STMC-15. INNATE IMMUNE SIGNALING VIA TLR4 SUPPRESSES SELF-RENEWAL IN GLIOBLASTOMA

Abstract

Multiple cellular and molecular components within the glioblastoma microenvironment work to maintain a balance between immune dampening and inflammation, effectively preventing immune rejection of the tumor. Therefore, revealing the mechanisms that dampen immune signaling is critically important. Immune response initiation can be activated by ligands recognized by the toll-like family of receptors (TLRs). TLR activation limits proliferation in developing neural systems, yet these pathways are generally drivers of proliferation in many tumors including glioblastoma. This functional shift is surprising, considering the number of developmental programs activated in self-renewing cancer stem cells (CSCs). While mechanisms responsible for TLR-mediated tumor progression are largely unknown, TLR ligands are currently under investigation as adjuvants for immunotherapy approaches. Using CSCs from glioblastoma patient-derived xenografts, we find that proliferation of non-CSCs is attenuated by TLR ligands, whereas CSC proliferation is unaffected by ligand exposure. Profiling revealed that CSCs have reduced TLR4 compared to non-CSCs and that low TLR4 expression correlates with aggressive disease progression. Overexpression of TLR4 in CSCs reduced stem cell signaling and attenuated proliferation, self-renewal, and tumor growth. Mechanistically, we found that TLR4 signaling is directly connected to the stem cell transcriptional program by way of tank-binding kinase 1 (TBK1) and retinoblastoma binding protein 5 (RBBP5). Further, RBBP5 is necessary and sufficient for CSC self-renewal via epigenetic regulation of core self-renewal transcription factors. The activation of TBK1 downstream of TLR4 is responsible for RBBP5 suppression. Our findings reveal a unique pathway through which TLR4 can potently suppress the stem cell state, linking innate immune signaling responses to stem cell maintenance. These data suggest that strategies to activate the immune system may also negatively impact stem cell programs in tumor cells, indicating that approaches that alter the immune system also target the treatment-refractory CSC population.