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The live attenuated yellow fever (YF) vaccine has an excellent record of efficacy and one dose provides long-lasting immunity, which in many cases may last a lifetime. Vaccination stimulates strong innate and adaptive immune responses, and neutralizing antibodies are considered to be the major effectors that correlate with protection from disease. Similar to other flaviviruses, such antibodies are primarily induced by the viral envelope protein E, which consists of three distinct domains (DI, II, and III) and is presented at the surface of mature flavivirions in an icosahedral arrangement. In general, the dominance and individual variation of antibodies to different domains of viral surface proteins and their impact on neutralizing activity are aspects of humoral immunity that are not well understood. To gain insight into these phenomena, we established a platform of immunoassays using recombinant proteins and protein domains that allowed us to dissect and quantify fine specificities of the polyclonal antibody response after YF vaccination in a panel of 51 vaccinees as well as determine their contribution to virus neutralization by serum depletion analyses. Our data revealed a high degree of individual variation in antibody specificities present in post-vaccination sera and differences in the contribution of different antibody subsets to virus neutralization. Irrespective of individual variation, a substantial proportion of neutralizing activity appeared to be due to antibodies directed to complex quaternary epitopes displayed on the virion surface only but not on monomeric E. On the other hand, DIII-specific antibodies (presumed to have the highest neutralizing activity) as well as broadly flavivirus cross-reactive antibodies were absent or present at very low titers. These data provide new information on the fine specificity as well as variability of antibody responses after YF vaccination that are consistent with a strong influence of individual-specific factors on immunodominance in humoral immune responses.

Immune senescence, defined as the age-associated dysregulation and dysfunction of the immune system, is characterised by impaired protective immunity and decreased efficacy of vaccines. Recent clinical, epidemiological and immunological studies suggest that Cytomegalovirus (CMV) infection may be associated with accelerated immune senescence, possibly by restricting the naïve T cell repertoire. However, direct evidence whether and how CMV-infection is implicated in immune senescence is still lacking. In this study, we have investigated whether latent mouse CMV (MCMV) infection with or without thymectomy (Tx) alters antiviral immunity of young and aged mice. After infection with lymphocytic choriomeningitis virus (LCMV) or Vaccinia virus, specific antiviral T cell responses were significantly reduced in old, old MCMV-infected and/or Tx mice compared to young mice. Importantly, control of LCMV replication was more profoundly impaired in aged MCMV-infected mice compared to age-matched MCMV-naïve or young mice. In addition, latent MCMV infection was associated with slightly reduced vaccination efficacy in old Tx mice. In contrast to the prevailing hypothesis of a CMV-mediated restriction of the naïve T cell repertoire, we found similar naïve T cell numbers in MCMV-infected and non-infected mice, whereas ageing and Tx clearly reduced the naïve T cell pool. Instead, MCMV-infection expanded the total CD8(+) T cell pool by a massive accumulation of effector memory T cells. Based on these results, we propose a new model of increased competition between CMV-specific memory T cells and any 'de novo' immune response in aged individuals. In summary, our results directly demonstrate in a mouse model that latent CMV-infection impairs immunity in old age and propagates immune senescence.