Abstract 7001: Tumor-intrinsic suppression of phagocytosis by RRP1 (ribosomal RNA processing 1) in glioblastoma

Abstract Glioblastoma (GBM) is the most aggressive and common malignant brain tumor in adults, with a median survival of just 14.6 months. Standard treatment, comprising maximal surgical resection followed by radiation and temozolomide, offers limited long-term benefit, highlighting the urgent need for more effective therapies. While other cancers have seen promising advances through T-cell immunotherapy, GBM has remained stubbornly resistant, largely due to its immunosuppressive microenvironment dominated by tumor-associated macrophages (TAMs). These macrophages, instead of engulfing tumor cells, are often hijacked to support tumor growth and evade immune clearance. GBM evades immune clearance in part by expressing “don’t eat me” signals such as CD47; however, clinical trials of anti-CD47 therapy in leukemia faced challenges, including toxicity and limited therapeutic efficacy. This gap in effective TAM-targeted therapies highlights an urgent need to uncover new, safer targets that could restore macrophage function. To address this, we performed a genome-wide CRISPR knockout screen in GBM cells (MGG18-RR) using human peripheral blood-derived macrophages polarized by GBM-conditioned media to identify tumor-intrinsic genes that inhibit macrophage phagocytosis. Our cytometry imaging results demonstrated that tumor-conditioned macrophages adopted distinct polarization profiles relative to unpolarized macrophages, thus establishing them as a more reliable experimental model. Our screen revealed RRP1 (ribosomal RNA processing 1) as a key regulator suppressing macrophage clearance of GBM cells. Proteomic data from the Clinical Proteomic Tumor Analysis Consortium (CPTAC) and the International Cancer Proteogenome Consortium (ICPC) showed that the RRP1 protein was increased in GBM tumors compared to normal brain tissue, indicating RRP1 upregulation as a feature of the GBM proteome. However, RRP1’s role in GBM phagocytosis remains unknown. Thus, to validate RRP1 as a therapeutic target in-vitro, we engineered RRP1 KO GBM cells (JX14P-RT) labeled with pHrodo dye which emits fluorescence when cells are phagocytosed. We performed this in-vitro phagocytosis assays with human peripheral blood-derived macrophages and RAW 264.7 macrophages, co-culturing a ratio of 200K macrophages per 1 million JX14P-RT cells for 20 hours. Co-cultures of CD47-KO-JX14P-RT tumor cells and macrophages were used as a positive control, and co-cultures of CD47-KO-JX14P-RT tumor cells and macrophages with Cytochalasin D, a known phagocytosis inhibitor, were used as negative control. Phagocytosis was quantified by the total Texas Red fluorescent signals. We found that loss of RRP1 significantly increased tumor cell engulfment (p < 0.05). Together, these findings highlight RRP1 as a promising immunotherapeutic target and support further investigation of RRP1 inhibition in vivo using syngeneic murine GBM models. Citation Format: Eshika Kudaravalli, Amr Elkholy, Mostafa Mohamed, Hasan Alrefai, Saeed Zakakhosravi, Satoru Osuka, Christopher D. Willey, Erin Ahn. Tumor-intrinsic suppression of phagocytosis by RRP1 (ribosomal RNA processing 1) in glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7001.