Discovery of Autoantibodies Targeting Nephrin in Minimal Change Disease Supports a Novel Autoimmune Etiology
Authors: Watts Andrew J.B. , Keller Keith H. , Lerner Gabriel , Rosales Ivy , Collins A. Bernard , Sekulic Miroslav , Waikar Sushrut S. , Chandraker Anil , Riella Leonardo V. , Alexander Mariam P. , Troost Jonathan P. , Chen Junbo , Fermin Damian , Yee Jennifer L. , Sampson Matthew G. , Beck Laurence H. , Henderson Joel M. , Greka Anna , Rennke Helmut G. , Weins Astrid 
Published: November 2021

Significance Statement
Although corticosteroids are an effective first-line therapy for minimal change disease, relapse, steroid dependence, and intolerance are common in this podocytopathy of unknown etiology. The efficacy of B cell–targeted therapies in some patients suggests an autoantibody-mediated etiology. This study describes the novel discovery in both adults and children with minimal change disease of autoantibodies targeting nephrin, a critical component of the podocyte slit diaphragm that ensures integrity of the glomerular filtration barrier. This observation aligns with the established proteinuric effect of antinephrin antibodies demonstrated in animal models. These findings identify an important autoimmune mechanism in a subset of patients with minimal change disease and provide a framework for the application and development of precision medicine strategies in this condition.

Background
Failure of the glomerular filtration barrier, primarily by loss of slit diaphragm architecture, underlies nephrotic syndrome in minimal change disease. The etiology remains unknown. The efficacy of B cell–targeted therapies in some patients, together with the known proteinuric effect of anti-nephrin antibodies in rodent models, prompted us to hypothesize that nephrin autoantibodies may be present in patients with minimal change disease.

Methods
We evaluated sera from patients with minimal change disease, enrolled in the Nephrotic Syndrome Study Network (NEPTUNE) cohort and from our own institutions, for circulating nephrin autoantibodies by indirect ELISA and by immunoprecipitation of full-length nephrin from human glomerular extract or a recombinant purified extracellular domain of human nephrin. We also evaluated renal biopsies from our institutions for podocyte-associated punctate IgG colocalizing with nephrin by immunofluorescence.

Results
In two independent patient cohorts, we identified circulating nephrin autoantibodies during active disease that were significantly reduced or absent during treatment response in a subset of patients with minimal change disease. We correlated the presence of these autoantibodies with podocyte-associated punctate IgG in renal biopsies from our institutions. We also identified a patient with steroid-dependent childhood minimal change disease that progressed to end stage kidney disease; she developed a massive post-transplant recurrence of proteinuria that was associated with high pretransplant circulating nephrin autoantibodies.

Conclusions
Our discovery of nephrin autoantibodies in a subset of adults and children with minimal change disease aligns with published animal studies and provides further support for an autoimmune etiology. We propose a new molecular classification of nephrin autoantibody minimal change disease to serve as a framework for instigation of precision therapeutics for these patients.