SO079ASYMPTOMATIC HYPERURICEMIA, A REGULATOR OF INNATE IMMUNITY IN CHRONIC KIDNEY DISEASE
Authors: Ma Qiuyue , Immler Roland , Prünster Monika , Romagnani Paola , Sperandio Markus , Anders Hans-Joachim , Steiger Stefanie 
Published: June 2020

Abstract

Background and Aims
Asymptomatic hyperuricemia (HU) is common among patients with chronic kidney disease (CKD) but the causative role of HU on CKD progression remains controversial. Two large multi-center randomized controlled trials (RCTs), the CKD-FIX and the PERL study, have now disproven a causal relation. On the other hand, a causative role of persistent HU exists with gouty arthritis but a rapid correction of HU with urate lowering therapy can also elicit acute gout attacks. This suggests a more complex role of HU in this context. Hence, we hypothesized that soluble uric acid (sUA) has immunomodulatory effects on leukocyte function during the immune response to monosodium urate (MSU) crystals.

Method
Alb-creERT2;Glut9lox/lox and Glut9lox/lox control mice were injected with tamoxifen and placed either on a chow or high-fat diet with inosine to induce HU with or without CKD. After two weeks, MSU crystals or vehicle were injected into air pouches or postcapillary venules in the cremaster muscle of transgenic mice. Leukocyte infiltration and the extent of inflammation were assessed by flow cytometry, intravital microscopy, ELISA, and colorimetric assays. Human blood neutrophils and CD14+ monocytes were isolated from CKD stage G2-4 (n=10) and CKD stage G5 (n=18) patients or healthy individuals (n=15). Neutrophil transwell migration assays were performed and the number of migrated human neutrophils towards chemoattractants determined by flow cytometry. Human blood CD14+ monocytes were treated with MSU crystals or lipopolysaccharide ex vivo, and the expression of inflammatory mediators determined by RT-PCR and flow cytometry.

Results
We found that the presence of HU impaired leukocyte recruitment into MSU crystal-injected air pouches of mice with or without CKD. Intravital microscopy revealed that HU specifically reduced leukocyte adhesion, extravasation, and leukocyte-related tissue inflammation. The CKD-mediated attenuation of MSU crystal-induced inflammation was fully reversible by treating HU with urate lowering therapy such as rasburicase. In blood neutrophils isolated from healthy individuals, sUA diminished β2 integrin activation and expression, and hence impaired neutrophil migration ex vivo. An impaired migratory capability was also observed in neutrophils from CKD stage G2-4 or G5D patients. Moreover, sUA attenuated Toll-like and TNFα receptor-induced human monocyte activation, a process depending on the intracellular uptake of sUA via the urate transporter SLC2A9 (GLUT9), the only urate transporter expressed by these immune cells.

Conclusion
We identify sUA as an endogenous modulator of innate immunity. HU modulates neutrophil migration by altering efficient β2 integrin activation and suppresses pattern recognition receptor-driven monocyte activation via SLC2A9 in gouty arthritis related or unrelated to CKD (see Figure). This process provides a molecular explanation for several previously unexplained clinical phenomena in the context of gouty arthritis and renal failure.