Pathogenic Mechanisms in Lupus Nephritis: Nucleosomes Bind Aberrant Laminin β1 With High Affinity and Colocalize in the Electron‐Dense Deposits

Abstract

Objective. Apoptotic nucleosomes are structurally and immunologically involved in lupus nephritis. The purpose of this study was to examine the expression and function of laminins and their interactions with nucleosomes in the kidneys of patients with lupus nephritis, using surface plasmon resonance (SPR) analysis.Methods. SPR interaction analysis was used to quantify the strength of laminin-nucleosome interactions. Electron microscopy techniques were used to determine in vivo colocalization of IgG, chromatin, and laminin ␤1, as well as to characterize nucleosomelaminin interactions in vitro.Results. Nucleosomes were found to possess high affinity for laminin ␤1-containing laminins and to have the potential to form stable molecular complexes with these structures. In vivo, laminin ␤1 was aberrantly expressed in the glomerular basement membrane (GMB) of lupus nephritis patients, and in situ, it acted as a nucleosome ligand, selectively colocalizing with nucleosomes within electron-dense deposits (EDDs). Normal adult laminin 11, which contains laminin ␤2, did not bind nucleosomes, and it did not colocalize in vivo with the nucleosomes in the nephritic GBM. In addition, TGF␤1 was expressed by the glomerular mesangium, glomerular endothelial cells, and by podocytes in patients with lupus nephritis. It was trapped in situ within EDDs by an as-yet-unknown ligand. As was recently described in a transgenic mouse model, paracrine kidney glomerular TGF␤1 may thereby contribute to the development of glomerulopathy via the induction of laminin ␤1 incorporation in the GBM, whereas systemic blood vessel-derived TGF␤1 could be trapped during filtration.Conclusion. Our findings of the specific highaffinity binding of nucleosomes to aberrantly expressed laminin ␤1 in the GBM and their colocalization in the GBM may explain important features of the initial steps in the pathogenesis of lupus nephritis, the planted antigen hypothesis.

Anti-alpha-actinin antibodies are part of the anti-cell membrane antibody spectrum that characterize patients with lupus nephritis

“…Indeed, anti-dsDNA Ab are suspected to deposit in the mesangial matrix (MM) of the glomeruli and to deposit, as the disease progresses, in the glomerular basement membrane (GBM) [28]. Anti-dsDNA Ab bind predominantly to exposed chromatin fragments released from apoptotic cells to form immune complexes present within electrondense structures in the MM and GBM [29]. Regarding anti-C1q Ab, another mechanism is proposed that involves deposition in the glomeruli together with C1q to induce renal inflammation that occurs only in the context of glomerular immune complex disease [30].…”

Elimination of Nucleoproteins in Systemic Lupus Erythematosus and Antinuclear Autoantibodies Production

“…In addition, after interaction with antinuclear antibodies, nucleosomecontaining apoptotic bodies, deposited on glomerular basement membrane or in mesangial space, turn into so-called electron-dense deposits, an atribute of IC-mediated nephritis. There is no immunoglobulin ixation in the kidneys outside these deposits [38].…”

The Pathogenesis of Lupus Nephritis

“…Transforming growth factor-β1, which is expressed by podocytes and mesangial cells in lupus nephritis, can aid in the replacement of expression of laminin-11 normally found in the mature glomerular basement membrane with laminin-1, which is normally only expressed in development [46,47]. Nucleosomes readily bind to laminin-1 through the β1 chain but do not bind laminin-11, which lacks the β1 chain, and the trapped nucleosomes can then be bound by autoantibodies that rev up t-cell-dependent autoimmune responses, adding insight into the early pathogenesis of lupus nephritis [46,47].…”

“…Nucleosomes readily bind to laminin-1 through the β1 chain but do not bind laminin-11, which lacks the β1 chain, and the trapped nucleosomes can then be bound by autoantibodies that rev up t-cell-dependent autoimmune responses, adding insight into the early pathogenesis of lupus nephritis [46,47]. Cross reactivity of monoclonal antibodies that can bind both double-stranded DNA and glomerular antigen α-actinin has high affinity; however, in kidney sections with lupus nephritis autoantibodies did not readily bind those components but preferentially bound to nucleosome-containing structures in the mesangial matrix or glomerular basement membrane, and nucleosomes most readily bound to laminin [48].…”