Dual MPO-ANCA and PLA2R positivity in crescentic glomerulonephritis with MPO-associated membranous nephropathy: a case report with pathogenic insights.

Recent studies have suggested that myeloperoxidase (MPO) may serve as a target antigen in cases of MPO-associated membranous nephropathy (MN) occurring concurrently with MPO-antineutrophil cytoplasmic antibody (ANCA)-associated crescentic glomerulonephritis (GN). We report the case of a 41-year-old Filipino man with MPO-ANCA-positive rapidly progressive glomerulonephritis (RPGN) complicated by the development of nephrotic syndrome. He exhibited no apparent signs of extra-renal vasculitis. Kidney biopsy demonstrated crescentic GN with MPO-associated MN, characterized by active cellular crescents and Churg stage II-III MN with positive staining for the phospholipase A2 receptor (PLA2R). Immunosuppressive therapy achieved concurrent remission of both RPGN and nephrotic syndrome, accompanied by a marked reduction in MPO-ANCA titers. This favorable response suggests that MPO-ANCA may contribute to the pathogenesis of both glomerular disease processes in this patient. Nonetheless, the possibility that the treatment was also effective against idiopathic MN cannot be excluded. The renal pathology showed early active crescentic GN without chronic changes, whereas the MN did not appear to represent an early stage. These findings suggest two possible mechanisms: first, that MPO-IgG immune complexes initially formed in MN, accompanied by bystander deposition of glomerular PLA2R, followed by the subsequent development of crescentic GN; or second, that MPO was deposited within preexisting PLA2R-positive immune complexes of idiopathic MN at the time crescentic GN emerged. Although the former mechanism appears more plausible given the increasing number of reports describing established MPO-associated MN, further studies are warranted to elucidate the pathogenic role of MPO in this condition.