This study investigates the use of a special type of immune cell treatment, called T-cell therapy, in children and young adults who have had an organ transplant and developed a condition known as post-transplant lymphoproliferative disorder (PTLD). PTLD is a complication that can happen after an organ transplant, where cells in the body grow out of control. The study focuses on a type of PTLD that is positive for Epstein-Barr virus (EBV) and a protein called CD20.

Participants in the study will receive a treatment involving T-cells that are specifically designed to target and fight the EBV virus. These T-cells are known as LMP-specific T-cells, which are engineered to recognize and attack cells expressing latent membrane proteins associated with the virus. The study will examine how these T-cells work in treating the disorder, and whether this approach is feasible for patients.

• Who can participate: Participants must have had a solid organ transplant, developed a specific type of PTLD that is CD20 and EBV positive, and have measurable disease. They must be medically unable to reduce immunosuppression without risk and must not have certain prior treatments or conditions like central nervous system involvement or hepatitis infections.
• Study details: Participants will receive T-cell therapy, using specially prepared cells to target the virus. Some participants may receive a placebo, an inactive substance that looks like the investigational medicine but does not contain any medicine.