PRTN3 and granulomatosis with polyangiitis: Granulomas contain fibroblasts interspersed with leucocytes, specifically T cells, and monocytes, with MGCs located at the core.11 The stimuli for monocyte and macrophage fusion are diverse and may be dependent on the persistence of particular antigens, as well as cytokine-driven macrophage activation12 13 and expression of particular macrophage molecules.12 In GPA, less is known about MGC formation or the reason for the close association with PR3-ANCA.