Possible IgG4-related kidney disease requiring a differential diagnosis of membranous lupus nephritis.

A 25-year-old woman presented with fever, arthralgia and proteinuria exhibiting leukopenia, hypocomplementemia, increased serum IgG and IgG4, and positive antinuclear and anti-double-stranded DNA antibodies. Renal biopsy revealed membranous nephropathy with tubulointerstitial nephritis. IgG subclass immunofluorescence revealed intense IgG4 expression in glomeruli, but no expression of IgG2. Observations resembled membranous lupus nephritis with tubulointerstitial nephritis; however, elevated IgG4, low titers of antinuclear and anti-double-stranded DNA antibodies, IgG4-bearing cell infiltration, and characteristic IgG subclass deposition in glomeruli prompted diagnosis of IgG4-related tubulointerstitial nephritis with membranous nephropathy. It is challenging but important to distinguish lupus nephritis from IgG4-related kidney disease.