Background: A rare and peculiar glomerulopathy has begun to be recognized in Japan. The Japanese Society of Nephrology has established a research working group and has collected cases from all over Japan in an attempt to understand the complete spectrum of this glomerulopathy. Method: The diagnostic criterion, which was needed to collect the cases, was proposed as a glomerulopathy showing microspheres or microtubular structures or both associated with podocytic infolding into the glomerular basement membrane (GBM) on electron microscopy. The lesion shows a non-argentaffin hole in the GBM with periodic acid methenamine silver staining and is similar to membranous glomerulonephritis. Results: Twenty-five cases were collected from 17 institutions. Patients were 20-69 years old (19 women, 6 men). Seventeen patients also had collagen diseases such as lupus nephritis and Sjögren's syndrome. All patients had proteinuria. Proteinuria showed a remission in 15 of 23 patients within 12 months, but proteinuria remained higher than 1.0 g/day in five patients despite different types of therapy. Podocytic infolding including microspheres showed either positive or negative staining for immunoglobulins. Cluster formation of microspheres was found in 4 of 17 patients with collagen disease, and in five out eight patients without collagen disease. Electron-dense deposits in the GBM were also found in 6 of 17 patients with collagen disease but were not found in eight patients without collagen disease. Conclusion: Some patients might have a subtype of lupus nephritis, class V, or membranous glomerulonephritis. However, we propose a new disease entity, podocytic infolding glomerulopathy, as a common basis of all 25 patients, because we suspect that microspheres or microtubular structures or both can be derived from podocytic infolding.
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