BackgroundLupus nephritis is characterized by deposition of chromatin fragment-IgG complexes in the mesangial matrix and glomerular basement membranes (GBM). The latter defines end-stage disease.Methodology/PrincipalsIn the present study we determined the impact of antibodies to dsDNA, renal Dnase1 and matrix metalloprotease (MMP) mRNA levels and enzyme activities on early and late events in murine lupus nephritis. The major focus was to analyse if these factors were interrelated, and if changes in their expression explain basic processes accounting for lupus nephritis.FindingsEarly phases of nephritis were associated with chromatin-IgG complex deposition in the mesangial matrix. A striking observation was that this event correlated with appearance of anti-dsDNA antibodies and mild or clinically silent nephritis. These events preceded down-regulation of renal Dnase1. Later, renal Dnase1 mRNA level and enzyme activity were reduced, while MMP2 mRNA level and enzyme activity increased. Reduced levels of renal Dnase1 were associated in time with deficient fragmentation of chromatin from dead cells. Large fragments were retained and accumulated in GBM. Also, since chromatin fragments are prone to stimulate Toll-like receptors in e.g. dendritic cells, this may in fact explain increased expression of MMPs.SignificanceThese scenarios may explain the basis for deposition of chromatin-IgG complexes in glomeruli in early and late stages of nephritis, loss of glomerular integrity and finally renal failure.

The accumulation of apoptotic cells has been suggested as a possible mechanism of nucleosome conversion into self-antigens that may both initiate autoimmune responses and participate in immune complex deposition in lupus nephritis. In this study, we analyzed both the rate of transcription of apoptosis-related genes and the presence of activated apoptotic factors within kidneys of lupus-prone (

Autoantibodies to components of chromatin, which include double-stranded DNA (dsDNA), histones and nucleosomes, are central in the pathogenesis of lupus nephritis. How anti-chromatin autoantibodies exert their nephritogenic activity, however, is controversial. One model assumes that autoantibodies initiate inflammation when they cross-react with intrinsic glomerular structures such as components of membranes, matrices or exposed nonchromatin ligands released from cells. Another model suggests glomerular deposition of autoantibodies in complex with chromatin, thereby inducing classic immune complex-mediated tissue damage. Recent data suggest acquired error of renal chromatin degradation due to the loss of renal DNasel enzyme activity is an important contributing factor to the development of lupus nephritis in lupus-prone (NZBxNZW)F1 mice and in patients with lupus nephritis. Down-regulation of DNasel expression results in reduced chromatin fragmentation and in deposition of extracellular chromatin-IgG complexes in glomerular basement membranes in individuals who produce IgG anti-chromatin autoantibodies. The main focus of the present review is to discuss whether exposed chromatin fragments in glomeruli are targeted by potentially nephritogenic anti-dsDNA autoantibodies or if the nephritogenic activity of these autoantibodies is explained by cross-reaction with intrinsic glomerular constituents or if both models coexist in diseased kidneys. In addition, the role of silencing of the renal DNasel gene and the biological consequences of reduced chromatin fragmentation in nephritic kidneys are discussed.

An acquired loss of renal DNaseI promotes transformation of mild mesangial lupus nephritis into membranoproliferative end-stage organ disease. In this study, we analyzed expression profiles of DNaseI in other organs of lupus-prone (NZB×NZW)F1 mice during disease progression to determine whether silencing of the renal DNaseI gene is an organ-specific feature or whether loss of DNaseI reflects a systemic error in mice with sever lupus nephritis. The present results demonstrate normal or elevated levels of DNaseI mRNA and enzyme activity in liver, spleen, and serum samples from (NZB×NZW)F1 mice throughout all the stages of lupus nephritis. DNaseI activity was dramatically reduced only in kidneys of mice with sever nephritis and was the only nuclease that was down-regulated, whereas six other nucleases (DNaseII1 to 3, caspase-activated DNase, Dnase2a, and endonuclease G) were approximately normally expressed in kidneys, liver, and spleen. Loss of renal DNaseI was not accompanied by changes in serum DNaseI activity, suggesting independent mechanisms of DNaseI regulation in circulation and in kidneys and an absence of compensatory up-regulation of serum DNaseI activity in the case of renal DNaseI deficiency. Thus, silencing of renal DNaseI is a unique renal feature in membranoproliferative lupus nephritis. Determining the mechanism(s) responsible for DNaseI down-regulation might lead to the generation of new therapeutic targets to treat and prevent progressive lupus nephritis.

Objective. Antinucleosome autoantibodies are pathogenic factors in lupus nephritis. Defects in apoptotic pathways may result in increased levels of apoptotic nucleosomes. The objectives of this study were 1) to determine whether low molecular weight oligonucleosomes are present in the kidneys of autoimmune (NZB ؋ NZW)F 1 mice, 2) to analyze whether the presence of glomerular membrane-associated TUNELpositive electron-dense structures reflect the existence of low molecular weight oligonucleosomes, and 3) to determine an eventual temporal relationship between glomerular electron-dense structures, oligonucleosomes, and proteinuria in these mice.Methods. DNA was isolated from mouse 111s34 hybridoma cells and from the kidneys of normal BALB/c mice in which apoptosis was induced by camptothecin and from the kidneys of (NZB ؋ NZW)F 1 mice at ages 4 weeks, 8 weeks, 20 weeks, and >26 weeks (nephritic mice). The DNA fragmentation pattern was determined with an Agilent bioanalyzer. An electron microscopybased TUNEL assay was performed to detect apoptotic chromatin in glomerular membranes, and immunoelectron microscopy was used to determine antibody binding. Transcription levels for nucleases associated with apoptosis and necrosis were determined by real-time polymerase chain reaction.Results. DNA from camptothecin-treated cell lines and BALB/c mouse kidneys, but not that from untreated (NZB ؋ NZW)F 1 mouse kidneys, demonstrated DNA cleavage consistent with apoptotic fragmentation. DNA from (NZB ؋ NZW)F 1 mice was devoid of apoptotic fragmentation, irrespective of the age of the mice, whereas TUNEL-positive chromatin particles were detected in glomerular membranes in nephritic mice. Renal DNase I transcription was reduced in nephritic mice. Nucleosomal DNA fragmentation in response to camptothecin exposure was highly reduced in (NZB ؋ NZW)F 1 mouse kidneys compared with that in their normal counterparts.Conclusion. The results of this study demonstrate that TUNEL-positive chromatin particles are deposited in the glomeruli of nephritic (NZB ؋ NZW)F 1 mice, due to reduced fragmentation and clearance of chromatin.