Arthritis Rheumatol. 2014 Feb;66(2):397-406.

Pathogenic mechanisms in lupus nephritis: Nucleosomes bind aberrant laminin β1 with high affinity and colocalize in the electron-dense deposits.

Olin AI, Mörgelin M, Truedsson L, Sturfelt G, Bengtsson AA.

Lund University and Lund University Hospital, Lund, Sweden.

 

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 β1, as well as to characterize nucleosome-laminin interactions in vitro.

RESULTS: Nucleosomes were found to possess high affinity for laminin β1-containing laminins and to have the potential to form stable molecular complexes with these structures. In vivo, laminin β1 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 β2, did not bind nucleosomes, and it did not colocalize in vivo with the nucleosomes in the nephritic GBM. In addition, TGFβ1 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 TGFβ1 may thereby contribute to the development of glomerulopathy via the induction of laminin β1 incorporation in the GBM, whereas systemic blood vessel-derived TGFβ1 could be trapped during filtration.

CONCLUSION: Our findings of the specific high-affinity binding of nucleosomes to aberrantly expressed laminin β1 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. Copyright © 2014 by the American College of Rheumatology.

PMID: 24504812

 

SUPPLEMENTS:

During lupus nephritis, the molecular pathogenesis of “planting” an antigen within the glomerular filtration apparatus had remained elusive. During lupus flares, nucleosomal and auto-antibody overload in the circulation and in the glomeruli itself is substantial. Besides from being immunologically involved in the pathogenesis of lupus, the nucleosome cause the kidney to both suffer from the systemic disease, and from disease-associated intraglomerular apoptotic cell damage and waste. During normal circumstances, the kidney would however have mechanisms and the capacity to clear the glomerular basement membrane from material trapped in transit due to filtration properties.

During lupus we however proposed that nucleosomes would likely have an intrinsic ligand expressed in the glomerular basement membrane, delaying and causing their permanent entrapment. This as one early step in forming the specific electron dense deposits that are diagnostic for lupus nephritis. We also hypothesised that this ligand could not be one of the naturally occurring proteins of a healthy membrane. In the screening process for such possible ligands we found out that laminin-beta1 chain of laminin had affinity for, and could trap nucleosomes to the sites were this protein chain was expressed. In other words, in vitro and from electron microscopy of pathological nephritic kidney samples, aberrant laminin-beta1 was found to co-localize with nucleosomes in contrast to healthy controls where laminin-beta1 was not present at all. This was expected since the expression is changed from beta-1 to beta-2 laminin during development in humans, with regard to the glomerular basement membrane content of full-length laminins.

During lupus, we also found that patients produce TGF-beta1 locally in their kidneys. This could in turn explain the switch from laminin-beta2 laminin production to the patients laminin-beta1, which has been a mechanism previously described as TGF-beta1-induced. These features would together also explain the protection of the healthy kidney from collecting nucleosomes originating from natural cell decay by affinity during normal filtration. When nucleosomal aggregates however have bound aberrant laminin during disease, these complexes are binding auto-antibodies from the circulation. In turn, furthermore immune components are attracted to the nephritic areas of the kidney.

Our findings are important in explaining one of the initial steps in the pathogenesis of lupus nephritis i.e. one of “the planted antigen hypothesis”. The TGF-beta1 mechanism could be an important target when aiming at discovering novel treatment regimes e.g. molecular mimicry concepts aiming at covering epitopes, or attempts to by the use of drugs breaking the cytokine induced expression of aberrant or fetal proteins during disease. Thereby, the kidney would be spared.

 

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