Polyclonal immunoglobulin G deposits with distinctive appearance in the tubular basement membrane: A report of two cases

Background: Tubulointerstitial diseases arise from various etiologies, including infections, medications, autoimmune conditions, and systemic disorders. The histological presentation of the tubulointerstitium in renal biopsies can vary considerably. When tubulointerstitial alterations are identified through light microscopy, additional evaluation with immunofluorescence and electron microscopy may assist in diagnosis. This report describes two slowly progressive atypical cases of tubulointerstitial disease. Renal biopsy in both cases revealed tubular atrophy and interstitial fibrosis with limited inflammatory cell infiltration, along with polyclonal linear immunoglobulin G (IgG) staining pattern along the thickened tubular basement membrane (TBM) and distinctive electron-dense deposits. Case Presentation: Case 1: A 65-year-old Japanese man with a history of chronic obstructive pulmonary disease, benign prostatic hyperplasia, and hypertension presented with anorexia and malaise 1 week prior to admission. Blood tests demonstrated marked renal impairment. Renal biopsy findings included relatively preserved glomeruli, widespread tubular atrophy, and significant interstitial fibrosis. Immunofluorescence showed linear deposition of IgG as well as kappa and lambda light chains along the TBM. Complement components C3 was also positive, and C1q were weakly positive. No IgG subclass restriction was noted. Electron microscopy revealed electron-dense deposits within the TBM and on the epithelial side of the TBM, exhibiting a mottled and speckled appearance. Case 2: A 52-year-old Japanese man who had undergone allogeneic hematopoietic stem cell transplantation for acute myeloid leukemia developed mild proteinuria and renal dysfunction 7 years post-transplant. Renal biopsy indicated thrombotic microangiopathy, with glomeruli showing diffuse mesangial expansion and focal mesangiolysis. There was also diffuse TBM thickening and interstitial fibrosis with scattered cellular infiltration. Immunofluorescence demonstrated linear staining of polyclonal IgG and both light chains along the TBM. C3 was also positive while C1q was negative. IgG subclass staining revealed positivity for IgG1 and IgG4. Electron microscopy again identified electron-dense deposits within the reticulated TBM. Conclusions: These two cases demonstrated linear IgG immunofluorescence and distinctive electron-dense deposits in the TBM characterized by a mottled and speckled pattern. Although the underlying pathophysiological mechanisms remain unclear, further research is necessary to elucidate the nature of this form of tubulointerstitial disease.