The target antigen of anti-tubular basement membrane antibody-mediated interstitial nephritis

Global analysis reveals the complexity of the human glomerular extracellular matrix

The glomerulus contains unique cellular and extracellular matrix (ECM) components, which are required for intact barrier function. Studies of the cellular components have helped to build understanding of glomerular disease; however, the full composition and regulation of glomerular ECM remains poorly understood. We used mass spectrometry-based proteomics of enriched ECM extracts for a global analysis of human glomerular ECM in vivo and identified a tissue-specific proteome of 144 structural and regulatory ECM proteins. This catalog includes all previously identified glomerular components plus many new and abundant components. Relative protein quantification showed a dominance of collagen IV, collagen I, and laminin isoforms in the glomerular ECM together with abundant collagen VI and TINAGL1. Protein network analysis enabled the creation of a glomerular ECM interactome, which revealed a core of highly connected structural components. More than one half of the glomerular ECM proteome was validated using colocalization studies and data from the Human Protein Atlas. This study yields the greatest number of ECM proteins relative to previous investigations of whole glomerular extracts, highlighting the importance of sample enrichment. It also shows that the composition of glomerular ECM is far more complex than previously appreciated and suggests that many more ECM components may contribute to glomerular development and disease processes. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium with the dataset identifier PXD000456.

Modified C-reactive protein might be a target autoantigen of TINU syndrome

BACKGROUND AND OBJECTIVES

The cross-reactive antigen(s) of tubulointerstitial nephritis and uveitis (TINU) syndrome from renal tubulointerstitia and ocular tissue remain unidentified. The authors' recent study demonstrated that the presence of serum IgG autoantibodies against modified C-reactive protein (mCRP) was closely associated with the intensity of tubulointerstitial lesions in lupus nephritis. The study presented here investigates the possible role of IgG autoantibodies against mCRP in patients with TINU syndrome.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

mCRP autoantibodies were screened by ELISA with purified human C-reactive protein in 9 patients with TINU syndrome, 11 with drug-associated acute interstitial nephritis, 20 with IgA nephropathy, 19 with minimal change disease, 20 with ANCA-associated vasculitis, 6 with Sjogren's syndrome, and 12 with amyloidosis. mCRP expression was analyzed by immunohistochemistry in renal biopsy specimens from the 9 patients with TINU syndrome and 40 from disease controls. Frozen normal human kidney and iris were used to demonstrate co-localization of human IgG and mCRP from patients with TINU syndrome with laser scanning confocal microscopy.

RESULTS

The mCRP autoantibodies were detected in all nine patients with TINU syndrome, significantly higher than that of those with disease controls (P < 0.05). The renal histologic score of mCRP in TINU syndrome was significantly higher than that in disease controls (P < 0.05). The staining of mCRP and human IgG were co-localized in renal and ocular tissues.

CONCLUSIONS

It is concluded that mCRP might be a target autoantigen in TINU syndrome.

Differential binding of cross-reactive anti-DNA antibodies to mesangial cells: the role of alpha-actinin

Target Ag display is a necessary requirement for the expression of certain immune-mediated kidney diseases. We previously had shown that anti-DNA Abs that cross-react with alpha-actinin may be important in the pathogenesis of murine and human lupus nephritis; in murine models, we had found that a significant proportion of pathogenic serum and kidney-deposited Igs are alpha-actinin reactive. Furthermore, a pathogenic anti-DNA/alpha-actinin Ab showed enhanced binding to immortalized mesangial cells (MCs) derived from a lupus prone MRL-lpr/lpr mouse as compared with MCs from BALB/c mice which are not susceptible to spontaneous lupus, suggesting that kidney alpha-actinin expression may be contributing to nephritis. In the current study, we established that two isoforms of alpha-actinin that are present in the kidney, alpha-actinin 1 and alpha-actinin 4, can both be targeted by anti-alpha-actinin Abs. We found novel sequence polymorphisms between MRL-lpr/lpr and BALB/c in the gene for alpha-actinin 4. Moreover, alpha-actinin 4 and a splice variant of alpha-actinin 1 were both expressed at significantly higher levels (mRNA and protein) in MCs from the lupus prone MRL-lpr/lpr strain. Significantly, we were able to confirm these differences in intact kidney by examining glomerular Ig deposition of anti-alpha-actinin Abs. We conclude that enhanced alpha-actinin expression may determine the extent of Ig deposition in the Ab-mediated kidney disease in lupus. Modulation of Ag expression may be a promising approach to down-regulate immune complex formation in the target organ in individuals with circulating pathogenic Abs.

Kimura disease with advanced renal damage with anti-tubular basement membrane antibody

Kimura disease (KD) is an autoimmune eosinophilic granulomatous disorder with generalized lymphadenopathy. A handful of pediatric patients with renal disease have been described, none of whom have been African-American (AA). We present an AA boy with KD and nephrotic syndrome (NS). Two months after stopping steroids, fever, asthma, eczema, and proteinuria recurred. His NS did not relapse but his platelet count decreased to 51,000/microl (x10(6)/l). On restarting prednisone, his platelet count normalized. A kidney biopsy revealed 23 of 37 glomeruli obsolescent and advanced damage with over 50% of cortical tissue replaced by interstitial fibrosis and chronic inflammation. Glomerular immunofluorescence was largely negative; very intense linear anti-tubular basement membrane (TBM) deposits of IgA, IgG, C3, and C4 were noted. At present, 36 months from onset, serum creatinine is 1.2 mg/dl (106 micromol/l). We present a 4-year-old AA boy with KD, NS, relapsing thrombocytopenia, and renal damage with anti-TBM antibody.

Cloning, characterization, and expression of the human TIN-ag-RP gene encoding a novel putative extracellular matrix protein

The human gene encoding a novel tubulointerstitial nephritis antigen (TIN-ag)-related protein (TIN-ag-RP) was isolated, and its genomic organization was determined. BLAST searches revealed the highest degree of homology to several mammalian TIN-ag orthologues, and a weak homology to cathepsin B-like proteases. The 12 kb gene was mapped by fluorescence in situ hybridization to chromosome 1p34.2-3, a locus neither related to that of the human TIN-ag (6p11.2-12) nor to that of cathepsin B (8p22-23.1). The TIN-ag-RP is encoded in ten exons with introns ranging from 83 bp to 4 kb. In addition, the gene contained one exon in the 5'UTR, but none in the 3'UTR. Five of the 10 splice sites of the TIN-ag-RP gene were fully conserved when compared to a related gene of C. elegans, whereas only one splice site was identical to those found in cathepsin B genes. Furthermore, human TIN-ag-RP tagged with the T7-epitope, was expressed in HeLa cells, and was found to be localized in vesicular compartments as well as secreted into the medium suggesting the involvement of the endosomal trafficking pathway. Based on the high degree of homology of the amino acid sequences and genomic organization between TIN-ag-RP and TIN-ag, we suggest that both molecules may form a distinct group or family of TIN-ag-like proteins.

Molecular cloning, expression, and chromosomal localization of a human tubulointerstitial nephritis antigen

Tubulointerstitial nephritis antigen (TIN-ag) is an extracellular matrix basement protein which was originally identified as a target antigen involved in anti-tubular basement membrane (TBM) antibody-mediated interstitial nephritis (TIN). Further investigations elucidated that TIN-ag plays a role in renal tubulogenesis and that TIN-ag is defected in hereditary tubulointerstitial disorder such as juvenile nephronophthisis. We previously isolated and characterized 54 kDa glycoprotein as TIN-ag. cDNA encoding rabbit and mouse TIN-ag has recently been identified. In the present study, the cDNA of the human homologue of TIN-ag was cloned and its nucleotide sequence was determined (Accession No. AB022277; the DDBJ nucleotide sequence database). Deduced amino acid sequence (476 aa) exhibited the presence of a signal peptide (1-18 aa), cysteine residues termed follistatin module, six potential glycosylation sites, and an ATP/GTP-binding site. Homology search revealed approximately 85% homology with both rabbit and mouse TIN-ag, and also some ( approximately 40%) similarity with C. elegans. Human TIN-ag contained a sequence similar to several classes of extracellular matrix molecules in amino terminal region and to cathepsin family of cysteine proteinases in the carboxyl terminal region. Northern blot analysis revealed exclusive expression of this molecule in human adult and fetal kidney tissues. Using a monoclonal antibody recognizing human TIN-ag, protein expression ( approximately 50 kDa) was identified in cultured COS-1 cells transfected with human TIN-ag cDNA. The human TIN-ag was mapped to chromosome 6p11.2-12 by fluorescence in situ hybridization. These results may provide further evidence for understanding TIN-ag molecule and clues for gene analysis of juvenile nephronophthisis.