Human Cytomegalovirus: Degranulation Is a Prominent Feature of Functionally Impaired pp65- and IE-1-Specific T-Cells in Patients after Allogeneic Stem Cell Transplantation

Human cytomegalovirus (CMV) reactivation and disease is still a frequent complication after allogeneic stem cell transplantation (allo SCT). It is well accepted that T-cell immunity is mandatory to control CMV infection and disease and much effort has been put into the development of cell-based monitoring assays. Nevertheless, no reliable marker for protective immunity has been established to date. Most studies use one CMV model antigen (pp65) to compare the frequencies of cytokine producers (mainly IFNg) or multimer-specific T-cells. Methods: In total, we recruited 16 patients after allo SCT, (7 high risk, 9 standard risk pts.). We used 8-colour flow cytometry to detect degranulation (mobilized CD107a/b), intracellular IFNg, TNFa, IL-2 production and CD28-expression in peptide pool stimulated pp65 and IE-1 specific CD8 T-cells. Results were compared to 7 healthy CMV exposed donors. Results: Degranulation identifies the highest percentage of CMV-specific T-cells in allo-transplanted patients (pp65: 0,94% degranulation and 0,31% IFNg; IE-1: 1,44% degranulation and 0,87% IFNg, mean frequency). These T-cells are relatively cytokine deficient compared to those in healthy donors (cytokine-production/degranulation ratio: SCT=0,42, healthy=0,72 for pp65, p=0,048; SCT=0,61, healthy= 1,00 for IE-1, p=0,133, U-test). The cytokine expression pattern differs between antigens used for stimulation, for example more IL-2-producers could be detected in the pp65 specific compartment (12,5% for pp65 and 4,5% for IE-1 of all activated CD8 T-cells, p=0,015).

Conclusion: This study demonstrates that degranulation is the most prominent marker of CMV-specific T-cells (pp65 and IE-1) in allo SCT patients. Looking at IFN-g producers only may underestimate the frequencies of CMV specific T-cells in this setting. Furthermore, these subsets have a divergent functionality in transplant recipients compared to healthy individuals. Our data challenge the concept of enumerating CMV specific T-cells to estimate immunity. We rather propose measuring functional differences in the T-cell response may help to identify patients with a high risk of CMV reactivation. A careful dissection of these differences is a prerequisite for the development of monitoring tools and adoptive T-cell transfer.