Monocyte Regulation By Soluble Uric Acid
Authors:   , McLaughlin René Joel 
Published: November 2021

<p>Hyperuricaemia is a chronic condition associated with diseases of the metabolic syndrome. However, the cause and effect relationship between increased serum uric acid (UA) levels and the pathophysiology of metabolic dysfunction is far from clear. From an immunological angle hyperuricaemia has been shown to modulate inflammatory signalling in both immune and nonimmune cell types. Blood monocytes are constantly exposed to soluble UA in the circulation but the direct effect of this exposure has not been examined. This research focuses on the how soluble UA alters blood monocyte responses to inflammatory stimuli using in vitro, in vivo and clinical manipulation of UA levels. The Harper group previously found that blood monocytes from hyperuricaemic individuals produced lower levels of inflammatory cytokines compared to monocytes from healthy controls when stimulated ex vivo with LPS. My research began by studying the direct effect of soluble UA on human blood monocytes in vitro. I found that soluble UA reduced monocyte production of pro-inflammatory cytokines and increased IL-10 in response to stimulation with LPS. I identified two inflammatory signalling pathways modulated by soluble UA that could be contributing to this suppressive monocyte phenotype: MAP kinase phosphorylation was reduced alongside increased expression of the regulatory protein DUSP10 and reduced ASC; there was a switch towards anti-inflammatory NFκB signalling illustrated by decreased p65 and increased p50 nuclear translocation. To study the modulation of soluble UA levels in a physiological context I raised serum UA levels in vivo with a model of acute hyperuricaemia and lowered serum UA using two clinically relevant medications: allopurinol and rasburicase. Consistent with in vitro UA treatment, raising serum UA levels in vivo suppressed pro-inflammatory cytokine responses to LPS, increased IL-10 and down-regulated monocyte MAP kinase and NFκB signalling pathways. Acute urate-lowering therapy (ULT) with allopurinol or rasburicase reversed this suppressive inflammatory cytokine and signalling pattern. The PLT2 mouse strain has had the purine metabolic pathway disrupted by random mutagenesis of the gene encoding 5-hiydroxyisourate hydrolase, the enzyme responsible for degradation of the molecule directly downstream of UA, 5-hydroxyisourate. I found that this mutation resulted in chronic hyperuricaemia with an average 2-fold increase in serum UA over C57 mice. LPS challenge resulted in increased IL-10 production in PLT2 mice compared to C57, however no differences in monocyte inflammatory signalling were observed between the two strains. Acute ULT with rasburicase reduced serum UA in the PLT2 strain and subsequent LPS challenge increased monocyte inflammatory signalling. Finally, I studied the effects of ULT on the inflammatory phenotype of human blood monocytes from patients with hyperuricaemia. ULT significantly reduced serum UA levels, which coincided with reduced blood monocyte percentages and adhesion molecule expression (CD11b and ICAM1). ULT increased the inflammatory potential of human blood monocytes: Monocytes stimulated with LPS produced less IL-10; MAP kinase phosphorylation increased alongside increased ASC expression; nuclear translocation of NFκB p65 was increased. ULT also increased expression of the NLRP3 inflammasome components procaspase1, pro-IL-1β and NLRP3. Taken together these results demonstrate a previously unidentified role for soluble UA in moderating monocyte immune responses to inflammatory stimuli. In vitro, in vivo and clinical experimentation all confirmed the immunosuppressive function of soluble UA. This potentially places UA in the centre of innate immune control through the dichotomy of its suppressive soluble effects, demonstrated herein, and the widely reported inflammatory crystalline effects. Importantly, this research illustrates that serum UA levels can be manipulated in a clinical setting to control the inflammatory phenotype of circulating immune cells.</p>