Daan Pieren

138 Chapter 5 in peripheral blood of older adults (62-83 years of age, n=36) suffering from a common respiratory viral infection ( Supplementary Table 1 ) ([27], Van Kaaijk et al . submitted). Blood samples were obtained at the acute phase of infection (within three days of fever onset) and during follow-up at 2 and 8 weeks. The proportion of KIR + RA + T cells within the total CD8 + T-cell pool remained relatively stable from the acute phase throughout the following eight weeks, as exemplified in influenza A virus infected individuals ( Figure 6A ) and also by other viral infections in older adults ( Supplementary Figure 7A ). However, we observed a strikingly increased frequency of activated CD69 + KIR + RA + T cells in older adults during the acute phase of influenza A virus infection compared to the follow-up at two and eight weeks, and compared to healthy older adults ( Figure 6B, Supplementary Figure 7B ). At the acute phase of influenza A virus infection, KIR + RA + T cells showed the highest frequencies of CD69 + cells compared to all other CD8 + T cells defined in our analyses ( Figure 6C, Supplementary Figure 7C ). Significant activation of KIR + RA + T cells was also found for individuals infected with a seasonal coronavirus ( Figure 6D ) and similar trends were found for other respiratory viruses included in the study ( Figure 6E-H ). To study whether KIR + RA + T cells are virus-antigen specific T cells, we assessed influenza-specificity of these cells by measuring the number of CD8 + T cells specific for the immunodominant influenza A-epitope GILGFVFTL [47] in influenza-A infected individuals (n=4). Whereas these specific cells could be detected in all other T cell subsets, most predominantly in the T CM and T EM subsets ( Figure 6I ), we observed no GILGFVFTL-specific cells within the KIR + RA + T-cell subset. These findings suggest that KIR + RA + T cells that are activated during an event of influenza are not influenza-specific T cells and therefore potentially are activated as bystander cells during infection. KIR + RA + T cells are highly activated in SARS-CoV-2 infected adults suffer- ing from COVID-19 Additionally, we had the unique opportunity to analyze the KIR + RA + T-cell subset in adults infected with SARS-CoV-2 (32-52 years of age, n=9, Supplementary Table 3 ) [31]. In these individuals, we also observed significant activation of KIR + RA + T cells during the early phase of COVID-19 compared to the follow-up at two and four weeks, and compared to SARS-CoV-2 negative controls ( Figure 6J ). Collectively, these findings show that activation of KIR + RA + T cells is a major phenomenon in respiratory viral infectious diseases.