Sara van den Berg

211 8 General discussion rate), CMV-specific T-cells will accumulate over time. Previously, a study in 3 individuals reported a lower turnover of CMV-specific T-cells in vivo compared to bulk memory T-cells, arguing for a model of accumulation of long-lived CMV-specific T-cells over time [43]. Our work investigated the phenomenon of memory inflation in two different studies; (1) by investigating the effect of duration of CMV infection on the T-cell response and (2) by investigating the underlying dynamics of T-cells ( Figure 1, Model A ). The role of duration of CMV infection and underlying dynamics of CMV- specific T-cells The association between age and the size of the CMV-specific T-cell response in human was previously assed in some longitudinal studies. These were often limited to a few years of follow-up and have reported conflicting results [44-47]. We followed CMV-specific T-cell numbers for up to 1.5 years, and found no increase over time over this timespan ( chapter 6 ). In a unique longitudinal study with 27 years of follow-up, we were able to assess time since CMV-seroconversion based on consecutive antibody analysis in healthy individuals. We found that large CMV-specific T-cell responses were already present shortly after CMV seroconversion ( chapter 7 ), which does not support a role for memory inflation in CD8 + T-cells. Furthermore, even in young children, i.e. short duration of CMV infection, high CMV-specific T-cell numbers have been observed [25], suggesting that already within a short period of time large numbers of CMV-specific T-cells are established. Thus, long-term infection with CMV seems to play a smaller role in the establishment of high CMV-specific T-cell numbers than previously anticipated based on mouse studies. Interestingly, we found that high CD4 + CMV-specific T-cell responses were present in elderly that seroconverted at older age, than in elderly that seroconverted at younger age. We expect that this may be explained by an impaired primary immune response in an aged immune system and subsequent a larger viral reservoir that is controlled by high CMV-specific immune responses ( Chapter 7 ). Thus, the reported associations between age and the magnitude of the CD4 + CMV-specific T-cell response will most likely not be explained by inflation of CD4 + CMV- specific T-cell numbers over time, but (at least partly) by increased CD4 + CMV-specific T-cell response when CMV-seroconverting older age. For CD8 + T-cells, no proof for an association between age at seroconversion and the magnitude of the CMV-specific T-cell response was found. We also investigated the dynamics of CMV-specific T-cells compared to bulk memory. We found lower expression of proliferation markers and reduced sensitivity for apoptosis of CMV- specific T-cells compared to bulk memory T-cells ( chapter 6 ). Also, CMV-specific T-cells expressed a late-stage differentiation state, which in turn was shown to be associated with a reduced turnover rate in vivo , probably reflecting homeostatic proliferation ( chapter 6 ). Despite this association and despite the observed indications based on snapshot markers like Ki-67 and Bcl-2, no significant differences in the in vivo turnover rate were observed between CMV-specific T-cells and bulk memory T-cells from CMV- and CMV+ donors ( chapter 6 ).