A previously unidentified immune-suppression pathway has been identified in castration-resistant prostate cancer, revealing how tumors modify the surrounding microenvironment to evade immune destruction and resist treatment. Combined inhibition of B7-H3 and MEK signaling demonstrated improved tumor control and prolonged survival in preclinical models, providing a potential therapeutic strategy for advanced prostate cancer.
Castration-resistant prostate cancer represents an advanced stage in which disease progression continues despite androgen-deprivation therapy. Treatment remains challenging because of resistance to conventional hormonal therapies and limited responses to several immunotherapeutic approaches. Suppression of antitumor immunity within the tumor microenvironment represents an important mechanism contributing to treatment resistance.
The tumor microenvironment contains myeloid-derived suppressor cells (MDSCs), which inhibit immune responses and prevent effective tumor elimination. Cancer-associated fibroblasts also contribute to treatment resistance by producing dense extracellular structures that restrict immune-cell infiltration and impede drug penetration. Interactions between these immune and stromal components create a protective environment that supports tumor progression. B7-H3, an immune checkpoint protein frequently expressed in several malignancies, has emerged as a potential therapeutic target. However, limited effectiveness following B7-H3 inhibition alone suggests the presence of compensatory mechanisms that preserve tumor protection.
Investigations using genetically engineered preclinical models of castration-resistant prostate cancer, single-cell sequencing and immune-profiling technologies revealed a previously unrecognized interaction between B7-H3 and monocytic MDSCs. This interaction activated a signaling pathway that induced cellular senescence, producing an aged-like state associated with the release of inflammatory molecules. The resulting inflammatory signals altered the surrounding microenvironment and promoted the recruitment of additional immunosuppressive cells. Immune activity against malignant cells became increasingly restricted, supporting tumor survival and progression.
B7-H3 inhibition reduced immunosuppressive MDSC populations and slowed tumor progression. However, compensatory changes subsequently increased the production of a protein capable of stimulating cancer-associated fibroblasts. This response promoted structural remodeling and strengthened the physical barrier surrounding the tumor, thereby limiting immune-cell access. To overcome this adaptive resistance mechanism, combined treatment with B7-H3-targeted therapy and trametinib was investigated. Trametinib, an approved MEK inhibitor, targets the signaling pathway implicated in the compensatory response.
The combination produced greater suppression of tumor growth and prolonged survival compared with either treatment administered individually. Further analyses demonstrated reductions in immunosuppressive MDSCs, diminished fibroblast-associated tissue remodeling and increased infiltration of T cells into the tumor microenvironment. These findings indicate that simultaneous targeting of immune suppression and stromal remodeling may provide a means of overcoming multiple protective mechanisms operating within advanced prostate tumors. The results also suggest that compensatory changes following immune checkpoint inhibition may contribute to treatment resistance, highlighting the importance of addressing interactions between immune and structural components of the tumor microenvironment.
Despite the encouraging findings, the therapeutic approach remains at the preclinical stage. Further investigations are required to establish its safety, effectiveness, optimal dosing and suitability for clinical application. Validation in additional experimental models and subsequent clinical studies will be necessary before potential benefits can be established in patients with advanced prostate cancer.
Further research into B7-H3-associated signaling and interactions between immune and stromal cells may also clarify the relevance of this pathway across other malignancies. The findings provide a foundation for developing combination-based therapeutic strategies aimed at overcoming tumor-mediated immune suppression and improving treatment responses.
References
- Shi W, Xu P, Gu Q, Fan Z, Liang X, Wang Y, et al. Dual functions of B7-H3-MAPK signaling orchestrate tumor immunosuppression via regulating the plasticity of myeloid-derived suppressor cells and fibroblasts. Cancer Discov. 2026. doi:10.1158/2159-8290.CD-25-1613.