PDCD1 and brain cancer: The molecular mechanism by which KIs modulate immune system is still not clear today but some studies show that KIs can contribute to a protective antitumor immune response by reducing the number and activity of various types of immune cells involved in the evasion mechanisms of the immune system in brain cancer such as regulatory T cells (T-reg), myeloid-derived suppressor cells (MDSC), and programmed cell death protein 1 (PD-1) reduction (Figure 3).