Sara van den Berg

104 Chapter 4 duration of symptoms, we found a significant positive correlation between the magnitude of the influenza-specific T-cell response and severity of symptoms 8 weeks after fever onset, and not early after fever onset or 2 weeks later. This might suggest that indeed a prolonged T-cell response, still present 8 weeks after fever onset, is associated with severity of symptoms of influenza virus infection. To the best of our knowledge, we are the first to investigate the potential effect of CMV infection on a heterologous infection in human in a larger group of individuals. A negative effect of CMV infection on the functioning of the immune system is often intuitively explained by competition between T-cells for ‘limited immunological space’. Indeed, CMV infection has a profound impact on the composition of the overall CD8+ T-cell pool, by increasing the number of highly differentiated memory cells, especially in older adults [49]. We show that CMV infection leads to a decrease in the frequency of memory influenza virus-specific T-cells, as measured by GILG-tetramer staining, in healthy older adults. Unfortunately, absolute T-cell numbers could not be investigated in this study, leaving the possibility that the decreased frequency of influenza virus-specific T-cells might only be relative and merely reflecting a relative increase in CMV-specific T-cells in older adults. This might explain why decreased influenza virus-specific T-cell frequencies in healthy older adults would not result in reduced responses in acute influenza virus infection. Alternatively, the influenza-virus specific T-cells are indeed lower, also in number, in CMV+ older adults compared to CMV- older adults, but can still probably be boosted by a influenza virus infection. A similar effect has previously been reported for EBV-specific T-cells [31]. Relatively low frequencies of influenza virus- specific T-cell in older adults compared to younger adults have been reported before [50, 51] and are considered to be a key determinant of a diminished T-cell response in influenza virus infection [52]. The T-cell response to influenza virus infection in the context of CMV infection was to the best of our knowledge not investigated before in humans. During influenza virus infection, we observed that influenza virus-specific T-cells in CMV+ and CMV- individuals respond equally well. Previously, other studies reported impaired influenza virus-specific T-cell responses to influenza vaccination [17, 53, 54]. In contrast, other groups showed a positive effect of CMV on the vaccine-induced influenza virus-specific T-cell response in humans [55]. In addition, in mice, MCMV infection was shown to enhance the diversity of the T-cell repertoire against an unrelated acute infection [56]. We also showed that CMV T-cell responses were positively associated with the magnitude of the T-cell response 2 weeks and 8 weeks after fever onset of influenza virus infection. Together, this suggests that CMV+ individuals have a sufficient amount of influenza virus-specific T-cells responding to influenza virus infection. As CMV infection was suggested to induce a more inflammatory environment [57], we also studied the potential association between CMV and inflammatory markers and how this might be related to influenza virus specific T-cell responses. Although the levels of pro-inflammatory cytokines and chemokines measured here were significantly increased in the acute phase of influenza virus infection (<72 hours after fever onset), they were similar between CMV+ and