Abstract 5862: ABCB5 knockdown suppresses tumor-intrinsic expression of immune checkpoint regulators PD-1 and PD-L1 in hepatocellular carcinoma (HCC)

Abstract

Abstract ATP-binding cassette member B5 (ABCB5) identifies cancer-initiating cells (CICs) in multiple malignancies, including malignant melanoma, colorectal cancer, glioblastoma, and hepatocellular carcinoma (HCC). In human melanoma, ABCB5-positive CIC subpopulations have been shown to preferentially express the immune checkpoint molecules PD-1 and PD-L1, and tumor-intrinsic PD-1 receptor functions have been found to promote tumor growth, even in mice lacking adaptive immunity. In HCC, where ABCB5 mediates CIC chemoresistance, PD-1 expression has also recently been identified. However, whether ABCB5 regulates tumor-intrinsic PD-1 expression in HCC or other malignancies is currently unknown. Here, we employed RNA interference in Hep G2 HCC cells with high endogenous ABCB5 expression to study the potential role of ABCB5 in regulating PD-1 expression, utilizing the ABCB5 shRNA target sequence GCTGGAAAGATAGCAACTGAA for ABCB5 knockdown (KD) as described previously. Endogenous ABCB5 levels in ABCB5-KD cells were successfully reduced by 92.8% compared to controls. Investigation of the effects of ABCB5-KD on tumor-intrinsic immune checkpoint molecule expression and on expression of the HCC-CIC marker GRN revealed that ABCB5-KD significantly reduced PD-1 expression by 98.0%, and PD-L1 expression by 62.8% compared to controls. Expression of GRN was also significantly reduced in ABCB5-KD cells by 78.8% compared to controls, suggesting ABCB5 KD-dependent loss of CIC phenotype. Injection of ABCB5-KD cells in NSG mice showed 32.3% reduction in tumor size at endpoint (6 weeks) compared to controls (728.7 ± 199.3mm3 vs 1076.0 ± 450.9 mm3, mean ± SD; p<0.005, repeated measures ANOVA) and 8.53% reduction in tumor weight at endpoint (0.47 ± 0.21g vs 0.51 ± 0.21g, mean ± SD). These results provide initial evidence in an HCC cell line model system that ABCB5 functionally regulates tumor-intrinsic PD-1 and PD-L1 expression. They therefore suggest that ABCB5 blockade represents a novel therapeutic strategy for targeted inhibition of pro-tumorigenic tumor-intrinsic PD-1/PD-L1 expression, with important additional implications for immune checkpoint-targeted cancer immunotherapy. Citation Format: Chun Philip Yeung, Brian J. Wilson, Yuzuru Sasamoto, Ana Maria Waaga-Gasser, Svetlana Karpova, Qin Guo, Natasha Y. Frank, Markus H. Frank. ABCB5 knockdown suppresses tumor-intrinsic expression of immune checkpoint regulators PD-1 and PD-L1 in hepatocellular carcinoma (HCC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5862.