Granuloma formation in ANCA‐associated vasculitides

Granuloma formation is a key pathologic finding in two of the anti‐neutrophil cytoplasmic autoantibody (ANCA)‐associated vasculitides: Wegener's granulomatosis (WG) and Churg–Strauss syndrome (CSS). So far, no animal models have been established convincingly reproducing both vasculitic and granulomatous features typical of WG and CSS. In biopsies, granulomatous lesions are found both at distant extravascular sites and in the vicinity of inflamed vessels, e.g. in the lung. Intriguingly, WG‐granulomata appear to display features of tertiary lymphoid tissue. Cartilaginous and osseous destruction is caused by granulomatous inflammation invading adjacent tissues. Rhinosinusitis is regularly encountered in WG and CSS. Septal perforation, saddle nose deformity, middle and inner ear symptoms, and granulomatous invasion of the palate, orbita, meninges, or the pituitary gland may complicate WG. Both common (e.g. FCGR3B copy number) and distinct (e.g. HLA‐DP, IL‐10.2 ) genetic factors have been identified in AAV potentially favouring inflammation and autoimmunity. The HLA‐DPB1 / RING1 / RXRB region constitutes a quantitative trait locus for ANCA‐positive WG with the strongest association to be reported up to now. A profound alteration of the T‐cell response including Th1 and Th17 responses, anomalously NK‐receptor‐expressing ‘NK‐like’ T cells, and dysfunctional regulatory T cells could facilitate and sustain granuloma formation and autoimmunity.