Epstein-Barr virus (EBV) infects almost all humans and establishes lifelong latency in B cells. Posttransplant lymphoproliferative disorder (PTLD) is a rare but serious complication after transplantation triggered by immunosuppression and often related to EBV infection. The aim of this thesis was to study the role of EBV in relation to clinical and histological features of PTLD, regulatory T cells (Tregs), and donor or recipient origin of PTLD.

EBV surveillance after allogeneic hematopoietic stem cell transplantation (allo-HSCT) showed that EBV reactivations were common, but that symptomatic EBV disease (including PTLD) only occurred in the high-risk group (unrelated or mismatched related grafts, reduced-intensity conditioning). A threshold of 1000 copies/ml plasma distinguished EBV disease from asymptomatic reactivations.

In a population-based cohort of 135 PTLDs/lymphomas after solid organ transplantation (SOT) almost half were EBV^–. EBV⁺ PTLDs were associated with B cell phenotype, non-germinal center subtype of diffuse large B cell lymphoma (DLBCL), early-onset, graft involvement, antithymocyte globulin treatment, and younger age. EBV^– PTLDs were associated with T cell phenotype, bone marrow involvement, and hepatitis C. Most PTLDs displayed few or no intratumoral Tregs with the marker FoxP3, possibly due to heavy immuno­suppres­sion. Half of both FoxP3⁺ and FoxP3^– PTLDs were EBV⁺. FoxP3⁺ PTLDs were associated with B cell phenotype and hepatitis C. All PTLDs for which tumor origin could be determined were recipient-derived and half of them were EBV⁺. Eight of twelve recipient-derived graft PTLDs were disseminated outside the graft. T cell PTLD and hepatitis C were independently associated with inferior overall survival, whereas subtype of DLBCL, FoxP3-expression, and EBV-status did not influence survival.

In conclusion, monitoring of EBV DNAemia in high-risk patients after allo-HSCT and pre-emptive therapy is valuable for prevention of PTLD. Use of anti­thymocyte globulin increases the risk for EBV⁺ PTLDs after allo-HSCT and SOT. With long follow-up time, a large proportion of PLTDs after SOT are EBV^– with a different clinical presentation. Tregs are rare in PTLD and do not affect survival. The vast majority of PTLDs after SOT is of recipient origin. Graft PTLDs are more likely recipient-derived if disseminated. EBV-status is not associated with intratumoral Tregs or PTLD of recipient origin.