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Diagnosis: Monoclonal IgM-kappa Glomerulonephritis

Proliferative glomerulonephritis (GN) with monoclonal immunoglobulin deposits (PGNMIDs) is a entity among the spectrum of monoclonal gammopathy of renal significance (MGRS). The disease manifests with chronic glomerular disease, altered renal function and albuminuria, sometimes in the nephrotic range. Acute nephritic syndrome is rare. PGNMID occurs more frequently from the sixth decade of life, but it may affect young adults. Histologically, PGNMID is characterized predominantly by membranoproliferative GN and less frequently by diffuse endocapillary GN, mesangioproliferative GN or atypical membranous GN. Immunofluorescence study is the cornerstone of diagnosis, showing deposits involving glomeruli only, and composed of monotypic immunoglobulin almost always IgG, with a single heavy chain subclass (most commonly IgG3) and light chain (LC) restriction (usually κ), admixed with complement deposits. PGNMID variants with monotypic LC-only, IgA or IgM deposits are uncommon, but they exist. Ultrastructurally, deposits are amorphous with predominant subendothelial and mesangial distribution. PGNMID should be distinguished from type 1 cryoglobulinemic GN and immunotactoid GN, which share some common pathological features. Contrary to other MGRS lesions, the rate of detection of the nephrotoxic monoclonal Ig in the serum or urine, and of an abnormal bone marrow B-cell clone, is only ~30%. Renal prognosis is poor, with progression to end-stage renal disease in 25% of patients within 30 months and frequent early recurrence on the renal allograft. The pathophysiology of PGNMID is unclear and its treatment remains challenging. However, recent studies indicate that clone-targeted chemotherapy may significantly improve renal outcomes, opening future perspectives for the management of this rare disease (All the above text was taken from: Bridoux F, et al. Proliferative glomerulonephritis with monoclonal immunoglobulin deposits: a nephrologist perspective. Nephrol Dial Transplant. 2021;36(2):208-215. [PubMed link]).

In our case a B cell small lymphocytic lymphoma (SLL) was diagnosed. In the differential diagnosis it is important to consider cryoglobulinemia, in this case type I, since only one Ig was found in the IF; glomerular involvement in this disease is usually in mixed cryoglobulinemias (type II and type III); in our patient several tests for cryoglobulins were negative and there were no other systemic manifestations of this disease. There were also no findings of Waldenström’s macroglobulinemia.

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