The adaptor ASC has extracellular and 'prionoid' activities that propagate inflammation.

Microbes or danger signals trigger inflammasome sensors, which induce polymerization of the adaptor ASC and the assembly of ASC specks. ASC specks recruit and activate caspase-1, which induces maturation of the cytokine interleukin 1ß (IL-1ß) and pyroptotic cell death. Here we found that after pyroptosis, ASC specks accumulated in the extracellular space, where they promoted further maturation of IL-1ß. In addition, phagocytosis of ASC specks by macrophages induced lysosomal damage and nucleation of soluble ASC, as well as activation of IL-1ß in recipient cells. ASC specks appeared in bodily fluids from inflamed tissues, and autoantibodies to ASC specks developed in patients and mice with autoimmune pathologies. Together these findings reveal extracellular functions of ASC specks and a previously unknown form of cell-to-cell communication.

Rewiring monocyte glucose metabolism via C-type lectin signaling protects against disseminated candidiasis.

Monocytes are innate immune cells that play a pivotal role in antifungal immunity, but little is known regarding the cellular metabolic events that regulate their function during infection. Using complementary transcriptomic and immunological studies in human primary monocytes, we show that activation of monocytes by Candida albicans yeast and hyphae was accompanied by metabolic rewiring induced through C-type lectin-signaling pathways. We describe that the innate immune responses against Candida yeast are energy-demanding processes that lead to the mobilization of intracellular metabolite pools and require induction of glucose metabolism, oxidative phosphorylation and glutaminolysis, while responses to hyphae primarily rely on glycolysis. Experimental models of systemic candidiasis models validated a central role for glucose metabolism in anti-Candida immunity, as the impairment of glycolysis led to increased susceptibility in mice. Collectively, these data highlight the importance of understanding the complex network of metabolic responses triggered during infections, and unveil new potential targets for therapeutic approaches against fungal diseases.

Insulin acutely activates metabolism of primary human monocytes and promotes a proinflammatory phenotype.

Increased glycolysis is a metabolic trait of activated innate immune cells and supports functional changes including cytokine production. Insulin drives glycolysis in nonimmune cells, yet its metabolic effects on human innate immune cells remain unexplored. Potential effects of insulin on immune cell metabolism may occur acutely after a postprandial increase in plasma insulin levels or as a consequence of chronically elevated insulin levels as observed in obese insulin-resistant individuals and patients with diabetes. Here, we investigated the effects of acute and chronic exposure to insulin on metabolism and function of primary human monocytes. Insulin acutely activated the PI3K/Akt/mTOR pathway in monocytes and increased both oxygen consumption and glycolytic rates. Functionally, acute exposure to insulin increased LPS-induced IL-6 secretion and reactive oxygen species production. To model chronically elevated insulin levels in patients with diabetes, we exposed monocytes from healthy individuals for 24 h to insulin. Although we did not find any changes in expression of metabolic genes that are regulated by insulin in non-immune cells, chronic exposure to insulin increased LPS-induced TNFα production and enhanced MCP-1-directed migration. Supporting this observation, we identified a positive correlation between plasma insulin levels and macrophage numbers in adipose tissue of overweight individuals. Altogether, insulin acutely activates metabolism of human monocytes and induces a shift toward a more proinflammatory phenotype, which may contribute to chronic inflammation in patients with diabetes.

Differences in Biomarkers of Inflammation Between Novel Subgroups of Recent-Onset Diabetes.

A novel clustering approach identified five subgroups of diabetes with distinct progression trajectories of complications. We hypothesized that these subgroups differ in multiple biomarkers of inflammation. Serum levels of 74 biomarkers of inflammation were measured in 414 individuals with recent adult-onset diabetes from the German Diabetes Study (GDS) allocated to five subgroups based on data-driven cluster analysis. Pairwise differences between subgroups for biomarkers were assessed with generalized linear mixed models before (model 1) and after (model 2) adjustment for the clustering variables. Participants were assigned to five subgroups: severe autoimmune diabetes (21%), severe insulin-deficient diabetes (SIDD) (3%), severe insulin-resistant diabetes (SIRD) (9%), mild obesity-related diabetes (32%), and mild age-related diabetes (35%). In model 1, 23 biomarkers showed one or more pairwise differences between subgroups (Bonferroni-corrected P < 0.0007). Biomarker levels were generally highest in SIRD and lowest in SIDD. All 23 biomarkers correlated with one or more of the clustering variables. In model 2, three biomarkers (CASP-8, EN-RAGE, IL-6) showed at least one pairwise difference between subgroups (e.g., lower CASP8, EN-RAGE, and IL-6 in SIDD vs. all other subgroups, all P < 0.0007). Thus, novel diabetes subgroups show multiple differences in biomarkers of inflammation, underlining a prominent role of inflammatory pathways in particular in SIRD.

Effects of oral butyrate supplementation on inflammatory potential of circulating peripheral blood mononuclear cells in healthy and obese males.

Sodium butyrate is well-known for its immune-modulatory properties. Studies until now only focused on the in vitro effects of butyrate or assessed local effects in the gut upon butyrate administration. In this trial, we studied the systemic anti-inflammatory effects induced by sodium butyrate supplementation in humans. Nine healthy (Lean) and ten obese (metabolic syndrome group, MetSyn) males were given 4 grams sodium butyrate daily for 4 weeks. PBMCs were isolated before and after supplementation for direct stimulation experiments and induction of trained immunity by oxidized low-density lipoprotein (oxLDL), ß-glucan, or Bacillus Calmette-Guérin vaccine (BCG). Butyrate supplementation moderately affected some of the cytokine responses in the MetSyn group. In the direct stimulation setup, effects of butyrate supplementation were limited. Interestingly, butyrate supplementation decreased oxLDL-induced trained immunity in the MetSyn group for LPS-induced IL-6 responses and Pam3CSK4-induced TNF-α responses. Induction of trained immunity by ß-glucan was decreased by butyrate in the MetSyn group for Pam3CSK4-induced IL-10 production. In this study, while having only limited effects on the direct stimulation of cytokine production, butyrate supplementation significantly affected trained immunity in monocytes of obese individuals with metabolic complications. Therefore, oral butyrate supplementation may be beneficial in reducing the overall inflammatory status of circulating monocytes in patients with metabolic syndrome.

Environmental Signals Influencing Myeloid Cell Metabolism and Function in Diabetes.

The environment induces metabolic reprogramming of immune cells via specific signaling pathways. Recent studies have revealed that changes in cell metabolism affect key immune cell functions including cytokine production and migration. In diabetes, these functions are either insufficiently or excessively activated, translating into diabetes-associated complications, including increased susceptibility to infection and accelerated cardiovascular disease. Diabetes alters the abundance of environmental signals, including glucose, insulin, and lipids. Subsequently, changes in environmental signals drive metabolic reprogramming, impair immune cell function, and ultimately contribute to diabetes-associated complications. We review here recent studies on changes in innate immune cell metabolism, especially in myeloid cells, that are driven by environmental signals relevant to diabetes, and discuss therapeutic perspectives of targeting metabolism of immune cells in diabetes.

In vitro and in vivo Effects of Lactate on Metabolism and Cytokine Production of Human Primary PBMCs and Monocytes.

Lactate, the end product of anaerobic glycolysis, is produced in high amounts by innate immune cells during inflammatory activation. Although immunomodulating effects of lactate have been reported, evidence from human studies is scarce. Here we show that expression of genes involved in lactate metabolism and transport is modulated in human immune cells during infection and upon inflammatory activation with TLR ligands in vitro, indicating an important role for lactate metabolism in inflammation. Extracellular lactate induces metabolic reprogramming in innate immune cells, as evidenced by reduced glycolytic and increased oxidative rates of monocytes immediately after exposure to lactate. A short-term infusion of lactate in humans in vivo increased ex vivo glucose consumption of PBMCs, but effects on metabolic rates and cytokine production were limited. Interestingly, long-term treatment with lactate ex vivo, reflecting pathophysiological conditions in local microenvironments such as tumor or adipose tissue, significantly modulated cytokine production with predominantly anti-inflammatory effects. We found time- and stimuli-dependent effects of extracellular lactate on cytokine production, further emphasizing the complex interplay between metabolism and immune cell function. Together, our findings reveal lactate as a modulator of immune cell metabolism which translates to reduced inflammation and may ultimately function as a negative feedback signal to prevent excessive inflammatory responses.

Proinflammatory Effects of Hypoglycemia in Humans With or Without Diabetes.

Severe hypoglycemic events have been associated with increased cardiovascular mortality in patients with diabetes, which may be explained by hypoglycemia-induced inflammation. We used ex vivo stimulations of peripheral blood mononuclear cells (PBMCs) and monocytes obtained during hyperinsulinemic-euglycemic (5.0 mmol/L)-hypoglycemic (2.6 mmol/L) clamps in 11 healthy participants, 10 patients with type 1 diabetes and normal awareness of hypoglycemia (NAH), and 10 patients with type 1 diabetes and impaired awareness (IAH) to test whether the composition and inflammatory function of immune cells adapt to a more proinflammatory state after hypoglycemia. Hypoglycemia increased leukocyte numbers in healthy control participants and patients with NAH but not in patients with IAH. Leukocytosis strongly correlated with the adrenaline response to hypoglycemia. Ex vivo, PBMCs and monocytes displayed a more robust cytokine response to microbial stimulation after hypoglycemia compared with euglycemia, although it was less pronounced in patients with IAH. Of note, hypoglycemia increased the expression of markers of demargination and inflammation in PBMCs. We conclude that hypoglycemia promotes mobilization of specific leukocyte subsets from the marginal pool and induces proinflammatory functional changes in immune cells. Inflammatory responses were less pronounced in IAH, indicating that counterregulatory hormone responses are key modulators of hypoglycemia-induced proinflammatory effects. Hypoglycemia-induced proinflammatory changes may promote a sustained inflammatory state.

Microbial stimulation of different Toll-like receptor signalling pathways induces diverse metabolic programmes in human monocytes.

Microbial stimuli such as lipopolysaccharide (LPS) induce robust metabolic rewiring in immune cells known as the Warburg effect. It is unknown whether this increase in glycolysis and decrease in oxidative phosphorylation (OXPHOS) is a general characteristic of monocytes that have encountered a pathogen. Using CD14+ monocytes from healthy donors, we demonstrated that most microbial stimuli increased glycolysis, but that only stimulation of Toll-like receptor (TLR) 4 with LPS led to a decrease in OXPHOS. Instead, activation of other TLRs, such as TLR2 activation by Pam3CysSK4 (P3C), increased oxygen consumption and mitochondrial enzyme activity. Transcriptome and metabolome analysis of monocytes stimulated with P3C versus LPS confirmed the divergent metabolic responses between both stimuli, and revealed significant differences in the tricarboxylic acid cycle, OXPHOS and lipid metabolism pathways following stimulation of monocytes with P3C versus LPS. At a functional level, pharmacological inhibition of complex I of the mitochondrial electron transport chain diminished cytokine production and phagocytosis in P3C- but not LPS-stimulated monocytes. Thus, unlike LPS, complex microbial stimuli and the TLR2 ligand P3C induce a specific pattern of metabolic rewiring that involves upregulation of both glycolysis and OXPHOS, which enables activation of host defence mechanisms such as cytokine production and phagocytosis.