Radiation can be used as a vaccine, to increase the effect of immunotherapy. We call this Radvax, which is promising. Even in melanoma patients who failed anti-PD-1 treatment, 44% of patients had their tumor shrink when radiation was added. We need to understand when and how to use radiation with other immunotherapies. One of the most important breakthroughs recently is the idea that the bacteria in our intestines can impact how a patient responds to immunotherapy. In this pilot study, we will study the effect of Radvax treatment on CD8 T cells that recognize these intestinal bacteria. The hope is that this information can eventually be used to predict who will benefit from immunotherapy and who will get immune side effects. We will study this in a Radvax clinical study where melanoma patients receive radiation and a combination of two immune checkpoint blockade antibodies targeting PD-1 and CTLA-4. Understanding how to combine radiotherapy with immunotherapy will give us new treatment options for cancer patients.