The pathophysiology of glomerular lesions of
membranous nephropathy (MN), including seldom-reported
IgG4-related disease, is still elusive. Unlike in idiopathic MN where
IgG4 prevails, in this patient
IgG3 was predominant in glomerular deposits in the absence of circulating anti-
phospholipase A2 receptor antibodies, suggesting a distinct pathologic process. Here we documented that
IgG4 retrieved from the serum of our propositus reacted against
carbonic anhydrase II (CAII) at the podocyte surface. In patient's biopsy, glomerular CAII staining increased and co-localized with subepithelial
IgG4 deposits along the capillary walls. Patient's
IgG4 caused a drop in cell pH followed by
mitochondrial dysfunction, excessive ROS production and cytoskeletal reorganization in cultured podocytes. These events promoted mitochondrial superoxide-dismutase-2 (SOD2) externalization on the plasma membrane, becoming recognizable by
complement-binding
IgG3 anti-SOD2. Among patients with
IgG4-related disease only sera of those with
IgG4 anti-CAII
antibodies caused low intracellular pH and mitochondrial alterations underlying SOD2 externalization. Circulating
IgG4 anti-CAII can cause podocyte injury through processes of intracellular acidification, mitochondrial oxidative stress and neoantigen induction in patients with
IgG4 related disease. The onset of MN in a subset of patients could be due to
IgG4 antibodies recognizing CAII with consequent exposure of mitochondrial neoantigen in the context of multifactorial pathogenesis of disease.