A case of lupus nephritis with masked polyclonal IgG presenting as severe AKI, successfully treated and withdrawn from hemodialysis: a case report.

Lupus nephritis (LN) is well-known as an immune-mediated glomerulonephritis characterized by the full-house pattern of immunoglobulin (Ig) and complement deposition. The "masked IgG" is a recently recognized concept in which IgG appears negative on standard frozen sections but becomes positive on formalin-fixed paraffin-embedded (FFPE) tissue sections treated with pronase. Typically, monoclonal IgGκ deposition is observed in such cases. We report a unique case of LN in a 69-year-old male with urinary abnormalities and renal dysfunction. Kidney biopsy revealed necrotizing glomerulonephritis and crescent formation with massive subepithelial and mild subendothelial electron-dense deposits, consistent with Class III(A/C) + V LN. However, immunofluorescence using a frozen sample showed pauci-immune features. In contrast, FFPE with pronase digestion showed strong positive for IgG and light chain-kappa and lambda indicating "masked polyclonal IgG". The patient developed AKI, which progressed to ESRD, necessitating the initiation of hemodialysis. Notably, 5 months after the initiation of treatment with steroids and mycophenolate mofetil, the patient experienced an improvement in renal function and ultimately achieved withdrawal from hemodialysis. A repeat kidney biopsy revealed progression to Class IV(A/C) + V lupus nephritis and Ig and complements were detected in normal immunofluorescence using a frozen sample. Further, mass spectrometry analysis of glomeruli in both biopsies identified a "high-temperature requirement A serine peptidase 1", HTRA1. In conclusion, this was a pathologically extremely rare case of lupus nephritis (LN) with masked polyclonal IgG, in which HTRA1 was detected in the kidney biopsy.