Sara van den Berg

189 7 Duration of CMV infection of CMV-specific antibody levels at endpoint and age at CMV-seroconversion turned out to play a major role. Duration of CMV infection was not related to the size and function of the memory CD8+ T-cell pool, suggesting that CD8+ T-cell memory responses do not further inflate over time. In contrast, CD4+ T-cell numbers, similar to CMV-specific antibody levels, were higher in individuals who seroconverted at older age. Furthermore, we did not find an association of frailty with CMV-serostatus or with duration of CMV infection in elderly individuals, although increased CMV-specific antibody levels were associated with higher prevalence of cardiovascular disease. We are the first showing the CMV seroconversion rate in an observational longitudinal cohort of healthy older adults. The average seroconversion rate we found in (0.56% per year) was similar to a previous estimate (0.55% per year) based on a large cross-sectional study in adults [46]. This thus provides a valuable estimate, although it is lower than that previously reported for individuals at higher risk like pregnant women, day-care providers and parents with young children (respectively 2.3%, 8.5% and 2.1%) that can be explained by the lower number of contacts with young children [47, 48]. Higher CMV-specific antibody levels in older adults are generally thought to reflect multiple experienced CMV reactivations during life [2, 3, 9, 11, 23]. However, little is known about how CMV-specific antibody levels are established and maintained during a lifetime in healthy individuals. We observed a small increase in CMV-specific antibodies over a substantial period of time (~27 years). Two other studies reported a small [13] or even no increase [49] in CMV-specific antibody levels over time, but these covered a much shorter follow-up time (5 years and 13 years, respectively). The slight increase in CMV-specific antibody levels we observed over time suggests that CMV reactivation, and probably to a lesser extent reinfection [3], indeed occurs in CMV-infected individuals. Antibody levels can be stable over prolonged periods of time as has been seen for other viruses [50], although CMV reactivation will play a role in maintaining CMV-specific antibody levels. The persistence of CMV antigen may contribute to activation of memory B-cells and the continuous replenishment of long- lived plasma cells and antibody production [50-52]. Furthermore, we show that variation of CMV antibody levels at end point is just for a minor part explained by changes in these levels over the preceding 27 years. Therefore, our results argue against the use of CMV-specific antibody levels as a surrogate marker for experienced CMV reactivation. Interestingly, while we found a positive relationship between age and CMV- specific antibody levels, short-term and long-term CMV+ individuals did not differ in their CMV-specific antibody levels at end point. In contrast, age, regardless of duration of CMV infection, was associated with increased CMV-specific antibody levels. Thus age-related effects might play a prominent role in the variation in antibody levels. Memory inflation, characterized by an expansion of the memory T-cell pool over time, is a hallmark for CMV infection, especially shown in CMV mouse models [24, 53, 54]. Memory