This study involves an investigational cellular therapy called Afamitresgene Autoleucel. The overall goal of this study is to find out how safe and effective this cellular therapy is in treating people that have been diagnosed with synovial sarcoma, malignant peripheral nerve sheath tumors (MPNST), neuroblastoma or osteosarcoma that has grown or returned after being treated.

This study enrolls patients that have been diagnosed with osteosarcoma or neuroblastoma that has come back (relapsed) or has not responded to standard treatments (refractory). This study involves an investigational cellular therapy called GD2CART. The study will be testing different doses of the study drug to find the safest dose.