The purpose of this study is to learn more about the ability of a substance called arginine to improve the functioning of the immune system in people with a certain type of brain tumor. This could lead to improvements in a type of treatment for brain tumors called immunotherapy. The immune system includes organs, cells, and substances in the body that fight infection and disease. Immunotherapy is a type of treatment that uses the immune system as a tool to seek out and destroy abnormal cells. Immunotherapy requires that the immune system be working properly. Arginine is a normal component of protein (an amino acid) that we all consume in foods such as red meat, poultry, fish, and dairy products and that our bodies can make. Arginine helps the immune system function normally. Recent research has shown that certain types of brain tumors decrease the amount of arginine in the body leading to impaired immune system function. This may interfere with the ability of immunotherapy to fight abnormal cells. We would like to see if giving people with brain tumors arginine in powder form will make their immune systems work better.