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Most cases of proliferative glomerulonephritis with monoclonal immunoglobulin deposits (PGNMID) are of the IgG class, particularly IgG3. PGNMID-IgM is very rare.
We studied the characteristics of 20 patients with PGNMID-IgM diagnosed at a large nephropathology laboratory over a 19-year period. Inclusion criteria included kidney biopsy findings of glomerulonephritis with glomerular monotypic granular IgM deposits and absence of clinicopathologic or laboratory evidence of cryoglobulinemic glomerulonephritis.
The patients were 80% male with a median age of 72 years (range 28-87) who presented with proteinuria (median 2.5 g/day), hematuria (85%), and renal insufficiency (median serum creatinine 1.9 mg/dL). The underlying hematologic condition was MGRS in all, including 2 (10%) with CLL/SLL. Monoclonal immunoglobulin was identified by serum and urine immunofixation in 44% and 22%, respectively, with abnormal sFLC in 19%, and a detectable bone marrow clone in 21% (CLL/SLL in 14% and plasma cell (<5%) in 7%, respectively). Five (25%) patients had solid organ malignancy. Renal biopsy showed a membranoproliferative pattern in most (75%) patients. By immunofluorescence, IgM deposits showed light chain restriction (κ in 45%) and C3 (95%), with granular appearance and subendothelial, mesangial and subepithelial distribution by electron microscopy. Most patients were treated with B cell therapy. After a median follow up of 53 months, 59% reached kidney failure and 18% died.
This is the largest series to date on PGNMID-IgM. It mainly affects elderly men, is associated with MGRS (B cell > plasma cell clones) and has a guarded prognosis.