Brain Tumor Immunotherapy

Glioblastoma, Microglia, Macrophages, T-cells, Combinatorial Immunotherapies, Translational Clincial Trials

Glioblastoma multiforme (GBM) is a fatal brain tumor that is resistant to all treatments. Besides other non-neoplastic stromal cells, the tumor microenvironment (TME) of GBM consists of a large, dynamic compartment of immune system related cells. Main players of the immune TME (iTME) are tumor infiltrating brain-resident microglia, macrophages, and T-cells. GBMs are capable of subverting the iTME to facilitate their own growth by re-educating it for their own purposes. Therefore, instead of targeting the tumor cells directly, modulation of the iTME is likely to be a promising approach to combat the disease. Multiple strategies that target either adaptive or innate compartments of the iTME are currently being evaluated. 

Previously, we designed novel orthotopic patient-derived xenograft mouse models with genetically color-coded macrophages and microglia, and modulated tumor-associated macrophages and microglia (termed tumor-associated macrophages, TAMs) of the GBM-iTME by pharmacological means. This in vivo modulation prompted both macrophage- and microglia-induced tumor cell phagocytosis. Moreover, it led to morphological changes in microglia, detectable with cranial in vivo imaging. The efficacy of our therapeutic intervention was preserved in mice lacking Ccr2, limiting macrophage recruitment to the brain. GBM phagocytosis by microglia was sufficient to lessen the tumor burden significantly. Under  treatment, macrophages changed their transcriptional profile towards a pro-inflammatory, M1-polarized signature, whereas microglia displayed a loss of M2 genes. Thus, microglia are a potential target of innate iTME modulation in GBM.

Genetic and pharmacological microglia modulation in syngeneic brain tumor models 

The interplay between the adaptive immune system and microglia in the setting of innate immune modulation has not been studied thus far. Based on the results of our previous research, we hypothesize that microglia reprogramming can elicit an adaptive immune response. To test this, we are going to determine the response of the adaptive immune system in GBM after genetic or local pharmacological microglia modulation in vivo. Furthermore, we will combine local microglia modulation with systemic T-cell checkpoint inhibitors to achieve therapeutic synergy.

Combinatorial tumor and microglia targeting 

Strategies to specifically target GBM neoplastic cells, e.g. with signaling pathway inhibitors, have failed for similar reasons. Therefore, we hypothesize that local reprogramming of microglia within the GBM-iTME facilitates the action of tumor-specific strategies. Among others, EGFRvIII is a promising tumor antigen currently used as a target in clinical trials. Thus, we are going to combine local  microglia modulation with intratumoral EGFRvIII-specific chimeric antigen receptor (CAR) T-cell application to induce better tumor control. 

Glioblastoma region-specific microglia heterogeneity

GBM tumor cells display a vast regional heterogeneity. However, the phenotypical differences of tumor-associated microglia in human GBM are unknown. We hypothesize that intratumoral microglia exhibit a region-specific functional heterogeneity, influenced by paracrine crosstalk with the tumor cells. This is especially important at the invasion zone of the tumor, where tumor recurrence prevails. To assess this microglial heterogeneity, we aim to characterize the region-specific phenotype of tumor-associated microglia in defined locations of resected GBM tissue obtained from a mouse model, and from patients.

Our lab has a direct connection to the Neurosurgery Department of the University Hospital Basel, and we are able to obtain, process and analyze tumor samples based on intraoperative neuronavigation, intraoperative fluorescence microscopy and excellent in house flow-cytometry facilities. We have a vast biorepository of clinically annotated glioma specimens. Mouse modeling of GBM is performed in patient-derived xenograft models and syngeneic mouse models of GBM. We will perform intravital cranial window imaging to visualize the interaction of important players of the immune microenvironment with GBM cells at baseline and under experimental treatments in vivo. We have setup various collaborations within the DBM: Brain Tumor Biology (Prof. Mariani); Cancer Immunology, (Prof. Zippelius, Rochlitz and Läubli); Tumor Hetereogeneity, Metastasis and Resistance (Prof. Mohamed Bentires-Alj); Tissue Engineering (Prof. Ivan Martin, Dr. Manuele Muraro); Embryology and Stem Cell Biology (Prof. Verdon Taylor, Dr. Claudio Giachino); Brain Ischemia and Regeneration (Prof. Guzman); Molecular Immune Regulation (Prof. Jeker); and with external partners to reach our goal to find more effective treatments against GBM. Our work is funded by the Swiss National Science Foundation (Grant PP00P3_176974) and the Department of Surgery. 

Figure 1

GBM tumors were single cell-dissociated and subjected to flow cytometry directly after surgery. Up to 50% of the cells in the GBM microenvironment are are macrophages and microglia. Reprogramming of tumor-associated, immunosuppressive macrophages/microglia might be an important strategy against GBM. Previously, we showed that interference with the don't eat me signal CD47 by anti CD47 antibodies in GBM xenografted mice reduced tumor load, and prolonged survival.  

Figure 2

GBM surgery is routinely performed using 5-amino levulinic acid, which accumulates in GBM cells and emits a red light under the fluoresecent operation microscope. Further, we perform our surgeries with a navigation system that locates the site of our biopsy. To assess region-specific microglia heterogeneity, microglia and tumor cells are sorted directly after navigated biopsy and subjected to downstream analysis.