Macrophage polarization refers to the distinct activation state of macrophages at a specific time and location (18–20). However, it should be noted that macrophage polarization is not a static or fixed state, as macrophages exhibit high plasticity and the ability to integrate diverse signals from damaged tissue, microorganisms, and normal tissue environments. This integration of signals leads to the development of dynamic and unstable polarization states. The regulation of macrophage polarization involves multiple pathways, including epigenetic and cell survival mechanisms that govern macrophage maturation and longevity. Furthermore, the tissue microenvironment and external factors such as microbial products and inflammation-related cytokines play crucial roles in macrophage polarization ( Figure 1 ). These pathways collectively determine the specific polarization state assumed by macrophages (21).

Macrophages possess the ability to differentiate into various phenotypes, commonly categorized as M1 and M2 types, which often exhibit contrasting characteristics ( Figure 1 ). The characteristics and regulation of macrophages are complex and interconnected, relying on the dynamic nature of their microenvironment (22). The incomplete characterization of macrophages and their functional polarization in many studies present significant challenges in their interpretation (23, 24). Pharmacological interventions targeting the polarization of macrophages towards an M2 phenotype have shown partial reversal of steatosis and hepatocyte apoptosis (25, 26). However, the efficacy of such interventions may vary depending on the specific microenvironment and the complexity of the underlying pathophysiology. Therefore, further investigation is warranted to gain a deeper understanding of the shared mechanisms governing macrophage polarization and to identify more effective therapeutic approaches for NAFLD/NASH (27). Laboratory studies have demonstrated that M2-type macrophages can induce apoptosis in M1-type macrophages through the activation of the enzyme arginase, mediated by the release of interleukin-10 (IL-10) (28) ( Figure 2 ). Animal studies suggest that macrophages exhibiting a pro-inflammatory phenotype contribute to the severity of NAFLD (29, 30). The activation of these macrophages can lead to the production of pro-inflammatory cytokines and the promotion of oxidative stress, ultimately driving the progression of liver fibrosis and other associated complications (31, 32).

The presence of macrophages exhibiting a reparative and anti-inflammatory phenotype in NAFLD has been associated with reduced hepatic injury. These macrophages have the ability to produce cytokines that reduce inflammation and promote tissue healing, thereby aiding in the mitigation of inflammation and improvement of liver function (33–38). An important study has indicated that patients with NASH exhibit higher expression of markers associated with M2 macrophages, suggesting the potential role of these macrophages in the regeneration and repair of liver tissue following hepatocyte damage (39). However, concerns have been raised regarding the potential risk of developing fibrosis as a result of this process (39). Selective targeting of macrophages has shown promise as a therapeutic approach for NASH. Combining the potent corticosteroid dexamethasone with a typical surface marker of M2 macrophages (CD163) has demonstrated enhanced reduction of necroinflammation and fibrosis in a rat model of fructose-induced NASH ( Figure 2 ). These positive results suggest that targeting macrophages based on surface markers could be a potential strategy for developing novel treatments for NASH (40). However, further investigation is needed to explore the safety and efficacy of this approach in human subjects. In summary, the examples mentioned above illustrate the diverse spectrum of macrophage polarization observed in NAFLD/NASH, with many of their in vivo roles still requiring comprehensive understanding.

Macrophages play versatile roles in the human body, including involvement in embryonic development, tissue repair, and inflammation (41). They exhibit remarkable plasticity and can adapt their physical characteristics based on their microenvironment and functional requirements (21, 42). Consequently, macrophages display a spectrum of activation states, characterized by variations in their transcriptome in response to stimuli such as fatty acids, cholesterol, and their metabolites (42, 43). Macrophages encounter a wide range of stimuli in their environment, leading to diverse phenotypes and functions. Traditionally, macrophages have been classified into two main categories: ‘classically activated’ or M1 macrophages and ‘alternatively activated’ or M2 macrophages (44–48), as mentioned earlier. M1 macrophages are responsible for secreting pro-inflammatory cytokines, while M2 macrophages exhibit an anti-inflammatory phenotype (49–51) ( Figure 1 ). Macrophages infiltration and Kupffer cells (KCs) activation were found to further express pro-inflammatory cytokines in the NASH model (52). Macrophages dynamically adjust their metabolic characteristics in response to the surrounding microenvironment, enabling them to perform their functions during both homeostasis and inflammation ( Figure 1 ). This metabolic adaptation helps maintain a delicate balance between pro-inflammatory and anti-inflammatory responses. The complex process of metabolic reprogramming in macrophages is regulated by factors such as cytokines, growth factors, and nutrient availability, and is crucial for their optimal functioning in both health and disease (53, 54). The focus of this review is specifically on the macrophage response to lipids and their metabolites. Lipid metabolism and the interaction between macrophages and lipid molecules play a significant role in the regulation of macrophage function and their involvement in various diseases and pathological conditions. Understanding the intricacies of how macrophages respond to lipids and their metabolites can provide valuable insights into the development of targeted therapeutic strategies for diseases such as NAFLD/NASH.

Macrophages, as integral constituents of the innate immune system, display remarkable heterogeneity within the hepatic environment, encompassing liver-resident KCs and recruited monocyte-derived macrophages (70, 71). Both liver-resident KCs and recently recruited monocyte-derived macrophages play pivotal roles in modulating inflammation, fibrogenesis, and fibrolysis in the context of NAFLD and NASH (72). A study has provided evidence supporting a positive correlation between the abundance of KCs in biopsy samples and the severity of NAFLD in patients (73). KCs possess the capacity to attract immune cells that undergo differentiation toward the M1 phenotype, thus eliciting the production of pro-inflammatory cytokines subsequent to liver injury (22, 74–76).

In addition to liver-resident KCs, macrophages derived from monocytes also play a significant role in the progression of NAFLD and NASH. Unlike KCs, recruited monocytes exhibit distinct morphological characteristics, providing evidence for the existence of one of the two major subpopulations of hepatic macrophages in NAFLD (77). The higher presence of CC-chemokine receptor 2⁺ (CCR2⁺) macrophages in patients with more severe NAFLD suggests that monocyte-derived macrophages, rather than KCs, contribute significantly to the pathogenesis of NAFLD (12). This notion is supported by the observation of increased infiltration of monocytes that rapidly differentiate into pro-inflammatory macrophages in an animal model, further highlighting the importance of monocyte-derived macrophages in NAFLD (78). In patients with NASH accompanied by fibrosis and cirrhosis, there was a notable increase in the number of pro-inflammatory macrophages expressing CCR2 in the portal areas, providing further evidence for the involvement of monocyte-derived macrophages in fibrosis development (79).

KCs play an important role in NAFLD/NASH progression and a promising target for intervention. Indeed, certain medications have the potential to indirectly influence NAFLD by targeting KCs. The farnesoid X receptor (FXR), also known as the bile acid receptor, has shown promising therapeutic benefits in the treatment of NAFLD ( Figure 2 ). Clinical trial data supports the effectiveness of obeticholic acid, an FXR agonist, in inhibiting hepatic glucose and lipid metabolism, as well as exhibiting anti-inflammatory and anti-fibrotic properties in NAFLD (95). FXR agonists have been found to decrease the production of pro-inflammatory cytokines in KCs by attenuating liver inflammation induced by LPS (96). Moreover, in both laboratory settings and living organisms, FXR agonists have been shown to transform macrophages into an anti-inflammatory phenotype (97, 98)( Figure 2 ). These findings suggest that the beneficial effects of FXR activators in NASH may be partially due to their impact on KCs. Currently, a comprehensive phase III clinical trial is underway to evaluate the efficacy of obeticholic acid in patients with NASH and fibrosis (4).

Indeed, the use of glucagon-like peptide-1 receptor agonists (GLP1RAs) has shown promise as a potential therapeutic strategy for NASH. GLP1RAs are agonists of the glucagon-like peptide-1 receptor (99–101)( Figure 2 ). In a clinical study, liraglutide demonstrated partial histological improvement in NASH (102). Furthermore, dipeptidyl peptidase 4 inhibitors, which indirectly activate the glucagon-like peptide-1 (GLP1) receptor, have been found to decrease the number of pro-inflammatory monocytes in the liver and shift macrophage polarization towards the M2 anti-inflammatory phenotype in mice fed a methionine-choline-deficient (MCD) diet (103). These findings suggest that GLP1RAs hold significant promise for the treatment of NASH by influencing inflammatory pathways and improving liver histology ( Figure 2 ). Furthermore, the stimulation of peroxisome proliferator-activated receptor gamma (PPARγ) through agonists like pioglitazone promotes the conversion of macrophages into an anti-inflammatory state. This conversion has been shown to alleviate hepatic steatosis by enhancing the uptake and breakdown of fatty acids (104–106). Similarly, peroxisome proliferator-activated receptor delta (PPARδ) plays a crucial role in controlling the polarization of KCs towards the anti-inflammatory M2 phenotype (104) ( Figure 2 ). Elafibranor, a dual agonist for PPARα and PPARδ, has shown improved effectiveness in treating NASH compared to a placebo without negatively impacting fibrosis progression (107) ( Figure 2 ). These findings collectively suggest that modulating the characteristics of macrophages may represent a viable target for the treatment of NASH. The use of GLP1RAs, PPARγ agonists like pioglitazone, and dual PPARα/PPARδ agonists like elafibranor hold promise in influencing macrophage polarization, inflammatory pathways, and improving liver histology in NASH.

The recruitment of monocytes plays a pivotal role in the advancement of NAFLD and offers a potential avenue for intervention. In an animal model of NASH, the administration of cenicriviroc, a dual antagonist targeting CCR2 and CCR5, exhibited significant improvements in fibrosis and inflammation (108). A subsequent study provided additional evidence of the beneficial effects of cenicriviroc on macrophage numbers and fibrosis in mouse models of NASH (109). In some clinical trials, it was observed that a significantly higher proportion of patients treated with cenicriviroc experienced improvements in fibrosis compared to those in the placebo group. Nevertheless, there was no discernible discrepancy observed among the groups with regards to the primary outcome of attaining a NAFLD activity without exacerbating fibrosis (110). This finding could potentially be ascribed to the presence of both CCR2 and CCR5 on HSCs, as the inhibition of chemokines may have hindered both detrimental and advantageous activation and recruitment of macrophages (111).

The proposition that macrophage involvement in the pathogenesis of NAFLD and NASH suggests that markers of macrophage activation could potentially serve as biomarkers for disease severity and treatment response. In a cohort study, the levels of soluble CD36, an indicative marker for macrophage lipid accumulation, were measured (112). The findings revealed elevated levels of soluble CD36 in individuals with impaired glucose regulation, metabolic syndrome, and an increased likelihood of fatty liver, as determined by noninvasive steatosis estimates (113, 114). Moreover, several studies have reported sCD163, a macrophage activation marker specific to certain lineages, as a promising biomarker for predicting liver disease severity (115–119). In two distinct cohorts comprising 195 participants each, sCD163 demonstrated a strong ability to predict advanced fibrosis in adults diagnosed with NAFLD, as evidenced by receiver operating characteristic (AUROC) values of 0.77 and 0.80. Hence, it is reasonable to propose that sCD163 may serve as a distinctive biomarker for macrophages, enabling the anticipation of NASH disease activity, fibrosis, and treatment response. Furthermore, circulating microparticles, originating from activated or apoptotic cells and retaining the surface characteristics of their parent cells, show significant potential as prognostic markers for histological NASH (120). Recent studies have witnessed a notable increase in the investigation of macrophage markers associated with NAFLD/NASH. However, thus far, none of these markers have been examined as predictive biomarkers with clinical outcomes in NAFLD. This observation emphasizes the need for extensive future research in this field.

Currently, the majority of therapeutic strategies targeting macrophages rely on receptor-mediated phagocytosis to achieve specificity (121–123). In this approach, compounds are custom-designed to encapsulate therapeutics and possess surface modifications that can be recognized by macrophage receptors (124). Although these receptors are not exclusive to macrophages, they enable the selective identification of cells exhibiting distinct phenotypes and activation states. This receptor-based approach offers a precise and targeted means of delivering therapeutic agents into macrophages, thereby minimizing off-target effects (125). Once inside macrophages, a variety of therapeutic interventions, including depletion, proliferation control, inflammation modulation, and gene silencing, are commonly employed ( Figure 2 ). An alternative strategy involves altering the signaling pathways responsible for macrophage-mediated inflammation. Introduction of anti-inflammatory agents into the macrophage cytoplasm allows for the modulation of inflammatory cytokine production and release (126, 127). By employing these therapeutic approaches and utilizing methods that specifically target different subsets of macrophages, a multitude of methodologies can effectively regulate macrophage numbers and improve the state of NASH.

Four primary delivery methods have been identified for targeted administration of therapeutic agents to macrophages: nanoparticles, liposomes, glucan shell microparticles, and oligopeptide complexes ( Figure 2 ). The rational approach of utilizing nanoparticles for macrophage-specific drug delivery has been conceptualized and implemented. Although nanoparticle technologies have been developed for targeting macrophage receptors in various diseases, their potential in treating NAFLD/NASH remains unexplored. Similar to nanoparticles, liposome shells can be modified to incorporate ligands or antibodies that selectively target specific macrophage phenotypes based on receptor specificity. Exploiting the inherent phagocytic properties of macrophages (128, 129), liposomes can gain entry into these cells. For instance, clodronate-loaded liposomes have been utilized to induce apoptosis in macrophages upon internalization, leading to their depletion. Insights from fields beyond metabolism in the realm of liposome delivery may offer valuable perspectives for addressing NAFLD/NASH. A recent investigation has explored the use of yeast-derived beta-glucans (Y-BGs) as a distinct encapsulation mechanism with the ability to target macrophages independently of their activation status (130). Administration of Y-BGs orally has been shown to promote the production of IL-10, an anti-inflammatory cytokine (131). Experimental studies have demonstrated that Y-BGs enhance anti-inflammatory activity in macrophages through an IL-10-mediated mechanism (132). Delivering genes to specific tissues without relying on viruses can be challenging, but the application of oligopeptides in conjunction with gene-modulating compounds offers a promising solution for such cases (133–135). This fortuitous finding holds potential for precise transportation of non-viral gene-modifying technology to adipose deposits and adipose tissue macrophages, facilitating targeted outcomes (136).