Tumor-Associated Microglia and Macrophages in the Glioblastoma Microenvironment and Their Implications for Therapy

SIMPLE SUMMARY: Patients with glioblastoma—the most frequent and malignant primary brain tumor—have a poor prognosis with a median survival of less than 15 months. Despite extensive research, treatment of glioblastoma has not improved since 2005. Most therapeutic strategies have focused on eliminating the glioblastoma cells themselves, but tumor-infiltrating immune cells are potential targets as well. Glioblastomas are highly infiltrated with tumor-associated microglia and macrophages (TAMs), which are known to support the glioblastoma cells and promote tumor progression. Although broad categories are used to describe TAM phenotypes, recent technological advances have allowed deeper insights into their phenotypical differences. A better understanding of how known and unknown TAM phenotypes interact with their neighboring cancer cells may be crucial for establishing effective treatment strategies for glioblastoma patients. In this review, we give an updated overview of the role of TAMs in glioblastoma, and we discuss TAM–glioblastoma crosstalk and review potential therapeutic TAM-targeting strategies. ABSTRACT: Glioblastoma is the most frequent and malignant primary brain tumor. Standard of care includes surgery followed by radiation and temozolomide chemotherapy. Despite treatment, patients have a poor prognosis with a median survival of less than 15 months. The poor prognosis is associated with an increased abundance of tumor-associated microglia and macrophages (TAMs), which are known to play a role in creating a pro-tumorigenic environment and aiding tumor progression. Most treatment strategies are directed against glioblastoma cells; however, accumulating evidence suggests targeting of TAMs as a promising therapeutic strategy. While TAMs are typically dichotomously classified as M1 and M2 phenotypes, recent studies utilizing single cell technologies have identified expression pattern differences, which is beginning to give a deeper understanding of the heterogeneous subpopulations of TAMs in glioblastomas. In this review, we evaluate the role of TAMs in the glioblastoma microenvironment and discuss how their interactions with cancer cells have an extensive impact on glioblastoma progression and treatment resistance. Finally, we summarize the effects and challenges of therapeutic strategies, which specifically aim to target TAMs.