Introduction

BC is the leading cause of disability and death among women globally [1]. The World Health Organization reports that approximately 2.26 million women are given a BC diagnosis every year [1]. Mosaic populations of tumor cells, immune cells, and stromal cells that have different genetic, epigenetic, and phenotypic traits make up breast malignancies. Four molecular subtypes of BC were categorized by gene expression sequence analysis; these include Luminal A, if estrogen receptor alpha-positive (ER) + and/or progesterone-receptor (PR) + , human epidermal growth factor receptor 2 (HER2) − , Ki67 < 14%), Luminal B (if ER + and/or PR + , HER2 overexpressed or Ki67 ≥ 14%), triple-negative breast cancer (TNBC) (if ER − , PR − , HER2 −), and HER2-enriched (if ER − , PR − and HER2 +) [2]. The specific receptors that cancer cells express (or do not express) act as biomarkers for therapy. Anti-estrogens and aromatase inhibitors, both of which disrupt ER activity, are effective against ER-α positive cancers [3]. Therapeutic agents directed at HER2, such as trastuzumab—an anti-HER2 antibody—demonstrate anticancer efficacy specifically in HER2-positive malignancies [4]. Hormone-responsive BC has been successfully treated with endocrine treatment. Regretfully, disease recurrence and relapse are caused by the emergence of drug resistance [5], TNBC has the poorest prognosis because of the high intra-tumor heterogeneity and absence of specific receptors [6]. Therefore, the outlook for women with BC remains grim. The immune and stromal cell subsets that compose the breast tumor ecosystem are extremely complicated, and their makeup, spatial arrangement, and functional orientation all significantly impact how the illness develops and how patients fare. Consequently, it is crucial to establish effective BC treatment techniques and identify new therapeutic targets. Cancer treatment has undergone a paradigm shift as a result of recent developments in immune checkpoint inhibitor (ICI) medicines [7].

Particular focus has been placed on the B7 family proteins due to its potential use as an ICI to cure cancer. Members of the B7 family closely regulate immunological responses [8] and tumor progression [9]. The 10 members of the B7 family that are now recognized include B7-1/CD80, B7-2/CD86, B7-H1/PD-L1, B7-DC/PD-L2, B7-H2/CD275, B7-H3/CD276, B7-H4/VTCN1, B7-H5/Vista, B7-H6/NCR3LG1, and B7-H7/HHLA2 [10]. It has been demonstrated that B7-H1/PD-L1 and B7-DC/PD-L2 interact with PD-1 (programmed death 1) and stimulate the growth of T cells via secreting IL-10 and interferon-γ [11]. In contrast, the T-cell response is inhibited and immune evasion is facilitated when PD-L1 is expressed on cancer-associated cells [12]. PD-1/PD-L1 pathway proteins have been targeted by antibodies to treat a variety of malignancies [13]. However, certain tumors that exhibit high PD-L1 proteins were found to respond to PD-L1 treatment with a low objective response rate (ORR), likely because the TME significantly affects how well the immune system responds to these inhibitors [14,15,16]. Just 40% of patients have clinically reacted to PD-1/PD-L1 blocking [17]. Thus, it is crucial for therapeutic purposes to find new biomarkers in patients who respond to ICIs.

Among B7 family members, B7-H3 has recently received attention because it is significantly expressed in several malignancies and predict a dismal prognosis [18,19,20,21,22]. The expression of B7-H3 on the surfaces of tumor cells stimulates the growth of tumors by allowing these cells to evade immunosurveillance [23]; Compared to normal tissues, tumor tissues have an excessive expression of B7-H3 [24, 25]. The American Joint Committee on Cancer evaluated B7-H3 expression in stage I to III primary breast cancer and normal breast specimens, results showed that 39% of initial breast cancers had B7-H3 mRNA expression, whereas normal breast tissues did not [26]. Moreover, B7-H3 was substantially linked with tumor formation and lymph node metastasis in primary breast cancers [26]. Elevated expression of B7-H3 was tied to a worse prognosis in a five-year examination of BC patients’ survival rates [27] and bad clinicopathological BC parameters [28]. According to another research, individuals with BC who have high levels of B7-H3 expression in their circulating epithelial tumor cells are more likely to develop metastases [29]. Hence, we propose that the B7-H3 immune checkpoint may be a promising target in BC immunotherapy.

B7-H3’s structure and physiological implications

B7-H3 is a dual-acting immunological checkpoint protein that is expressed on cancer cells and antigen-presenting cells (APCs) including dendritic cells and macrophages. It is effective in both soluble and membrane-associated forms [30]. The soluble form can be produced by selective splicing [31] or, more commonly, by cleavage of B7-H3 present on the surfaces of monocytes, DCs, and T cells by membrane metalloproteinases [32]. The membrane-associated form has an extracellular Ig-like structural domain, a transmembrane part, and a shorter intracellular region [33]. The number of extracellular Ig-like domains that each of the two membrane-bound B7-H3 isoforms, 2IgB7-H3, and 4IgB7-H3, contains serves to distinguish them from one another; the former contains a single IgV (variable) domain and a single IgC (constant) domain, due to exon duplication, the latter has tandemly duplicated IgV and IgC domains [34]. B7-H3 has both stimulatory and inhibitory properties to increase or decrease the activity of T cells, possibly due to its interaction with various receptors that have different functions in specific contexts. However, the B7-H3 receptor’s identification is up for debate. Certain putative receptors, including phospholipase A2 receptor 1, interleukin-20 receptor subunit α, and the trigger receptor expressed on myeloid cells-like transcript 2, have not been conclusively verified [153, 154]. FOXP3 is crucial for Treg function [155,156,157]. Treg cells that express FOXP3 are thus effective peripheral immunological tolerance mediators. B7-H3 expression and the quantity of FOXP3 + Treg cells have a strong positive connection [158], indicating that the recruitment of Treg cells may be a partial mediator of the immunosuppressive action of B7-H3.

CAFs

Many stromal variables either repress or encourage genetic epithelial alterations to impact the complex ecosystems that makeup tumors. While normal fibroblasts suppress tumor formation [159], Cancer-associated fibroblasts (CAFs) promote tumor characteristics such as ECM remodeling, inflammation, and cancer cell proliferation and invasiveness [160,161,162]. It has been reported that different CAF populations produce various cytokine patterns in malignancies [163, 164]. CAFs produce alpha-smooth muscle actin (α-SMA) [165]. The development of several malignant tumors is strongly correlated with α-SMA expression [166, 167]. Increased stromal myofibroblasts in human BC are linked to aggressive adenocarcinomas and foretell disease recurrence [168]. Some tumor subtypes have also been linked to CAF subtypes, and CAFs that are positive for these CAF-associated markers have been predominantly found in HER2 and TNBC [169]. As mentioned earlier, BC often metastasizes to bone. It has been shown that CAFs play a crucial role in develo** characteristics that enable cells in the original TME to metastasize to bone [170]. One study showed that primary tumor stroma enriched in CAFs could imitate the CXCL12-rich bone metastatic niche and promote the preselection of cancer cells that possess the potential to metastasize to bone [171].

Using an orthotopic xenograft tumor model they established in nude mice, Zhang et al. confirmed that B7-H3 + CAFs play a significant role in tumor growth and metastatic progression [172]. Another research revealed that the lack of B7-H3 reduced the release of cytokines, including interleukin (IL)-6 and vascular endothelial growth factor (VEGF), as well as the capacity of CAFs to migrate and invade [173]. In a subgroup of breast cancers, high B7-H3 expression on CAFs was shown to alter T-cell activity toward more regulatory activities [174]. Hence, more research is required into the role of B7-H3 expression in immune cell-connected fibroblasts.

The above observations, considered together, reiterate how crucial the immunological environment is for influencing clinical outcomes. Develo** more effective treatment plans for BC will undoubtedly need combination therapy that targets both tumor cells and TME.

B7-H3 as an attractive immunotherapy target

The ability to target B7-H3 via diverse effector pathways has recently been made available by developments in molecular biology and antibody design. Most of these tactics have been examined in mice and in vitro, and the testing has yielded safety and/or antitumor data, laying the foundation for clinical trials targeting B7-H3. It is regrettable that, as of now, no targeted drug has received FDA approval. Table 1 lists the current therapeutic studies being conducted to treat B7-H3.

Table 1 A list of the medications chosen for clinical trials against B7-H3 [175]
Full size table

Targeting B7-H3 with monoclonal antibodies

Strong justification exists for using B7-H3-specific inhibitory monoclonal antibodies (mAbs) in the management of solid tumors due to the substantial alterations in cancer cells brought about by silencing of B7-H3 and the remarkable therapeutic outcomes of mAbs that block checkpoint molecules. It has been shown that using mAbs to block B7-H3 activity increases CD8 + T and NK cell tumor infiltration, prevents tumor growth, and/or lengthens life [176]. A mouse IgG1 mAb targeting B7-H3, 8H9, was shown to effectively against primary brain cancers [177]. 8H9 is currently being tested in phase 1 clinical studies to treat advanced CNS malignancies and desmoplastic small round cell tumors [178]. When the Fc part of an antibody interacts with immune cells to assault targets, the process is known as antibody-dependent cellular cytotoxicity (ADCC) [179]. Enoblituzumab (MGA271), a monoclonal antibody targeting the Fc region of B7-H3 with the potential to activate killer T cells through FcR binding, has demonstrated potent Antibody-Dependent Cellular Cytotoxicity (ADCC) against various xenograft tumors. It is currently undergoing clinical trials for the treatment of resistant malignancies (NCT02982941, NCT02923180, NCT02381314, NCT04630769, NCT02475213 and NCT01391143) [180].

Targeting B7-H3 with bispecific antibodies

Nisonoff and his colleagues originally introduced the idea of a bispecific antibody (bsAb), a synthetic antibody-based molecule with two distinct antigen-binding sites, more than 60 years ago [181]. The ensuing conceptual and technical developments in the production of bsAbs evolved in tandem with groundbreaking developments in antibody design and physiology disciplines [182]. BsAbs’ ability to allow dual-targeting ideas holds significant therapeutic potential. For example, the anti-CD3 mAb scFv was combined with the anti-B7-H3 mAb scFv to create obrindatamab [183]. Obrindatamab instructs T lymphocytes to attack B7-H3 + tumor cells by attaching simultaneously to CD3 and B7-H3. Obrindatamab demonstrated an enhancement in T-cell cytotoxicity by stimulating the production of IL-2, TNF-α, and IFN-γ. This resulted in a substantial reduction in tumor development, leading to increased survival in immunodeficient animals [183]. The B7-H3-targeting bispecific antibody now undergoing clinical review, is being investigated for its potential synergy with anti-PD-1 treatment, although no results have been made public as of yet. Recently, Huang et al. created a BiTE-based mRNA therapy by encasing the mRNA that codes for B7-H3CD3 BiTE inside brand-new ionizable lipid nanoparticles (LNPs). These findings imply that treatment approaches based on B7-H3 × CD3 BiTE mRNA expression may be beneficial and have good clinical application possibilities [184].

Targeting B7-H3 through ADC therapies

Antibody–drug conjugates (ADCs), hybrid molecules designed for targeted therapy, have demonstrated considerable promise in facilitating a paradigm change in cancer therapy through antibody-antigen interactions [185]. ADCs comprise a potent cytotoxic payload, a humanized antibody that targets tumors, and a linker that connects them [186]. Antibody–drug conjugation systems are sophisticated, cutting-edge strategies that can deliver the best outcomes in BC therapy. MGC018 is a DNA-alkylating anti-B7-H3 ADC that has been studied in phase 1 dose-expansion trials and has been shown to have robust anticancer efficacy in various cancer models (NCT03729596) [187]. In a more recent clinical study, DS-7300a, an ADC that combines a humanized anti-B7-H3 antibody that contains an inhibitor of DNA topoisomerase I, has shown to be secure and reliable in the treatment [188]; the published interim results show good tolerability in patients with advanced tumors. Scientists have been immensely enthused by the DS-7300a’s early achievements, and a fresh trial testing DS-7300a’s efficiency has started (NCT05280470).

Targeting B7-H3 with CAR T cells and CAR NK cells

Two types of immune cells, CD8 + cytotoxic T and NK cells, destroy their target cells through similar cytotoxic processes. While HLA class I antigen expression is not required to detect tumor cells by Chimeric Antigen Receptor (CAR) T cells, the CAR T cells detect tumor cells quickly and with solid cytotoxicity [189]. B7-H3 CAR T cells with different B7-H3-specific scFvs exhibit potent in vitro antitumor efficacy against several tumor types [190,191,192,193]. In the case of reports, B7-H3-targeted CAR-T cells exhibited excellent tolerance in patients with relapsed basal cell carcinoma, glioblastoma, and recurrent anaplastic meningioma [194]. Combinatorial approaches that increase CAR-T cell antitumor efficacy and the vulnerability of tumor cells to the effector mechanism are being studied. Regarding cost-effectiveness, while CAR-T therapy has shown remarkable clinical outcomes, its economic implications, including manufacturing costs, accessibility, and long-term sustainability, need careful consideration.

As a crucial component of the innate immune response against malignancy, NK cells are capable of directly destroying tumors [195]. Nonetheless, it has been demonstrated that the cytotoxicity of NK cells is functionally compromised by the immunosuppressive characteristics of B7-H3 in several cancers [196]. It is possible to obtain CAR with distinct specificity for cancer immunotherapy and use it to enhance NK cell function in malignancy. Several clinical scenarios have demonstrated the superior safety of CAR-NK cell immunotherapy and shown that it has a lower risk of causing neurotoxicity and cytokine release syndrome [197, 198]. Findings from the first large-scale study using CAR-NK cells in individuals with CD19 + chronic lymphocytic leukemia and B-cell lymphoma demonstrated safety and showed encouraging clinical efficacy [199]. Tumor heterogeneity, the disappearance of the targeted antigen, and antagonistic TME are the insurmountable difficulties that CAR-NK cell therapy now confronts. Several strategies should be taken into consideration in the future to optimize the efficacy of CAR-based NK cell treatment.

Radiotherapy

Radioimmunotherapy slows tumor growth by attaching radionucleotides to tumor-targeting antibodies, producing radiation-induced cytotoxicity [199]. The carrier most often utilized in radioimmunoconjugates is omburtamab. In phase I trials, intrathecal omburtamab was well tolerated by patients treated for metastatic central nervous system neuroblastoma and intraperitoneal 131I-mAb 8H9 in desmoplastic small round cell tumors (NCT04022213) [200]. Delivering 124I-mAb 8H9 to diffuse pontine glioma through convection-enhanced brainstem caused low systemic exposure and no harm (NCT01502917) [175]. Control of radiotoxicity remains a significant obstacle that must be overcome when attempting to treat other solid tumors using radioimmunotherapy against B7-H3.

B7-H3 small-molecule inhibitors

By combining computational modeling with an in silico technique, synthetic chemical libraries can be screened to identify compounds with apparent inhibitory effects on B7-H3. These compounds provide various observable advantages; their small size and solubility allow them to readily cross membrane barriers such as the blood–brain barrier, allowing precise penetration into different tissues, including TMEs. This makes them particularly helpful for the treatment of central nervous system cancers. Compared to antibody-based or CAR therapy, the cost of producing small-molecule inhibitors is minimal, and the conditions required for their storage are less rigorous [201]. Thus, targeting B7-H3 with small-molecule inhibitors might be an appealing alternative or supplementary treatment approach.

Application of B7-H3 in tumor imaging

B7-H3 has shown promise for therapeutic use in tumor imaging in addition to being a prognostic marker and an immunotherapy target. The first line of defense in BC screening programs is mammography. The median size of lesions identified with mammography screening is 1.5 cm; however, the median size identified through clinical detection is 2.6 cm [202], and digital mammogram analysis greatly boosts screening sensitivity [203]. Unfortunately, mammograms frequently lead to overdiagnosis and pointless biopsies, and half of the women who receive multiple screenings report experiencing false-positive results [204].

It has been established that B7-H3 is a target for BC molecular ultrasound imaging. As molecular targeting contrast agents, microbubbles functionalized with B7-H3-targeted affibodies [205] or antibodies [206] have shown excellent promise. While nontargeted microbubbles produced lower imaging signals in normal mammary tissues and malignancies that block B7-H3, Strong imaging signals were obtained in tumors expressing hB7-H3 by microbubbles conjugated to the B7-H3-targeted affibody (MBABY-B7-H3) [205], proving the B7-H3’s diagnostic utility in BC imaging. Spectroscopic photoacoustic imaging is a new focused approach [207]. Using an affibody or antibody that is specific for B7-H3 and conjugated to indocyanine green, researchers can detect BC [208], assess the tumor’s grade [209], and direct the resection during surgery.

Conclusion

BC is the primary cancer-related killer of women worldwide and is regarded as a lethal malignant tumor in most countries. The threat of BC lies not only in its widespread incidence but also in its cunning ability to relapse and metastasize. The BC patient’s treatment journey is often accompanied by multiple treatment modes such as surgery, radiotherapy, chemotherapy. Given the strain on the patient’s body and the fact that conventional procedures may not always appear sufficient, new effective and gentle therapeutic approaches are especially required.

Within this context, the stable high expression of B7-H3 in a variety of cancers is of great interest to researchers, especially in BC. The close correlation between elevated expression levels of B7-H3 and an unfavorable prognosis provides compelling evidence for its potential as a promising therapeutic target. Furthermore, preclinical studies and early trials have also shown the value of B7-H3 as a serum marker for use in BC diagnosis and prognosis. Its integration into breast ultrasound imaging further underscores its potential as a non-invasive tool for early disease detection and monitoring.

Overall, while B7-H3 shows promise in BC treatment and may serve as a therapeutic target, continued research is needed to fully understand its complex receptor interactions and overcome barriers to develo** potent B7-H3 inhibitors. By overcoming these challenges, new therapeutic approaches may be developed, instilling renewed hope in BC patients worldwide.