Immune checkpoint modulation: Tenets and implications in glioblastoma

Glioblastoma (GBM) is the most common primary central nervous system tumor, and despite advances made in traditional chemotherapy and radiation, it continues to carry a poor prognosis. The discovery of the profound immunosuppressive microenvironment created by GBM has given insight on the aggressiveness of this recalcitrant disease. This has led many to believe that immune therapy may yield the improvement in survival that the neuro-oncology community is seeking. In other cancers, the targeting of immune checkpoints has been the most promising immunotherapeutic strategy to date. Immune checkpoints modulate the function of the immune system by increasing or decreasing immune activity. Checkpoint inhibitors and more recently agonists target molecules that regulate immune response to increase immune function either directly or by removal of inhibitory signals. These molecules modulate immunity in the physiologic state to maintain homeostasis, but they are co-opted by cancer to avoid immune detection and attack. The use of checkpoint inhibition to improve cancer therapy has revolutionized the field of oncology, leading to unprecedented improvements in survival from many systemic malignancies. Utilizing PubMed and ClinicalTrials.gov to compile published findings and ongoing trials, we review immune checkpoints and their modulators from bench to bedside over several decades. In this review, the discovery of different checkpoint molecules and the development of drugs used to target them are addressed. In addition, the current state of checkpoint inhibition in GBM, presenting completed and ongoing preclinical and clinical studies utilizing these therapies, is discussed. Finally, we conclude by reviewing the current limitations and potential future directions for the use of checkpoint blockade in the treatment of GBM.