High level of perforin expression in T cells: an early prognostic marker of the severity of herpesvirus reactivation after allogeneic stem cell transplantation in adults.

Background. Epstein-Barr virus (EBV) and cytomegalovirus reactivations are frequent complications of hematopoeitic allogeneic stem cell transplantation (SCT) because of a lack of T cell control after immunosuppression. Early diagnosis of reactivation and subsequent preemptive therapy relies on frequent viral load measurement. Additional virus-specific T cell reconstitution data could improve the predictive value of viral load detection for viral complications after transplantation. Here, we studied perforin expression in CD8 + T cells as a measure of cytotoxic T cell capacity in relation to the occurrence of viral reactivation. Methods. In a prospective study, we monitored 40 patients during the first 3 months after transplantation and measured viral loads in combination with intracellular perforin expression in CD8 + T cells. Results. Median perforin expression in CD8 + T cells throughout follow-up was higher in patients with viral reactivations than in patients without viral reactivations (4.9% vs 2.3%; P = .001). The median percentage of perforin-expressing CD8 + T cells in patients with high viral reactivations exceeding 1000 copies/mL (10.7%) was statistically significantly higher than that in patients with minor reactivations of 50-1000 copies (4.0%), that in patients with detectable EBV loads that did not exceed the detection limit of 50 copies/mL (2.9%), and that in patients without reactivations (0.8%). Patients with high viral reactivations reached a high percentage of perforin-expressing CD8 + T cells (>10.2%) more often and faster than did patients with low viral loads (≤1000 copies/ mL) or without viral reactivations. High perforin expression preceded high viral loads. Conclusion. Perforin-expressing CD8 + T cells may be useful as an easy-to-measure prognostic marker for identifying patients at risk for severe viral reactivation very soon after SCT.