According to the 2020 World Cancer Report, prostate cancer (PCa) ranks the sixth in the incidence of male cancers [1]. PCa is becoming a common reproductive system disease that affects the health of middle-aged and elderly men in China [2]. B7-H3 is a new member of the B7 family of costimulatory proteins, which is an important component of the tumor microenvironment. The expression of B7-H3 is low in normal tissue cells, but is high in a variety of malignant solid tumors such as PCa [3,4,5,6]. B7H3 induces immune evasion in tumor cells, immune cells, and tumor-associated vasculature, thereby increasing the survival, proliferation, metastasis, and drug resistance of cancer czhaoells [7, 8]. In this review, we summarize current understanding of molecular mechanism of B7-H3 in PCa and discuss the potential of B7-H3 as a novel therapeutic target for PCa.

1.1 Structure and function of B7-H3

B7-H3 is B7 homolog 3, also known as CD276, and belongs to the B7-CD28 family. The extracellular domain of B7-H3 molecule is similar to that of another B7 family member programmed death receptor 1 (PD-L1). Both of them can act as immune checkpoint molecules on T cells, which express costimulatory/coinhibitory molecules that bind to their receptors and regulate T cell activation and tolerance [3].

B7-H3 is a type I transmembrane protein composed of an extracellular domain, a transmembrane domain, and a short intracellular domain, and is expressed on antigen presenting cells (APC), which is involved in cell stable growth, motility, signal transduction and adhesion, and tumor-related immune response [9]. The human B7-H3 protein has two isoforms: 4IgB7-H3 and 2IgB7-H3. 4IgB7-H3 consists of two pairs of identical IgV-like and IgC-like domains, and 2IgB7-H3 consists of a pair of identical IgV-like and IgC-like domains (Fig. 1). 4IgB7-H3 is more common in human cells and has been shown to have a background dependent, pleiotropic, immunomodulatory function in autoimmunity, transplantation, and malignancy [10,11,12,13].

Fig. 1

Domain structure of B7-H3 isoforms 4IgB7-H3 and 2IgB7-H3

B7-H3 is expressed on Dendritic cells (DCs), and the anti-tumor mechanisms of DCs have been studied in many ways, including: DC can initiate CD4 + response and anti-tumor immune response through high expression of major histocompatibility complex(MHC) molecules and co-stimulatory molecules; DC can up-regulate the expression of cytokines, increase the activity of Cytotoxic T lymphocyte (CTL), and enhance the toxicity of natural killer cells (NK); DC secrete Chemotactic Cytokines (CCK) to enhance the stimulation of T cells and then promote DC activation; Affect tumor blood vessels by anti-angiogenic substances; Directly presents antigens to CD8 + T cells to enhance immune responses. These immune functions may be directly related to the anti-tumor immunity of B7-H3 [10].

The tumor microenvironment (TME) is complex and constantly evolving. Malignant tumor cells can functionally shape their TME by secreting various cytokines, chemokines, and other factors. B7-H3 can affect tumor cells through co-stimulation, co-inhibition or other immune pathways in the TME. In addition, B7-H3 also has non-immunogenic effects, promoting tumor cell migration, invasion, angiogenesis, drug resistance and epithelial-mesenchymal transition (EMT) to regulate cell metabolism [14]. These results indicate that B7-H3 is a potential target for tumor therapy, and understanding the mechanism of B7-H3 in PCa can provide ideas for better control of PCa [15].

1.2 Expression and significance of B7-H3 in PCa

The expression of B7-H3 is very low in normal tissues, but B7-H3 is widely expressed in malignant solid tumors, such as the lung, ovary, glioblastomas, colon, rectum, stomach, prostate, urothelium, brain, pancreas, breast cancer, bile duct, liver cells, oral cavity and kidney cancer cells [16]. B7-H3 can be induced to be expressed on APC, and then induce immune evasion through the effects on tumor-infiltrating immune cells, cancer cells, cancer-related vasculature and stroma, thereby increasing the survival, proliferation, invasion and drug resistance of cancer cells [14, 17]. Previous studies have shown that B7-H3 is overexpressed by PCa cells, and high expression of B7-H3 is significantly associated with more aggressive phenotype and worse prognosis [18].

1.3 Mechanism of B7-H3 in PCa immunity

Due to the dynamic nature of immune cell types, the impact of the immune system on cancer cells is complex. Below we describe the role of immune checkpoints, non-immunogenic functions and other functions of B7H3 in PCa (Fig. 2).

Fig. 2

Mechanisms of action of B7-H3 in prostate cancer. B7-H3 plays both immune and non-immune related role in prostate cancer and regulates a variety of cellular activities of prostate cancer cells. B7H3 regulates signaling pathways such as PI3K/AKT, NF-κB, Ras/Raf/MEK/MAPK, JAK2/STAT3 and others by increasing or decreasing CD4 + , CD8 + T cells or cytokines including IFN-γ, IL-8 and TNF-α, therefore promoting the migration and invasion, angiogenesis, and epithelial-mesenchymal transition of prostate cancer cells

First, neoantigens produced by prostate tumors are captured by APCs such as macrophages or dendritic cells. The cytotoxic T lymphocytes were then identified in combination with MHC I, MHC II and costimulatory molecules to stimulate T cells. This is followed by the infiltration of effector T cells to recognize and kill tumor cells. Nevertheless, tumor cells develop a variety of mechanisms to escape the killing by T cells, and B7-H3 is involved in immune evasion of PCa cells [18]. One of the possible mechanisms is that the tumor may lack surface antigens or costimulatory /MHC molecules that could induce CTL. Tumor cells could express immunosuppressive cytokines such as transforming growth factor beta (TGF-β), or other programmed cell death ligands. These molecules interact with T cells to attenuate their immune response and facilitate immune evasion [19].

Moreover, regulatory myeloid-derived suppressor cells (MDSCs) in tumor microenvironment could suppress innate and T cell-mediated anti-tumor immune response. Tumor-infiltrating lymphocytes (TILs) could upregulate CTLA-4 to compete with the costimulatory molecule CD-28 for binding B7 on APC and reduce T cell immune function.

In the presence of MHC, the antigens bind to T cell receptors (TCRs) to initiate signaling cascades, but the activation of T cells depends on costimulatory molecule CD28, and other interactions between ligands and activating or inhibitory receptors are critical for further regulation of T cell activation and tolerance. In addition, B7-H3 can promote the binding of MHC I molecules to inhibitory killer cell (NK) receptors, thereby counteracting the activation signal and causing NK cell dysfunction. Therefore, cancer cells can escape the adaptive immune response mediated by NK cells [20].

On the other hand, intra-tumor co-stimulation effect of B7-H3 has been demonstrated [21]. B7-H3 was initially identified as a costimulatory molecule that activates T cells and increases the secretion of cytokines such as interferon-γ (IFN-γ). However, recent studies reported the co-inhibitory effect of B7-H3 on T cells, demonstrating its role in reducing IFN-γ secretion and promoting tumor cell immune escape [22]. It is suggested that the mechanism of B7-H3 in immune response is really questionable, and may be determined by the structure of B7-H3 domain.

1.4 Mechanism of costimulatory action of B7-H3

The first identified function of B7-H3 is costimulatory action and is thought to be a co-stimulatory molecule of T cells that promotes cytokine production. A recent study has also shown that blocking activated T cells with bispecific anti-CD3x, anti-B7-H3 antibody can enhance the cytotoxicity of T cells and greatly increase the production of interferon-γ and other cytokines [23]. In the presence of anti-CD3 antibody mimicking TCR signaling, human B7-H3-Ig fusion protein increased the proliferation of CD4 + and CD8 + T cells and enhanced CTL activity. B7-H3 selectively stimulates the secretion of IFN-γ, IL-8 and TNF-α, and enhances the cytotoxic effect of CD8 + T cells [24].

1.5 Mechanism of co-inhibitory action of B7-H3

However, more recent studies have shown that B7-H3 mainly plays a co-inhibitory role [25]. Co-inhibition of B7-H3 overexpression in prostate cancer is associated with adverse disease outcomes [3]. The co-inhibitory mechanism of B7-H3 is used as an immune escape pathway by prostate cancer. B7-H3 can inhibit the activation and expansion of CD4 + and CD8 + T cells by inhibiting the secretion of cytokines such as interferon-γ, IL-2, IL-4, IL-10 and IL-13.

1.6 Other immune functions of B7-H3

In the regulation of tumor immunity, B7-H3 can regulate T cells and NK cells to reduce their toxicity, but the receptor bound by NK cells is still not clear. In addition, B7-H3 can regulate osteoblasts, and plays an important role in bone immunity and bone growth [21].

1.7 B7-H3 has non-immunogenic function in PCa

In addition to potential immunological effects on the tumor environment, B7-H3 plays non-immune-mediated roles in tumor progression, for example by inducing EMT or promoting the Warburg effect [26]. B7H3 may induce EMT of tumor cells by resisting NK cell killing activity [27]. At the same time, B7-H3 can regulate glucose metabolism through ROS-mediated HIF1a stabilization, which help enhance the Warburg effect and promote tumor growth [28].B7-H3 also promotes migration and invasion, angiogenesis, chemoresistance and metabolism of tumor cells, which has attracted more attention recently.

B3-H7 can affect multiple signaling pathways in tumor cells, including PI3K/AKT, NF-κ B, Ras/Raf/MEK/MAPK, JAK2/STAT3 signaling pathways, and can also regulate glucose metabolism pathway. Moreover, B3-H7 plays an important role in cell migration, invasion, angiogenesis and EMT by downregulating the expression of MMP-2, MMP-9 and VEGF. Upon activation of EMT, tumor cells lose epithelial-specific E-cadherin or have reduced expression levels, whereas N-cadherin and vimentin expression levels increase as cells acquire mesenchymal activity. Loss of B7-H3 would result in a significant inhibition of vimentin and N-cadherin expression. Therefore, B7H3 may facilitate the activation of EMT [29, 30].

1.8 B7-H3 as target of immunotherapy for PCa

Tumor immunotherapy includes inhibition of immune checkpoints and improvement of tumor immune microenvironment, which is an effective means to treat tumors. More effective and alternative targets of inhibition are essential for the success of immune checkpoint blockade therapy. Interactions of the B7 family of immunomodulatory ligands with their corresponding receptors induce or inhibit T cell responses by sending costimulatory and coinhibitory signals, respectively. As an immune checkpoint molecule, the strong expression of B7-H3 in many cancer types makes it a promising target for immunotherapy. Anti-B7-H3 approaches currently in preclinical and clinical trials include B7-H3 antibodies (such as 8H9) and antibody–drug conjugates (DX-8951 derivative, DXd), CAR T-cell therapy, and combination therapies [31]. The details of some of these trials have been summarized in recent reviews [32,33,34].

According to clinicaltrials.gov, at present there are 101 clinical projects of B7-H3 under research worldwide, of which 14 are related to prostate cancer, mainly in the United States, as shown in Table 1. Therapies that overcome the co-inhibition of T cells with antagonistic monoclonal antibodies are emerging as effective tumor immunotherapies. Therapies that block B7-H3 should be developed, either as monotherapy or in synergy with conventional therapies. B7-H3 targeting is being tested in a variety of cancer therapeutic strategies, mainly based on anti-B7-H3 monoclonal antibodies or derivatives. These treatment options include (I) use of blocking mAb, (II) mAb dependent cytotoxicity (ADCC), (III) mAb drug conjugate, (IV) mAb targeted radiotherapy, (V) bisspecific mAb, and (VI) chimeric antigen receptor (CAR) T cells and NK cells.

Table 1 Summary of clinical trials of B7-H3 and prostate cancer1.9 Use monoclonal antibodies to block the binding of B7-H3 to the receptor

Using monoclonal antibody blocking B7—H3 is an ideal way to block the cytotoxicity. However, the receptor for B7-H3 has not yet been identified [35]. Receptor uncertainty may be related to its complex immunomodulatory activity, or it may be due to the structure of B7-H3 consisted of one or two pairs of identical domains. Although TLT-2 is currently reported to be a potential receptor for B7-H3, its usability is questionable since TLT-2 may not be the sole receptor of B7-H3.

1.10 B7-H3 monoclonal antibody ADCC

The fully human monoclonal antibody Enoblituzumab is the first monoclonal antibody to be tested against B7-H3 in the treatment of tumors. Enoblituzumab is a humanized Fc optimized B7-H3 targeting antibody that induces antibody-dependent cytotoxicity (ADCC). Treatment with this single agent is currently in phase II clinical studies showing antitumor activity in both localized moderate and high-risk PCa. Phase I studies of anti-B7-H3 bispecific antibody in PCa patients enhanced T cell activation and proliferation and cytokine and mediator (granzyme/perforin) production, thereby eliminating tumor cells and making B7-H3 a potential target for PCa immunotherapy.

1.11 Antibody-coupled drug (ADC)

B7-H3 antibodies bind to specific radioactive elements or toxic substances and release them when they come into contact with tumor antigens, killing them. For example, MacroGenics' MGC-018 (humanized B7-H3 monoclonal antibody duocarmycin) delivers duocarmycin to tumors. ADCs consisted of mAbs allow direct delivery of potent cytotoxic drugs to target cells to improve therapeutic efficacy. Deruxtecan (DXd) is a HER2-targeted ADC consisted of tetrapeptide based adaptor and topoisomerase 1 (TOP1) inhibitor.

1.12 CAR-T cell therapy

CAR-T cells are another approach to overcome immunosuppressive mechanisms and they are consisted of three components: an extracellular component for the recognition of the antigens, a transmembrane component, and an intracellular component for signal transduction [36]. B7-H3 CAR-T/NK cells have potent in vitro antitumor activity against different tumor types. Currently, several studies have employed humanized scFv to construct the second-generation CAR targeting B7-H3 and confirmed the efficacy of B7-H3 CAR-T therapy both in vitro and in vivo, suggesting that B7-H3-targeted CAR-T therapy might be a novel immunotherapy strategy for PCa [37].

Epitopes that bind antibodies to tumor targets are of great significance for the activity of CAR-T in CAR-T cell therapy. Nevertheless, it remains ambiguous which epitopes are the most efficacious against tumor targets. A research group isolated nanoantibodies capable of recognizing all major domains on B7-H3 by using a large dromedary VHH phage library, identifying a previously undescribed epitope that enables CAR-T cells to display extraordinary activity against large tumors [38]. This provides a new direction for B7-H3 CAR-T therapy.

1.13 Radioimmunotherapy

B7-H3 specific monoclonal antibodies have been used as carriers for selective targeted delivery of radioisotopes to tumor sites. Radiotherapy (RT) is currently the main method for the treatment of prostate cancer. The damage of radiation is accomplished by ionizing reactions in vivo and initiating biochemical reactions, which first occur in cells and nuclei, and then cause necrosis and apoptosis. The cellular response to radiation is divided into physical, chemical and biological reaction phases. Radiation induced reactive oxygen species (ROS) could cause chemical changes of DNA such as base disruption, single and double strand breaks (DSBS), or cross-linking. Despite cell repair mechanisms, complex DSBS cannot be corrected, leading to cell death and increased recognibility of tumor antigens [39].

The combination of RT and immune checkpoint therapy (ICT) could be explored in future studies to enhance the advantages of RT, namely enhancing immune activation and suppression effects.

1.14 HHLA2 and B7 scores

HERV-H LTR (B7-H7/B7-H5) related 2 molecule (HHLA2) is a recently identified member of B7 family. B7-H3 and HHLA2 have extremely similar immune function, and most of the reports show a significant association of B7-H3 and HHLA2 [40]. B7-H3 and HHLA2 molecules were reported to be expressed in 31% of PCa cases and 18% of PCa cases, respectively, indicating that they are rarely co-overexpressed. However, high B7 scores were associated with poor clinical symptoms and prognosis of PCa [40]. Therefore, B7-H3 combined with HHLA2 is better for the prediction PCa prognosis. Among them, overexpression of B7-H3 or HHLA2 may inhibit T cell activation and proliferation through a non-redundant/exclusive inhibitory pathway.

It has been demonstrated that extensive expression of B7-H3 and HHLA2 is significantly correlated with CD8 and TILs [41]. Both B7-H3 and HHLA2 are involved in dual stimulatory and inhibitory immune microenvironment in PCa. Therefore, we can only make a preliminary speculation that B7-H3 and HHLA2 play the paradoxical dual functions of co-stimulation and co-inhibition in heterogeneous malignant tumors, which may be caused by the differential expression of the corresponding activating or inhibiting ligands on immune cells or the differential infiltration of immune cells. The correlation and mechanism remain to be explored.

With the development of precision medicine, many immune classifications have been proposed to guide immunotherapy. However, this immunophenotype does not apply to certain "cold" cancers, especially PCa, where PD-L1 expression is as low as none. To optimize this immunophenotype, PD-L1 expression was replaced with a new B7 score, and a combination of B7 score and CD8 + TIL status was used to predict the prognosis of PCa patients. There was a significant association between stratified CD8 + TIL based on B7 score and OS and CSS [40]. Patients with high B7 scores and low CD8 + TIL infiltration could be less sensitive to clinically available immunotherapies and have the worst prognosis, while patients with low B7 scores and high CD8 + TIL infiltration may not need chemoradiotherapy because of favorable prognosis. The association of B7-H3 with PCa remains to be investigated in order to determine the availability of B7-H3 blockers in PCa therapy. Further studies may provide new information for diagnosis and treatment options.