Macrophages and Myeloid-Derived Suppressor Cells in the Tumor Microenvironment: Unraveling Molecular Pathways, Immunometabolic Processes, and Their Significance in Immunotherapy for Hepatocellular Carcinoma (HCC)

Abstract

Hepatocellular carcinoma (HCC) is a major global health concern, and understanding the complex interplay of immune cells within the tumor microenvironment is crucial. This review explores the roles of myeloid-derived suppressor cells (MDSCs) and macrophages in HCC, focusing on their molecular pathways, immunometabolic processes, and implications for immunotherapy. We begin by elucidating the origin, expansion, and immunosuppressive mechanisms of MDSCs, emphasizing the importance of molecular pathways and immunometabolism in regulating their functions. In parallel, we delve into the dual nature of tumor-associated macrophages (TAMs) and discuss the molecular and metabolic cues governing their plasticity. Tumor metabolism is a central theme, with a comprehensive overview of altered metabolic processes in cancer cells and their impact on immune cells in the tumor microenvironment. We examine the metabolic crosstalk between tumor cells, MDSCs, and macrophages, shedding light on how tumor metabolism contributes to immune evasion. Furthermore, we discuss the challenges and limitations faced in the clinical application of immunotherapy in HCC. In conclusion, this review highlights the intricate web of molecular pathways and immunometabolic processes shaping the functions of MDSCs and macrophages in HCC. Understanding these dynamics is essential for the innovative immunotherapeutic interventions in HCC, improving outcomes of this devastating disease.

1. Introduction

Hepatocellular carcinoma (HCC) stands as a formidable global health challenge, representing the most prevalent type of liver cancer and posing a substantial burden on public health systems worldwide [1, 2]. Its formidable nature arises from its insidious progression, often remaining asymptomatic until reaching an advanced stage, limiting therapeutic options and yielding a high mortality rate [3, 4, 5]. Therefore, it is imperative to unravel the intricacies of HCC pathogenesis and discover innovative therapeutic strategies.

The tumor microenvironment (TME) constitutes an indispensable player in the context of HCC progression [6, 7]. Beyond the tumor cells themselves, the TME is an intricate ecosystem comprising a multitude of cell types, extracellular matrix components, and signaling molecules. This dynamic milieu serves as a critical regulator of disease evolution, influencing tumor growth, metastasis, and the response to therapy [8].

Within the TME, two key players come into focus: myeloid-derived suppressor cells (MDSCs) and macrophages. These immune cell populations, although integral components of the immune system, paradoxically wield immunosuppressive capabilities that can be harnessed by HCC to its advantage. Their recruitment, activation, and subsequent functions within the TME significantly contribute to the evasion of immune surveillance and immune-mediated tumor destruction.

Myeloid-derived suppressor cells, a heterogeneous population of immature myeloid cells, expand in response to chronic inflammation and are potent inhibitors of T-cell responses. Through various mechanisms, including the production of immunosuppressive cytokines and the induction of regulatory T cells, MDSCs suppress the antitumor immune response, thereby facilitating tumor progression. Understanding the molecular pathways governing MDSC activation and function is pivotal for the development of targeted immunotherapies [9, 10, 11].

Similarly, macrophages, versatile immune cells, adopt diverse phenotypes in response to cues from the TME. They can polarize into M1 macrophages with pro-inflammatory and antitumor properties or M2 macrophages, which promote an anti-inflammatory environment and support tumor growth [12, 13, 14]. The balance between these macrophage subsets can significantly impact HCC progression [9, 15]. Molecular signaling pathways and immunometabolic processes play a crucial role in determining macrophage polarization within the TME.

In this review, we embark on a comprehensive exploration of the roles of MDSCs and macrophages in the context of HCC, shedding light on their contributions to immunosuppression and tumor progression. We will delve into the intricate molecular pathways and immunometabolic processes that govern their activities within the TME. Additionally, we will discuss the implications of these findings for the development of innovative immunotherapeutic strategies in HCC, offering a glimpse of hope for improved outcomes for patients grappling with this formidable disease.

2. Myeloid-derived suppressor cells (MDSCs)

MDSCs represent a heterogeneous population of myeloid cells with potent immunosuppressive properties. They are characterized by their ability to suppress the immune response, particularly the activation and effector functions of T cells and natural killer (NK) cells [16, 17]. MDSCs play a significant role in promoting tumor immune evasion and have been extensively studied in the context of cancer immunology [18, 19, 20].

2.1 MDSC subsets

MDSCs can be broadly categorized into two main subsets: granulocytic MDSCs (G-MDSCs) and monocytic MDSCs (M-MDSCs) [21]. G-MDSCs are morphologically similar to neutrophils and are characterized by the expression of cell surface markers such as CD11b + and Ly6G+. M-MDSCs share phenotypic similarities with monocytes and are typically identified as CD11b + and Ly6C+ cells. These subsets may exhibit distinct mechanisms of immunosuppression and differentially impact tumor progression.

2.2 Origin and expansion of MDSCs in cancer

MDSCs originate from myeloid precursor cells in the bone marrow. Under pathological conditions, such as cancer, chronic inflammation, or infection, a dysregulated myelopoiesis process leads to the accumulation and expansion of MDSCs. Factors within the tumor microenvironment, including tumor-derived soluble mediators (cytokines and growth factors), can drive the recruitment and expansion of MDSCs. MDSC expansion is also influenced by the crosstalk between tumor cells and immune cells, leading to an immunosuppressive feedback loop [20, 22, 23].

2.3 Immunosuppressive mechanisms of MDSCs

MDSCs employ various mechanisms to suppress the immune response, primarily targeting T cells [19, 24, 25]: T-cell suppression: MDSCs inhibit T-cell activation and proliferation through direct cell-cell contact or the production of immunosuppressive molecules. Cytokine production: MDSCs secrete immunosuppressive cytokines such as interleukin-10 (IL-10), transforming growth facto beta (TGF-β), and interleukin-6 (IL-6), which dampen T-cell functions and promote regulatory T-cell (Treg) expansion. Induction of T-cell anergy: MDSCs induce T-cell dysfunction and exhaustion, leading to reduced cytotoxicity and cytokine production.

2.4 Molecular pathways and immunometabolic processes regulating MDSC functions

MDSC functions are tightly regulated by molecular pathways and immunometabolic processes. Key aspects include [26, 27, 28] signal transducer and activator of transcription 3 (STAT3) and nuclear factor-kappa B (NF-κB) signaling: these transcription factors are activated in MDSCs, driving the expression of immunosuppressive molecules and cytokines. Metabolic reprogramming: MDSCs exhibit metabolic changes, such as increased glycolysis and upregulated arginase-1 activity, which contribute to their immunosuppressive functions. Reactive oxygen species (ROS) production: MDSCs generate ROS, which not only suppress T-cell responses but also modulate immune cells in the TME [16, 19, 29]. Understanding these molecular and metabolic processes is critical for the development of targeted therapies aimed at disrupting MDSC-mediated immunosuppression in cancer, including hepatocellular carcinoma (Figure 1). Strategies to inhibit MDSC recruitment, differentiation, or function hold promise for enhancing the efficacy of immunotherapies in HCC and other cancers.

Figure 1.

MDSCs establish an immunosuppressive milieu within tumors by impeding the activity of T effector cells and NK cells while enhancing the functionality of Tregs. Macrophages polarized into M1 and M2 macrophages, producing an anti-inflammatory and pro-inflammatory microenvironment within the tumor.

3. Macrophages

Macrophages are versatile immune cells that play a pivotal role in the tumor microenvironment (TME) [30, 31, 32]. They exhibit remarkable plasticity, allowing them to adopt different phenotypes and functions depending on local cues. In the context of cancer, macrophages can be broadly categorized into two polarized states: M1 (pro-inflammatory) and M2 (anti-inflammatory) phenotypes, each with distinct roles and functions [33, 34, 35].

3.1 Polarization of macrophages

3.1.1 M1 macrophages (pro-inflammatory)

M1 macrophages are classically activated and driven by pro-inflammatory signals, such as interferon-gamma (IFN-γ) and lipopolysaccharide (LPS). They are characterized by M1 macrophages release cytokines, such as interleukin-1 beta (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α). These cytokines promote inflammation and activate immune responses. M1 macrophages have an increased ability to present antigens to T cells, thereby stimulating cytotoxic T-cell responses. This is crucial for combating infections and initiating adaptive immunity. M1 macrophages play a key role in host defense by efficiently clearing pathogens and exhibiting cytotoxic activity against tumor cells [35, 36, 37, 38, 39].

3.1.2 M2 macrophages (anti-inflammatory)

M2 macrophages are alternatively activated and influenced by anti-inflammatory signals such as interleukin-4 (IL-4) and interleukin-13 (IL-13). They exhibit: M2 macrophages are characterized by the secretion of anti-inflammatory cytokines, including IL-10 and transforming growth factor-beta (TGF-β). These cytokines help to suppress inflammatory responses, thereby promoting tissue repair and resolution of inflammation. M2 macrophages play a crucial role in tissue repair and remodeling. They contribute to the resolution of inflammation by promoting the clearance of cellular debris and facilitating tissue regeneration. Additionally, M2 macrophages produce factors that stimulate the proliferation and differentiation of fibroblasts, aiding in the deposition of extracellular matrix components necessary for tissue healing. M2 macrophages are involved in angiogenesis, the process of forming new blood vessels. They release angiogenic factors, such as vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF), which promote the growth of blood vessels. Angiogenesis is essential for supplying oxygen and nutrients to tissues undergoing repair and regeneration. M2 macrophages possess immunosuppressive properties, which help to dampen immune responses and maintain tissue homeostasis. They inhibit the activity of immune cells, such as T cells and NK cells, and contribute to the establishment of an immunosuppressive microenvironment. This immunosuppression can be exploited by tumors to evade immune surveillance and promote tumor growth. M2 macrophages play a significant role in promoting tumor growth and metastasis. They create an immunosuppressive microenvironment that supports tumor cell survival and proliferation. Additionally, M2 macrophages contribute to tumor angiogenesis, extracellular matrix remodeling, and the suppression of antitumor immune responses, thereby facilitating tumor growth and metastatic spread. Understanding the functions of M2 macrophages in promoting tissue repair, angiogenesis, immunosuppression, and tumor progression is crucial for developing therapeutic strategies targeting these cells in various pathological conditions, including cancer. Modulating the activity of M2 macrophages holds promise for improving clinical outcomes and reducing the progression of diseases characterized by chronic inflammation and tissue damage.

3.2 Plasticity of macrophages and their diverse roles in cancer

Macrophages are highly plastic and can transition between M1 and M2 states in response to changing microenvironmental signals. In cancer, macrophages can exert both pro-tumorigenic and antitumorigenic effects:

3.2.1 Pro-tumorigenic roles

M2-like macrophages are often associated with tumor promotion. They can stimulate angiogenesis, suppress antitumor immunity, and support tumor invasion and metastasis. Angiogenesis promotion: M2-like macrophages can secrete pro-angiogenic factors, such as VEGF and matrix metalloproteinases (MMPs), promoting the formation of new blood vessels that support tumor growth and metastasis. M2-like macrophages produce anti-inflammatory cytokines, such as IL-10 and TGF-β, dampening antitumor immune responses and creating an immunosuppressive microenvironment that facilitates tumor evasion from immune surveillance. Tumor invasion and metastasis: M2-like macrophages contribute to tumor invasion and metastasis by promoting extracellular matrix remodeling, facilitating cancer cell migration, and enhancing the establishment of metastatic niches in distant organs [35, 40, 41].

3.2.2 Antitumorigenic roles

Conversely, M1-like macrophages have the potential to inhibit tumor growth through various mechanisms: M1-like macrophages produce pro-inflammatory cytokines, such as TNF-α and interleukin-1 beta (IL-1β), promoting inflammation within the tumor microenvironment and activating antitumor immune responses. M1-like macrophages possess enhanced phagocytic activity, allowing them to engulf and eliminate cancer cells directly, thereby inhibiting tumor growth. M1-like macrophages express increased levels of major histocompatibility complex (MHC) molecules and co-stimulatory molecules, facilitating the presentation of tumor antigens to T cells and enhancing antitumor immune responses.

3.3 Molecular pathways and immunometabolic processes shaping macrophage phenotypes

Macrophage polarization is intricately regulated by molecular pathways and immunometabolic processes:

3.3.1 Signal transduction pathways

Key signaling pathways, such as signal transducer and activator of transcription 1/signal transducer and activator of transcription 6 (STAT1/STAT6), NF-κB, and janus kinase (JAK)/STAT, play crucial roles in driving M1 or M2 polarization in response to cytokines and other signals: activation of the STAT1 pathway by interferon-gamma (IFN-γ) promotes M1 polarization, leading to the expression of pro-inflammatory genes. Conversely, activation of the STAT6 pathway by interleukin-4 (IL-4) and interleukin-13 (IL-13) induces M2 polarization, resulting in the expression of anti-inflammatory and tissue repair genes. NF-κB signaling is involved in M1 polarization in response to pro-inflammatory stimuli, such as lipopolysaccharide (LPS) and TNF-α. Activation of NF-κB leads to the expression of pro-inflammatory cytokines and antimicrobial effectors characteristic of M1 macrophages. The JAK/STAT pathway is essential for transmitting signals from cytokine receptors to the nucleus, regulating gene expression in response to extracellular stimuli. Cytokines, such as IFN-γ and IL-4, activate JAK/STAT signaling pathways, driving M1 or M2 polarization, respectively.

3.3.2 Immunometabolism

Metabolic reprogramming is crucial for macrophage polarization, with distinct metabolic profiles characterizing M1 and M2 macrophages: M1 macrophages exhibit enhanced glycolytic metabolism to meet the energy demands of their pro-inflammatory functions. Glycolysis supports the rapid production of adenosine triphosphate (ATP) and provides intermediates for the biosynthesis of pro-inflammatory cytokines and effector molecules. In contrast, M2 macrophages rely on oxidative phosphorylation and fatty acid metabolism for energy production. Oxidative phosphorylation is more efficient in generating ATP and supports the biosynthetic requirements of M2 macrophages involved in tissue repair and immunosuppression.

3.3.3 Epigenetic regulation

Epigenetic modifications, including DNA methylation and histone acetylation, play critical roles in controlling gene expression patterns that define macrophage phenotypes: DNA methylation regulates gene expression by altering chromatin structure and accessibility. Changes in DNA methylation patterns can influence macrophage polarization by modulating the expression of genes involved in inflammatory responses and metabolic pathways. Histone acetylation is associated with transcriptional activation, promoting an open chromatin conformation that facilitates gene expression. Histone acetylation patterns are dynamically regulated during macrophage polarization, influencing the expression of genes associated with M1 or M2 phenotypes. Understanding the molecular underpinnings of macrophage polarization and their immunometabolic adaptations is vital for targeting them effectively in cancer therapy. Strategies to modulate macrophage polarization, inhibit pro-tumorigenic functions, and enhance anti-tumorigenic properties are under investigation to improve cancer treatment outcomes, including those in hepatocellular carcinoma (HCC). Metabolism plays a central role in cancer biology, influencing various aspects of tumor development and progression. Altered metabolic processes in cancer cells not only support their rapid proliferation but also have profound effects on the tumor microenvironment (TME), including immune cells. Here, we delve into the key metabolic changes in cancer cells, the metabolic crosstalk within the TME, and the role of tumor metabolism in immune evasion and immunosuppression [39, 42, 43, 44].

4. Altered metabolic processes in cancer cells

Cancer cells often exhibit enhanced glycolysis, even in the presence of oxygen, a phenomenon known as the Warburg effect. This increased glucose metabolism provides a rapid source of energy and biosynthetic precursors for cell growth (Figure 2). Key features of glycolysis in cancer cells include Refs. [45, 46, 47].

Figure 2.

T cells and cancer cells disrupt glucose and amino acids. T cell loss effector function and glycolytic metabolism are elevated in the tumor microenvironment.

4.1 Glycolysis

Cancer cells upregulate glucose transporters, such as glucose transporter type 1 (GLUT1), to enhance glucose uptake from the extracellular environment. Glycolytic enzymes are upregulated in cancer cells, leading to increased flux through the glycolytic pathway. This results in the production of ATP and metabolic intermediates necessary for cellular proliferation. Cancer cells convert a significant portion of glucose-derived pyruvate to lactate, even under aerobic conditions. Lactate production contributes to the acidic microenvironment of tumors and may promote cancer cell invasion and metastasis.

4.2 Glutaminolysis

Glutaminolysis is upregulated in cancer cells to support the synthesis of amino acids and nucleotides. Glutamine, a non-essential amino acid, is catabolized to provide carbon and nitrogen for biosynthesis. Key aspects of glutaminolysis in cancer cells include: many cancer cells exhibit dependence on glutamine for survival and proliferation. Glutamine serves as a major nitrogen donor for nucleotide biosynthesis and a carbon source for the tricarboxylic acid (TCA) cycle. Enzymes involved in glutamine metabolism, such as glutaminase and glutamate dehydrogenase, are upregulated in cancer cells to facilitate glutamine catabolism. Glutamine-derived metabolites contribute to the synthesis of amino acids, nucleotides, and other macromolecules essential for cancer cell growth and proliferation.

4.3 Fatty acid metabolism

Lipid metabolism is rewired in cancer cells, with increased fatty acid synthesis and lipid droplet formation to support membrane biogenesis and energy storage. Key features of fatty acid metabolism in cancer cells include: cancer cells upregulate enzymes involved in de novo fatty acid synthesis, such as fatty acid synthase (FASN), to meet the demands for membrane biogenesis and lipid signaling. Cancer cells accumulate lipid droplets, which serve as reservoirs for stored energy and essential lipids. Lipid droplets play roles in maintaining lipid homeostasis and supporting cancer cell survival under metabolic stress. Lipids and lipid-derived signaling molecules regulate various cellular processes in cancer, including proliferation, migration, and survival. Alterations in lipid metabolism contribute to the dysregulated signaling pathways observed in cancer cells.

4.4 Metabolic crosstalk between tumor cells and immune cells

4.4.1 Metabolic competition

Tumor cells compete with immune cells, such as T cells, for essential nutrients like glucose and amino acids within the TME. This competition can impair immune cell function and contribute to immunosuppression. Tumor cells can consume large amounts of glucose and amino acids, depriving nearby immune cells of these essential nutrients necessary for their activation and effector functions. Nutrient competition within the TME can impair the function of immune cells, including T cells and antigen-presenting cells (APCs), leading to reduced antitumor immune responses and immune evasion by the tumor [48, 49, 50].

4.4.2 Metabolic reprogramming of immune cells

Immune cells within the TME can undergo metabolic reprogramming. For example, T cells can shift toward glycolytic metabolism when exposed to the TME, which can limit their effector functions. Immune cells within the TME can undergo metabolic reprogramming in response to environmental cues. For example: T cells exposed to the TME often undergo metabolic reprogramming, shifting from oxidative phosphorylation to glycolytic metabolism. This metabolic shift can limit their effector functions, including cytokine production and cytotoxicity, thereby compromising antitumor immune responses. Immune cells, such as macrophages and dendritic cells, may also undergo changes in amino acid metabolism within the TME, impacting their polarization and function.

4.4.3 Metabolites as signaling molecules

Metabolites produced by cancer cells (e.g., lactate and kynurenine) can act as signaling molecules that modulate the behavior of immune cells, promoting an immunosuppressive environment. Metabolites produced by cancer cells, such as lactate and kynurenine, can act as signaling molecules that modulate the behavior of immune cells. Accumulation of lactate within the TME can suppress the function of immune cells, including T cells and NK cells, leading to immunosuppression. Lactate can also promote the polarization of immune cells toward immunosuppressive phenotypes, such as M2-like macrophages. Metabolism of tryptophan by indoleamine 2,3-dioxygenase (IDO) in cancer cells results in the production of kynurenine, which has immunosuppressive effects. Kynurenine can inhibit the proliferation and function of T cells and promote the generation of regulatory T cells (Tregs), contributing to immune evasion by the tumor.

4.4.4 Role of tumor metabolism in immune evasion and immunosuppression

Cancer cells can produce metabolites that inhibit immune responses. For example, the production of adenosine within the TME can suppress T-cell activity. Altered tumor metabolism can lead to an acidic and hypoxic TME, which impairs immune cell function and promotes immune evasion. Metabolic changes can influence the expression of immune checkpoint molecules on both cancer cells and immune cells, regulating immune responses. Targeting specific metabolic pathways in cancer cells is being explored as a strategy to improve immunotherapy responses. For example, inhibiting glycolysis or glutaminolysis may enhance the antitumor immune response [51]. Understanding the intricate relationship between tumor metabolism and immune responses is essential for developing novel therapeutic approaches. Targeting metabolic vulnerabilities in both cancer cells and immune cells may provide new avenues for cancer treatment, with the potential to overcome immune evasion and immunosuppression in hepatocellular carcinoma (HCC) and other cancers. Immunotherapy has emerged as a promising approach for the treatment of hepatocellular carcinoma (HCC), offering the potential to harness the patient’s immune system to target and destroy cancer cells. Here, we provide an overview of the current state of immunotherapeutic approaches for HCC, including immune checkpoint inhibitors and adoptive cell therapies, while also addressing the associated challenges and limitations. We further discuss potential strategies to enhance the efficacy of immunotherapy in HCC by targeting myeloid-derived suppressor cells (MDSCs) and macrophages within the tumor microenvironment (TME) [52, 53, 54, 55, 56].

5. Immunotherapy

5.1 Current state of immunotherapeutic approaches for HCC

5.1.1 Immune checkpoint inhibitors (ICIs)

Immune checkpoint inhibitors (ICIs), such as anti-PD-1/PD-L1 and anti-CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) antibodies, have shown promise in HCC. Immune checkpoint inhibitors, such as anti-PD-1/PD-L1 and anti-CTLA-4 antibodies, have shown promise in HCC treatment. ICIs work by blocking inhibitory signals that dampen T-cell activity, thereby allowing the immune system to mount a more effective antitumor response against HCC cells. Clinical trials evaluating ICIs, both as monotherapy and in combination with other agents, have demonstrated encouraging results in terms of tumor response rates and overall survival in patients with advanced HCC [57, 58].

5.1.2 Adoptive cell therapies

Adoptive cell therapies, particularly chimeric antigen receptor (CAR) T-cell therapy and tumor-infiltrating lymphocyte (TIL) therapy, are being investigated for HCC. These therapies involve the infusion of engineered or expanded immune cells with enhanced tumor-targeting capabilities. This approach involves the engineering of T cells to express chimeric antigen receptors (CARs) targeting specific antigens expressed on HCC cells. CAR T cells are then infused into patients to selectively target and kill tumor cells. Tumor-infiltrating lymphocyte (TIL) therapy involves the isolation and expansion of T cells from tumor tissue, followed by infusion back into the patient to enhance antitumor immune responses [59, 60].

5.1.3 Cytokine-based therapies

Interleukin-2 (IL-2) and interferon-alpha (IFN-α) have been used in HCC treatment to stimulate immune responses, although their efficacy can be limited by toxicity. IL-2 stimulates the proliferation and activation of T cells and NK cells, enhancing antitumor immune responses. However, its use in HCC may be limited by toxicity, including cytokine release syndrome and vascular leak syndrome. IFN-α has antiproliferative and immunomodulatory effects, including activation of immune effector cells and inhibition of angiogenesis. However, its efficacy as a monotherapy in HCC is modest, and it is often used in combination with other agents [61, 62].

5.2 Challenges and limitations of immunotherapy in HCC

5.2.1 Heterogeneity of HCC

HCC is a highly heterogeneous disease, with variations in tumor biology, genetics, and microenvironment among patients [63, 64, 65]. This heterogeneity can influence treatment response and contribute to differences in patient outcomes following immunotherapy.

5.2.2 Tumor microenvironment

The immunosuppressive tumor microenvironment (TME) in HCC poses a significant challenge to immunotherapy. MDSCs are immune cells that suppress antitumor immune responses and promote tumor growth. Their accumulation in the TME can hinder the effectiveness of immunotherapy by inhibiting the activity of T cells and other immune effector cells. M2-like macrophages, which exhibit anti-inflammatory and immunosuppressive properties, are often enriched in the HCC TME. Their presence can contribute to immune evasion and resistance to immunotherapy. Various other factors present in the HCC TME, such as regulatory T cells (Tregs), cytokines, and chemokines, contribute to immune suppression and create a hostile environment for antitumor immune responses.

5.2.3 Resistance mechanisms

Some HCC tumors develop resistance to immunotherapy through various mechanisms, including HCC tumors may upregulate alternative immune checkpoint molecules, such as T-cell immunoglobulin-3 (TIM-3), lymphocyte-activation (LAG-3), and V-domain immunoglobulin g (Ig)-containing suppressor of T-cell activation (VISTA), which can bypass inhibition by conventional immune checkpoint inhibitors (ICIs) like anti-PD-1/PD-L1 antibodies. Tumor cells may downregulate major histocompatibility complex (MHC) molecules or antigen presentation machinery, limiting the recognition of cancer cells by T cells and reducing the effectiveness of immunotherapy. Some HCC tumors may exhibit intrinsic resistance to immunotherapy due to genetic alterations, tumor cell plasticity, or other tumor-intrinsic factors.

5.3 Enhancing immunotherapy efficacy by targeting MDSCs and macrophages

5.3.1 Targeted therapies

5.3.1.1 Myeloid-derived suppressor cell targeting

Inhibitors of chemokine receptors involved in MDSC recruitment, such as CXCR2 (CXC motif chemokine receptor 2) inhibitors, can reduce the influx of MDSCs into the tumor microenvironment [18, 19, 66, 67]. Agents targeting MDSC immunosuppressive mechanisms, such as arginase-1 inhibitors or inhibitors of reactive oxygen species (ROS) production, can neutralize MDSC-mediated suppression of antitumor immune responses. Differentiation-inducing agents, such as all-trans retinoic acid (ATRA) or inhibitors of STAT3 signaling, can promote the differentiation of MDSCs into less suppressive phenotypes, such as mature dendritic cells or granulocytes.

5.3.1.2 Macrophage repolarization

Efforts to repolarize tumor-associated macrophages (TAMs) toward a pro-inflammatory M1 phenotype represent a promising strategy in cancer immunotherapy. Macrophage repolarization strategies: Targeted therapies aim to specifically modulate signaling pathways and molecules involved in macrophage polarization. Signal transducer and activator of transcription 3 (STAT3) signaling is often associated with the polarization of TAMs toward an M2 phenotype. Inhibiting STAT3 activation can shift TAMs toward an M1 phenotype. Various small molecule inhibitors of STAT3 signaling are under investigation for their potential in repolarizing TAMs. Nuclear factor-kappa B (NF-κB) signaling promotes the expression of genes associated with M2 macrophage polarization. Inhibiting NF-κB activation can promote the repolarization of TAMs toward an M1 phenotype.

5.3.2 Immunomodulatory agents

Immunomodulatory agents can stimulate the immune system and promote the repolarization of TAMs toward an M1 phenotype. Interferon-gamma (IFN-γ): IFN-γ is a cytokine known for its potent pro-inflammatory effects. It can activate macrophages and promote their polarization toward an M1 phenotype. Administration of IFN-γ or strategies to enhance IFN-γ signaling in the tumor microenvironment can repolarize TAMs and enhance antitumor immune responses. Toll-like receptor (TLR) agonists: TLR agonists are compounds that activate toll-like receptors, which play a crucial role in initiating innate immune responses. Activation of TLRs on macrophages can promote their polarization toward an M1 phenotype and enhance their antitumor activity.

5.3.2.1 Combination therapies

Combining immunotherapy with agents that target MDSCs and TAMs may synergistically enhance antitumor immune responses.

5.3.2.1.1 Targeting MDSCs and TAMs

Agents that target MDSC recruitment, function, or differentiation can reduce their immunosuppressive effects within the tumor microenvironment. This can be achieved using CXCR2 inhibitors, arginase-1 inhibitors, or differentiation-inducing agents. Strategies aimed at repolarizing TAMs toward a pro-inflammatory M1 phenotype can enhance their antitumor activity. This can involve targeted therapies directed at specific signaling pathways or immunomodulatory agents such as interferon-gamma (IFN-γ) or toll-like receptor (TLR) agonists.

5.3.2.1.2 Immunotherapy

Immune checkpoint inhibitors (ICIs) such as anti-PD-1/PD-L1 antibodies can unleash the antitumor immune responses by blocking inhibitory signals that dampen T-cell activity. They can enhance the activation and function of cytotoxic T cells within the tumor microenvironment. Adoptive cell therapies, such as chimeric antigen receptor (CAR) T-cell therapy or tumor-infiltrating lymphocyte (TIL) therapy, involve the infusion of engineered or expanded immune cells with enhanced tumor-targeting capabilities. These therapies can directly target and kill cancer cells.

5.3.2.1.3 Synergistic effects

Overcoming immunosuppression: Combining immunotherapy with agents targeting MDSCs and TAMs can alleviate immunosuppression within the tumor microenvironment. This can enhance the efficacy of immunotherapy by overcoming resistance mechanisms and promoting antitumor immune responses.

Enhancing tumor recognition: repolarization of TAMs toward an M1 phenotype can improve antigen presentation and promote the activation of cytotoxic T cells. This can enhance the recognition and elimination of tumor cells by the immune system.

Augmenting immune activation: inhibition of MDSCs and repolarization of TAMs can create a more favorable immune microenvironment, allowing for increased activation and function of immune effector cells, including T cells and NK cells. Immunotherapy holds promise as a treatment option for HCC, but challenges related to tumor heterogeneity and the immunosuppressive TME need to be addressed. Strategies to enhance immunotherapy efficacy, such as targeting MDSCs and macrophages, are actively being explored and may unlock the full potential of immunotherapy in HCC, offering hope for improved outcomes in patients with this challenging malignancy.

6. Conclusions

In this comprehensive review, we have explored the intricate interplay between myeloid-derived suppressor cells (MDSCs), macrophages, tumor metabolism, and immunotherapy in the context of hepatocellular carcinoma (HCC). The key findings and insights from this review can be summarized as follows [68, 69, 70]:

1. MDSCs and macrophages in HCC: MDSCs and macrophages are pivotal components of the tumor microenvironment in HCC, where they play multifaceted roles. MDSCs promote immunosuppression through diverse mechanisms, while macrophages exhibit plasticity, shifting between pro-inflammatory (M1) and anti-inflammatory (M2) phenotypes, influencing tumor progression.

2. Molecular pathways and immunometabolism: molecular pathways and immunometabolic processes profoundly impact the functions of MDSCs and macrophages. These immune cells undergo dynamic changes in response to the tumor microenvironment, shaping their immunosuppressive or pro-inflammatory functions. Understanding these pathways is essential for therapeutic targeting.

3. Tumor metabolism: altered metabolic processes in cancer cells, including increased glycolysis, glutaminolysis, and fatty acid metabolism, drive tumor growth and influence the immune response within the TME. Metabolic crosstalk between tumor cells and immune cells further impacts immune function.

4. Immunotherapy in HCC: immunotherapy has emerged as a promising strategy for HCC treatment, including immune checkpoint inhibitors and adoptive cell therapies. However, challenges such as tumor heterogeneity and the immunosuppressive TME must be addressed to maximize therapeutic efficacy.

5. Targeting MDSCs and macrophages: targeting MDSCs and macrophages within the TME represents a potential avenue for improving immunotherapy outcomes in HCC. Strategies to inhibit MDSC recruitment, suppress MDSC immunosuppressive functions, and repolarize macrophages toward pro-inflammatory phenotypes are actively under investigation.

7. Future directions

As we continue to explore the complexities of hepatocellular carcinoma (HCC) and its interaction with the immune system, several areas for future research and development of therapeutic strategies emerge. These directions are essential to advance our understanding of HCC immunotherapy and translate preclinical findings into clinical applications effectively.

1. Identification of novel targets: microenvironmental targets: investigate additional immune and stromal components of the TME in HCC, such as cancer-associated fibroblasts and regulatory T cells, to identify new therapeutic targets.

2. Metabolic targets: explore metabolic vulnerabilities in HCC cells and immune cells to identify novel metabolic inhibitors that can be combined with immunotherapy.

3. Epigenetic regulation: investigate the epigenetic modifications that govern MDSC and macrophage functions in the TME, with a focus on potential druggable targets.

4. Combination therapies: immunotherapy combinations: explore rational combinations of immunotherapies, such as combining immune checkpoint inhibitors with MDSC- and macrophage-targeted therapies, to overcome resistance mechanisms.

5. Metabolic modulators: evaluate the efficacy of combining metabolic inhibitors with immunotherapy to enhance antitumor immune responses while mitigating the impact of metabolic competition in the TME.

6. Patient stratification: biomarker discovery: identify reliable biomarkers that can predict response to immunotherapy in HCC, enabling patient stratification for personalized treatment approaches.

7. Genomic profiling: conduct comprehensive genomic profiling of HCC tumors to uncover genetic alterations that influence immunotherapy response.

8. Translational studies: clinical trials: design and conduct well-designed clinical trials that incorporate the insights gained from preclinical studies. Investigate the safety and efficacy of combination therapies in diverse patient populations.

9. Biomarker validation: validate promising biomarkers in clinical settings to guide treatment decisions and monitor therapeutic responses.

10. Patient-derived models: develop and utilize patient-derived models (organoids, xenografts, and patient-derived xenografts) to bridge the gap between preclinical and clinical research and assess therapeutic efficacy in a more relevant context.

11. Immunotherapy resistance mechanisms: investigate the molecular and cellular mechanisms underlying immunotherapy resistance in HCC. This includes studying the role of immune checkpoints, immune cell dysfunction, and TME alterations. Develop strategies to overcome resistance mechanisms, such as combining immunotherapy with targeted therapies or immune-potentiating agents.

In summary, the future of HCC immunotherapy research lies in the identification of novel therapeutic targets, the development of combination therapies, improved patient stratification, and a strong focus on translational studies. Bridging the gap between preclinical findings and clinical applications will be essential to bring innovative immunotherapies to HCC patients, offering them more effective and personalized treatment options in the fight against this challenging malignancy.

Acknowledgments

This study was supported by grants from the Ministry of Science and Technology (MOST), Taiwan (nos. 108-2320-B-010-045-MY3, 110-2320-B-002-080-MY3, MOST 111-2314-B-A49-072, NSTC 112-2314-B-A49-028-MY3, NSTC 113-2321-B-A49-014 , NSTC 113-2740-B-A49-003 ), Yen Tjing Ling Medical Foundation (nos.CI-110-22 and CI-111-24 to L.L.W), and the TYGH-NYCU Joint Research Program (no. PTH110001) and Ministry of Health and Welfare (No. 11210). We thank the Taiwan Mouse Clinic Office for their technological assistance and National Laboratory Animal Center (NLAC) (NARLabs, Taiwan) for technical assistance regarding isolators. The authors acknowledge the research collaboration and technical service supported by National Human Microbiome Core Facility, Taiwan. The authors acknowledge the use of Quillbot for language polishing of the manuscript.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.