Abstract PD13-02: Cytomegalovirus reactivation drives brain metastasis in breast cancer via immunosuppressive microglial reprogramming

Abstract Background: Brain metastases (BM) are a major cause of mortality in metastatic breast cancer, often arising in advanced disease and carrying a dismal prognosis. While current research emphasizes tumor-intrinsic drivers, the role of latent viral infections has been largely overlooked. Notably, cytomegalovirus (CMV) reactivation has been detected in 90% of BM lesions from breast cancer patients, implicating it as a potential but underexplored contributor to metastasis. Methods: We developed a CMV-infected breast cancer BM mouse model and evaluated the therapeutic efficacy of the anti-CMV drug foscarnet—alone and in combination with capecitabine—in survival studies. Mechanistically, we performed single-cell spatial transcriptomics and multiome (ATAC + RNA) profiling on brain tissues from three groups: non-infected controls, CMV-infected vehicle-treated, and CMV-infected foscarnet-treated mice. Clinical relevance was further assessed through survival analysis and immune profiling of brain metastases from 20 breast cancer patients, stratified by CMV reactivation status. Results: CMV infection significantly reduced survival in vehicle-treated mice compared to non-infected controls (P = 0.0063). Foscarnet monotherapy significantly prolonged survival (P = 0.026), and the combination of foscarnet and capecitabine yielded an even greater survival benefit (P 0.0001). CMV-infected, vehicle-treated mice exhibited a significant peritumoral enrichment of CXCR3+C1q+ microglia, a population reduced in both non-infected and foscarnet-treated groups. Transcriptomic and in vitro analysis revealed that CMV infection reprograms microglial into an M2-like, immunosuppressive phenotype, characterized by upregulation of TGF-β, ARG1, IL-10, PPARG, and CHIL3. Additionally, CMV-infected 4T1 tumor cells expressed high levels of CXCL9 (P=4×10-10), a ligand for CXCR3, and CCL2 (P=6 ×10-6), a key inducer and recruiter of M2-microglial polarization. Breast cancer BM patients with high CMV-reactivation showed remarkably shorter survival in comparing to those patients without or low CMV-reactivation (P=0.005) Conclusion: Our findings reveal that CMV reactivation promotes brain metastasis by reprogramming microglia into an M2-like, immunosuppressive state. Infected tumor cells further amplify this effect by recruiting and sustaining immunosuppressive microglial populations, fostering a pro-metastatic brain microenvironment. Although anti-CMV therapy is not currently part of the standard-of-care for breast cancer brain metastases, our data—supported by emerging clinical evidence—identify CMV as a key, underrecognized driver of disease progression and therapeutic resistance. To translate these findings into clinical impact, we have initiated a Phase II trial at our cancer center to assess the benefit of adding anti-CMV therapy to standard treatment in CMV-reactivated BM patients. Targeting this viral-immunologic axis may represent a paradigm shift in the management of brain metastases. Citation Format: W. DongS. WangA. PuriM. VasquezJ. ChangH. ZHAOS. Wong. Cytomegalovirus reactivation drives brain metastasis in breast cancer via immunosuppressive microglial reprogramming abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PD13-02.