Granulomatous inflammation supports plasma cell survival in GPA. (A through D) Immunohistochemistry displaying neutrophils (A, red arrows) forming a microabscess and strong expression of membrane-associated APRIL (B, brown arrows) in the area corresponding to the neutrophilic microabscess. Immunohistochemistry showing macrophages and giant cells forming a granuloma (C, brown) and strong expression of secreted APRIL (D, brown arrows) by macrophages and giant cells. (E through G) Double immunohistochemistry displaying BCMA (E, brown) in close vicinity to CD138 (E, red). Cells positive for BCMA and CD138 are marked (arrows). Immunohistochemistry indicating different types of RANKL-expressing cells, close to bone (b) (F, brown arrows) and with a plasma cell-like morphology (G, brown arrows). (H through J) Immunofluorescence staining demonstrating that CD138 (H, green) is co-localized with secreted APRIL (I, red color) on plasma cells (J, merger of H + I, arrow). Nuclei are stained in blue with 4′,6-diamidin-2-phenylindol (DAPI). Scale bar, 5 μm. (K, Double immunohistochemistry depicting that secreted APRIL (red) is expressed in direct vicinity of IgG+ cells (brown arrows). (L) Immunohistochemistry showing that TACI+ cells (brown arrows) exhibit a plasma cell-like appearance.