The excessive generation of reactive oxygen species (ROS) significantly contributes to maladaptive renal inflammation and fibrosis. While most attention has been directed towards superoxide, hydrogen peroxide (H2O2), and other ROS, our recent work has focused on hypobromous acid (HOBr) as an important ROS. Animal heme peroxidases produce HOBr using H2O2 and Br- as substrates. HOBr, a chemical cousin of bleach (HOCl), is a powerful oxidant capable of damaging proteins, lipids, and nucleic acids and typically promotes tissue injury. We recently discovered that an extracellular matrix (ECM) protein known as peroxidasin (Pxdn) also generates HOBr. Pxdn uses HOBr as an anabolic, reactive intermediate to form novel sulfilimine (S=N) cross- links in collagen IV, a prominent constituent of basement membranes (BM), such as glomerular BM (GBM). In Drosophila, we found that loss of Pxdn and sulfilimine cross-links compromised basement membrane and tissue integrity leading to larval lethality. However, Pxdn knock-out (KO) mice are viable but demonstrate reduced sulfilimine cross-links and BM strength. Based on this phenotype, we expected that unilateral ureteral obstruction (UUO) and increased intratubular pressures would lead to greater tubular BM stretch and mechanical stress on tubular cells in Pxdn KO mice. Consequently, Pxdn KO mice would exhibit increased renal inflammation and fibrosis. Paradoxically, Pxdn KO mice demonstrated less renal inflammation and fibrosis with diminished HOBr mediated oxidative damage. Based on these data, we hypothesize that Pxdn normally uses HOBr to cross-link collagen IV and support tissue integrity, but when dysregulated, Pxdn generates excessive HOBr leading to oxidative damage and tissue injury. To test this hypothesis, in Aim 1, we will determine whether Pxdn loss of function protects against renal injury that varies in extent of inflammation and localization contrasting glomerular and tubulointerstitial disease. Aim 2 will examine how HOBr alters renal cell behavior to promote tubulointerstitial fibrosis and inflammation, including the use of mass spectrometry to identify HOBr mediated oxidative modifications of proteins. In Aim 3, we will mechanistically address how Pxdn uses HOBr to cross-link collagen IV yet avoid collateral damage to other biomolecules. Taken together, this proposal aims to define a novel role for HOBr in renal inflammation and fibrosis. These studies hope to rationalize a strategy to target HOBr to treat chronic kidney disease.