Trial of Nivolumab Following Partially Human Leukocyte Antigen (HLA) Mismatched BMT in Children & Adults With Sarcoma

This research is being done to find out if an investigational drug, Nivolumab, can be safely administered after a "half-matched" (haplo) bone marrow transplant (BMT), and if the investigational drug will help to prevent or delay relapse or progression of sarcomas. In this study investigators will also be trying to learn more about how the investigational drug changes blood and/or tumors. Participants are eligible for this trial if they have recently undergone a "half-matched" (haplo) bone marrow transplant and have either relapsed or are at high risk to relapse.