Background: Giant cell arteritis (GCA) is an immune-mediated vasculitis affecting the large- and medium-sized arteries. The role of CD8+ T cells in the pathogenesis of GCA is poorly understood despite the potential association with viral infections, HLA-class I genes and the involvement of CD8+ T cells in other vasculitides. Therefore, we performed a comprehensive phenotypic, transcriptomic and functional analysis of CD8+ T cells in GCA patients. Methods: To assess the phenotype of circulating CD8+ T cells, we measured the frequencies of CD8+ T-cell differentiation subsets for markers of activation (CD69 and CD25) and proliferation (Ki-67+) in 14 newly diagnosed GCA patients and 18 age- and sex-matched healthy controls (HCs). Next, we assessed the proliferative capacity of CD8+ T cells upon anti-CD3 or anti-CD3/CD28 stimulation in vitro. To gain more mechanistic insights, we performed single cell (sc) RNA sequencing on peripheral blood mononuclear cells from three GCA patients and HCs. Lastly, immunohistochemistry was performed on GCA-affected arteries to detect CD3, CD8, Ki-67, TNF-α and IFN-γ expression whereas colocalization of Ki-67/ CD8 and IFN-γ/ CD8 was determined with immunofluorescence. Results: The absolute number of circulating effector memory CD8+ T cells was decreased in GCA patients compared to HCs, whereas the percentages of Ki-67+ effector memory CD8+ T cells were increased. Circulating CD8+ T cells from GCA patients had reduced activation thresholds after in vitro stimulation and a gene expression profile concurrent with increased proliferation as assessed by scRNA sequencing. CD8+ T cells in GCA-affected tissues were present in all layers of the temporal artery biopsies whereas their location in the aorta was confined to the adventitia. The large majority of CD8+ T cells in the tissues expressed IFN-γ but not Ki-67. Conclusions: In GCA, circulating CD8+ T cells demonstrate a proliferation-prone phenotype. Their lower activation thresholds could render CD8+ T cells more prone to react to antigens and to differentiate into pro-inflammatory cells. The finding that CD8+ T cells in tissues were Ki-67 negative suggests that these cells were recruited to the site of inflammation, rather than the result of local expansion. Vascular CD8+ T cells produce IFN- γ which could contribute to the local inflammation. Disclosures: AB was a consultant for Grünenthal Gmbh until 2017. EB and KSMvdG as employees of the UMCG received speaker/consulting fees from Roche paid to the UMCG.