Src
Low-grade B cell lymphoma of mucosa-associated tissue type (MALToma)
rarely may involve the kidney. Membranoproliferative glomerulonephritis (MPGN) is an uncommon complication of B
cell lymphoma and may be related to cryoglobulin
and/or immunoglobulin synthesis by a secretory B cell
clone. We report 2 patients with the novel renal biopsy findings of coexistent MALToma and MPGN. Both subjects presented with nephrotic proteinuria and renal
insufficiency. One patient had a serum M protein (IgG
K) but neither individual had any other clinical or serologic evidence of
systemic disease, including hematolymphoid
malignancy, autoimmune disease, cryoglobulinemia, or
hepatitis C viral infection. Both renal biopsies demonstrated MPGN type I with
immunoglobulin deposits that in 1 case showed light chain restriction (IgM K). Electron microscopy disclosed corresponding glomerular electron dense deposits in subendothelial
locations. Both biopsies also contained atypical interstitial lymphoid
infiltrates comprising marginal zone (centro-cyte-like)
cells that infiltrated tubules and showed extra-capsular extension. Immunostains demonstrated a predominantly B cell population
that lacked expression of CD5 and cycline
D1, and gene rearrangement studies confirmed the presence of a monoclonal B
cell population in both cases. These findings indicate that low-grade B cell
lymphoma in the kidney may be an unexpected finding in patients with nephrotic syndrome related to MPGN. Immunophenotypic
and gene rearrangement studies are important ancillary tools for the evaluation
of atypical lymphoid infiltrates in kidney biopsies.
