This thesis focuses on the interaction of herpes virus reactivations and immune reconstitution after hematopoietic stem cell transplantation (HSCT). HSCT is increasingly used as treatment for malignant and non-malignant hematological and immunological diseases. Due to the pre-transplant conditioning treatment and immunosuppressive therapy, HSCT recipients are severely immuno-suppressed, resulting in an increased susceptibility for opportunistic infections (e.g. viral reactivations and disease). Herpes viruses, which establish life-long persistence in the host upon primary infection, rarely cause severe disease in healthy individuals. Herpes virus reactivations, especially cytomegalovirus and Epstein–Barr-virus, are severe post-transplant threats and are associated with acute Graft-versus-host-Disease (aGvHD), allograft rejections and increased mortality. However, the role of human herpes virus type 6 (HHV6) reactivation after HSCT has not been investigated into detail. Although HHV6 rarely causes severe disease in healthy individuals, we observed that 67% of paediatric patients developed HHV6 reactivation early after HSCT and was strongly associated with aGvHD and increased mortality both in paediatric and adult HSCT recipients. In line with the general accepted hypothesis of the pathogenesis of aGvHD, early occurrence of viral infections/reactivations after HSCT may result in tissue damage and enhanced inflammation, resulting in the development of allo-reactive T-cells and consequently in the clinical syndrome of aGvHD. HHV6 reactivation, occurring very early after HSCT may have such a role as well: host antigen presenting cells may be activated and induce the development of allo-reactive T-cells and consequently aGvHD. To gain insight in absolute T-cell numbers during severe immunosuppression, immune reconstitution early after HSCT in children was prospectively monitored. Delayed T-cell reconstitution in recipients after cord blood (CB) transplantation compared to bone marrow transplantation was observed. However, increased proportions of preferable naïve T-cells instead of effector/memory T-cells were observed 1 year after CB HSCT compared to bone marrow HSCT. Especially in children this may indicate improved immune responses and decreased risks of infections at adult ages after CB compared to bone marrow HSCT. To gain more insight in the specific features of antiviral immune responses after HSCT, we prospectively analyzed perforin-expression as a surrogate marker for ‘effector’ function of T-cells. Remarkably, although absolute CD8+T-cells did not differ, a peak in perforin expressing CD8+T-cells coincided with a decline in herpes virus load and control of herpes virus reactivation. We also observed an increased number of HHV6 specific T-cells in HSCT recipients after HHV6 reactivation compared to recipients without HHV6 reactivation which is promising for future HHV6-specific immunotherapy. Moreover, we observed that regulatory T-cells, which are suggested to play a role in preventing aGvHD, are present at lower levels in pediatric HSCT recipients with aGvHD. Altogether, the data suggest an important interaction between viral reactivation, its immune response and the consequent development of aGvHD. Several (bio)markers (e.g. perforine, regulatory T-cells) may be used to identify patients at risk for complications and to guide immunosuppressive therapy. The most important future challenge is to improve immune reconstitution and develop attractive adjuvant (immuno)therapies to prevent or treat viral reactivations after HSCT. Increasing the safety of the HSCT procedure will eventually cure more patients
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