Daan Pieren

17 General introduction and scope of this thesis sustained antigen exposure over a lifetime. Currently, multiple markers for cellular exhaustion in addition to PD-1 have been reported to increase in aged mice: Lymphocyte-activation gene 3 (LAG-3), T-cell immunoglobulin and mucin-domain containing-3 (TIM-3) expression by CD8 + T cells have been shown to mark populations of exhausted cells that increase with age in mice [102,106,107]. In contrast to findings in mice, expression of co-inhibitory receptors by human T cells is less evident. Age-related increased expression of LAG-3 by CD4 + and CD8 + T cells in blood of individuals aged 61 years or older was shown, whereas expression of PD-1 and TIM-3 were not found to increase [108]. Recently, the proportion of CD8 + T cells that expressed the co-inhibitory receptor T-cell immunoglobulin and ITIM domain (TIGIT) was shown to increase with age in humans [108]. To date, TIGIT is one of the few co-inhibitory markers of which it convincingly has been shown in humans that its expression increases with age. TIGIT-expression by antigen-specific CD8 + T cells have been shown to mark exhausted CD8 + T cells in the context of viral infection and cancer [109-111]. TIGIT competes with the co-stimulatory receptor CD226 for their shared ligands CD155 and CD112 expressed by APCs [109,112,113]. The balance between expression of TIGIT and CD226 on a single CD8 + T cell is important, as signaling through TIGIT results in T-cell inhibition, whereas signaling through CD226 results in T-cell activation. In line with this, TIGIT + CD8 + T cells have been reported to be functionally impaired compared to TIGIT - CD8 + T cells [108,109]. The age-related increase of TIGIT + CD8 + T cells is therefore thought to diminish CD8 + T-cell responses in the elderly, potentially explaining their increased susceptibility to infectious diseases. This is currently also speculated to occur during severe COVID-19 disease, caused by SARS-CoV-2 infection [114] and may be applicable to other infectious diseases as well. Inflammaging In addition to T-cell senescence and exhaustion, another phenomenon that has been reported to occur at older age is so-called ‘inflammaging’ ( Figure 2 ). Whereas intermittent increases in the level of pro-inflammatory cytokines are vital in responses against infections to survive, inflammaging is characterized by the presence of a constant low-grade inflammatory state [115-117]. This state is characterized by increased circulating levels of pro-inflammatory factors, including TNF- α , IL-6, and C-reactive protein (CRP). Inflammaging is thought to develop over time as a result of multiple factors, including lifestyle factors and environmental factors, such as exposure to antigens over a lifetime [116,118], which lead to increased susceptibility to communicable and non- 1