P-01.06 ICMT inhibition as a targeted approach for increasing MHC-I expression and blocking immune evasion in ovarian cancer

<h3>Background</h3> Ovarian cancer is the most lethal gynecological malignancy worldwide. Despite advanced treatments, its prognosis remains poor. Immunotherapy using programmed cell death 1 (PD-1) or PD-ligand 1 (PD-L1) immune checkpoint blockades (ICBs) has demonstrated significant promise in treating various cancers, including endometrial and cervical cancers. However, its effectiveness in treating ovarian cancer is unexpectedly low. Loss of antigen presentation that impairs CD8+ T cell recognition is a common mechanism of ICB resistance. Major histocompatibility complex class I (MHC-I) molecules present intracellular peptide antigens on the cell surface for CD8+ T cell recognition. Enhancing MHC-I expression may improve the efficacy of ICBs. Interestingly, interferon-γ secreted by tumor-infiltrating T cells can induce expression of both MHC-I and the immunosuppressive regulator PD-L1, which counteracts the effect of MHC-I. Therefore, it is crucial to identify genes that specifically increase MHC-I expression without elevating PD-L1 levels, termed MHC-I-specific negative regulators. <h3>Materials and Methods</h3> A dual-label FACS-based genome-wide CRISPR knockout library screening approach was employed to identify novel MHC-I-specific negative regulators in ovarian cancer. RNAi knockdown and pharmacological inhibition techniques were utilized to validate the candidate gene identified from the screening process. Flow cytometry was used to check MHC-I and PD-L1 expression. Candidate gene knockdown ovarian cancer OVCAR4, ES2, and RMG1 cells were co-cultured with T cells to assess the role on T cell-mediated killing of ovarian cancer cells. Multiplex immunohistochemistry (mIHC) was used to correlate the candidate gene expression with T cell activation signature in ovarian clinical samples. RNA-seq was conducted to analyze the underlying molecular mechanism behind MHC-I regulation. <h3>Results</h3> This screening revealed isoprenylcysteine carboxylmethyltransferase (ICMT) as an MHC-I-specific negative regulator in ovarian cancer. RNAi knockdown and pharmacological inhibition of ICMT significantly increase MHC-I expression without inducing PD-L1 expression, thereby enhancing CD8+ T cell-mediated killing of ovarian cancer cells. Besides, ICMT expression was negatively correlated with GZMB+CD8+ cytotoxic T cells in ovarian clinical samples in tissue microarray by mIHC. RNA-seq revealed ICMT down-regulated ubiquitination-related genes TSG101 and CXCR4, but had no significant effect on MHC-I mRNA expression. <h3>Conclusions</h3> ICMT was identified as an MHC-I-specific negative regulator through ubiquitination pathway that is associated with CD8+ T cell cytotoxicity in ovarian cancer. Targeting ICMT could enhance MHC-I expression and improve immune responses, offering a potential therapeutic strategy to enhance the efficacy of immune checkpoint blockades in ovarian cancer. R. Zhang: None. M.K. Siu: None. J. He: None. R. Long: None. X. He: None. H. Lu: None. K. Liu: None. A.N. Cheung: None. H.Y. Ngan: None. K.K. Chan: None.