Abstract P022: Reciprocal influence of immune response and tumor hypoxia during glioblastoma progression

Tumor hypoxia is linked to poor outcome for glioblastoma (GBM), a highly malignant brain cancer, but underlying mechanisms and instigators that initiate tumor hypoxia remain unclear. We tracked tumor hypoxia in GBM in mice using a sensitive fluorescent reporter. We revealed that tumor hypoxia functions as a critical link between immune cells and tumor cells that drives malignant potency and immunosuppression in GBM. Single-cell RNA sequencing analysis revealed that hypoxic GBM cells are quiescent, display a mesenchymal transition, are more represented in recurrent GBM and predict worse patient outcome. Interestingly, the in vivo GBM hypoxia gene signatures surprisingly showed an enrichment for immune pathways. We unveiled two potential mechanisms of hypoxia-induced immunosuppression: by sequestrating activated immune cells in hypoxia zones, thus limiting inflammatory spread and cutting off immune cell communication, and by reprograming entrapped immune cells towards an immunotolerant state. Reciprocally, entrapped TAMs release CCL8 and IL1β as hypoxic niche factors that not only reinforce immune cell retainment in hypoxic cores, but also shape the transcriptional response of hypoxic GBM cells. Contrary to the conventional viewpoint that hypoxia arises from rapid tumor expansion outstripping vascular supply, we discovered anticancer immunity as an important driving force of tumor hypoxia; attenuating immune responses by implanting GBM in host mice with immunodeficiency or IL1β deletion greatly decreased GBM hypoxia. Altogether, our study revealed a reciprocal influence of anticancer immunity and tumor hypoxia, which has significant ramifications for prognosis and immunotherapy for GBM.

Citation Format: Anirudh Sattiraju, Valerie Marallano, Zhihong Chen, Sangjo Kang, Concetta Brusco, Aarthi Ramakrishnan, Li Shen, Dolores Hambardzumyan, Roland H. Friedel, Hongyan Zou. Reciprocal influence of immune response and tumor hypoxia during glioblastoma progression [abstract]. In: Abstracts: AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2021 Oct 5-6. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(1 Suppl):Abstract nr P022.

TAMI-59. RECIPROCAL IMPACT OF CANCER IMMUNITY AND TUMOR HYPOXIA DURING GLIOBLASTOMA PROGRESSION

Tumor hypoxia is linked to poor outcome for glioblastoma (GBM), a highly malignant brain cancer, but the underlying mechanisms and the environmental factors that initiate tumor hypoxia are poorly understood. We tracked tumor hypoxia in GBM in immunocompetent mice with a hypoxia sensitive fluorescent reporter combined with single cell transcriptomics. We found that hypoxic GBM cells are quiescent, immunosuppressive and display a mesenchymal transition, all of which are linked to malignant potency. We also captured in vivo hypoxia gene signature, which is more represented in recurrent GBM and predicts worse outcome. Interestingly, hypoxic GBM cells is a diverse population, consisted of four subclusters, and enriched for immune pathways. Concordantly, our reporter highlighted a distinct geographic pattern of immune cells in hypoxic regions, with phagocytic tumor-associated macrophages (TAMs) and CD8+ cytotoxic T cells (CTLs) congregated in hypoxic cores confined by hypoxic GBM cells in pseudo-palisading patterns. Mechanistically, this is a dynamic temporospatial process, requiring cytokine CCL8. Remarkably, the sequestered TAMs also experience hypoxia, and they are reprogrammed to express immunotolerant markers by factors released from hypoxic GBM cells. Contrary to the conventional viewpoint that hypoxia arises from rapid tumor expansion outstripping vascular supply, we discovered anticancer immunity as an important driving force of tumor hypoxia; attenuating immune responses by implanting GBM in host mice with immunodeficiency or IL1β deletion greatly decreased GBM hypoxia. Analyses of human patient GBM samples highlighted a connection of mesenchymal subtype, immune response, and tumor hypoxia, all contributing to poor survival. Altogether, our study revealed a reciprocal influence of anti-tumor immunity and tumor hypoxia, which has significant ramifications for prognosis and immunotherapy for GBM.

TAMI-61. EXAMINING THE ROLE OF HYPOXIA INDUCED GENES CXCR4 AND NXPH4 IN INVASION OF HYPOXIC GLIOBLASTOMA CELLS

Hypoxia (low oxygen) has been associated with adverse effects in tumor biology by exaggerating the capabilities of invasion, proliferation, and survival of tumor cells within the tumor microenvironment. We engineered glioblastoma (GBM) proneural cells with a novel hypoxia reporter, HRE-UnaG, to study areas of tumor hypoxia and the effects that these hypoxic cells have on tumorigenesis. Single cell RNA-seq analysis from a mouse intracranially injected with our HRE dUnaG GBM cells revealed a shift to a mesenchymal state upon hypoxia (detected by expression of UnaG). Two genes, CXCR4 and NXPH4, were identified as being specifically induced in the hypoxic population. Our studies focus on the hypothesis that these two hypoxia induced genes, CXCR4 and NXPH4, are upregulated in hypoxic GBM cells, which may allow tumor cells to become more aggressive and resistant to conventional forms of therapies. GBM cells will be transduced with lentiviral vectors for Dox inducible shRNA knockdown of CXCR4 or NXPH4 to test specific contribution of these genes to the phenotype of the hypoxic population, with particular focus on the change in invasion and overall tumor burden upon gene silencing.

TAMI-18. SINGLE-CELL SEQUENCING AND GENETIC LABELING TO CHARACTERIZE HYPOXIC GBM CELLS IN THEIR MICROENVIRONMENT

Glioblastoma (GBM) recurrence, arising from treatment-resistant hypoxic cells, is a major contributor to patient mortality. We have engineered GBM cell lines with a novel genetic reporter system (“HRE-UnaG”), which sensitively labels hypoxic cells with green fluorescence. Using murine GL261 HRE-UnaG GBM cells in intracranial transplants in immunocompetent hosts, we detected hypoxic cells primarily around pseudopalisading structures. Here, we report our new results of single cell RNA-seq sequencing of intracranial GL261-HRE-dUnaG GBM tumors, which revealed in hypoxic UnaG+ GBM cells upregulation of more than 60 genes, including canonical hypoxia response genes such as Slc2a1, Bnip3, Ldha and Vegfa, and downregulation of a smaller number of genes, such as Rgcc, Cdc20 and Hist1h2ap, which regulate cell cycle progression. This novel GBM hypoxia signature was in TCGA patient samples mostly upregulated in recurrent GBM and associated with worse prognosis, especially within proneural and mesenchymal subtypes. Gene set enrichment analysis revealed that the GBM hypoxia signature was positively correlated with hypoxia, glycolysis, TNFα and MTORC1 signaling, and negatively correlated with oxidative phosphorylation, Myc and DNA repair pathways. Subclustering revealed four distinct subpopulations of hypoxic GBM cells, which were distinguished by differential expression of pathway genes for proliferation, angiogenesis, stress response, and type I/II interferon signaling. Intercellular communication between hypoxic GBM cells and the stromal cells were predicted using NicheNET package. To spatio-temporally label in future studies hypoxic GBM cells and their progeny, we have engineered GBM cells with an inducible, hypoxia-sensitive lineage-tracing reporter (HRE-Cre; floxed-tdTomato). In combination with the HRE-UnaG hypoxia reporter, we will be able to reveal the role of “hypoxia memory” in GBM expansion and recurrence after treatment. Identifying key markers of hypoxic cells in respect to treatment resistance and relapse will lead to improvements in GBM detection and new options for therapeutic treatments.

TMIC-59. INVESTIGATING PHYSIOLOGY OF THERAPY RESISTANT, TUMOR-INITIATING GBM CELLS IN HYPOXIC NICHES USING SPATIALLY-SENSITIVE HYPOXIA REPORTER SYSTEM AND SINGLE-CELL RNA SEQUENCING

Glioblastoma (GBM) is the most common and lethal brain cancer that invariably recurs after therapy due to presence of resistant GBM cells within hypoxic and peri-necrotic regions. Eradicating such GBM cells, which constitute a major source of tumor recurrence, is important to curb disease relapse. An endogenously expressed, spatially sensitive hypoxia reporter would therefore be a valuable tool to evaluate hypoxic zones in GBM in detail, and to measure the efficacy of hypoxia-activated drugs. For this purpose, we engineered a lentiviral vector that carries a hypoxia reporter, consisting of HIF response elements (HRE) that drive expression of UnaG fluorescent protein, which fluoresces independent of oxidative maturation. We validated the sensitivity of our reporter in vitro using U87MG, GBM2, and patient-derived GBM stem cell lines, and we performed intracranial transplantations of GBM cells in SCID mice to identify cells undergoing hypoxic stress in in vivo microenvironment. In addition, GL261 murine GBM cells with hypoxia reporter were intracranially implanted in C57BL/6 mice as syngeneic model for studies on immune responses. Brains from our transplant studies were dissociated and single-cell RNA sequencing (Drop-Seq) was performed to investigate heterogeneity in response to hypoxia within GBM cells and the cellular composition of microenvironment. We will also apply a hypoxia-activated prodrug, Evofosfamide (Evo), in our ongoing studies that can potentially eradicate hypoxic tumor cells and increase T cell infiltration and reverse immune suppression. As hypoxic niches are thought to confer resistance to radiation therapy (XRT), combining XRT with Evo could thus improve therapy efficacy. Our hypoxia gene reporter, combined with single-cell transcriptomics, could therefore serve as an effective tool to enable fundamental investigation of GBM microenvironment and could be used to evaluate therapies targeting tumor microenvironment to enhance GBM patient survival.

Dephosphorylation of threonine-821 of the retinoblastoma tumor suppressor protein (Rb) is required for apoptosis induced by UV and Cdk inhibition

The Retinoblastoma protein (Rb) is important in the control of cell proliferation and apoptosis. Its activity is controlled by reversible phosphorylation on several serine and threonine residues. When Rb is hypophosphorylated, it inhibits proliferation by preventing passage through the G 1- S phase transition. Hyperphosphorylated Rb promotes cell cycle progression. The role of Rb phosphorylation in the control of apoptosis is largely unknown, although several apoptotic stimuli result in dephosphorylation of Rb. It may be that dephosphorylation of specific amino acids signals apoptosis vs. cell cycle arrest. Using glutamic acid mutagenesis, we have generated 15 single phosphorylation site mutants of Rb to alter serine/threonine to glutamic acid to mimic the phosphorylated state. By calcium phosphate transfection, mutant plasmids were introduced into C33A Rb-null cells, and apoptosis was induced using UV. Apoptosis was measured by ELISA detection of degraded DNA and by immunoblotting to assess proteolytic cleavage of PARP. Our results show that only mutation of threonine-821 to glutamic acid (T821E) blocked apoptosis by 50%, whereas other sites tested had little effect. In Rb-null Saos-2 and SKUT-1 cells, the T821E mutation also blocked apoptosis induced by the cdk inhibitor, Roscovitine, by 50%. In addition, we show that endogenous Rb is dephosphorylated on threonine-821 when cells are undergoing apoptosis. Thus, our data indicates that dephosphorylation of threonine-821 of Rb is required for cells to undergo apoptosis.