Abstract 1602: Quaking regulates macrophage MHC II-mediated antigen presentation and immune function in glioblastoma

Abstract Glioblastoma (GBM) remains one of the most lethal brain tumors, in part due to profound immunosuppression driven by tumor-associated macrophages (TAMs). A central deficit within the GBM microenvironment is the failure of macrophages to execute effective MHC class II (MHC II)-mediated antigen presentation, a process essential for priming anti-tumor CD4+ T cell responses. We identified the RNA-binding protein Quaking (Qki), frequently lost or mutated in GBM, as a critical regulator of macrophage antigen processing and presentation. Across human GBM datasets and the QPP (Qki-/-; Trp53-/-; Pten-/-) glioma model, Qki expression positively correlates with MHC II and its master regulator CIITA. Using bone marrow-derived macrophages, we found that Qki deficiency does not impair IFNγ-driven induction of MHC II transcripts. However, Qki knockout (KO) macrophages were unable to effectively activate CD4+ T cells in OT-II co-culture assays, indicating a defect downstream of transcription. Functionally, phagocytosis and DQ-OVA assays revealed that Qki KO macrophages display reduced phagocytosis and proteolytic activity, suggesting that Qki mediates multiple steps of antigen processing and presentation. To define Qki-dependent programs, we performed RNA sequencing in wild-type and Qki-KO macrophages. Qki loss led to coordinated downregulation of pathways involved in extracellular matrix interactions, focal adhesion signaling, and protein uptake. Using qPCR, we confirmed decreased expression of key regulators, including Cav1 and Thbs1, demonstrating that Qki orchestrates post-transcriptional programs that enable cytoskeletal dynamics, vesicle trafficking, and antigen processing required for MHC II function. To explore therapeutic potential, we tested the Qki co-activator agonist KD3010, which enhanced MHC II expression and CD4+ T cell activation in vitro, and improved survival in QPP mice. Proteomic analysis identified osteopontin (OPN), a protein elevated in GBM, as a tumor-derived factor suppressing Qki. In vitro, OPN treatment reduced Qki expression in macrophages, suggesting that it acts as a mechanism of immune suppression. In conclusion, our findings identify Qki as a central regulator of macrophage antigen processing and MHC II functionality and uncover a GBM-driven pathway that disrupts this axis. Restoring Qki represents a promising strategy to reawaken macrophage immunity in glioblastoma. Citation Format: Spring Yewon Hwang, . Quaking regulates macrophage MHC II-mediated antigen presentation and immune function 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 1602.