Cubilin and Amnionless Protein are Novel Target Antigens in Anti-Brush Border Antibody Disease

Autoimmune Tubulopathies

The renal tubule and collecting duct express a large number of proteins, all having putative immunoreactive motives. Therefore, all can be the target of pathogenic autoantibodies. However, autoimmune tubulopathies seem to be rare, and we hypothesize that they are underdiagnosed. This review summarizes the current knowledge on autoimmune tubulopathies. We elected to classify tubulopathies according to the segment that is targeted because this determines, at least in part, the phenotypic presentation. In the proximal tubule, autoantibodies can cause anti-brush border antibody disease, renal Fanconi syndrome, renal proximal tubular acidosis, or tubulointerstitial nephritis and uveitis syndrome. Autoantibodies targeting the thick ascending limb of the loop of Henle can cause either acquired Bartter syndrome or hypomagnesemia with hypercalciuria, whereas autoantibodies targeting the distal convoluted tubule can cause acquired Gitelman syndrome. Finally, renal distal tubular acidosis or nephrogenic diabetes insipidus can be caused by autoantibodies targeting the collecting duct. In most instances, the characterization of the autoantibodies remains incomplete and the pathogenesis of the disease obscure. We believe it is important to increase the awareness of physicians regarding autoantibody-mediated tubular diseases to have a better estimation of the prevalence and to improve the care to patients. A research effort to increase the understanding of the pathogenesis of autoantibodies-mediated tubular diseases is also hoped for.

Antibrush Border Antibody Disease: A Case Series

Antibrush border antibody (ABBA) disease is a rare cause of kidney disease characterized by progressive renal tubular injury associated with immune complex deposition along the basement membranes of the proximal tubule and circulating autoantibodies to brush border antigens. Several antigens have been identified as targets of autoantibodies in this disease, including low-density lipoprotein receptor related protein 2 (LRP2), cubilin, and amnionless proteins. We present 9 patients from 2 academic medical centers and describe the clinicopathologic characteristics and outcome data. All patients presented with acute kidney injury and proteinuria. Pathology confirmed immune complex deposition along proximal tubular basement membranes in all patients, but the majority (6/8) also showed segmental glomerular subepithelial immune complexes. Two of 3 patients treated with rituximab demonstrated stabilization of kidney function; 1 of these patients had mantle cell lymphoma. One patient with lung cancer showed stabilization of disease after treatment of the malignancy. The remaining patients progressed to end-stage kidney disease with either conservative therapy (3 patients) or immunosuppression with glucocorticoids (2 patients). This series highlights the poor prognosis of ABBA disease, but a potential benefit of anti-B cell therapy or treatment of an underlying malignancy in some cases.

Discovery of seven novel putative antigens in membranous nephropathy and membranous lupus nephritis identified by mass spectrometry

Multiple autoantigens have been identified in membranous nephropathy (MN) by tissue-based proteomics. However, antigenic targets of disease are unknown for over 10% of patients with MN and over half of those with membranous lupus nephritis (MLN). Here, we identified multiple new targets in PLA2R-/THSD7A-/EXT-/NELL1-quadruple negative MN biopsies through mass spectrometry of immune complexes recovered from biopsy tissue of patients with MN. Patients with MN negative for these four antigens were identified from Arkana Laboratories case archives. Protein G immunoprecipitation recovered immune complexes from frozen biopsy tissue from 142 quadruple-negative cases and 278 cases of known antigen type, followed by interrogation by mass spectrometry. Potential putative antigens were confirmed through paraffin immunofluorescence and co-localization with IgG within immune deposits. Consecutive series of 165 cases of PLA2R-negative MN and 142 MLN biopsies were screened to determine the frequency for each potential antigen. Seven putative antigens were discovered within immune complexes from biopsies of patients with MN including FCN3, CD206, EEA1, SEZ6L2, NPR3, MST1, and VASN. Peptides from these proteins were not enriched in the 278 cases of known antigen type. Between three to 30 unique peptides were detected for each new target. Frequencies of each biomarker, determined by staining consecutive case series, ranged from under 1 to 4.9%. NPR3 and CD206 were only positive in index cases. All cases showed co-localization of IgG within the immune deposits. Thus, seven putative antigens were newly identified in MN and MLN. Due to the number of antigens identified, it is becoming impractical to type PLA2R-negative MN or MLN cases through immunostaining alone. A multiplex approach is needed for subtyping of these diseases.

Case report: Isolated tubular basement membrane deposits in a patient with systemic lupus erythematosus - A diagnostic challenge

Lupus nephritis is one of the most serious and frequent manifestations of systemic lupus erythematosus. It usually presents in the first years of the disease, which suspicion should be raised in cases of elevated serum creatinine, presence of proteinuria above 500 mg/day or active urinary sediment, in the absence of other apparent causes such as urinary tract infection and use of nephrotoxic drugs. In most cases, it affects the glomerulus, and its presentation is rare in the form of isolated tubulo-interstitial disease. In this report, we describe a case of lupus nephritis diagnosed after 2 years of illness, in the form of atypical isolated tubular disease, characterized by massive deposits in the tubular basement membrane. Clinically, there were altered renal function, subnephrotic proteinuria, and evolution to a complete clinical response after immunosuppressive treatment.