Extracellular histones cause vascular necrosis in severe glomerulonephritis

Crescentic glomerulonephritis is characterized by glomerular necrosis. Dying cells release intracellular proteins that act as danger-associated molecular patterns to activate the innate immune system. Previously, we have demonstrated that dying tubular cells release histones, which can kill endothelial cells and activate the toll-like receptor 2/4 (TLR2/4). This drives tubulointerstitial inflammation in septic or post-ischemic acute kidney injury (AKI). Furthermore, other groups have also reported that extracellular histones cause organ damage during acute lung injury, stroke, peritonitis and retinal dysfunction, and that blocking extracellular histones represents a beneficial approach during the disease progression. In this thesis, we investigated whether extracellular histones can elicit similar pathogenic effects during necrotizing glomerulonephritis. To do so, we used an animal model based on the necrotizing type of severe glomerulonephritis. Necrotic glomerulonephritis was induced in mice by a single intravenous injection of 100µl sheep anti-GBM antiserum. The impact of histone neutralization was studied by using an antibody isolated from the BWA-3 clone, which had the capacity to neutralize released extracellular histones in-vivo and in-vitro. After 7 days, mice were sacrificed and kidneys were collected for further data analysis. Proteinuria was assessed in spot urine samples. Anti-GBM treated mice showed increased proteinuria (albumin/creatinine ratio), plasma creatinine and BUN levels. This was associated with a reduced number of podocytes, increased crescentic glomeruli and the infiltration of neutrophils and macrophages into the kidney. Interestingly, neutralization of extracellular histones significantly reduced proteinuria leading to less podocyte damage. This was linked to an improved renal function defined by lower plasma creatinine and BUN levels, and with a decrease in neutrophil and macrophage infiltration and activation in kidney. Histone blockade also significantly reduced renal mRNA expression of TNF-α and fibrinogen in the glomerular capillaries, which was associated with less glomerulosclerosis, crescents and tubular atrophy. In-vitro studies demonstrated that extracellular histones and NETs-related histones kill glomerular endothelial cells, podocytes and parietal epithelial cells in a dose-dependent manner. Histone-neutralizing agents such as anti-histone IgG, activated protein C or heparin prevented this cytotoxic effect. Stimulation of BMDCs with histones upregulated the expression of the activation marker including MHC-II, CD48, CD80 and CD86 significantly as well as increased the production of TNF-α and IL-6. It has been previously reported by others including us that in biopsies from patients with ANCA-associated vasculitis showed an over-expression of the TLR2/4 receptor compared to the healthy glomeruli. Histone toxicity on glomeruli ex-vivo was also dependent on the TLR2/4 receptor axis given that the lack of TLR2/4 attenuated histone-induced renal thrombotic microangiopathy and glomerular necrosis in mice. Anti-GBM glomerulonephritis involved NET formation and vascular necrosis, while blocking NET formation via PAD inhibitor or pre-emptive anti-histone IgG injection significantly reduced all parameters of glomerulonephritis including vascular necrosis, podocyte loss, albuminuria, cytokine induction, recruitment and activation of glomerular leukocytes, and glomerular crescent formation. Finally, to evaluate histones as a therapeutic target, mice with established glomerulonephritis were treated with three different histone-neutralizing agents such as anti-histone IgG, recombinant activated protein C and/or heparin. Interestingly, all agents were equally effective in abrogating severe glomerulonephritis, while combination therapy had no additive effect. In summary, the results of this thesis indicate that NET-related histones released during glomerulonephritis elicit cytotoxic and immunostimulatory effects and that neutralizing extracellular histones, therefore, represents a potential therapeutic approach when applied during already established glomerulonephritis