Investigating the systemic immune landscape of thymectomised paediatric transplant patients at risk of EBV-associated post-transplant lymphoproliferative disease

Abstract

Epstein-Barr virus-associated post-transplant lymphoproliferative disease (EBV-PTLD) are rare but life-threatening lymphoid malignancies that disproportionately affect paediatric heart transplant recipients. Recent discourse has focussed on the role of early removal of the thymus gland, which is critical for normal antiviral immunity, in the aetiology of EBV-PTLD after heart transplantation. This Fellowship aimed to investigate the systemic immune landscape of thymectomised paediatric transplant patients at risk of EBV-PTLD. Fifty-four patients awaiting transplant were recruited to the Immunology of THymectomy And childhood CArdiac transplant (ITHACA) study for immune profiling using full spectrum flow cytometry before transplant and at 3-, 6-, 12-and 24-months post-transplant. Patients were stratified into early (<6 months old) and late (≥6 months old) thymectomy for comparison to age-matched non thymectomy controls. Induction immunosuppression with anti-thymocyte globulin was associated with diminished antiviral CD16+ monocytes compared to Basiliximab induction. Failed expansion of CD16+ monocyte subsets was notable among patients with EBV DNAemia alongside a negative correlation between EBV DNA load and their expression of activation markers. Mycophenolate Mofetil was associated with an overall reduction in NK cells and a phenotypic shift towards immunosenescence. EBV DNAemia was associated with a marked increase in CD16dim CD56dim NK cells but a failure to expand the CD56dim NKG2A+ KIR- subset required for effective EBV control. Early thymectomy was associated with the depletion of regulatory, naïve CD4+ and CD8+ T-lymphocytes and expansion of senescent CD57+ memory T-lymphocytes. Most of these changes persisted after transplant and included the emergence of terminal effector memory subsets with increased expression of immune checkpoint proteins within the memory compartment of early thymectomy patients. Increased immunosenescence and exhaustion of EBV-specific T-lymphocytes were associated with early thymectomy and persistent EBV DNAemia. These data demonstrate that both early thymectomy and choice of immunosuppression are associated with distinct features of immunosenescence, exhaustion and impaired EBV control, all likely contributing to the risk of developing to EBV-PTLD.