TLR9 has been shown to be an important mediator in AAV: stimulation of peripheral blood mononuclear cells from GPA patients with a TLR9 ligand increased ANCA production [32], TLR9 expression is increased on B lymphocytes in patients with AAV; when these B lymphocytes are cultured with a TLR9 ligand they produce ANCAs [33] and when neutrophils are stimulated by TLR9 ligands MPO release and neutrophil degranulation is induced after Proteinase 3 (PR3)-ANCA stimulation [34]. Here, PRTN3 is linked to granulomatosis with polyangiitis.