The relevance of B7-H3 and tumor-associated macrophages in the tumor immune microenvironment of solid tumors: recent advances

Abstract

B7 homolog 3 (B7-H3) is a member of the B7 ligand family. It is highly expressed in various human cancers, especially mesenchymal malignancies. B7-H3 regulates cancer progression through multiple signaling pathways such as JAK2/STAT3, NF-κB, PI3K/AKT, and ERK. It also has the ability to downregulate CD8⁺ T-cell infiltration and drive immune evasion. Tumor-associated macrophages (TAMs) are the primary immune infiltrating cells in diverse solid tumors, dominating the immune environment of these malignancies. B7-H3 may have connections to TAMs through the induction of polarization and immunosuppression by the CCL2-CCR2-M2 macrophage axis. This mechanism can inhibit antitumor immunotherapy and promote tumor progression in non-small cell lung cancer, ovarian cancer, colorectal cancer, and osteosarcoma. The inducibility of B7-H3 in TAMs provides novel insight into the targeting of checkpoints for tumor immunotherapy. In general, B7-H3 represents a promising immune therapeutic target and should be considered an immunologic adjuvant for activating the tumor immune microenvironment. Therefore, combination therapies based on anti-B7-H3 agents hold great potential for improving the solid tumor microenvironment to enhance the initiation of the cancer-immunity cycle.

Introduction

In recent years, immune checkpoint inhibitors (ICIs) have emerged as a highly promising treatment option, enabling the clinical cure of unresectable solid tumors by producing a lasting response. Nevertheless, the objective response rates to anti-PD-1/PD-L1 or anti-CTLA-4 drugs remain somewhat unsatisfactory and limited in many solid tumors [1]. Therefore, efforts have been directed to identify effective tumor targets that cover a broader range of tumors and exhibit wider expression patterns. B7-H3, a member of the B7 ligand family, represents an attractive target because it is commonly overexpressed on various malignant cells. For decades, the precise mechanism of B7-H3 in immune evasion has remained ambiguous, due to an unknown receptor for its extracellular domain, which consequently hampers our understanding of its biological function. Some researchers have found that B7-H3 has close interactions with TAMs, suggesting its important role in modulating suppressive myeloid immune cells in the tumor immune microenvironment (TIME). Thus, further studies are urgently needed to provide additional insight for this theory and explore key molecules involved in these signaling pathways to guide clinical practice. The aim of this article is to review the histologic literature on the specific interactions between B7-H3 and TAMs, which act to modulate the TIME.

Basic profiles of B7-H3

B7-H3, also known as CD276 or B7RP-2, contains 316 amino acids, and its gene is located on 15q24.1 in human cells. It is a type I transmembrane glycoprotein with two isoforms, namely 2IgB7-H3 and 4IgB7-H3, where 4IgB7-H3 is the most frequent in humans, while mice only express 2IgB7-H3 (Figure 1) [2,3]. B7-H3 is highly expressed on the surfaces of T cells, antigen-presenting cells (APCs), and tumor cells, and it can affect immune inflammation, material metabolism, angiogenesis, and composition of TIME, to promote the growth of tumor cells and metastasis. The receptor for B7-H3 has not been identified, but possible candidates include TLT-2, IL-20RA, and PLA2R1 [4]. Previous studies have shown that the binding affinity between B7-H3Ig and TLT-2 is similar to that of PD-1 and PD-L1, leading to enhanced T-cell response and IFN-γ production [4]. The function of IL20RA toward B7-H3 is still under investigation, but it is possible that the interaction between IL20RA and B7-H3 negatively regulates inflammatory and immune responses [5]. B7-H3 has initially been considered a T-cell positive regulator that stimulates the proliferation of CD4⁺ and CD8⁺ T cells, enhances cytotoxic lymphocyte (CTL) function, and induces IFN-γ production [6]. So far, B7-H3 has been deemed to play an influential immunosuppressive role in most solid tumors. B7-H3 may affect immune function by modulating glycolysis, leading to alterations in HK2 and HIF-α levels, which promotes lactate production, ultimately contributing to the regulation of the tumor immunosuppressive microenvironment [7]. Furthermore, under the influence of B7-H3, CD14⁺ monocytes can promote tumor-associated vessel (TAV) formation and differentiate into vascular endothelial cells [8].

Figure 1.

The genetic background and molecular structure of B7-H3. B7-H3 is a type I transmembrane glycoprotein with two isoforms, namely 2IgB7-H3 and 4IgB7-H3. 2IgB7-H3 consists of IgV₁ and IgC₂ while 4IgB7-H3 consists of IgV₁, IgC₁, IgV₂ and IgC₂. Both also have a transmembrane domain, a cytoplastic tail and a signal peptide. 4IgB7-H3 is the most frequent in humans while mice express only 2IgB7-H3. Its gene is located in 15q24.1 in human cells. sB7-H3: soluble B7-H3.

In addition to its role in immune function, recent studies have demonstrated that B7-H3 can directly affect the growth and metastasis of tumor cells. B7-H3 has been shown to facilitate endothelial-to-mesenchymal transition (EMT), thereby expediting tumor progression. Furthermore, B7-H3 has been reported to modulate the effects of 5-Fluorouracil (FU) in colorectal cancer [9] and gemcitabine in pancreatic cancer [10] by inhibiting autophagy, apoptosis, and DNA-break repair in baseline cells, ultimately leading to enhanced radioresistance [11]. The available evidence highlights a significant role of B7-H3 in tumor initiation, progression, and immune evasion.

Distinction from other B7 members

Comparison with PD-L1

PD-L1, also known as B7-H1, is an important protein for maintaining immune homeostasis [12]. The PD-1/PD-L1 signaling pathway can inhibit excessive activation of immune cells to prevent autoimmune diseases [13], and may even be hijacked to assist tumor to escape immune surveillance [14]. The combination of PD-L1 on cancer cells and PD-1 on tumor-infiltrating lymphocytes (TILs) activates Src homology region 2 domain-containing phosphatase (SPH2), leading to the suppression of T-cell receptor (TCR) and T-cell function [15]. Furthermore, the transcription of PD-L1 can be regulated by a large number of signaling pathways, such as mitogen-activated protein kinase (MAPK), anaplastic lymphoma kinase (ALK), phosphatidylinositol 3-kinase (PI3K)/AKT/mTOR, Janus kinase 2/signal transducer and activator of transcription 3 (JAK/STAT3), and nuclear factor-κB (NF-κB). Unlike PD-L1, expression of B7-H3 on tumor cells is wider, whether in non-small cell lung cancer, melanoma, gastrointestinal cancers, or sarcomas. Nevertheless, while the function and mechanism of PD-L1 have been relatively clear, the function and mechanism of B7-H3 are still under investigation. Currently, suppression of the PD-1/PD-L1 signaling pathway is a significant strategy to restore the normal regulatory condition of TIME [16]. As B7-H3 has both immunosuppressive and immunostimulatory functions, it is important to explore its mechanism.

Comparison with other B7 members

In addition to B7-H3 and PD-L1, there are eight more members of the B7 ligand family, namely B7-1, B7-2, B7-H2, B7-DC, B7-H4, B7-H5, B7-H6, and B7-H7 (Figure 2) [17]. B7-1 and B7-2 are primarily expressed on professional APCs such as dendritic cells and macrophages. They can specifically interact with ligands on the T cell surface such as CD28 or CTLA-4, which play a key role in T-cell activation, maintenance of steady state, and self-tolerance [18]. For example, it was reported that the interaction between B7-1 and PD-L1 expressed by dendritic cells (DCs), T cells, B cells, and macrophages could suppress the activation of T cells and the production of cytokines [19]. Some investigators have begun to investigate chemical agents (e.g., belatacept) and monoclonal antibodies (e.g., galiximab) that target B7-1 to modulate immunity [20,21]. B7-H2 is a ligand of the inducible co-stimulator (ICOS), serving as a co-stimulatory signal to induce T-cell proliferation and B-cell differentiation into plasma cells while also facilitating the secretion of antitumor cytokines. B7-DC, also known as PD-L2, primarily contributes to tumor’s evasion of the immune system through the interaction with PD-1. However, it is less efficient than PD-L1, whose function on tumor progression and prognosis in TIME in osteosarcoma has been studied before [2]. V-domain Ig-containing suppressor of T-cell activation (VISTA), also known as B7-H5, is a member of the B7 family of negative checkpoint regulators and represents a new target for immunotherapy. VISTA shows the highest expression in myeloid cells, including macrophages, conventional DCs, monocytes, and circulating neutrophils. Among conventional T cells, expression of VISTA is the highest in naïve cells and FoxP3+ regulatory T cells (Tregs), and memory CD4⁺ T cells also express VISTA to some extent. This unique surface expression pattern suggests that VISTA may function to restrict T-cell immunity at different stages compared with the PD-1/PD-L1 and CTLA-4 axes [22]. B7-H6 can facilitate or suppress immune reaction by interacting with TMIGD2 or KIR3DL3, respectively [23]. The B7 family members have immune regulatory functions, specifically in the interaction with T cells. B7-H3, in contrast to other B7 family members, is expressed in a wide range of tumor cells and plays a dual role in immune response modulation, both stimulatory and inhibitory.

Figure 2.

The B7 family members and its receptor. This picture provides a summary of research findings on the B7 family members and their corresponding receptors. The family includes B7-1, B7-2, B7-H1 (also known as PD-L1 or Programmed Death-Ligand 1), B7-DC (also referred to as PD-L2 or Programmed Death-Ligand 2), B7-H3, B7-H4, B7-H5, B7-H6, and B7-H7. Notably, B7-H3, B7-H4, and B7-H5 have both membrane surface expression and soluble forms, although the identity of their ligands remains unclear. CTLA4: The cytotoxic T-lymphocyte-associated antigen 4; PD-1: Programmed cell death protein 1; NCR3: Natural cytotoxicity receptor 3; TMIGD2: Transmembrane and immunoglobulin domain containing 2.

Mechanisms of B7-H3 in cancer signaling pathways

So far, many immune signaling mechanisms of B7-H3 in tumors have been explored, highlighting its critical and debated role in TIME (Figure 3). B7-H3 has been shown to promote immune evasion, thereby inhibiting antitumor responses. The depletion or inhibition of B7-H3 is likely to hinder the progression, invasion, angiogenesis, and metastasis of gastrointestinal tract (GIT) tumors, weakening their resistance. The overexpression of B7-H3 can activate a large number of signaling pathways to inhibit apoptosis genetically and control the activity of cell cycle-controlling proteins and DNA-repair mechanisms [24].

Figure 3.

Control factors of B7-H3 expression. In this picture, we summarize the existing literature on the signaling pathways influencing B7-H3 expression, covering aspects such as transcription, translation, and immune cell involvement. Throughout the picture, the arrow-headed lines denote induction, the circles-headed line denote ubiquitination degradation, and crosses denote blockade. mTORC1: The mechanistic/mammalian target of rapamycin complex I; S6K: S6 kinase; YY2: Yin yang 2; Smurf1: SMAD specific E3 ubiquitin protein ligase 1; AP-1: Activating protein-1; FOSL1: Fos-like antigen 1; SEs: Super-enhancers; TNF-α: Tumor necrosis factor α; SP20H: SPT20 homolog; MAPK: Mitogen-activated protein kinases; MNK1/2: MAPK interacting protein kinases 1 and 2; eIF4E: eukaryotic translation initiation factor 4E; eIF4A: eukaryotic translation initiation factor 4A; eIF4G: eukaryotic translation initiation factor 4G; eIF4F: eukaryotic translation initiation factor 4F; GM-CSF: Granulocyte/macrophage colony stimulating factor; LPS: Lipopolysaccharide; IFN-γ: Interferon-γ; CMTM6: The CKLF-like MARVEL transmembrane domain-containing protein 6; PD-1: Programmed cell death protein 1.

It is suggested that FOSL1, YY2, eIF4F complex, and inflammatory cytokines can directly influence the expression of B7-H3. A key component of FOSL1, AP-1, binds to the superenhancer (SE) region upstream of B7-H3 to promote B7-H3 transcription. The expression of FOSL1 is influenced by B7-H3 through the activation of AP-1, creating a positive feedback loop between B7-H3 and FOSL1 [25]. mTORC1 regulates B7-H3 expression by directly phosphorylating the transcription factor YY2. More specifically, the downstream effector of mTORC1, S6K, directly phosphorylates Thr336 on YY2. This phosphorylation inhibits the ubiquitin-mediated degradation of YY2, leading to increased stability and enhanced expression of YY2 protein. In mice with mTORC1 hyperactivity, inhibition of B7-H3 increases CD38⁺CD39⁺CD4⁺ cytotoxic T-cell infiltration, activates the IFN-γ response, and enhances the expression of MHC-II [26]. Eukaryotic translation initiation factor 4E (eIF4E) plays a crucial role in the translation of B7-H3 by assembling an eIF4F complex with eIF4A and eIF4G. Phosphorylation of PI3K/AKT/mTOR and extracellular signal-regulated kinase (ERK) or p38MAPK-MNK1/2 regulates the expression of B7-H3, and loss of SP20H and stimulation of TNF-α in cancer upregulates it. For example, deficiency of S20H can induce an eIF4E-dependent translation of B7-H3, enhancing the antitumor activity of APCs. Some inflammatory cytokines such as GM-CSF, LPS, and IFN-γ can also induce DCs and monocytes to express B7-H3 [27]. In addition, there are some genes that can affect the expression of B7-H3, but the mechanism is still unclear. CMTM6, a regulator of PD-L1, can affect tumorigenesis and interfere with the maintenance of cancer stem cell (CSC) and TGF-β-induced EMT through the Wnt/β-catenin signaling pathway. Knocking down CMTM6 in TILs results in downregulation of B7-H3, leading to an antitumor action [28], and overexpression of B7-H3 may adjust the level of HK2 and HIF-α to reprogram cancer glucose metabolism.

TAMs in the TIME

The TIME comprises a variety of immune and nonimmune cells, including CTLs, NK cells, cancer-associated fibroblasts (CAFs), and endothelial cells (ECs), among others. Additionally, the TIME contains significant non-tumor stromal cells that have an effect on tumor immune evasion and the immune response [29]. Macrophages are a type of versatile immune cell capable of phagocytosis, antigen presentation, and secretion of signaling molecules [30-34], and they can also modulate tissue homeostasis and promote wound healing [35]. In the tumor, TAMs can magnify immune signals and start the antitumor immune response, and can also restore homeostasis and auxiliary tumor tissue remodeling and repair.

Generation of macrophages

Macrophages can be induced to two contrasting groups through polarization, namely M1 macrophages, which are activated classically, and M2 macrophages, which are activated alternatively [36]. M1 macrophages are typically induced by bacterial products and proinflammatory factors, whereas M2 macrophages are often induced by immunoregulatory cytokines. Commonly, M1 macrophages produce antitumor angiostatic factors such as IL-12 and CXCL10, and M2 macrophages generate tissue-remodeling factors and pro-angiogenic factors such as MMP and VEGF. Nevertheless, the binary classification of macrophages to M1 and M2 phenotypes is becoming less clear, leading to the more frequent use of terms such as “M1-like” or “M2-like” [37]. There is another view that macrophages can be regarded as “beneficial”, “harmful”, and “bystander” toward the immune system. Because M2 and a minority of M1 macrophages are involved in tumor initiation, progression, angiogenesis, and metastasis [38], they are collectively known as TAMs.

Relationship with the TIME

The interaction between TAMs and immune cells enables an immunosuppressive microenvironment. T cells, myeloid cells, and tumor cells can regulate TAMs. Tregs also have some influence on TAMs. For example, Tregs can adjust lipid metabolism of M2-like TAMs by preventing IFN-γ secreted by CD8⁺ T cells from cutting of the activation of sterol regulatory element by binding ptotein-1 (SREBP-1)-mediated fatty acid synthesis, and sustain survival, metabolic fitness, and mitochondrial integrity of M2-TAMs [39]. LILRB4 is a myeloid inhibitory receptor from leukocyte Ig-like receptor superfamily. It is usually expressed on myeloid cells, especially on CD45⁺ cells, and is expressed more than PD-1 in the TIME. The PD-1/PD-L1 axis also affects the antitumor ability of TAMs. Specifically, when PD-1/PD-L1 signaling is blocked by anti-PD-1 antibodies or PD-L1 inhibitors, antitumor responses are observed even in the absence of T cells, B cells, and NK cells. The interactions between MHC II and macrophages-LILRB1 can affect the function of TAMs. Through combination with LILRB1, MHC I causes cancer cells to resist phagocytosis [40].

TAMs can regulate the TIME by releasing cytokines and regulating membrane receptors. For example, macrophages can express CSF1R to activate the CSF1-CSF1R signaling pathway, accelerating initiation and proliferation of macrophage precursors, and recruitment and retention within inflamed sites [37,41]. Moreover, TAMs upregulate PD-L1 and PD-L2 on their surface, leading to T-cell exhaustion. TAMs produce IL-10 and TGF-β, directly inhibit the function of CD8⁺ T cells, and stimulate CD4⁺ T cells to differentiate into Tregs [39]. TAMs-derived IL-10 activates PD-L1(+) TAMs to induce CD8⁺ T-cell dysfunction in hepatocellular carcinoma (HCC) [42,43], and TREM-1(+) TAMs-derived CCL-20 facilitates CC6⁺ FOXP3⁺ Tregs infiltration through the ERK/NF-κB signaling pathway in response to tumor metabolites and hypoxia [44,45]. PI3Kγ is located in a signaling bottleneck in TAMs, integrating external reprogramming signals to control the switch from immune stimulation to immune suppression. In the case of PI3Kγ gene deletion or inhibition, the lack of catalytic subunit p110 promotes the degradation of IκBα, thereby enhancing the activity of NF-κB, which results in the increased expression of immunostimulatory molecules such as MHC II and IL-12, and decreased expression of immunosuppressive molecules such as IL-10 and arginase [46].

Interaction between B7-H3 and TAMs in cancer

During the development and progression of cancer, there are abundant interactions between B7-H3 and TAMs to support or impede tumor cells jointly (Supplementary Table 1). Tumor-expressed B7-H3 can influence the polarization of human monocytic cell line THP-1 toward an anti-inflammatory M2-like phenotype, thereby creating an immunosuppressive and tumor-promoting TIME (Figure 4). The expression of B7-H3 in tumor spheroids is closely associated with the infiltration of macrophages. Depletion of B7-H3 can lead to the differentiation of monocytes into macrophages in the presence of GM-CSF and M-CSF. Activated monocytes and macrophages can upregulate B7-H3, but this does not aid in the differentiation and polarization of monocyte-derived macrophages (MDMs). B7-H3 may promote tumor progression by inducing immunosuppression through the CCL2-CCR2-M2 macrophage axis. αCD276-based immunotherapy can selectively act on macrophages, reducing B7-H3 and facilitating macrophages infiltration [47].

Figure 4.

Effect of B7-H3 on macrophage polarization. B7-H3 can affect the CCL2-CCR2-M2 pathway through STAT3, promoting the recruitment and polarization of monocytes to macrophages. Additionally, B7-H3 can indirectly influence macrophage polarization through its effects on T cells or lactate production. Stat3: Signal transducer and activator of transcription 3; CCL2: C-C motif ligand 2; CCR2: C-C chemokine receptor type 2; IFN-γ: Interferon-γ; TNF-α: Tumor necrosis factor α; IL-4: Interleukin-4; IL-13: Interleukin-13; IL-10: Interleukin-10; TGF-β; Transforming growth factor β.

Pathways significant for cell polarization

CCL2-CCR2-M2 macrophage axis

CCL2 (also known as MCP-1) is a chemokine that attracts monocytes to sites of inflammation and tumor by binding to its receptor CCR2. Once monocytes are recruited to these regions, they may further differentiate into M2 macrophages, forming an immunosuppressive microenvironment conducive to tumor progression. Recent studies have shown that spondin 2 (SPON2), high-mobility gene group A2 (HMGA2), cytoplasmic polyadenylation element binding protein 3 (CPEB3), and miR-106b produced by tumor cells can affect the secretion of CCL2. SPON2, an extracellular matrix glycoprotein, can activate TAM migration and infiltration and induce M2 polarization indirectly by upregulating several cytokines such as IL-10, CCL2, and CSF1, perhaps through the ERK/AP1 pathway activated by integrin β1. HMGA2 is an overexpressed oncoprotein, and it can stimulate recruitment and M2 polarization of macrophages in the TIME by inducing CCL2 secretion by binding directly to STAT3 promoter [48]. A decrease of CPEB3 is associated with fewer CD86⁺ TAMs and more CD163⁺ TAMs, and CPEB3 can suppress M2-TAMs polarization by downregulating CCL2 and inhibit TAMs-induced EMT via IL-6 to impair tumor progression [49]. Conversely, EMT-Exos-derived miR-106b can induce M2 polarization to activate the PI3Kγ/AKT/mTOR signaling cascade by decreasing PDCD4 at the posttranscription level, which assists M2 polarization, thereby forming a vicious circle to push the migration, invasion, and metastasis [50]. B7-H3 can affect the CCL2-CCR2-M2 macrophage pathway by partially affecting STAT3.

B7-H3 indirectly affects macrophage polarization

B7-H3 has an effect on the infiltration and function of CD4⁺ T cells, particularly Tregs, Th1 cells, and Th2 cells. Research on prostate cancer has demonstrated that B7-H3 influences the invasion of all three cell types, especially regarding the presence of Tregs in B7-H3_high TAMs to facilitate tumor accumulation [51]. Additionally, B7-H3 affects the function of these three T-cell types, with the deletion of B7-H3 resulting in increased granzyme B, IFN-γ, and IL-2 production in CD4⁺ T cells, indicating an enhanced cytotoxic effect [51]. B7-H3 also plays a role in negatively regulating Th1-driven immune responses and affecting the production of Th2 cytokines (IL-4, IL-5, and IL-13) [52,53]. These T cells influence the polarization of macrophages, with Th1 cytokines such as IFN-γ and TNF-α inducing M1 polarization and Th2 cytokines such as IL-4 and IL-13 inducing M2 polarization [54]. Tregs can induce M2 macrophage polarization through arginase, IL-10, and TGF-β pathways [54,55]. Although there is no direct evidence showing that B7-H3 can affect macrophage polarization through CD4⁺ T cells, there is a strong association between them. In addition, B7-H3 can affect the glycolytic process of tumor cells and increase the formation of lactate [7]. High levels of lactic acid can promote M2 macrophage polarization [56].

Relationship between B7-H3 or TAMs in several common cancers

Non-small cell lung cancer (NSCLC)

NSCLC accounts for the majority of lung cancer cases. It is usually treated with anti-PD-1 or anti-PD-L1 agents, which have a response rate of less than 20% given that only 25%-37% of tumor cells are positive for PD-L1 in NSCLC [57]. However, about 76% of NSCLC cases are positive for B7-H3 [58]. Even in pulmonary sarcomatoid carcinoma (PSC), an NSCLC with a high frequency of driver mutations, B7-H3 is positive in 73% of PSC and in 100% of sarcomatoid components [59]. B7-H3 is associated with prognosis in NSCLC, which may be due to its immunomodulatory and immune-independent function. Overexpression of B7-H3 is associated with tumor-infiltrated cytotoxic lymphocytes and pDC upregulation in NSCLC. The proportion of CD3^- CD45⁺ macrophages is downregulated in B7-H3-high tumor tissues and non-tumor tissues [60]. In samples which have close proximity and interaction between malignant cells and T cells, macrophages expressing B7-H3 and B7-H4 actively interact with malignant cells, which may increase the risk of tumor recurrence [61].

Ovarian cancer (OC)

OC is a highly aggressive tumor that is closely linked to the immune checkpoint protein B7-H3, and B7-H3 is significantly associated with tumor subtypes and TIME [62]. B7-H3 is predominantly expressed on M2 macrophages and MACRO+ macrophages, which have a tumor-promoting phenotype in OC [62]. According to the TCGA, B7-H3 expression in mesenchymal and proliferative subtypes is greater than that in immunoreactive subtype [63]. Analysis of the learning resource (LR)-network suggests that B7-H3 may play a role in anchoring macrophage clusters to carry out its functions within TIME through macrophage signaling [62]. Compared to normal tissues, the expression level of B7-H3 in OC is significantly enhanced and correlates with immune cell infiltration. It positively correlates with M0 and M2 macrophages and negatively correlates with infiltrating memory B cells and plasma cells [62]. Similar results have been found for high-grade serous OC (HGSOC). For example, in patients with HGSOC, B7-H3 expression is positively correlated with the abundance of M2 macrophages [63]. This phenomenon may be attributed to the positive correlation between B7-H3 expression and various immuno-related genes, immune checkpoint genes, and mismatch repair (MMR) genes in OC [62]. Additionally, B7-H3 may promote tumor progression by inducing immunosuppression through the CCL2-CCR2-M2 macrophage axis. Specifically, CCL2 produced by tumor cells by the STAT3 pathway is inhibited by B7-H3, thereby suppressing the polarization and migration of M2 macrophages and monocytes [63]. These findings indicate that B7-H3 may be a promising therapeutic target, particularly in tumors with high M2 infiltration and B7-H3 expression [64].

Colorectal cancer (CRC)

CRC is tightly associated with B7-H3 and TAMs. CRC tumor cells can affect SPON2, HMGA2, CPEB3, and miR-106b to influence the CCL2-CCR2-M2 pathway, which affects macrophage polarization and distribution. It has also been shown that B7-H3 is related to M2 macrophage polarization. Factor 1 from principal component analysis (PCA) for cytokines with M2 macrophages is positively associated with the percentage of B7-H3 immunohistochemistry (IHC) expression and B7-H3 concentrations in tumor tissue homogenates. B7-H3 can support tumor progression by forming an immunosuppressive TIME induced by M2-like macrophages and anti-inflammatory cytokines, and CD163 expression and the immune scores of macrophage regulation both improve in B7-H3_high CRC [65].

Glioma

In glioma, the expression of B7-H3 is significantly higher than that of normal tissue, with approximately 97% of cases showing detection. Also, in supratentorial ependymoma, a rare form of glioma, B7-H3 is positive in TIME. The expression of B7-H3 in glioma is positively associated with the WHO grade of glioma, patient age, greater recurrence, and a poor prognosis. In gliomas, B7-H3 has a strong association with immune cells, especially TAMs. The risk score of glioma established by five screened immune gene-related lncRNAs is tightly associated with B7-H3 [66]. While there is no significant difference in B7-H3 expression and TAMs abundance between pediatric diffuse gliomas (pDGs) and adult diffuse gliomas (aDGs), there is a strong correlation between TAMs and B7-H3 expression and PD-L1 levels in patients with aDGs and pDGs. Additionally, the expression of B7-H3 in pDGs may affect the anatomic location and prognosis of the tumors. High expression of B7-H3 is found in patients with pDGs located in the posterior capsule, and external capsule is composed of white matter fibers, which are thought to assist malignant behavior, such as the proliferation of glioma [67]. Thus B7-H3 may be a therapeutic target for glioma.

Breast cancer

B7-H3 is expressed on tumor cells in over 70% of breast cancer cases [47], and plays an important role in breast cancer. Since B7-H3 is overexpressed in different subtypes of breast cancer, it may be a potential target for ultrasound molecular imaging of breast cancer. For example, affibody (ABY)-based ultrasound molecular imaging of vascular B7-H3 by using the ABY-B7-H3 to recognize B7-H3 tissue sections of breast cancer can improve cancer detection [68]. B7-H3 has also been implicated in immune regulation of breast cancer. Previous research has shown that high expression of B7-H3 on TAMs is associated with poor prognosis and exerts a significant pro-metastatic function in triple-negative breast cancer (TNBC). B7-H3_high TAMs aggravate the immunosuppressive TIME by supporting the accumulation of myeloid-derived suppressor cells (MDSCs) and pro-tumoral Tregs, and suppressing the infiltration of NK cells and CD8⁺ T cells. This increases tumor angiogenesis and mediates extracellular matrix (ECM) degradation. Furthermore, blocking B7-H3 can improve the therapeutic efficacy of many drugs such as anti-PD-1 and paclitaxel in TNBC, indicating that targeting B7-H3 can combine with other antitumor therapies to reduce TAMs-related ineffective treatment toward TNBC [69]. Although there is little evidence for the interaction between B7-H3 and TAMs in breast cancer, it is likely of great significance in immunotherapy for breast cancer.

Relationship between B7-H3 or TAMs in sarcomas

B7-H3 is expressed in diverse sarcomas, such as osteosarcoma, rhabdomyosarcoma, liposarcoma, chondrosarcoma, and synovial sarcoma, promoting tumor growth and metastasis. B7-H3-targeting monoclonal antibodies (mAbs) can improve the cytotoxicity and degranulation of NK cells almost in all sarcoma cell lines, and increase IFN-γ, TNF, IL-4, IL-10, and granzyme A and B [70]. Research on the relationship between TAMs and B7-H3 is less abundant in sarcomas than in solid cancers, which may be related to the relative rarity of sarcomas.

Osteosarcoma (OS)

B7-H3 is expressed in 91.8% of OS lesions, and its expression intensity is higher than in adjacent normal tissues. The expression of B7-H3 negatively correlates with the density of infiltrating CD8⁺ T lymphocytes [71]. In addition, the expression of B7-H3 is associated with tumor Ennecking stage/metastasis status. B7-H3-sr39tk and B7-H3 CAR-T cells both have antitumor effects on 143B OS cells naturally expressing B7-H3 [72]. Anti-B7-H3 monoclonal antibody conjugated with FITC can penetrate tumor tissue and bind with 143B OS tumor cells, enhancing their recognition by CAR-T cells and preventing their suppression and exhaustion. This interaction can also enhance the antitumor activity of anti-FITC CAR-T cells, which can be stimulated to migrate to OS, leading to upregulation of IFN-γ and TNF-α, and ultimately improving antitumor efficacy. Another study has shown that M1-like macrophages activated by IFN-γ can inhibit the growth of OS cells [73]. There is no direct evidence that TAMs and B7-H3 are related, but both are associated with IFN-γ. Further studies are needed to verify the relationship between the two.

Rhabdomyosarcoma (RMS)

B7-H3 has been shown to be a specific target in RMS [74]. High expression of B7-H3 is associated with low density of CD8⁺ T cells. M1 macrophages and NK cells are enriched in tumors with high B7-H3 expression. In PAX3/7-FOXO1 fusion-negative RMS, B7-H3 correlates positively and significantly with M1 macrophages and neutrophils, and moderately negatively correlates with M2 macrophages and monocytes [75]. B7-H3 CAR-T cells incubated with fLuc⁺ RD and Rh4 cells can almost eradicate all fLuc⁺ RMS cells and release substantial IL-2, IFN-γ, granzyme B, and B7-H3, particularly [76]. Canine MGA271 CARs, a type of B7-H3 CAR-T cells, have extremely lethal function on tumor spheroids in RMS through increasing serum ALP and ALT and upregulating CCL2 after lymphodepletion [77]. There is not yet enough conclusive evidence to support an immune connection between B7-H3 and TAMs in the TIME.

Ewing sarcoma (ES)

B7-H3 mRNA is also highly expressed in ES, but lower than in OS and Wilms tumor [78]. There is a higher B7-H3 expression in low-risk than in high-risk ES, probably associated with the immune-related 11-lncRNA. In ES, plasma cells positively correlate with M1 macrophages and negatively correlate with M2 macrophages, infiltration of which generally implies a poor prognosis [79]. It has been shown that the population of macrophages in ES is less abundant than that of OS and RMS, and ES has a tendency of low M2 macrophage gene signatures. However, the population of CD163/CD68 double-positive M2 macrophages in metastatic ES is higher than that of local recurring ES [80]. At present, diverse mAbs of immune checkpoints including B7-H3 are applied in immunotherapy of sarcomas [81], which may provide new targeting opportunities for ES.

Others

In numerous cancers, B7-H3 is associated with TAMs. Fibrolamellar carcinoma (FLC) is a liver carcinoma, belonging to HCC. About 91% of FLC cases have B7-H3-positive TAMs in the TIME, and B7-H3 expression significantly correlates with high abundance of CD8⁺ T cells [82]. In sarcomatoid HCC (sHCC), B7-H3 expression in peritumor tissue is lower than that in tumor tissue, and B7-H3 expression in the sarcomatoid component is higher than that of the conventional HCC component. However, there is no difference in CD68⁺ TAMs abundance between the sarcomatoid and conventional components [83]. Furthermore, B7-H3 CAR-T cells play an efficient antitumor therapeutic role in many solid tumors [84].

Concluding remarks

B7-H3 and M2 TAMs have a consistent immunosuppressive tendency in tumor immunotherapy. B7-H3 is overexpressed in a number of cancers, inducing macrophages to polarize from M1 to M2 and facilitating pro-tumor roles of TAMs in the TIME. Various molecules are tightly associated with B7-H3 expression and its specific effect in many cancers, so it is important to search for their binding ligands or regulatory sites to enable drug development for immunotherapy.

T cells and macrophages are both immune cells, and thus implanting the technology of chimeric antigen receptor into TAMs and other immune cells may be useful. Although we have collected and summarized the probable complex characteristics and mechanisms of B7-H3, its exact action is still unclear, preventing optimization of a strategy of combining drugs. The M2 polarization of TAMs in the TIME induced by B7-H3 has been discussed in diverse tumors and involves various signaling pathways, mostly NF-κB and STAT3. However, there is still no unified signaling pathway or clear downstream receptor. It is worth pondering whether those receptors exist on the surface or inside of immune cells, or endocytosis is involved in the corresponding process of the interactions between B7-H3 and TAMs, making receptors or targets blurry. Furthermore, the pro-tumor function of M2 TAMs mainly matters not at the terminal of cancer-immunity cycle, but during antigen presentation, the most critical stage. This may explain why clinical trials of B7-H3 mAbs have not been very successful. As an immune checkpoint, B7-H3 does not have an equal therapeutic efficacy of single drugs or mAbs, as enabled by PD-L1 in many solid tumors, indicating that the function of this checkpoint is rather auxiliary and supplementary. In other words, it is not the final part of the cancer-immunity cycle. In our opinion, it probably exists at the beginning of this cycle, which mainly enables the presence of antibodies as well as modulates the TIME into a “cold” state. B7-H3 dominates targets on the surface of many sarcomas and requires more reasonable drug combinations to play an immune regulatory role. If this is true, it is even more necessary to clarify the signaling pathways and receptors managed by this checkpoint. Furthermore, the above research mainly focuses on the influence of myeloid cells on TIME, but perhaps the secretion of anti-inflammatory cytokines and the infiltration of Tregs caused by M2 polarization are the chief reasons for immunosuppression. Thus, further cell and mouse experiments should demonstrate whether the increase of M1 macrophages in TIME by inhibition of B7-H3 could lead to more infiltration of CD8⁺ T cells and more cytotoxicity of NK cells. An immune checkpoint that works on the myeloid system and is highly expressed on the cell membrane surface will have great prospects. Accordingly, we can predict that target inhibition and blockade of B7-H3, combined with mAbs against CD47, PD-1/PD-L1, or CTLA-4, can reform the distribution of cytokines, reverse the situation of drug insensitivity or low response rate of the PD-1/PD-L1 axis, and bring revolutionary immunotherapeutic efficacy, only if toxic side effects (such as anemia and cytokine release syndrome) are acceptable. In conclusion, B7-H3 has many complicated and close relationships with TAMs in cancers, especially sarcomas. B7-H3 may mediate M2 polarization of TAMs in TIME to support tumor progression through diverse molecular signaling pathways. Thus, both B7-H3 and TAMs are promising immune therapeutic targets for combination therapy.

Disclosure of conflict of interest

None.

Abbreviations

B7-H3

B7 homolog 3

TAMs

tumor-associated macrophages

ICIs

immune checkpoint inhibitors

TIME

tumor immune microenvironment

APCs

antigen-presenting cells

CTL

cytotoxic lymphocyte

TAV

tumor-associated vessel

EMT

endothelial-to-mesenchymal transition

FU

fluorouracil

TILs

tumor-infiltrating lymphocytes

SPH2

Src homology region 2 domain-containing phosphatase

TCR

T-cell receptor

MAPK

mitogen-activated protein kinase

ALK

anaplastic lymphoma kinase

PI3K

phosphatidylinositol 3-kinase

JAK/STAT3

janus kinase 2/signal transducer and activator of transcription 3

NF-κB

nuclear factor-κB

DCs

dendritic cells

ICOS

inducible co-stimulator

VISTA

V-domain Ig-containing suppressor of T-cell activation

Tregs

regulatory T cells

GIT

gastrointestinal tract

ERK

extracellular signal-regulated kinase

SE

superenhancer

eIF4E

eukaryotic translation initiation factor 4E

CSC

cancer stem cell

CAFs

cancer-associated fibroblasts

ECs

endothelial cells

SREBP-1

sterol regulatory element by binding ptotein-1

HCC

hepatocellular carcinoma

MDMs

monocyte-derived macrophages

SPON2

spondin 2

HMGA2

high-mobility gene group A2

CPEB3

cytoplasmic polyadenylation element binding protein 3

NSCLC

non-small cell lung cancer

PSC

pulmonary sarcomatoid carcinoma

OC

ovarian cancer

LR

learning resource

HGSOC

high-grade serous OC

MMR

mismatch repair

CRC

colorectal cancer

PCA

principal component analysis

IHC

immunohistochemistry

pDGs

pediatric diffuse gliomas

aDGs

adult diffuse gliomas

ABY

affibody

TNBC

triple-negative breast cancer

MDSCs

myeloid-derived suppressor cells

ECM

extracellular matrix

mAbs

monoclonal antibodies

OS

osteosarcoma

RMS

rhabdomyosarcoma

ES

Ewing sarcoma

FLC

fibrolamellar carcinoma

sHCC

sarcomatoid HCC