Induction of trained immunity (innate immune memory) is mediated by activation of immune and metabolic pathways that result in epigenetic rewiring of cellular functional programs. Through network-level integration of transcriptomics and metabolomics data, we identify glycolysis, glutaminolysis, and the cholesterol synthesis pathway as indispensable for the induction of trained immunity by β-glucan in monocytes. Accumulation of fumarate, due to glutamine replenishment of the TCA cycle, integrates immune and metabolic circuits to induce monocyte epigenetic reprogramming by inhibiting KDM5 histone demethylases. Furthermore, fumarate itself induced an epigenetic program similar to β-glucan-induced trained immunity. In line with this, inhibition of glutaminolysis and cholesterol synthesis in mice reduced the induction of trained immunity by β-glucan. Identification of the metabolic pathways leading to induction of trained immunity contributes to our understanding of innate immune memory and opens new therapeutic avenues.

Background: Elevated lipoprotein(a) [Lp(a)] is a prevalent, independent cardiovascular risk factor, but the underlying mechanisms responsible for its pathogenicity are poorly defined. Because Lp(a) is the prominent carrier of proinflammatory oxidized phospholipids (OxPLs), part of its atherothrombosis might be mediated through this pathway. Methods: In vivo imaging techniques including magnetic resonance imaging, ¹⁸ F-fluorodeoxyglucose uptake positron emission tomography/computed tomography and single-photon emission computed tomography/computed tomography were used to measure subsequently atherosclerotic burden, arterial wall inflammation, and monocyte trafficking to the arterial wall. Ex vivo analysis of monocytes was performed with fluorescence-activated cell sorter analysis, inflammatory stimulation assays, and transendothelial migration assays. In vitro studies of the pathophysiology of Lp(a) on monocytes were performed with an in vitro model for trained immunity. Results: We show that subjects with elevated Lp(a) (108 mg/dL [50–195 mg/dL]; n=30) have increased arterial inflammation and enhanced peripheral blood mononuclear cells trafficking to the arterial wall compared with subjects with normal Lp(a) (7 mg/dL [2–28 mg/dL]; n=30). In addition, monocytes isolated from subjects with elevated Lp(a) remain in a long-lasting primed state, as evidenced by an increased capacity to transmigrate and produce proinflammatory cytokines on stimulation (n=15). In vitro studies show that Lp(a) contains OxPL and augments the proinflammatory response in monocytes derived from healthy control subjects (n=6). This effect was markedly attenuated by inactivating OxPL on Lp(a) or removing OxPL on apolipoprotein(a). Conclusions: These findings demonstrate that Lp(a) induces monocyte trafficking to the arterial wall and mediates proinflammatory responses through its OxPL content. These findings provide a novel mechanism by which Lp(a) mediates cardiovascular disease. Clinical Trial Registration: URL: http://www.trialregister.nl . Unique identifier: NTR5006 (VIPER Study).

Traditionally, the innate and adaptive immune systems are differentiated by their specificity and memory capacity. In recent years, however, this paradigm has shifted: Cells of the innate immune system appear to be able to gain memory characteristics after transient stimulation, resulting in an enhanced response upon secondary challenge. This phenomenon has been called trained immunity. Trained immunity is characterized by nonspecific increased responsiveness, mediated via extensive metabolic and epigenetic reprogramming. Trained immunity explains the heterologous effects of vaccines, which result in increased protection against secondary infections. However, in chronic inflammatory conditions, trained immunity can induce maladaptive effects and contribute to hyperinflammation and progression of cardiovascular disease, autoinflammatory syndromes, and neuroinflammation. In this review we summarize the current state of the field of trained immunity, its mechanisms, and its roles in both health and disease.

Innate immune memory, or trained immunity, has recently been described to be an important property of cells of the innate immune system. Due to the increased interest in this important new field of immunological investigation, we sought to determine the optimal conditions for an in vitro experimental protocol of monocyte training using three of the most commonly used training stimuli from the literature: β-glucan, the bacillus Calmette-Guérin (BCG) vaccine, and oxidized low-density lipoprotein (oxLDL). We investigated and optimized a protocol of monocyte trained immunity induced by an initial training period with β-glucan, BCG, or oxLDL, followed by washing and resting of the cells and, thereafter, restimulation with secondary bacterial stimuli. The training and resting time intervals were varied to identify the optimal setting for the long-term induction of trained immunity. Trained immunity was assessed in terms of the secondary cytokine response, the production of reactive oxygen species, cell morphology, and induction of glycolysis. Monocytes primed with β-glucan, BCG, and oxLDL showed increased pro- and anti-inflammatory cytokine responses upon restimulation with nonrelated stimuli. Also, all three stimuli induced a switch to glycolysis (the Warburg effect). These effects were most pronounced when the training interval was 24 h and the resting time interval was 6 days. Training with BCG and oxLDL also led to the increased production of reactive oxygen species, whereas training with β-glucan led to the decreased production of reactive oxygen species. We describe the optimal conditions for an in vitro experimental model with human primary monocytes for study of the induction of trained innate immunity by microbial and metabolic stimuli.