment by direct or indirect mechanisms, their role in altering immune-modulating effectors on tumor cells, especially CRC cells, are poorly understood. Thus, elucidating the role of ECs in regulating immune-modulators in CRC cells and understanding the underlying mechanisms by which this occurs will provide important insights into the role of ECs on altering the immune-microenvironment of CRC tumors that likely affects tumor progression and, possibly, response to immunotherapies. Methods: A panel of CRC cells lines were exposed to conditioned media (CM) from CRC or multiple primary or immortalized liver parenchymal endothelial cells (LPECs), and alterations in some of the known immune-inhibitory ligands were measured by western blotting or flow cytometry. Molecular size-based fractionation of CRC-CM or LPEC-CM were performed and various sized fractions were utilized to detect fractions harboring inducers of immune-inhibitory ligands. Mass spectrometry was performed to identify possible factors in LPEC-CM. Western blotting of CRC cell lysates was performed to identify possible signaling changes following LPEC-CM treatment. Results: Our studies demonstrated that ECs were more efficient in inducing PD-L1 expression in CRC cells compared to other cell types present in the CRC tumor microenvironment. LPEC-CM induced PD-L1 expression in multiple CRC cells regardless of their genetic mutations or MSI-H/MSS status. Initial studies indicate that LPEC-CM can also enhance MHC-1 expression in CRC cells. Size-fractionation studies indicate that inducer/s of PD-L1 expression are present in a fraction >100kDa. Mass-spectrometry of this fraction has identified prospective candidates that can possibly induce PD-L1 on CRC cells. CRC cells treated with LPEC-CM led to enhanced levels of pSTAT3, a known PD-L1 transcription factor. Conclusions: Our studies, for the first time, demonstrate that ECs, through paracrine signaling, can educate CRC cells to overexpress immune-modulatory ligands. Further studies are being conducted to elucidate the molecular factors involved in these processes. These studies not only shed light on the complex role played by ECs in modulating the TME, but also warrants further investigations into the effects of these changes in CRC tumor progression and responses to therapies. Citation Format: FanFan, Lee M. Ellis, Rajat Bhattacharya. Endothelial cells mediated paracrine signaling alters immune cell modulators on colorectal cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB121.