SAT-192 LOSS OF MACROPHAGE-SPECIFIC TLR4 INHIBITS OBSTRUCTIVE RENAL FIBROSIS BY BLOCKING MACROPHAGE-TO-MYOFIBROBLAST TRANSITION VIA A BAMBI-DEPENDENT MECHANISM

Abstract

Macrophages are a key inflammatory cell that plays a critical role in acute and chronic kidney diseases. Toll-like receptor (TLR) 4 is highly expressed on inflammatory macrophages and has been shown to trigger acute renal inflammation. However, the role of macrophage-specific TLR4 in progression of chronic kidney disease remains unclear, which was investigated in the present study in a mouse model of unilateral ureter ligation (UUO). A mouse model of UUO was induced in Tlr4flox/floxand lysozyme Mpromoter-driven cre(LysM-cre) transgenic mice (C57BL/6). Role and mechanisms of TLR4 in macrophage-mediated renal fibrosis were studied. UUO led to remarkably F4/80+macrophage and CD3+/CD4+T cell infiltration and progressive renal fibrosis in Tlr4flox/floxmouse kidney. In contrast, deletion of macrophage-specific TLR4 blunted these inflammatory responses in LysM-cre/Tlr4flox/flox animals. Mechanistically, these inhibitory effects on renal inflammation and fibrosis in LysM-cre/Tlr4flox/floxmice were associated with the shift of macrophages from M1 (F4/80+/iNOS+) to M2 phenotype (F4/80+/CD206+). Most importantly, we also found that deletion of TLR4 from macrophage largely suppressed progressive renal fibrosis by inhibiting macrophage-to-myofibroblast transition (MMT) as identified by F4/80+αSMA+cells. This novel observation suggests that macrophage-TLR4 may trigger inflammatory M1 macrophage activation and subsequent transition to the fibrogenic phenotype via the MMT process. Further studies revealed that BMP and activin membrane-bound inhibitor (Bambi), a TGF-β1 pseudoreceptor that competitively inhibits the binding of active TGF-β1 to receptor, was also highly upregulated in LysM-cre/Tlr4flox/floxmouse kidney after UUO. Thus, macrophages lacking TLR4 showed an anti-inflammatory and anti-fibrotic phenotype as demonstrated by high levels of Bambi but low levels of TGF-b/Smad3 signaling, MMT, and renal fibrosis in vivoand in vitro. Deletion of macrophage-specific TLR4 ameliorates obstructive renal injury by promoting anti-inflammatory macrophage polarization and inhibiting MMT-mediated renal fibrosis via a Bambi-dependent mechanism. Results from this study suggest that targeting the macrophage-specific TLR4 pathway may represent a novel therapeutic intervention for renal inflammation and fibrosis.