AIMS: CD155 is an immune checkpoint protein expressed in tumor cells that interacts with its ligand T cell immunoreceptor with immunoglobulin and ITIM domain (TIGIT) on natural killer (NK) cells and T cells, mediating inhibitory regulation on immune cells. Blockade of the CD155-TIGIT interaction has demonstrated clinical benefits in patients with advanced cancers. The transcriptional and post-translational mechanisms governing CD155 expression remain largely unknown.
METHODS: To identify regulators of CD155, we conducted a genome-wide CRISPR-Cas9 screen in cancer cells. Surface CD155 protein levels were analyzed via flow cytometry. The role of candidate regulators was validated through loss- and gain-of-function experiments with flow cytometry, Western blot, quantitative PCR, and chromatin immunoprecipitation (ChIP) assays. Additionally, ubiquitination assay was performed to examine post-translational modifications. Functional studies, including NK and T cell cytotoxicity assays, were conducted to assess the immune modulatory effects of CD155 regulation. Clinical relevance was evaluated by analyzing Cyclin C (CCNC) and CD155 expression in datasets of cancer patients who underwent immune checkpoint blockade therapy.
RESULTS: The CRISPR-Cas9 screen identified CCNC as a transcriptional suppressor of CD155. CCNC knockout led to increased surface CD155 expression in cancer cell lines. Mechanistically, CCNC inhibited CD155 transcription by suppressing the activity of the transcription factor FOSL2. Furthermore, CCNC was found to be ubiquitinated and degraded by the E3 ubiquitin ligase FBXO11, suggesting a post-translational regulatory mechanism. Functionally, loss of CCNC promoted CD155 upregulation, thereby enhancing tumor immune evasion from NK and T cell-mediated responses. Clinically, CCNC expression was negatively correlated with CD155 levels in cancer patients, particularly those receiving immune checkpoint blockade therapy.
CONCLUSION: This study identifies a previously unrecognized master regulator CCNC that functions as a suppressor of CD155-mediated cancer immune evasion. The findings of this study suggest that tumors with low CCNC expression may be resistant to monotherapy and highlight a combination immunotherapy (TIGIT/PD-1 co-blockade) as a promising anti-cancer therapeutic strategy to overcome immune evasion in CCNC-deficient tumors.
