Background: Hyperuricemia (HU), defined by elevated serum uric acid (UA) levels, is strongly associated with gouty arthritis. Gouty arthritis is triggered by the formation of monosodium urate (MSU) crystals in joints. These MSU crystals induce an acute inflammatory response, which is characterized by the infiltration of neutrophils. On the other hand, a rapid correction of HU with urate-lowering therapy can also elicit acute gout attacks. Despite profound HU, only a minority of patients with advanced chronic kidney disease (CKD) experiences gouty arthritis, but why ? We hypothesized that HU affects neutrophil migration in acute gouty arthritis during renal failure and speculated on a suppressive effect of soluble UA (sUA) in this context.
Methods: Mice study: Six weeks old Alb-creERT2/Glut9lox/lox (ki/ki) or Alb-creERT2/ Glut9lox/lox without active cre (+/+, control) mice were injected with tamoxifen and placed on a chow or high-fat diet with inosine. After 22 days, all mice received an intrascrotal injection of MSU crystals 4 hours prior to exteriorization of the cremaster muscle. Rolling flux fraction, leukocyte adhesion and leukocyte rolling velocities were analyzed. Extravasated leukocytes were counted on H&E stained cremaster muscle tissue. Human study:
human neutrophils were isolated from healthy individuals and pre-incubated with or without 10 mg/dL
soluble UA (sUA) prior to stimulation with human CXCL8. The expression levels of LFA-1, MAC-1 and mAB24 (β2 integrin activation marker) were quantified by flow cytometry. Transwell migration assays were carried out and the number of migrated neutrophils determined.
Results: Mice study: After 22 days, the ki/ki mice fed a chow with inosine diet developed HU (serum UA: 9-12mg/dL), whereas the ki/ki mice on high-fat diet with inosine developed HU and CKD (serum BUN: 100mg/dL, creatinine: 2.0mg/dL). Intravital microscopy revealed that HU mice had an increased leukocyte rolling velocity in vessels, a reduced leukocyte intravasal adhesion and extravasation towards MSU crystal-induced inflammation compared to healthy mice. These effects were even more pronounced in HU+CKD mice. Human study: in blood neutrophils isolated from healthy individuals, sUA significantly reduced the expression of LFA-1 and MAC-1 compared to CXCL-8-stimulated neutrophils alone. In addition, CXCL-8 induced β2 integrin activation (increased mAB24 binging and MFI) that was significantly impaired in the presence of sUA. As a consequence, the presence of sUA significantly impaired the ability of neutrophils to migrate towards CXCL-8.
Conclusions: Our in vivo data show that HU and CKD synergistically modulate leukocyte rolling, adhesion, and extravasation towards MSU crystal-induced inflammation. This impaired migratory potential of neutrophils is mediated by sUA resulting in impaired β2 integrin expression and preventing neutrophil activation. Taken together, sUA impairs neutrophil recruitment and might therefore be an explanation for the previously unexplained clinical phenomenon of an unexpectedly low prevalence of gouty arthritis despite persistent HU in CKD.