Significance Antigen recognition by the immune system triggers rapid, specific, and protective responses, which are counterbalanced by inhibitory checkpoints to minimize potentially harmful immunity. The programmed death-1/ programmed death ligand-1 (PD-1/PD-L1) checkpoint is overreactive in cancer patients, curbing antitumor immunity. Whether a failing PD-1/PD-L1 checkpoint contributes to spontaneous autoimmune disease in humans is unknown. Here, we found that in patients with the autoimmune vasculitis giant cell arteritis, antigen-presenting cells provide insufficient negative signaling; unleashing highly activated T cells to infiltrate and damage the walls of large arteries. Thus, immunoinhibitory signals protect large arteries against inflammatory attack and checkpoint activation may be a suitable strategy to treat autoimmune vasculitis.