A 67 year old man presented with several months of progressive fatigue, edema and diuretic resistant weight gain. About 6 months prior to developing these symptoms he had recovered from a COVID-19 infection that did not require hospitalization, and 3 months after that he received 2 doses of the Moderna COVID-19 vaccine. Upon presentation serum creatinine was elevated at 1.50 mg/dL (eGFR 46 mL/min/1.73m2). Urinalysis revealed markedly elevated protein (2779 mg/dl), 3-10 red blood cells, 4-10 WBCs, free fatty casts and oval fat bodies; a random albumin to creatinine ratio was above the assay analytical measuring range (>5565 mg/g). ANA was positive with a homogenous pattern and a titer of 1:640. Serum protein electrophoresis revealed a small monoclonal protein of 0.1g/dL, a decreased albumin and gamma-globulin fraction along with an increased alpha-2 globulin fraction suggestive of nephrotic syndrome. Bone marrow biopsy revealed no evidence of a plasma cell neoplasm. A diagnostic kidney biopsy revealed membranous nephropathy (MN, Figure 1). Phospholipase A2 receptor (PLA2R) immunostaining of the biopsy was negative (Figure 2), and serum anti PLA2R IFA and ELISA antibody tests were both negative.
The correct answer is ...
An antibody against an antigen other than PLA2R is responsible for the MN.
Membranous nephropathy (MN) is a glomerular disease which can occur at all ages and represents the most common cause of nephrotic syndrome in adults. In approximately 80% of cases there is no underlying cause (primary MN), whereas the remaining 20% occur in association with disorders such as systemic lupus erythematosus (SLE), hepatitis B, hepatitis C, or malignancy, or medications such as NSAIDS (1,2). MN appears to be caused by antibodies that target podocyte antigens resulting in antigen antibody complexes in the sub epithelial space of the glomerular basement membrane (GBM) (1,3).
Circulating antibodies directed towards M-type Phospholipase A2 receptor (PLA2R) can be detected in approximately 70% of adult patients with primary MN. Serum titers of anti-PLA2R antibodies can be used to both diagnose and monitor treatment response of patients with PLA2R-positive MN. A smaller percentage of MN cases (<5%) are associated with antibodies to thrombospondin type-1 domain-containing 7A (THSD7A). More recently, neural EGF-like-1 protein (NELL1) and exostosin 1/exostosin 2 (EXT1/EXT2) have been identified as a target antigens in MN (4,5). Interestingly, both were identified utilizing laser-capture microdissection and mass spectrometry to identify the target antigen in a series of PLA2R negative MN cases. The underlying target antigen for approximately 10% of MN remains to be identified. Although associated malignancy or autoimmune disorders have been identified for all forms of MN, malignancy appears more commonly associated with NELL1 positive MN, while autoimmune disorders appear more commonly associated with EXT1/EXT2 positive MN (5).
Antibodies targeting NELL1 appear to be the second most common cause of MN accounting for up to 23% of cases (4). On kidney biopsy NELL1-associated MN is characterized by incomplete capillary loop staining, IgG1 predominance, and is clinically associated with malignancy more often than other types of MN (2). NELL1-antigen antibody complexes within the kidney biopsy of this patient could be seen by immunohistochemistry (Figure 3) and electron microscopy (Figure 4). Work is ongoing to develop an assay to detect anti-NELL1 antibodies in serum.
Patrick Vanderboom, Ph.D.
Fellow, Clinical Chemistry
John Lieske, M.D.
Consultant, Nephrology and Hypertension
Professor of Medicine
Mayo Clinic College of Medicine and Science