Daan Pieren

122 Chapter 5 Here we identified a novel human CD8 + T-cell subset that is characterized by expression of KIR and CD45RA, and the lack of NKG2A. We provide evidence that these “KIR + RA + CD8 + T cells” (KIR + CD45RA + NKG2A - CD8 + T cells) accumulate with age and are therefore a previously unrecognized hallmark of aging. In contrast to previously described virtual memory cells, we show that KIR + RA + and NKG2A + RA + CD8 + T cells are distinct subsets. KIR + RA + T cells express high levels of the age-related exhaustion marker TIGIT, and appear to be a regulatory CD8 + T-cell subset as we observed that these cells suppress the proliferation of other CD8 + T cells. Importantly, we show that KIR + RA + T cells are activated during respiratory disease caused by viruses such as influenza viruses and coronaviruses, including SARS-CoV-2. Our data collectively show that these KIR + RA + cells are a major activated T-cell population in older adults suffering from respiratory viral infection and provide evidence that activation of this T-cell subset is a correlate of prolonged respiratory disease in influenza A virus infected older adults. MATERIALS AND METHODS Study design Older adults with Influenza-Like-Illness and matched controls Part of the samples investigated in the current study were embedded in a trial that monitored Influenza-Like-Illness (ILI) in community-dwelling older adults (ILI cohort; Netherlands Trial Register NTR4818) ([27], Van Kaaijk et al. submitted). The study was performed according to Good Clinical Practice, the Declaration of Helsinki. The study was approved by the ethical committee METC Noord-Holland and written informed consent was obtained from all participants. There were no exclusion criteria for this study. Participants were 60 years and older, and were instructed to report ILI-associated symptoms according to the Dutch Pel criteria [28] (fever > 37.8 °C with at least coughing, myalgia, nasal congestion, sore throat, difficulty breathing, or headache) as soon as possible after the symptoms started. Blood samples were drawn within 72 hours after the symptom report (acute phase of infection), as well as approximately two and eight weeks after the initial report. The presence or absence of ILI-associated symptoms were monitored during each visit. Nasopharyngeal and oropharyngeal swabs were taken at all three visits to identify the pathogen causing the ILI symptoms by qPCR as described below. Symptomatic participants reported on in this study are a subset of the ILI cohort. The selection was based on the presence of the type of respiratory virus as a single infection at the acute phase of infection and absence of all ILI-causing pathogens at two and eight weeks