Interleukin (IL)-33 expression in temporal arteries of patients with giant cell arteritis (GCA). Representative photomicrographs showing 5-μm-thick paraffin-embedded sections of artery biopsy specimens obtained from controls (A) and patients with GCA (B-F), stained for IL-33. IL-33 expression was observed in inflamed arteries displaying small vessel vasculitis (SVV) (B) and transmural inflammation (C). In SVV IL-33 expression was observed among infiltrating mononuclear cells (arrows) and smooth vascular muscle cells (B). In arteries displaying transmural inflammation IL-33 expression was observed among infiltrating inflammatory mononuclear cells (D) and giant cells scattered through the inflamed wall of inflamed arteries (E). Microvessels distributed through the inflammatory infiltrates also showed intense IL-33 positivity (arrows) (F). (A, original magnification ×100; B-C, original magnification ×250; D-F, original magnification ×400).