Daan Pieren

121 Regulatory KIR+RA+ CD8+ T cells accumulate with age INTRODUCTION Aging dramatically increases the risk for developing prolonged and more severe disease after acute viral respiratory infections [1-5]. This has been shown for viruses such as influenza virus, respiratory syncytial virus (RSV), and coronaviruses [1,2,5,6]. Moreover, susceptibility of older adults for respiratory infections is currently being highlighted by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic [7,8]. The risk of severe disease is in part explained by decreasing functionality of CD8 + T cells during the process of aging [9-12] resulting in impaired viral clearance [13-16]. To better understand the role of the impaired CD8 + T-cell response during viral infection in the development of prolonged and severe disease in older adults, it is important to identify CD8 + T-cell subsets that contribute to the decline of T-cell mediated responses. The decline of CD8 + T-cell-mediated immune responses with age is in part associated with changes in CD8 + T-cell phenotype and defects in proliferation, often referred to as T-cell exhaustion and/or senescence (reviewed in [17,18]). T-cell exhaustion is characterized by increased expression of inhibitory receptors by CD8 + T cells [19]. In humans, expression of the co-inhibitory receptor T-cell immunoglobulin and ITIM domain (TIGIT) by CD8 + T cells has been shown to increase with age and is associated with diminished CD8 + T-cell responses in the elderly [20]. Recent work demonstrated that effector memory (T EM ) and terminally differentiated effector cells (T EMRA ) were the most predominant CD8 + T-cell subsets expressing TIGIT during aging [20]. Recently, so-called “virtual memory cells” have been identified in mice and humans [21-23] as a new CD8 + T-cell subset that accumulates with age [22- 24]. It is currently assumed that virtual memory cells are antigen-naïve T cells that acquire a memory phenotype through homeostatic proliferation [22,25,26] and may act as bystander cells during infection [22,26]. In aged mice, virtual memory cells express a senescent phenotype with diminished capacity to participate in primary immune responses [23]. In humans, virtual memory CD8 + T cells are CD45RA + cells that have been defined by expression of killer- cell immunoglobulin-like receptors (KIR) and/or the NK-cell receptor NKG2A. However, further phenotypical and functional characterization as well as the clinical relevance of these cells in humans is currently unknown. We therefore aimed to address the significance of KIR + and/or NKG2A + CD45RA + CD8 + T cells in aging by defining their phenotype and functionality, and their relevance for respiratory viral infections in older adults. 5