Abstract 1371: Rho-GEF hampers CD8+T cells-mediated immunosurveillance by reducing antigen presentation
Authors: Eyvani Haniyeh , Zhang Xinna 
Published: June 2022

Abstract
Immune response is a critical anti-tumor response to overcome tumorigenesis. In line with this, immune checkpoint blockade shows tremendous anti-tumor efficacy by enhancing cytotoxicity of CD8+ cytotoxic T lymphocytes (CTLs). However, cancer cells develop different mechanisms of immune evasion, among which suppression of immune recognition of somatic mutations via tumor antigen is prevalent. In fact, 65% to 90% of the cancers show reduced MHC-I antigen presentation. There is, nonetheless, little achievement toward the therapeutic approaches to restore antigen presentation on tumor cells. Colorectal cancer (CRC), the second deadliest cancer in U.S., shows increasing incidence and mortality rates over the past years. Standard conventional CRC treatments, surgery, chemotherapy and radiotherapy, often lead to many side effects due to their non-specificity and cytotoxicity. While immunotherapy is very promising, only a small population of CRC patients respond to the current immune checkpoint inhibitors. In this study, our objective was to harness a novel immunological approach to target CRC tumors. We developed a bioinformatics tool to create a pool of genes whose expression negatively correlates with cytotoxicity of CD8+ CTLs in the tumor microenvironment of CRC patients. Given the central role of antigen presentation in activating CD8+ CTLs, we further screened these genes based on the scope of their effects on antigen presentation. To this end, we developed stable clones of OVA expressing MC38 cells (MC38OVA) where OVA-derived peptide, SIINFEKL, will be bound to MHC-I complex for cell surface presentation. 4 genes with highest capacity to alter MHC-I cell surface levels were identified. Our consequent in vivo experiments and in vitro cytotoxicity assays identified that “Rho-GEF” gene was the top actionable target with striking suppressive effects on tumor growth. TCGA database analysis showed Rho-GEF is upregulated in 22% of CRC patients accompanied by a poor prognosis. Rho-GEF knockdown (KD) significantly inhibited tumor growth in immunocompetent mice whereas it only showed minor effects on immunocompromised mice, suggesting that the Rho-GEF may modulate tumor growth by regulating the interaction of tumor cells with non-tumor cells such as immune cells. Consistent with the major contribution of antigen presentation in promoting CD8+ CTLs activity, TNFα, IFNγ, and GZMB levels in CD8+ T cells isolated from Rho-GEF KD MC38-derived tumors showed a significant increase compared to the wild type tumors. Taken together, we introduced Rho-GEF as a potential clinical target involved in CRC immune evasion by modulating antigen presentation and CD8+ CTLs anti-tumor responses.
Citation Format: Haniyeh Eyvani, Xinna Zhang. Rho-GEF hampers CD8+T cells-mediated immunosurveillance by reducing antigen presentation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1371.