Abstract 5218: RGS4+aSMA+CAF subsets promote tumor immune escape by regulating the formation of immune-exhaustive mesenchymal networks

Abstract Solid tumors consist of tumor cells and the tumor microenvironment (TME), a dynamic system comprising various immune cells, mesenchymal cells, cytokines, microvesicles, extracellular matrix, and vascular lymphatic networks. The recruitment of lymphocytes into tumors is essential for effective anti-tumor immunity and successful immunotherapy. The tumor stroma serves as the initial site for effector immune cell infiltration, and the mesenchymal network directly influences immune cell function. Cancer-associated fibroblasts (CAFs) are the largest proportion of TME intrinsic stromal cells, with significant heterogeneity and multiple origins, and perform different biological functions. CAFs can not only induce PD-L1 expression in tumor cells, but also promote PD-L1 expression in TME immune cells through the release of cytokines and vesicles, thereby facilitating tumor immune escape. In this study, we found that in some breast cancer patients, PD-L1 in tumor stroma was abundantly expressed and associated with immunosuppression and poor prognosis, and the distribution of tumor stromal PD-L1 was also closely related to the activation status of CAFs. Furthermore, we performed high-throughput RNA sequencing on PD-L1low and PD-L1high stroma cells, respectively. We found a CAF subset in high PD-L1 expression group, which was characterized by immune exhaustion and hyperactivation. Notably, Regulator of G protein signaling 4 (RGS4) pathways were significantly upregulated in this subset. Furthermore, we confirmed that RGS4 induces fibroblasts to adopt a PD-L1 immunodepleting phenotype and establishes an immune-exhaustive mesenchymal network that modulates anti-tumor immunity. In addition, we validated in humanized murine models that targeting RGS4 significantly enhances the efficacy of immunotherapy in animal models. The spatial heterogeneity of PD-L1 expression in TME is a primary factor contributing to the limited efficacy of PD-L1/PD-1 antibody therapies. Currently, numerous studies aim to enhance the immune response by inhibiting or degrading PD-L1. A comprehensive understanding of the molecular mechanisms governing PD-L1's spatial distribution could fundamentally enhance its clinical application. This study explored the mechanism governing the spatial distribution difference of PD-L1, and provided more possibilities for the development of targeted intervention in CAFs. Citation Format: Huiping Chen, Wenfeng Zeng. RGS4+aSMA+CAF subsets promote tumor immune escape by regulating the formation of immune-exhaustive mesenchymal networks [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5218.