IgG4-related membranous nephropathy with high blood and low urine IgG4/IgG ratio: a case report and review of the literature.

Membranous nephropathy (MN) is a rare manifestation of IgG4-related disease. Interestingly, the significance of IgG4 has also been documented in idiopathic MN (IMN). Previous studies reported that urine IgG4/IgG ratios were significantly higher in IMN compared with other kinds of nephropathy, indicating that impairment of charge selectivity barrier seemed to be an obvious characteristic of IMN. Although high blood concentration of IgG4 is very common in IgG4-related MN, no study about the urine IgG4 has been described before. Here, we present a 55-year-old male with IgG4-related MN. Complete remission of proteinuria was promptly achieved by glucocorticoid treatment without immunosuppressant. Consistent with previous reports, the serum antibody against M-type phospholipase A2 receptor was negative. Surprisingly, although the blood concentration of IgG4/IgG reached as high as 36 %, the urine concentration of IgG4/IgG was only 5 %. The calculated ratio of the renal clearance of IgG4 to IgG of this patient (0.15) was obviously lower than that of five patients with IMN (0.53∼0.81). We speculated that this phenomenon might be a clue of the different pathogenesis between IgG4-related MN and IMN.