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.