Gut-derived lipopolysaccharide contributes to pathogenesis of lupus nephritis

Abstract

Pathogenesis of lupus nephritis is complex, and involves both genetic and environmental factors. Gut microbiota and their components have been implicated in the etiology of autoimmune diseases. Bacterial products from the gut may enter the circulation and induce inflammatory responses. Lipopolysaccharide (LPS) is a component of the outer wall of Gram-negative bacteria. This study investigated serum LPS level in patients and mice with lupus nephritis, and its role in kidney inflammation and fibrosis. Methods: Serum LPS level was determined in patients and mice with lupus nephritis using commercially available limulus amebocyte lysate test, with healthy subjects and BALB/c mice as Controls. Intestinal mucosal permeability in NZB/W F1 mice was investigated with LPS-FITC administration and ZO-1 expression. Qualitative and quantitative changes to the gut microbiota were assessed by 16S rRNA sequencing. Expression of LPS-binding protein (LBP) was investigated in renal specimens using cytochemistry. The effect of LPS on cytokine secretion and matrix protein expression was investigated in cultured HK-2 cells. Results: Serum LPS level was significantly higher in patients and mice with lupus nephritis compared to Controls. 16S rRNA analysis showed a progressive decrease in Gram-positive bacteria phyla Actinobacteria and Firmicutes and a concomitant increase in Gram-negative bacteria Bacteroides and Proteobacteria as lupus nephritis progressed in severity. NZB/W F1 mice with active nephritis, but not BALB/c mice, showed increased gut permeability and markedly reduced mucosal ZO-1 expression, and also the presence of LPS-FITC in renal proximal tubules. Kidney specimens from lupus nephritis patients and NZB/W F1 mice showed markedly increased LBP expression in the proximal tubules. LPS induced IL-6, IL-8 and MCP-1 secretion and fibronectin, laminin and collagen I expression in HK-2 cells, mediated in part through increased MAPK and AKT/PI3K signaling. Conclusions: Our data demonstrate that in lupus nephritis there is increased gut-derived Gram-negative bacteria and translocation of LPS from the gut to the circulation and kidney, and this may contribute to inflammation and fibrosis in the renal tubulointerstitium.