Cancer Discovery | B7-H3 Dual Regulation of Myeloid Cell and Fibroblast Plasticity Drives Tumor Immunosuppression

This study reveals the dual immunosuppressive mechanism of B7-H3 in the prostate cancer microenvironment, providing critical experimental evidence for designing clinical strategies combining MEK and B7-H3 targeting.

 

Literature Overview

The article titled "Dual Functions of B7-H3-MAPK Signaling Orchestrate Tumor Immunosuppression via Regulating the Plasticity of Myeloid-derived Suppressor Cells and Fibroblasts," published in the journal Cancer Discovery, systematically explores how the immune checkpoint B7-H3 regulates the senescence and subset transition of myeloid-derived suppressor cells (MDSCs) via non-canonical signaling pathways. This process subsequently influences the phenotypic remodeling of cancer-associated fibroblasts (CAFs), ultimately leading to immune evasion and therapeutic resistance in prostate cancer.

Background Knowledge

1. The key challenge addressed by this study in castration-resistant prostate cancer (CRPC) is the limited efficacy of single-agent immune checkpoint inhibitors and the complex immunosuppressive network within the tumor microenvironment, particularly the often-overlooked synergistic interaction between MDSCs and CAFs. 2. Current bottlenecks in B7-H3 research stem from insufficient understanding of its downstream signaling pathways and cell-cell interaction mechanisms, resulting in suboptimal monotherapy outcomes and difficulty in overcoming acquired resistance. 3. The research entry point leverages single-cell transcriptomics and genetically engineered mouse models to elucidate the mechanism by which B7-H3 induces senescence in monocyte-derived MDSCs and secretes chemokines to recruit granulocyte-derived MDSCs. Furthermore, it investigates how B7-H3 deficiency leads to compensatory activation of the EGFR/MEK/ERK pathway via SPINK1, promoting extracellular matrix (ECM) remodeling.

 

 

Research Methods and Core Experiments

The authors constructed a genetically engineered mouse model with concurrent deletion of Pten and Trp53, alongside conditional knockout of Cd276 (B7-H3). Combined with the DX1 syngeneic transplant model, single-cell RNA sequencing (scRNA-seq) and multiplex immunohistochemistry (mIHC) were utilized to comprehensively analyze the impact of B7-H3 deficiency on the cellular composition of the tumor microenvironment. Key evidence demonstrates that B7-H3 directly binds to monocyte-derived MDSCs and activates the MAPK signaling pathway, inducing their entry into a senescent state and secretion of CXCL2/3, which subsequently recruits granulocyte-derived MDSCs.

In terms of mechanistic validation, the study confirmed via in vitro culture of THP-1 cells and human primary monocytes that B7-H3 protein induces MEK/ERK phosphorylation and p21 expression in a dose-dependent manner. Simultaneously, using SPINK1 neutralizing antibodies and a MEK inhibitor (Trametinib), the researchers blocked the conversion of CAFs to an ECM-remodeling phenotype caused by B7-H3 deficiency in both in vitro and in vivo experiments, proving the compensatory activation mechanism of the SPINK1-EGFR-MEK-ERK axis.

In preclinical efficacy evaluation, the team tested a combination regimen of B7-H3 monoclonal antibody and a MEK inhibitor in a castration-resistant prostate cancer model. Results showed that combination therapy not only significantly inhibited MDSC infiltration but also effectively reduced collagen deposition and restored the cytotoxic function of CD8+ T cells, demonstrating the synergistic anti-tumor effect of the dual-blocking strategy.

Key Conclusions and Perspectives

• B7-H3 directly induces senescence in monocyte-derived MDSCs by activating the MEK/ERK/STAT1 signaling pathway, rather than directly inhibiting their proliferation. This finding reveals a novel mechanism by which immune checkpoints regulate myeloid cells.
• Senescent monocyte-derived MDSCs recruit granulocyte-derived MDSCs by secreting CXCL2 and CXCL3, forming a dual immunosuppressive barrier. This suggests that targeting CXCR2 could serve as an adjunctive therapeutic strategy.
• Deficiency or inhibition of B7-H3 leads to increased SPINK1 secretion by MDSCs, which subsequently activates the EGFR/MEK/ERK pathway in CAFs, promoting ECM remodeling and fibrosis. This represents a critical compensatory mechanism responsible for resistance to B7-H3 monotherapy.
• Combining B7-H3 monoclonal antibodies with MEK inhibitors (e.g., Trametinib) effectively blocks the aforementioned dual immunosuppressive mechanisms, demonstrating sustained anti-tumor activity in castration-resistant prostate cancer models, thereby providing theoretical support for clinical combination therapies.

Research Significance and Prospects

This discovery holds significant implications for drug development, indicating that immunotherapies targeting B7-H3 must be combined with agents targeting the MAPK pathway or ECM remodeling factors to overcome adaptive resistance within the tumor microenvironment.

In terms of clinical monitoring, the study suggests that SPINK1 and ECM remodeling markers could serve as biomarkers for assessing B7-H3 treatment response and resistance risk, aiding in the selection of patient populations most likely to benefit from combination therapy.

Regarding disease modeling, this study emphasizes the necessity of fully considering the dynamic interactions between myeloid cells and fibroblasts when constructing prostate cancer models, as genetic modification of a single cell type may not fully replicate the evolution of the authentic tumor microenvironment.

 

 

Conclusion

This study provides an in-depth analysis of the dual regulatory network of B7-H3 in the prostate cancer microenvironment, revealing its complex mechanism of inducing senescence in monocyte-derived MDSCs and promoting ECM remodeling via activation of the SPINK1-EGFR-MEK-ERK axis. This discovery not only elucidates the molecular basis for the limited efficacy of single-agent B7-H3 targeting but also offers a new breakthrough for treatment strategies in castration-resistant prostate cancer. Translating from the laboratory to the clinic, this research underscores the necessity of combined targeting of immune checkpoints and stromal remodeling pathways, laying a solid foundation for optimizing immunotherapy regimens, overcoming resistance, and improving patient prognosis. In the future, combination therapies based on this mechanism are expected to become a key approach for addressing refractory prostate cancer.

 

Wei Shi, Ping Xu, Qianlin Gu, Nicholas E Navin, and Di Zhao. Dual Functions of B7-H3-MAPK Signaling Orchestrate Tumor Immunosuppression via Regulating the Plasticity of Myeloid-derived Suppressor Cells and Fibroblasts. Cancer discovery.