Membranous nephropathy is characterized by deposition of
immune complexes along the glomerular basement membrane. PLA2R and THSD7A are target
antigens in 70% and 1-5% of primary
membranous nephropathy cases, respectively. In the remaining cases, the target
antigen is unknown. Here,
laser microdissection of glomeruli followed by mass spectrometry was used to identify novel
antigen(s) in PLA2R-negative
membranous nephropathy. An initial pilot mass spectrometry study in 35 cases of PLA2R-negative
membranous nephropathy showed high spectral counts for neural tissue encoding
protein with
EGF-like repeats, NELL-1, in six cases. Mass spectrometry failed to detect NELL-1 in 23 PLA2R-associated
membranous nephropathy and 88 controls. NELL-1 was localized by immunohistochemistry, which showed bright granular glomerular basement membrane staining for NELL-1 in all six cases. Next, an additional 23 NELL-1 positive cases of
membranous nephropathy were identified by immunohistochemistry in a discovery cohort of 91 PLA2R-negative
membranous nephropathy cases, 14 were confirmed by mass spectrometry. Thus, 29 of 126 PLA2R-negative cases were positive for NELL-1. PLA2R-associated
membranous nephropathy and controls stained negative for NELL-1. We then identified five NELL-1 positive cases of
membranous nephropathy out of 84 PLA2R and THSD7A-negative cases in two validation cohorts from France and Belgium. By confocal microscopy, both
IgG and NELL-1 co-localized to the glomerular basement membrane. Western blot analysis showed reactivity to NELL-1 in five available sera, but no reactivity in control sera. Clinical and biopsy findings of NELL-1 positive
membranous nephropathy showed features of primary
membranous nephropathy. Thus, a subset of
membranous nephropathy is associated with accumulation and co-localization of NELL-1 and
IgG along the glomerular basement membrane, and with anti-NELL-1
antibodies in the serum. Hence, NELL-1 defines a distinct type of primary
membranous nephropathy.