Monocyte reprogramming and trained immunity: Linking metabolism to inflammation in non-alcoholic fatty liver disease

Abstract

Non-alcoholic fatty liver disease (NAFLD) represents a global clinical challenge, largely due to the liver’s central role as a key immunometabolic organ. Recent research underscores the systemic immunometabolic nature of NAFLD. It has been shown that peripheral blood immune cells of NAFLD patients exist in a primed state, which aligns with the concept of long-term functional reprogramming of innate immune cells in metabolic diseases. This functional reprogramming - encompassing priming and trained immunity - represents a recently described facet of innate immunity. While evolutionarily beneficial for host defense, these mechanisms are now recognized as contributors to the pathogenesis of various chronic non-communicable diseases. It is hypothesized that monocyte reprogramming, induced by chronic exposure to metabolic signals such as lipotoxicity and hyperglycemia, fosters a hyperactive pro-inflammatory phenotype. This phenotype significantly contributes to disease pathogenesis and the development of systemic immunometabolic disturbances. Understanding the role of immunometabolic reprogramming opens new prospects for the search of biomarkers and the development of therapeutic strategies aimed at modulating the metabolism of immune cells in NAFLD.

Key Words: Non-alcoholic fatty liver disease; Innate immune system; Immunometabolism; Metabolic reprogramming; Priming; Trained immunity; Monocytes

Core Tip: Non-alcoholic fatty liver disease is a systemic immunometabolic disorder. Chronic exposure to metabolic factors such as lipotoxicity and hyperglycemia induces long-term functional reprogramming of innate immunity, underlying phenomena like priming and trained immunity. The resulting hyperactive pro-inflammatory phenotype of monocytes and macrophages serves as a key mechanism linking obesity, insulin resistance, and chronic liver inflammation, opening new avenues for therapeutic intervention.

INTRODUCTION

Non-alcoholic fatty liver disease (NAFLD) or, in the new classification, metabolic dysfunction-associated steatotic liver disease is a global health problem affecting up to 20% of the adult population and is the leading cause of end-stage liver disease, transplantation, and hepatocellular carcinoma[1-3]. Understanding of the pathogenesis of NAFLD has improved significantly in recent years and has evolved from a simple classical hepatocentric theory to the recognition of the disease as a complex multifactorial process involving insulin resistance, oxidative stress, chronic inflammation, and other immune disorders occurring at the whole-body level[4-7]. At the same time, there is a growing understanding of the importance of immune disorders and their complex cross-links with metabolism. A recent study by Zeber-Lubecka et al[8] emphasizes the systemic immune nature of NAFLD. It has been shown that even in the absence of peripheral blood mononuclear cells stimulation, adolescents with NAFLD have a transcriptional profile indicative of systemic immune activation and functional cell reprogramming. These data contribute to the understanding that NAFLD is a systemic immunometabolic disease and once again highlight the complexity of the immune system’s interactions[9]. Thus, the aim of this mini-review is to examine the role of long-term functional reprogramming of monocytes and macrophages - key innate immune cells, as central players in NAFLD pathogenesis, linking metabolic disturbances to persistent inflammation.

METABOLIC REPROGRAMMING AND MONOCYTE IMMUNOLOGICAL FUNCTION

The key mechanism linking metabolic disorders to chronic inflammation in NAFLD and other associated conditions (such as atherosclerosis) is considered to be dysfunction of the innate immune system[10-12]. The mammalian immune system has a variety of defense mechanisms and is believed to include two evolutionarily conserved components: Innate and adaptive immunity. The innate immune system relies on phylogenetically ancient mechanisms and is inherent in all multicellular organisms, including humans. Innate immunity is characterized by a rapid and nonspecific response to pathogens, including biochemical and cellular reactions[13]. Evidence indicates that impaired mechanisms of the innate immune system, in particular a disproportionate pro-inflammatory response, underlie many chronic non-infectious diseases in humans, such as atherosclerosis[14-16].

It is believed that invertebrates have only an innate immune system and do not have an adaptive immune system, which, however, has not prevented them from being a very numerous group of animals that have successfully colonized various ecological niches. Thus, only one innate immune system can provide them with the necessary protection against pathogens. For a long time, it was believed that the innate immune system has no immunological memory and is unable to respond differentially to pathogens, while the adaptive immune system retains memory of the pathogen and can act more selectively. Data obtained in recent years have made it possible to revise some of these principles of immunology, as it has been shown that the innate immune system is also capable of retaining some immunological memory, which has been termed “trained immunity” and has been convincingly demonstrated in monocytes, macrophages, and natural killer cells, as well as in numerous experiments on invertebrates[17-19]. It is important to note that the functional reprogramming of innate immune cells is diverse and includes four distinct functional states: Differentiation, priming, tolerance, and trained immunity[20]. Differentiation is associated with long-term changes in innate immune cells necessary for specialized functions. Priming is characterized by a sustained activated state of the immune cell after contact with a stimulus without returning to a quiescent state, whereas trained immunity is mediated by sustained epigenetic and metabolic changes that persist after the cell returns to a quiescent state and cause an enhanced response upon re-stimulation, i.e., unlike priming, trained immunity is characterized by a return to a quiescent state before reactivation[20]. In turn, immune tolerance is characterized by sustained hyporeactivity of cells to repeated exposure to a stimulus (e.g., endotoxin)[21-23].

It has been established that trained immunity is based on epigenetic mechanisms and the switching of cellular metabolism[24-27]. In particular, it has been shown that Candida albicans and β-glucans from the fungal cell wall cause functional reprogramming of monocytes, leading to enhanced cytokine production. Training of monocytes with β-glucans was associated with stable changes in histone H3K4 trimethylation, confirming the involvement of epigenetic mechanisms in this phenomenon[28,29]. The induction of trained immunity causes changes in the cellular metabolism of myeloid cells, including a shift towards glycolysis induced via the protein kinase B - mammalian target of rapamycin - hypoxia-inducible factor 1-alpha signaling pathway, as well as glutaminolysis and cholesterol synthesis[30-34]. Induction of trained immunity reprograms cellular metabolism not only in mature cells, but also in hematopoietic stem cells, transferring immunological memory to cells[35].

Metabolic reprogramming in trained immunity corresponds to the broader concept of immunometabolism, which involves switching metabolic pathways in immune cells between different states that provide the cell with energy or substrates for biochemical processes necessary for the immune response[25]. Thus, macrophages can demonstrate different roles in inflammation or even switch from one role to another by changing their cellular metabolism. The classic model of immunometabolism demonstrated in mice includes two key macrophage phenotypes: M1 and M2 polarization (it is important to note that this classification is greatly simplified compared to the actual, more complex picture of macrophage polarization, but this model is useful for understanding the role of metabolism in immune cell function). M1 polarization of macrophages is pro-inflammatory. In these cells, glycolysis is activated (similar to the Warburg effect in highly proliferative tumor cells) and oxidative phosphorylation activity is reduced. M2 macrophages are involved in resolving inflammation and rely heavily on oxidative phosphorylation as a key source of energy[36-38]. The disrupted tricarboxylic acid cycle in M1 phenotype macrophages contributes to the accumulation of succinate, which stabilizes hypoxia-inducible factor 1-alpha, leading to increased interleukin 1 beta production via NLR-family pyrin domain-containing protein 3 inflammasome. An anti-inflammatory mechanism is also induced through the synthesis of itaconate, which inhibits glycolysis and suppresses the production of pro-inflammatory cytokines[39-41]. Itaconate is considered one of the key metabolites involved in the formation of innate immune tolerance and exerting an indirect effect on the adaptive immune response[42].

Thus, the concept of trained immunity suggests that monocytes circulating in the bloodstream can come into contact with pathogens or danger signals, such as lipopolysaccharides of Gram-negative bacteria or oxidized low-density lipoproteins[43]. Essentially, trained immunity is a state of hyperreactivity of innate immune cells[44]. In this regard, trained innate immunity is considered a new mechanism linking inflammation and the development of autoimmune and metabolic diseases, including atherosclerosis and NAFLD[45,46]. It has also been shown that even brief exposure of monocytes to low concentrations of oxidized low-density lipoproteins induces a long-lasting proatherogenic macrophage phenotype through epigenetic modifications of histones, characterized by increased production of proinflammatory cytokines and the formation of foam cells[47].

Evidence suggests that systemic metabolic dysregulation in NAFLD reprograms innate immune cells[48]. Monocytes circulating in an environment rich in free fatty acids and glucose can switch their metabolism from oxidative phosphorylation to aerobic glycolysis. This reprogramming creates the conditions for their hyperinflammatory phenotype. This immunometabolic reprogramming may serve as a link between visceral obesity, insulin resistance, and chronic inflammation in the liver and other metabolic disorders. Once in the liver, reprogrammed monocytes differentiate into macrophages that produce pro-inflammatory cytokines, which directly damage hepatocytes and promote fibrogenesis[48]. Macrophages are key innate immune cells of the liver that play a central role in the pathogenesis of NAFLD. Their population is highly heterogeneous and changes dynamically during the course of the disease. In a healthy liver, embryonic Kupffer cells dominate, but as NAFLD develops, their pool is depleted and replaced by macrophages derived from monocytes. Earlier studies suggested that this pool of macrophages can polarize into M1 and M2 phenotypes depending on various signals and inflammatory factors[49]. Macrophage polarization plays a key role in the development of inflammation and liver fibrosis[50]. It has been shown that prolonged feeding with a high-fat diet, including saturated fatty acids, increases the number of Kupffer cells with the M1 phenotype, which corresponds to increased production of pro-inflammatory cytokines[51]. M1 macrophages contribute to the development of hepatic steatosis, the recruitment of inflammatory cells, and the activation of fibrosis, while M2 macrophages have anti-inflammatory and reparative functions but have a profibrogenic phenotype[52,53]. It should be noted that, in contrast to this simplified binary model, modern studies, including single-cell RNA sequencing, reveal several functionally different subpopulations of macrophages in NAFLD, and the functional specialization of these macrophage subpopulations is closely related to their metabolic reprogramming[10].

The gut microbiota plays an important role in regulating immunological memory, which can be disrupted by obesity and NAFLD. For example, it has been shown that microbial dysbiosis in the neonatal period also contributes to the training of the sensitivity of innate immune cells in the intestine, bone marrow, and liver, which leads to the programming of NAFLD development in children[54]. It has been established that the intestinal bacterium Limosilactobacillus reuteri causes an atypical phenotype similar to the memory phenomenon in human dendritic cells in vitro[55]. Interestingly, small extracellular vesicles secreted by the gut microbiota support memory-like inflammatory responses in mouse neutrophils[56].

In the context of NAFLD, functional reprogramming of innate immune cells can play a negative role, but at different stages of disease development. On the one hand, priming and trained immunity exacerbate systemic and hepatic inflammation, contributing to the progression of steatohepatitis and fibrosis. On the other hand, the formation of tolerance in resident liver macrophages (Kupffer cells) may be an adaptation to the chronic, low-intensity influx of bacterial products from the intestine in dysbiosis. At the same time, “tolerant” macrophages lose their ability to effectively phagocytose and clear bacteria. As a result, subsequent damage to hepatocytes due to lipotoxicity or acute infectious exposure can lead to a loss of tolerance and uncontrolled release of accumulated pro-inflammatory mediators, which contribute to the progression of the disease[57,58].

DISCUSSION AND PERSPECTIVES

Thus, the innate immune system, which relies on evolutionarily conserved mechanisms, has a long-term and ambiguous effect on the pathogenesis of chronic non-infectious diseases. In this context, the immunometabolic reprogramming of monocytes and macrophages can be considered a key link in the pathogenesis of NAFLD. The concepts of trained immunity and immunometabolic reprogramming expand the understanding of NAFLD development mechanisms, shifting the focus from hepatocentric models to a systemic view of the disease, where immune cells act as integrators and long-term carriers of metabolic damage.

However, when interpreting these data clinically, it is crucial to consider the existing limitations. A significant portion of the data linking trained immunity to NAFLD has been obtained from experimental animal models or in vitro studies, and their direct translation to humans requires caution. Secondly, many observations are correlational, and establishing causal relationships necessitates further longitudinal and interventional studies. Thirdly, there is considerable inter-individual variability in the formation of immunological memory, driven by genetic factors, metabolic profile, and lifestyle characteristics. Finally, the pathogenic pathways of NAFLD are closely intertwined with those of atherosclerosis, insulin resistance, and obesity, complicating the isolation of liver-specific immunometabolic disturbances.

Despite these limitations, the concept of immunometabolic reprogramming opens new clinical possibilities (Figure 1). Promising directions for future research include the search for biomarkers reflecting the state of immune cell reprogramming (e.g., specific plasma metabolites, epigenetic marks, or transcriptional signatures in circulating monocytes), which could lead to new tools for the early diagnosis and monitoring of NAFLD progression. Furthermore, immunometabolism represents a promising target for pharmacological intervention. Potential therapeutic strategies targeting trained immunity include epigenetic and metabolic modulators[59]. Some non-pharmacological interventions also possess the potential to reprogram the immune response. For example, it has been shown that physical exercise promotes a shift of Kupffer cells towards an anti-inflammatory phenotype with trained immunity through metabolic reprogramming via itaconate metabolism[60].

Figure 1 Schematic diagram illustrating the role of innate immune immunometabolic reprogramming in non-alcoholic fatty liver disease pathogenesis. PAMPs: Pathogen-associated molecular patterns; LPS: Lipopolysaccharide; mTOR: Mechanistic target of rapamycin.

CONCLUSION

Thus, trained immunity, which evolved as a mechanism to enhance the body’s defense against reinfection by pathogens or tissue damage, may be one of the mechanisms of immunometabolic diseases. The role of the innate immune system in general and trained immunity in particular in the pathogenesis of NAFLD is a promising topic for future research and will improve our understanding of the complex links between NAFLD and other diseases, thereby increasing the effectiveness of diagnosis and treatment. In this regard, the concept of trained immunity represents a paradigm shift in understanding the pathogenesis of NAFLD. It shifts the focus from a purely hepatocentric model to a systemic immunometabolic approach, in which innate immune cells, such as monocytes and macrophages, act as long-term integrators of metabolic dysfunction and inflammatory priming. Chronic functional reprogramming of these cells, based on epigenetic and metabolic changes, creates a self-sustaining cycle of inflammation that fuels the progression of the disease from steatosis to steatohepatitis and fibrosis. This new paradigm positions trained immunity as a novel therapeutic target, which is a promising avenue for future research.