The role of myeloperoxidase and myeloperoxidase–antineutrophil cytoplasmic antibodies (MPO-ANCAs) in the pathogenesis of human MPO-ANCA-associated glomerulonephritis

It is well known that antineutrophil cytoplasmic antibodies (ANCAs) are pathogenic and have a diagnostic value for ANCA-associated vasculitis. We demonstrated that a rise in myeloperoxidase (MPO)-ANCA titers during remission is often predictive of a future relapse in MPO-ANCA-associated vasculitis. Pathological examination of renal biopsies indicated that not only MPO-ANCAs, but also extracellular MPO, an in situ immune complex composed of MPO and MPO antibodies, may play important roles in the pathogenesis of glomerular capillary injury in MPO-ANCA-associated vasculitis.

MPO release from neutrophils and sensitivity to formyl-methionyl-leucyl-phenylalanine (FMLP) The release of MPO from neutrophils in patients with MPO-ANCA-associated GN was higher than that in healthy controls. The sensitivity of MPO release to FMLP of neutrophils in patients with MPO-ANCA-associated GN was significantly higher than in patients whose GN was not associated with MPO-ANCA and in healthy controls [2].
Serum MPO and serum cytokines in MPO-ANCAassociated GN Serum MPO was detected in patients with MPO-ANCAassociated GN and the amounts of MPO were especially high in the cellular crescent stage and correlated with MPO-ANCA [3]. Tumor necrosis factor-alpha and interleukin (IL)-6 were also detected in the sera in parallel with disease activity and MPO-ANCA titers [3]. IL-8 was also increased in the active stage of MPO-ANCA-associated GN [4].

Relationship between MPO-positive cells and MPO on the glomerular capillary wall
MPO existed along the glomerular capillary walls near the infiltrated MPO-positive cells in active (Fig. 1a-c) and early-phase necrotizing GN (NGN) (Fig. 2a, b). CD34 staining was decreased on the adjacent area of the same glomerulus (Fig. 2c, d). Many MPO-positive cells and MPO along the glomerular capillary wall were detected in active and more severely damaged NGN (data not shown) [5]. MPO-positive cells and MPO were not detected on the glomerular capillaries during inactive and chronic-phase NGN [5].

Double immunofluorescence staining (MPO and CD34)
MPO was detected along the glomerular capillary wall near MPO-positive cells which was accompanied by decreased staining of CD34 in some areas of the glomerulus suggesting capillary injuries (Fig. 3). In other areas, double staining of MPO and CD34 was seen [5,6].

Triple immunofluorescence staining (MPO, immunoglobulin (Ig) G and CD34)
IgG was associated with MPO along the CD34-negative glomerular capillary walls but was also detected alone in other areas near the capillaries [5,6].
Relationship between C3, IgG and MPO on the glomerular capillary wall MPO, IgG and C3 staining was seen on the same area during the early stage of GN [6].

Conclusion
We demonstrated that serum MPO, MPO release, and sensitivity to FMLP from neutrophils increased in patients with MPO-ANCA-associated GN [2,3]. Clinically, a rise in MPO-ANCA titers during remission was often predictive of a future relapse in MPO-ANCA-associated vasculitis. Histological examination showed many MPOpositive cells and MPO along the glomerular capillary wall in early-phase and in more active and severely damaged MPO-ANCA-associated NGN. MPO, IgG and C3 deposition on the same area was detected mainly during the early phase [6], suggesting that immune complexes containing MPO may be pathogenetically important especially in the early phase of the disease. Recently, Kessenblock et al. [7] reported that neutrophil extracellular traps, which contained MPO and nuclear fragments in the chromatin fibers and are released from ANCA-stimulated neutrophils, result in glomerular capillary necrosis in ANCA-associated GN. We concluded that extracellular MPO released from activated MPO-positive cells, and in situ immune complexes composed of MPO and MPO antibody, may play a pathogenic role in glomerular capillary injury in the early stage of MPO-ANCA-associated NGN.
Conflict of interest All the authors have declared no competing interest.
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