Sara van den Berg

119 5 Hallmarks of CMV-specific T-cells Costimulatory and inhibitory receptors The advanced differentiation state of CMV-specific T-cells is also marked by the lack of expression of the costimulatory receptors CD27 and CD28, which are otherwise constitutively expressed on naïve T-cells [17]. This is in contrast to other virus-specific CD8 T-cells. For example, EBV and hepatitis C virus (HCV)-specific T-cells more often display expression of CD27 and CD28, and HIV-specific CD8 T-cells, despite advanced loss of CD28, still express CD27 [17], although this may also depend on the disease state [18]. Acute HCMV infections frequently occur in CMV-negative transplant recipients receiving a CMV-positive organ. In these individuals, the CMV-specific T-cell response consists of mainly CD27 + CD28 - CD45RA - CD45RO + memory T-cells shortly after the peak of CMV infection [19]. In time, expression of CD27 is lost and CD45RA is re-expressed on the majority of the cells [20, 21]. The gradual loss of CD27 is also observed in mouse models, and is likely caused by chronic antigenic triggering [22]. In mouse models, the functional role of CD27 and CD28 has been studied in CMV infection and indicated that CD28 costimulation is especially important during primary infection to enhance CMV-specific T-cell expansions while CD27 and its ligand CD70 seem to play an activating role during both the primary and latent phase of infection [22-26]. The costimulatory receptor OX40 is transiently upregulated upon activation, and is important during the latent phase [27]. Programmed cell death 1 (PD-1), cytotoxic T-lymphocyte antigen 4 (CTLA-4), T-cell immunoglobulin domain and mucin domain protein 3 (TIM-3), lymphocyte activation gene 3 (LAG-3) and CD160 are inhibitory receptors associated with the exhaustion phenotype of T-cells [28]. PD-1 was identified to be abundant on chronic lymphocytic choriomeningitis virus (LCMV)-specific T-cells in mice models [29] and was next shown to be upregulated on T-cells in a number of chronic viral infections including HIV [30, 31], hepatitis B virus (HBV) [32] or HCV [33]. In addition, PD-1 and other inhibitory molecules are abundantly found on T-cells in the tumor-microenvironment and this aspect forms the basis for reinforcing exhausted T-cells by blocking these inhibitory molecules [34]. Indeed, as demonstrated by variants of LCMV eliciting either acute or chronic infection, the induction of the exhausted phenotype is caused by strong chronic antigenic triggering [35], and is elevated by the lack of CD4 T-cell help [29, 36]. Interestingly, during the latent phase, circulating CMV-specific T-cells express relatively low levels of inhibitory receptors [37, 38]. PD-1 expression on CMV-specific T-cells is lower compared to chronic virus-specific T-cells against HBV [32], HIV [30, 38-40] and EBV-specific T-cells [37, 38]. Likewise, also TIM-3, CD160 and 2B4 are expressed at lower levels in CMV- specific T-cells compared to HIV-specific T-cells [40]. Nonetheless, the inter-individual variation of PD-1 and 2B4 expression observed for CMV-specific T-cells can be substantial [33, 40]. This heterogeneity of PD-1 expression could reflect different differentiation phenotypes of