The liver’s immune tolerance is primarily attributed to its unique cellular composition. Kupffer cells, the liver’s resident macrophages, play a dual role in maintaining immune tolerance by both inducing Treg differentiation through PD-L1 expression (MFI = 285 ± 35) and IL-10 secretion (1200 ± 150 pg/mL), and clearing gut-derived antigens (11, 12). Notably, intestinal microbiota critically modulates this Treg-inducing capacity of Kupffer cells. Clinical studies show that gut microbiota perturbations (e.g., reduced diversity with Shannon index dropping from 3.8 to 2.9 in HCC patients) correlate with impaired Treg function and disrupted hepatic tolerance (13). Liver sinusoidal endothelial cells (LSECs) possess a unique fenestrated structure that enables them to efficiently capture antigens from the bloodstream, exhibiting an antigen-presenting capability that is 8–10 times greater than that of conventional endothelial cells (14). Through the cross-presentation of antigens, LSECs induce tolerance in CD8+ T cells (15). Furthermore, the gut microbiota plays a pivotal role in the maintenance of liver immune tolerance. It achieves this by regulating the gut-liver axis, modulating Treg function, preserving the tolerant state of Kupffer cells, and preventing the excessive activation of natural killer T (NKT) cells (16). Disruption of the intestinal barrier may permit the translocation of gut microbiota and their metabolites into the liver, thereby directly initiating liver immune tolerance.

Clinical evidence reveals three key microbiota-derived metabolites regulating liver tolerance. 1.Secondary bile acids: Elevated DCA (2.8-fold increase in HCC) activates TGR5 on Kupffer cells, increasing IL-10 production by 40% but simultaneously inducing PD-L1 expression (17). 2.Tryptophan metabolites: Kynurenine/AhR signaling upregulates hepatic PD-L1 while suppressing CD8+ T cell IFN-γ production by 65 ± 8% (18). 3.SCFAs: Butyrate (optimal 2-4mM) maintains Treg function via GPR43, but concentrations <1mM (as in HCC) reduce Foxp3+ cells by 35 ± 5% (19).

The gut-liver axis functions as a bidirectional regulatory system that connects the intestine and liver, playing a crucial role in maintaining the body’s immune homeostasis through various mechanisms. The primary anatomical foundation of this axis is formed by the portal vein and bile acid circulation, while metabolite diffusion serves as a key communication pathway. Bacterial translocation and immune surveillance contribute to establishing a defense barrier. The integrity of the intestinal barrier is essential for preserving this balance. Clinical studies demonstrate intestinal barrier dysfunction as a pivotal event in HCC progression. The diminution of intestinal tight junction integrity results in the formation of leaky gut, facilitating the translocation of various microbial metabolites from the intestine into the bloodstream, which subsequently elicits an immune response. For instance, a 60% reduction in occludin, a tight junction protein, results in the detection of 10^3 copies/mL of bacterial 16S rRNA in portal vein blood (20). Elevated portal SCFA levels (butyrate +220%) paradoxically indicate barrier breach (21), while reduction in occludin correlates with increase in hepatic IL-6.

Kupffer cells efficiently eliminate these foreign substances in a CD14-dependent manner; however, persistent activation of the Toll-like receptor 4 (TLR4)/MyD88 pathway may lead to pathological fibrosis (22). In patients with chronic liver diseases, such as liver cancer, alterations in the gut microbiota are closely associated with the severity of liver disease, with reduced microbial diversity being linked to increased liver inflammation (23).

The primary metabolic products of the gut microbiota encompass SCFAs, secondary bile acids (including deoxycholic acid [DCA] and lithocholic acid [LCA]), trimethylamine N-oxide, tryptophan metabolites, and LPS, among others. These metabolites not only contribute to energy metabolism but also serve as pivotal regulators of the immune response within the gut-liver axis. They modulate immune responses through various pathways, exhibiting both pro-carcinogenic and anti-carcinogenic properties (28, 29). Collectively, these metabolites interact in multifaceted ways to maintain immune homeostasis, thereby influencing the onset and progression of diseases such as liver cancer. The mechanisms through which gut microbiota metabolites exert their effects on the gut-liver axis are intricate and varied, suggesting potential novel targets for future liver disease treatments. Further investigation is warranted to elucidate the specific mechanisms by which these metabolites impact liver health and disease, with the objective of developing innovative therapeutic strategies centered on gut microbiota metabolites.

Pathogen mimicry refers to the phenomenon whereby pathogens emulate host biomolecules through their molecular structures, thereby evading the host’s immune response. This mechanism is pivotal in the pathogenesis of various infectious diseases and is particularly significant in the progression of malignancies such as liver cancer, with substantial clinical implications (30). PAMPs are integral to immune activation, as they are recognized by pattern recognition receptors (PRRs), which subsequently initiate intracellular signaling pathways. These pathways lead to the production of pro-inflammatory molecules, thereby instigating the host’s initial response to infection and facilitating the subsequent activation of adaptive immunity (31). Nevertheless, pathogens can modulate the host’s immune response by mimicking host molecules, enabling their survival and replication within the host. A comprehensive understanding of the interplay between PAMPs, pathogen mimicry, and immune activation not only enhances our insight into the mechanisms underlying infectious diseases and tumorigenesis but also identifies potential targets for the development of novel vaccines and therapeutic interventions.

Recent studies on liver cancer have increasingly elucidated a direct communication process between microbes and immune cells, which is essential for the maintenance of immune homeostasis. Gut microbes can modulate immune cell function through direct interactions or by releasing metabolic products (32). Furthermore, these microbes influence immune responses by regulating the differentiation and functionality of immune cells. The interactions between microbes and immune cells also encompass the regulation of various cytokines and signaling pathways (33). Cytokines, as critical mediators of intercellular communication, play a pivotal role in the interaction between microbes and immune cells; they are instrumental in regulating the proliferation, differentiation, and function of immune cells, thereby shaping the intensity and nature of immune responses.

The integrity of the gut barrier is essential for maintaining intestinal homeostasis and immune equilibrium. Its disruption is intricately linked to immune activation in liver cancer (34). A compromised gut barrier allows for the translocation of gut microbes and their metabolites into the bloodstream, facilitating the movement of PAMPs to the liver. This process modulates hepatic immune responses through mechanisms such as immune cell activation and systemic inflammation. The gut-liver immune axis maintains tolerance through balanced microbiota composition and intact intestinal barrier. Increased intestinal permeability (evidenced by 60% occludin reduction) allows microbial translocation (10^3 copies/mL bacterial 16S rRNA in portal blood), which dysregulates hepatic immune tolerance via two mechanisms: 1.Direct modulation of Kupffer cell function: LPS from translocated bacteria activates TLR4/NF-κB pathway in Kupffer cells, shifting their phenotype from IL-10-producing tolerogenic to pro-inflammatory. 2.Treg modulation: Microbial metabolites like butyrate (at physiological 2-4mM) maintain Treg suppressive function via HDAC inhibition, while dysbiosis-induced reduction (to <1mM in HCC) impairs Foxp3+ Treg stability. A reduction in the expression of the tight junction protein occludin in the gut permits bacterial products, such as LPS, to access the liver via the portal vein. The LPS-TLR4 axis is crucial in the pathogenesis of leaky gut syndrome. The translocation of LPS, resulting from increased intestinal permeability, activates TLR4 on hepatic Kupffer cells, thereby initiating the NF-κB signaling pathway and promoting the release of inflammatory mediators (35). LPS, in conjunction with signaling molecules such as adenosine triphosphate (ATP), can activate the NLRP3 inflammasome, thereby facilitating the maturation of interleukin-1β (IL-1β) and exacerbating hepatic inflammation (36). Antigens originating from the gut are transported to the liver by dendritic cells, which subsequently activate CD4+ T cells, including T helper 17 (Th17) cells, and CD8+ T cells, thereby enhancing specific immune responses (37). Pro-inflammatory factors derived from the gut, such as transforming growth factor-beta (TGF-β), can activate hepatic stellate cells, resulting in collagen deposition and fibrosis.

A randomized double-blind controlled trial (RCT) focusing on irritable bowel syndrome (IBS) demonstrated that a composite probiotic formulation containing Lactobacillus and Bifidobacterium significantly decreased intestinal permeability in IBS patients (38). Additional studies have suggested that supplementation with probiotics can reduce serum LPS levels in patients with NAFLD, thereby mitigating intestinal leakage (39). Nonetheless, the impact of these probiotics on gut barrier function in patients with liver cancer remains unclear, necessitating further investigation.

The initial activation of Kupffer cells in the liver is mediated through TLRs recognizing LPS. Kupffer cells, the primary resident macrophages in the liver, are adept at recognizing and responding to gut-derived bacterial components via various PRRs, with a particular emphasis on TLRs. Empirical studies have demonstrated a significant correlation between the activation of Kupffer cells and the pathogenesis of liver cancer, particularly under conditions of chronic inflammation. Upon recognition of LPS via TLRs, Kupffer cells trigger a cascade of signaling pathways, notably the NF-κB and MAPK pathways, which culminate in the secretion of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and IL-6, thereby intensifying hepatic inflammation. Furthermore, LPS stimulation results in an elevated expression of hypoxia-inducible factor 1-alpha (HIF-1α) in Kupffer cells, indicating its potential role in facilitating adaptation to hypoxic conditions and modulating metabolic processes (40). In the pathophysiological development of liver cancer, Kupffer cells play a dual role by not only participating in the inflammatory response but also modulating hepatocyte proliferation and fibrosis through the secretion of various cytokines and chemokines, thereby contributing to the progression of liver cancer to a certain degree (41). The initial immune activation mechanism of Kupffer cells, which involves the recognition of LPS through TLRs, constitutes a fundamental aspect of the liver’s immune response. This mechanism also provides a critical framework for understanding liver inflammation and regeneration. Future research should focus on elucidating strategies to enhance the prognosis of liver-related diseases by regulating the functionality of Kupffer cells, thereby offering novel insights for precision treatment approaches.

The involvement of SCFAs in the pathogenesis and progression of HCC constitutes a complex and multifaceted area of research. This field encompasses various mechanisms, including immune regulation, metabolic intervention, and the protection of the intestinal barrier. In the context of hepatic innate immune cells, SCFAs, particularly butyrate, exert regulatory effects on the polarization and functionality of liver macrophages through multiple mechanisms. Hu et al. demonstrated that gut-derived SCFAs can attenuate hepatic inflammatory responses by promoting the polarization of macrophages towards the M2 phenotype, thereby mitigating liver damage. Within the milieu of liver cancer, an optimal concentration of SCFAs is crucial for maintaining macrophage activity against tumors; however, a reduction in SCFA levels is associated with an increase in pro-tumorigenic M2 macrophages (42). Furthermore, SCFAs modulate inflammatory gene expression at the epigenetic level by inhibiting histone deacetylase (HDAC) activity, which influences their function within the liver cancer microenvironment (43). Recent investigations have revealed that butyrate modulates the activation of the NLRP3 inflammasome in liver macrophages via G protein-coupled receptors GPR43 and GPR109A. This modulation plays a dual role in the progression of liver cancer: initially suppressing excessive inflammatory responses and subsequently potentially facilitating immune suppression (44). Furthermore, research conducted by Ma et al. indicates that dysbiosis results in diminished SCFA levels, which are associated with a decrease in liver NKT cell populations and compromised immune surveillance. SCFAs have the capacity to modulate NKT cells through the CXCL16-CXCR6 signaling pathway, thereby facilitating the progression of liver cancer (4). Additionally, Hu et al. identified that gut-derived SCFAs augment the anti-liver cancer efficacy of type 3 innate lymphoid cells (ILC3s) by enhancing IFN-γ production. SCFAs not only exert a direct influence on the functionality of ILC3s but also regulate their interaction with NKT cells, collectively contributing to immune surveillance in HCC. Moreover, SCFAs impact the maturation and functionality of liver DCs. Zheng et al. demonstrated that butyrate induces the transformation of liver DCs into a tolerogenic phenotype via the GPR109A signaling pathway, thereby reducing their secretion of pro-inflammatory cytokines and altering their antigen presentation capabilities (45). Within the liver cancer microenvironment, fluctuations in SCFA levels can modulate the capacity of DCs to activate T cells, consequently affecting the efficacy of anti-tumor immune responses.

Regarding hepatic adaptive immune cells, SCFAs particularly butyrate, can enhance the anti-tumor activity of CD8+ T cells through various mechanisms, including metabolic reprogramming, epigenetic regulation, and immune checkpoint modulation. Specifically, butyrate facilitates the transition of CD8+ T cells from glycolysis to oxidative phosphorylation, thereby improving mitochondrial function and promoting T cell persistence and memory formation (46). Additionally, SCFAs can upregulate the expression of effector and memory-associated genes, such as IFN-γ, Granzyme B, and T-bet, by inhibiting HDACs, which in turn enhances the cytotoxic function of CD8+ T cells (47). Furthermore, butyrate has been shown to inhibit HDAC3 activity by 78 ± 5% at a concentration of 10 μM, while propionate reduces IL-1β secretion from Kupffer cells from 1250 ± 150 to 380 ± 50 pg/mL via the GPR43 receptor (48). SCFAs have been shown to downregulate the expression of PD-1 in CD8+ T cells within the liver cancer microenvironment, thereby mitigating T cell exhaustion and enhancing the efficacy of anti-PD-1/PD-L1 immunotherapy. Furthermore, SCFAs play a significant role in the differentiation and functional modulation of CD4+ T cell subsets. Notably, SCFAs, particularly butyrate, have been found to enhance the expression of Foxp3 and facilitate the differentiation and stability of Tregs by promoting histone acetylation in the promoter region of the Foxp3 gene (49). In the context of the liver cancer microenvironment, an optimal level of Tregs is crucial for regulating excessive inflammatory responses; however, an overabundance of Tregs may lead to immune suppression and contribute to tumor progression. The effects of SCFAs on Th17 cells are more complex; while SCFAs can inhibit Th17 cell differentiation and thereby reduce inflammatory responses, under certain conditions, they may also enhance Th17 cell functionality, thereby participating in anti-tumor immunity (50).

In the context of immune-suppressive cells, SCFAs have been shown to attenuate the recruitment of MDSCs by downregulating the expression of chemokines such as CCL2 within the liver cancer microenvironment (51). Additionally, SCFAs diminish the capacity of MDSCs to produce immunosuppressive molecules, including arginase 1 (Arg1) and inducible nitric oxide synthase (iNOS), via the GPR43 signaling pathway, thereby mitigating their suppression of T cells and NK cells. SCFAs also facilitate the differentiation of MDSCs into M1 macrophages, thereby converting them into tumor-fighting cells. Research conducted by Bi et al. has demonstrated that SCFAs modulate the function of neutrophils in the liver cancer microenvironment by influencing their recruitment and polarization (52). Specifically, SCFAs decrease the proportion of N2 (pro-tumor) neutrophils while increasing N1 (anti-tumor) neutrophils, which is advantageous for the immune-mediated control of liver cancer. Given the dual regulatory role of SCFAs, future investigations should aim to optimize their dosage and administration strategies to enhance their anticancer efficacy while minimizing potential adverse effects.

Secondary bile acids can promote the establishment of an immunosuppressive microenvironment in the liver through the activation of the TGR5 and FXR receptor pathways, consequently influencing tumor immune surveillance and evasion. Secondary bile acids, particularly elevated levels of DCA, have been shown to induce the polarization of Kupffer cells towards the M2 phenotype, which is immunosuppressive, via the TGR5 and FXR receptor signaling pathways. This polarization results in the secretion of inhibitory cytokines, including IL-10 and TGF-β, thereby suppressing anti-tumor immune responses (53). Furthermore, DCA modulates the inflammatory response of Kupffer cells through the NF-κB and STAT3 signaling pathways, promoting inflammation in the early stages of liver cancer and potentially facilitating immune suppression in later stages. The regulation of NK and NKT cells by secondary bile acids is crucial within the liver cancer immune microenvironment. Research conducted by Ma et al. demonstrated that dysregulated bile acid metabolism results in increased DCA levels, which inhibit the expression of CXCL16 in the liver, thereby reducing the recruitment of CXCR6+ NKT cells and diminishing the liver’s immune surveillance against tumors (43). Additionally, DCA impairs the cytotoxic function of NK cells against liver cancer cells by altering the expression of activation receptors and the release of cytotoxic molecules, such as perforin and granzyme B (54). Elevated concentrations of secondary bile acids have been shown to inhibit the secretion of IFN-γ by CD8+ T cells, while simultaneously promoting the proliferation of MDSCs, inducing apoptosis and functional exhaustion in CD8+ T cells, and upregulating inhibitory receptors such as PD-1 and TIM-3. These effects collectively diminish the anti-tumor efficacy of CD8+ T cells. Furthermore, secondary bile acids facilitate the differentiation of Tregs and suppress the function of Th17 cells via the FXR and TGR5 signaling pathways, thereby establishing an immunosuppressive milieu within the liver cancer microenvironment. Elevated levels of DCA have been implicated in the increased accumulation of MDSCs in the liver through the COX2-PGE2 pathway, thereby augmenting their immunosuppressive capabilities (55). Some studies propose that the strategic use of secondary bile acids in conjunction with liver cancer immunotherapy could represent a promising therapeutic avenue. Specifically, modulating the metabolism of secondary bile acids may restore the anti-tumor function of CD8+ T cells, thereby enhancing the efficacy of immunotherapeutic interventions.

Various metabolites, including TMAO, have been implicated in exacerbating liver inflammatory responses. TMAO achieves this by activating the NLRP3 inflammasome, which in turn stimulates Kupffer cells to secrete pro-inflammatory cytokines such as IL-1β and IL-18. Additionally, TMAO facilitates macrophage polarization towards the M1 phenotype through the NF-κB signaling pathway, potentially exerting anti-tumor effects during the early stages of liver cancer. However, prolonged inflammation may contribute to the progression of liver cancer (56). TMAO also influences T cell differentiation and function by promoting Th17 cell differentiation while inhibiting Treg cell function, thereby intensifying liver inflammatory responses. Furthermore, TMAO can promote the expansion and activation of MDSCs, enhancing their immunosuppressive capabilities and facilitating liver cancer progression. In parallel, tryptophan metabolites significantly impact T cell differentiation via the aryl hydrocarbon receptor (AhR) signaling pathway (57). Specifically, kynurenine has been shown to promote tumor cell expression of PD-L1 through the AhR-NF-κB pathway.

In the pathogenesis of liver cancer, the gut microbiota is pivotal in facilitating the interaction between metabolic processes and immune responses. While there is presently insufficient research to establish a direct causal link between metabolic disorders induced by abnormal microbiota and the development of HCC, it is well-established that an imbalance in gut microbiota can result in metabolic abnormalities. These abnormalities are associated with the onset and progression of NAFLD, which is a significant risk factor for HCC. Furthermore, small intestinal bacterial overgrowth and dysbiosis of gut microbiota are recognized as critical factors in the progression of NAFLD to non-alcoholic steatohepatitis (NASH) and decompensated liver disease (58). Dysbiosis can influence tryptophan metabolism, which may promote liver cancer through the upregulation of sterol regulatory element-binding protein 2 (SREBP2) (59). Tryptophan metabolites are capable of modulating the expression of PD-1 via the AhR pathway, consequently impacting T cell functionality. As previously discussed, metabolites derived from the gut microbiota, including short-chain fatty acids, bile acids, and indole, exert significant effects on hepatic immune and metabolic functions through the gut-liver axis. For example, succinate can enhance the infiltration of cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) positive T cells by interacting with its receptor SUCNR1, thus modifying the immune milieu of the tumor microenvironment (60). The potential implications of this research for immunotherapy in liver cancer are significant. Alterations in the liver microbiota composition may reduce the liver’s metabolic capacity for drugs, thereby influencing the effectiveness of liver cancer treatments. Empirical evidence suggests that modulating the gut microbiota can enhance the efficacy of immunotherapy for liver cancer. Interventions such as probiotics, prebiotics, antibiotics, and fecal microbiota transplantation may emerge as crucial adjuncts to liver cancer immunotherapy. These strategies enhance immunotherapy by modulating the host immune system, minimizing adverse effects, and improving survival rates among liver cancer patients.

Recent investigations have elucidated the role of gut microbiota in modulating immune evasion mechanisms in liver cancer through multiple pathways. Primarily, gut microbiota can facilitate immune evasion by modulating the host immune system. Dysbiosis within the gut microbiota can precipitate immune system disorders, thereby impairing the efficacy of anti-tumor immune responses. For instance, metabolites produced by specific gut bacteria have been shown to inhibit immune cell activity, consequently diminishing their capacity to identify and eliminate tumor cells. Furthermore, the gut microbiota can influence immune evasion by modulating immune cells within the liver cancer microenvironment, such as by altering the activity of T cells and natural killer cells, which in turn promotes tumor growth and metastasis. Additionally, the gut microbiota may contribute to immune evasion in liver cancer by regulating the expression of immune checkpoint molecules. Empirical evidence suggests that gut microbiota can modulate the expression of PD-L1 via its metabolites, thereby suppressing anti-tumor immune responses and facilitating tumor immune evasion (61). The mechanisms of immune evasion facilitated by microbiota present significant challenges in the treatment of liver cancer.