Neurobiological pathways underlying brain tumor progression: basis for oncogenicity and opportunities for immunotherapeutic intervention

Both primary and metastatic brain tumors rely on signals from the surrounding environment for their survival and progression. In particular, the most common and lethal brain cancer, glioblastoma (GBM), derived from glial cells (astrocytes or microglia), has been shown to integrate into synaptic networks and to receive paracrine signals from neighbouring tumor microenvironment (TME) cells. There is increasing evidence that metastatic disease in the brain exhibits similar behavior. The TME both maintains malignant cells and is maintained by them, a process that relies on cancer stem cells (CSCs). These stem cells and their signaling mechanisms, including in the case of GBM, “GSCs,” provide possible novel targets for immunotherapy. In this review, we will discuss the integration of primary and malignant brain tumors into normal synaptic networks, the role of tumor stem cells and the TME in this integration, and the potential for immunotherapeutic targeting of these processes.