Glomerulonephritis With Masked Monotypic Immunoglobulin Deposits and Concurrent Lymphomatous Infiltration

The evaluation of monoclonal gammopathy of renal significance: a consensus report of the International Kidney and Monoclonal Gammopathy Research Group

The term monoclonal gammopathy of renal significance (MGRS) was introduced by the International Kidney and Monoclonal Gammopathy Research Group (IKMG) in 2012. The IKMG met in April 2017 to refine the definition of MGRS and to update the diagnostic criteria for MGRS-related diseases. Accordingly, in this Expert Consensus Document, the IKMG redefines MGRS as a clonal proliferative disorder that produces a nephrotoxic monoclonal immunoglobulin and does not meet previously defined haematological criteria for treatment of a specific malignancy. The diagnosis of MGRS-related disease is established by kidney biopsy and immunofluorescence studies to identify the monotypic immunoglobulin deposits (although these deposits are minimal in patients with either C3 glomerulopathy or thrombotic microangiopathy). Accordingly, the IKMG recommends a kidney biopsy in patients suspected of having MGRS to maximize the chance of correct diagnosis. Serum and urine protein electrophoresis and immunofixation, as well as analyses of serum free light chains, should also be performed to identify the monoclonal immunoglobulin, which helps to establish the diagnosis of MGRS and might also be useful for assessing responses to treatment. Finally, bone marrow aspiration and biopsy should be conducted to identify the lymphoproliferative clone. Flow cytometry can be helpful in identifying small clones. Additional genetic tests and fluorescent in situ hybridization studies are helpful for clonal identification and for generating treatment recommendations. Treatment of MGRS was not addressed at the 2017 IKMG meeting; consequently, this Expert Consensus Document does not include any recommendations for the treatment of patients with MGRS.

This Expert Consensus Document from the International Kidney and Monoclonal Gammopathy Research Group includes an updated definition of monoclonal gammopathy of renal significance (MGRS) and recommendations for the use of kidney biopsy and other modalities for evaluating suspected MGRS

Paraffin Immunofluorescence: A Valuable Ancillary Technique in Renal Pathology

Immunofluorescence on frozen tissue is the gold standard immunohistochemical technique for evaluation of immune deposits in the kidney. When frozen tissue is not available or lacks glomeruli, immunofluorescence can be performed on paraffin tissue after antigen retrieval (paraffin immunofluorescence). Excellent results can be obtained by paraffin immunofluorescence in most immune complex-mediated glomerulonephritides and dysproteinemia-associated kidney lesions, and thus this technique has become a valuable salvage technique in renal pathology. Furthermore, new data have emerged suggesting that paraffin immunofluorescence can be used as an unmasking technique, as it is more sensitive than frozen tissue immunofluorescence in some kidney lesions, such as crystalline light chain proximal tubulopathy and is needed to establish the diagnosis of certain unique lesions, such as membranous-like glomerulopathy with masked IgG kappa deposits and membranoproliferative glomerulonephritis with masked monotypic Ig deposits. However, it is important to recognize and be aware of the limitations and pitfalls associated with paraffin immunofluorescence. These include poor sensitivity for detection of C3 deposits and for the diagnosis of primary membranous nephropathy. Here, we summarize the available techniques of paraffin immunofluorescence, review its role and performance as a salvage and unmasking technique in renal pathology, address its limitations and pitfalls, and highlight unusual forms of glomerulopathy that require paraffin immunofluorescence for diagnosis.