Sara van den Berg

123 5 Hallmarks of CMV-specific T-cells and CD56, yet the cells are nevertheless functional with respect to cytokine production and cytotoxicity ( Figure 1 ). However, the advanced differentiated phenotype is not entirely exclusive as also other viruses can elicit CD8 T-cells with a similar differentiation status. Less attention is given to this since either only a small subset among the total memory pool has this phenotype (e.g. upon infection with EBV or HIV) or the frequencies of the late-stage differentiated CD8 T-cells is generally lower compared to these frequencies in CMV infection (e.g. infection with herpes simplex virus-1 (HSV-1) [71] and parvoviruses B19 and PARV4 [72]. One clear feature is that high doses of adenovirus-based vaccine vectors can actually induce a comparable phenotype (and transcriptome) to CMV [56], which is also accompanied with a high frequency of the cells, which makes this a vaccine platform with great potential. As variation exists in the differentiation state of CMV-specific T-cells between individuals, we will next discuss factors that influence the T-cell differentiation. ESTABLISHMENT OF THE ADVANCED DIFFERENTIATION PHENOTYPE The phenotype of the CMV-specific CD8 T-cells is strongly connected with the magnitude of the CMV-specific T-cell response. Cross-sectional human studies show that in both healthy and immunosuppressed individuals a high HCMV-specific T-cell response is associated with a high percentage of advanced differentiated T-cells within the total specific T-cell population [44, 73-75]. Nevertheless, the association between the differentiation state and level of CMV-specific T-cells is shown in experimental mouse models [74, 76]. Low dose inoculums elicit fewer circulating CMV-specific CD8 T-cells, and these cells have a less advanced differentiation phenotype. Accordingly, interference with an established mouse CMV infection by antiviral treatment reduces the frequency of the CMV-specific CD8 T-cell response, and also in this setting the CD8 T-cells acquired a lesser differentiated phenotype compared to CMV-infected mice that are untreated [77]. Differences in the infectious dose of primary CMV infection may be instrumental in causing the large variation of the advanced-stage differentiation status of CMV-specific T-cell that exists between individuals. CMV-specific CD8 T-cells may reach an advanced differentiation phenotype already early after infection, and then maintain this status stably over time. In young individuals and even in children an advanced differentiated CMV-specific T-cells can appear [78-80]. Thus, the (primary) infectious dose might determine the viral setpoint (the initial balance between virus and host after primary infection)[81] and thereby subsequently influencing the level and amount of viral reactivation episodes and consequent antigen triggering of CMV-specific T-cells. Notably, within the inflationary epitope-specific memory T-cell population not all CMV- specific T-cells acquire the late-stage differentiation. Depending on the viral dose, a significant portion can attain a central-memory (CM) phenotype [76]. These CM-like CD8 T-cells produce more IL-2 and are probably dominantly contributing to T-cell expansion upon re-challenge [82]. Also, within the total pool of CMV-specific T-cells also non-inflationary