Glioblastoma (GBM) is the most lethal form of brain cancer in adults, with a median survival of one year following diagnosis. Unfortunately, both conventional and targeted therapies have failed to improve GBM patient survival over the last 40 years. Immune cells in the tumor microenvironment (TME) are genetically stable, and have emerged as promising therapeutic targets. Tumor-associated macrophages/microglia (TAMs) are the most abundant immune cells infiltrating the GBM TME (which can account for up to 50% of total live cells), where they exhibit an important role in promoting tumor progression and inducing immunotherapy resistance. However, the promise of TAM-targeted therapy or immunotherapy in general has not yet been realized in GBM, due in part to a limited understanding of the molecular mechanisms underlying TAM behavior and function in GBM. My postdoctoral work in the DePinho lab revealed novel mechanisms governing the recruitment of macrophages and microglia into the GBM TME. Notably, I determined that targeting macrophage/microglia infiltration via inhibition of lysyl oxidase (LOX) or circadian regulator CLOCK represents a promising therapeutic approach for GBM (Chen et al., Cancer Cell 2019; Chen at al., Cancer Discovery, under revision). Upon recruitment, macrophages/microglia exhibit a spectrum of phenotypes, including the immunostimulatory M1 phenotype and immunosuppressive M2 phenotype. It is well known that TAMs in GBM are usually polarized toward an M2 phenotype, and reprogramming TAMs from M2 to M1 phenotype could be a promising therapeutic strategy for GBM. My preliminary studies show that TANK binding kinase 1 (TBK1) is uniformly expressed by TAMs in GBM and that this druggable kinase can control macrophage polarization switch between M1 and M2 phenotypes. Both genetic and pharmacological inhibition of macrophage TBK1 impaired M2 polarization and inhibited GBM progression in multiple GBM mouse models (Chen et al., manuscript in preparation). In the K99 phase, this proposal will further investigate how macrophage TBK1 is regulated/activated in GBM and how TBK1 controls macrophage M2 polarization. Since TAMs are immune suppressive cells, in the R00 phase this proposal will investigate whether inhibition of TAM infiltration (LOX or CLOCK inhibition) and/or M2 polarization (inhibition of TBK1 and its related signaling pathways) can alter anti-tumor responsiveness to immune checkpoint blockade (ICB), i.e., I will test potential combination therapeutic strategies targeting TAMs and immune checkpoints in GBM. Finally, the proposed studies will identify the key factors from TBK1-regulated TAMs which might contribute GBM progression. I propose to employ an integrated strategy combining gain- and loss-of-function approaches, in vitro and in vivo systems, as well as proteomic and transcriptomic analysis to identify and characterize these factors. Together, this project will uncover novel mechanisms of GBM progression and offer new therapeutic targets for GBM.
The project will reveal the underlying molecular mechanisms governing how TAMs are reprogrammed and how these TAMs contribute to tumor progression in GBM. Moreover, these mechanistic insights will illuminate possible combination immunotherapy strategies targeting TAMs (including TAM infiltration and polarization) and immune checkpoints in GBM.