The renal biopsy shown here has an MPGN pattern of injury by light microscopy. Routine immunofluorescence on the fresh (unfixed) tissue is negative for immune reactants but repeat staining on the formalin fixed paraffin embedded tissue after protease retrieval shows positive staining for IgG and kappa. A subsequent bone marrow biopsy revealed a small clonal B-cell population. A recent case series (see reference below) detailed a cohort of patients with renal biopsy findings similar to those shown here, namely an MPGN pattern by LM and false negative staining for immunoglobulins by routine IF that are ‘unmasked monoclonal deposits’ by performing IF on the formalin-fixed paraffin embedded tissue after protease digestion. This case series found that this combination of findings is frequently associated with the presence of an underlying hematologic disorder. Since these cases are best considered a paraprotein-associated glomerulopathy, treatment directed against the underlying clonal hematologic process is likely warranted to treat the glomerulonephritis.
Monoclonal gammopathy of renal significance (MGRS) is a recently conceived description of kidney injury resulting from a non-malignant plasma cell/B cell clone that, by definition, does not fulfill criteria for multiple myeloma or lymphoma. As a result, the disorder is otherwise consistent with MGUS. In addition to the entity shown here, there are a number of MGRS lesions that can be seen on renal biopsy including amyloidosis, monoclonal immunoglobulin deposition disease, cryoglobulinemic glomerulonephritis, proliferative glomerulonephritis with monoclonal IgG deposits, and monoclonal membranous glomerulopathy, among others.
Reference: Kidney Int. 2015 Oct;88(4):867-73.
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