#944 Soluble uric acid inhibits neutrophil functions during infections in patients with kidney disease

Abstract

Abstract Background and Aims Chronic kidney disease (CKD) is one of the most common diseases affecting 8-16% of the world population. Beside cardiovascular complications, infections including sepsis are the second most common cause of death in patients with acute kidney injury (AKI) and CKD. However, the underlying pathomechanisms that contribute to the secondary immunodeficiency related to kidney disease [1] are not well understood. Recent evidence suggests that CKD-related hyperuricemia characterised by elevated serum uric acid (UA) levels suppresses immune cell functions during sterile inflammation [2]. Whether this is also the case during host defense is subject of our current investigation. In this study, we hypothesized that soluble UA inhibits neutrophil effector functions and therefore contributes to the increased infection risk in patients with kidney disease. Method Blood neutrophils were isolated from healthy individuals as well as from patients with kidney diseases, and incubated ex vivo in the presence or absence of 10 mg/dl soluble UA prior to stimulation with bacterial peptides including LPS. Immune cell functions including cytoskeletal changes, immune cell activation, maturation, endo- and phagocytosis (pHrodo Dextran particles, IgG-FITC beads, pHrodo E. coli bioparticles), ROS production, and neutrophil extracellular trap (NET) formation were analysed and quantified using flow cytometry, fluorescence and colorimetric assays, and fluorescence microscopy. In addition, to verify our results we stimulated neutrophils from healthy individuals with sera from hyperuricemic CKD patients. Results Our results show for the first time that soluble UA significantly inhibits the ability of neutrophils to endocytose small particles and phagocytose beads in neutrophils from healthy individuals comparable to the inhibitory effect of Cytochalasin D, an inhibitor of endo- and phagocytosis. Interestingly, unlike neutrophils from AKI patients, neutrophils from CKD patients were significantly less able to phagocytose beads compared to healthy controls with and without stimulation. To mimic the uptake of pathogens more physiologically, pHrodo E. coli bioparticles were used. We found that soluble UA inhibited the phagocytosis of such bioparticles in neutrophils. In addition, our investigation into the formation of ROS showed that soluble UA inhibited specifically nitric oxide, peroxynitrite, hydroxyl radicals and hydrogen peroxide, but to a lower extent the formation of superoxides in neutrophils from healthy individuals, similar to the effectiveness of the radical scavenger N-acetylcysteine. These data were comparable with our cohort study of neutrophils from CKD patients. To examine the mechanism of reduced ROS production, we inhibited NADPH oxidase (DPI) and MPO (4-ABAH) in neutrophils and found that soluble UA has similar inhibitory effects on ROS production like DPI whereas 4-ABAH was more potent. This indicated that soluble UA modulates NADPH oxidase and ROS production, which subsequently resulted in reduced NET formation in neutrophils. Conclusion Our data identify soluble UA as potential immune regulator of the secondary immunodeficiency in patients with kidney disease by inhibiting the endo- and phagocytosis of particles, NADPH-mediated ROS production and NET formation in neutrophils, processes that are important to kill pathogens and fight an infection. Thus, specifically targeting UA with urate-lowering therapy might overcome the suppressed host defence against infection in patients with kidney disease.