Objectives The study of the proinflammatory role of uric acid has focused on the effects of its crystals of monosodium urate (MSU). However, little is known whether uric acid itself can directly have proinflammatory effects. In this study, we investigate the priming effects of uric acid exposure on the cytokine production of primary human cells upon stimulation with gout-related stimuli. Methods Peripheral blood mononuclear cells (PBMCs) were harvested from patients with gout and healthy volunteers. Cells were pretreated with or without uric acid in soluble form for 24 h and then stimulated for 24 h with toll-like receptor (TLR)2 or TLR4 ligands in the presence or absence of MSU crystals. Cytokine production was measured by ELISA; mRNA levels were assessed using qPCR. Results The production of interleukin (IL)-1β and IL-6 was higher in patients compared with controls and this correlated with serum urate levels. Proinflammatory cytokine production was significantly potentiated when cells from healthy subjects were pretreated with uric acid. Surprisingly, this was associated with a significant downregulation of the anti-inflammatory cytokine IL-1 receptor antagonist (IL-1Ra). This effect was specific to stimulation by uric acid and was exerted at the level of gene transcription. Epigenetic reprogramming at the level of histone methylation by uric acid was involved in this effect. Conclusions In this study we demonstrate a mechanism through which high concentrations of uric acid (up to 50 mg/dL) influence inflammatory responses by facilitating IL-1β production in PBMCs. We show that a mechanism for the amplification of IL-1β consists in the downregulation of IL-1Ra and that this effect could be exerted via epigenetic mechanisms such as histone methylation. Hyperuricaemia causes a shift in the IL-1β/IL-1Ra balance produced by PBMCs after exposure to MSU crystals and TLR-mediated stimuli, and this phenomenon is likely to reinforce the enhanced state of chronic inflammation.

Trained immunity is a process in which innate immune cells undergo functional reprogramming in response to pathogens or damage‐associated molecules leading to an enhanced non‐specific immune response to subsequent stimulation. While this capacity to respond more strongly to stimuli is beneficial for host defense, in some circumstances it can lead to maladaptive programming and chronic inflammation. Gout is characterized by persistent low‐grade inflammation and is associated with an increased number of comorbidities. Hyperuricemia is the main risk factor for gout and is linked to the development of comorbidities. Several experimental studies have shown that urate can mechanistically alter the inflammatory capacity of myeloid cells, while observational studies have indicated an association of hyperuricemia to a wide spectrum of common adult inflammatory diseases. In this review, we argue that hyperuricemia is a main culprit in the development of the long‐term systemic inflammation seen in gout. We revisit existing evidence for urate‐induced transcriptional and epigenetic reprogramming that could lead to an altered functional state of circulating monocytes consisting in enhanced responsiveness and maladaptive immune responses. By discussing specific functional adaptations of monocytes and macrophages induced by soluble urate or monosodium urate crystals and their contribution to inflammation in vitro and in vivo, we further enforce that urate is a metabolite that can induce innate immune memory and we discuss future research and possible new therapeutic approaches for gout and its comorbidities.

Objective: Autophagy has recently been shown to modulate the production of pro-inflammatory cytokine production and to contribute to antigen processing and presentation through the major histocompatibility complex. Genetic variation in the autophagy gene ATG16L1 has been recently implicated in Crohn's disease pathogenesis. The mechanisms underlying this association are not yet known, although experimental models suggest an inhibitory effect of autophagy on interleukin 1β (IL-1β) responses. Here, the effect of ATG16L1 genetic variation on cytokine responses has been assessed in humans. Design and setting: Peripheral blood mononuclear cells from healthy individuals and patients with Crohn's disease with different ATG16L1 genotypes were stimulated with ligands for Toll-like receptor 2 (TLR2), TLR4 and nucleotide-binding oligomerisation domain 2 (NOD2), with or without the autophagy inhibitor 3-methyladenine. Induction of cytokine production and related factors were measured at the mRNA and protein level. Furthermore, protein levels of ATG16L1 were assessed by western blot. Results: The present study demonstrates that cells isolated from individuals bearing the ATG16L1 Thr300Ala risk variant, which is shown to affect ATG16L1 protein expression upon NOD2 stimulation, display increased production of the pro-inflammatory cytokines IL-1β and IL-6, specifically after stimulation with NOD2 ligands. In contrast, no differences were found when cells were stimulated with TLR2 or TLR4 agonists. These findings were confirmed in two independent cohorts of volunteers and in a group of patients with Crohn's disease. The increased production could be ascribed to increased mRNA expression, while processing of pro-IL-1β by caspase-1 activation was not affected. The effect of the ATG16L1 polymorphism was abrogated when autophagy was blocked. Conclusions: The present study is the first to link the ATG16L1 polymorphism with an excessive production of IL-1β and IL-6 in humans, which may explain the effects of this polymorphism on the inflammatory process in Crohn's disease.

Autophagy is a cell housekeeping mechanism that has recently received attention in relation to its effects on the immune response. Genetic studies have identified candidate loci for Crohn's disease susceptibility among autophagy genes, while experiments in murine macrophages from ATG16L1 deficient mice have shown that disruption of autophagy increases processing of IL-1β and IL-18 through an inflammasome-dependent manner. Using complementary approaches either inducing or inhibiting autophagy, we describe modulatory effects of autophagy on proinflammatory cytokine production in human cells. Inhibition of basal autophagy in human peripheral blood mononuclear cells (PBMCs) significantly enhances IL-1β after stimulation with TLR2 or TLR4 ligands, while at the same time reducing the production of TNFα. In line with this, induction of autophagy by starvation inhibited IL-1β production. These effects of autophagy were not exerted at the processing step, as inflammasome activation was not influenced. In contrast, the effect of autophagy on cytokine production was on transcription level, and possibly involving the inhibition of p38 mitogen activated protein kinase (MAPK) phosphorylation. In conclusion, autophagy modulates the secretion of proinflammatory cytokines in human cells through an inflammasome-independent pathway, and this is a novel mechanism that may be targeted in inflammatory diseases.

Significance: Hyperuricemia is a metabolic condition intrinsic to gout pathogenesis but also associated with other common metabolic inflammatory diseases. The soluble uric acid-induced priming of monocytes determines a unique shift in cytokine production consisting of high interleukin (IL)-1β but low IL-1 receptor antagonist (IL-1Ra). We investigated metabolic sensors and scanned for transcriptomic pathways and kinase activation to provide mechanistic leads for this process. We describe that uric acid suppresses autophagy and diminishes the antiinflammatory capacity of the cell. The consequences of uric acid exposure are broad and can impact a larger spectrum of inflammatory diseases from gout to metabolic, renal, or cardiovascular diseases or cancer.