Phagocytosis Checkpoints in Glioblastoma: CD47 and Beyond

Abstract

Glioblastoma multiforme (GBM) is one of the deadliest human cancers with very lim-ited treatment options available. The malignant behavior of GBM is manifest in a tu-mor which is highly invasive, resistant to standard cytotoxic chemotherapy and is strongly immunosuppressive. Immune checkpoint inhibitors have recently been in-troduced in the clinic and have yielded promising results in certain cancers. GBM however is largely refractory to these treatments. The immune checkpoint CD47 has recently gained attention as potential target for intervention as it conveys a “don’t eat me” signal to tumor associated macrophages via the inhibitory SIRP alpha protein. In preclinical models, administration of anti-CD47 monoclonal antibodies have shown impressive results with GBM and other tumor models. Several well characterized on-cogenic pathways have recently been shown to regulate CD47 expression in GBM cells and Glioma Stem cells (GSCs) include EGFR, beta catenin and LRIG2. Other macro-phage pathways involved in regulating phagocytosis including TREM2 and glycan binding proteins are discussed as well. Finally, Chimeric Antigen Receptor Macro-phages (CAR)-M could be leveraged for greatly enhancing phagocytosis of GBM and repolarization of the microenvironment in general. Here we comprehensively review the mechanisms that regulate about macrophage phagocytosis of GBM cells.