Abstract
[Case] A man in his 60s was diagnosed by renal biopsy as having segmental membranous nephritis associated with Cronkite–Canada syndrome. IgG and C3c were negative in unfixed frozen specimens as determined by the immunofluorescence method, but anti-C3d antibody immunostaining using formalin-fixed paraffin-embedded immunoperoxidase (FFPEIP)-stained specimens revealed that C3d was found in the glomerular basement membrane with a clearly recognized segmental form. C3d-positive findings in the renal tubules and interstitium, which were observed in unfixed frozen specimens by the immunofluorescence method, disappeared with FFPEIP. [Discussion] C3c observed by renal biopsy with the immunofluorescence method indicates recent complement activation and disappears over time. Therefore, the absence of C3c does not mean the absence of immune complexes. Additionally, C3c is not suitable for PEIP owing to its antigenicity, being inactivated by fixation. C3d, on the other hand, remains in place upon complement activation and is an essential component of immune complexes, making it ideal for observing them. Additionally, its antigenic deactivation due to fixation is minimal, making it suitable for PEIP. [Conclusion] C3d immunostaining using formalin-fixed paraffin-embedded specimens is a valuable tool for examining renal biopsies, particularly in membranous nephritis with negative IgG staining in frozen specimens or with focal segmental glomerular lesions, and in cases where glomeruli are not found in frozen specimens.
