Abstract PS2-09-12: Splenomegaly and response to chemo-immunotherapy in Triple-Negative Breast Cancer: Findings from MMTV-R26Met mouse model and patients

Abstract Background: Triple-negative breast cancer (TNBC) is marked by pronounced heterogeneity in immune microenvironments and variable response to immunotherapies. Preclinical models often fail to capture this diversity, limiting translational progress and biomarker discovery for patient stratification. Method: We developed the immunocompetent MMTV-R26 Met mouse model, which mimics key TNBC features. Multi-omics characterization (histology, proteomics, genomics, immunophenotyping, scRNA-seq) enabled direct comparison with human TNBC. We generated syngeneic orthotopic transplants that preserved tumor-immune heterogeneity across passages. Treatment studies evaluated the effects of epirubicin, anti-PD-1 and combination therapy. Clinical validation involved 125 stage II-III TNBC patients receiving neoadjuvant pembrolizumab plus chemotherapy, with comprehensive clinical, pathological, and imaging data collection. Predictive models used machine learning approaches (logistic regression). Results: Our multi-omics analysis revealed strong similarities between the MMTV-R26 Met mouse model and TNBC patients, such as high-grade tumors, inter-tumoral diversity, and distinct molecular subtypes. Additionally, immune profiling revealed four distinct immune subtypes (depicting enrichment in macrophages, neutrophils, dendritic cells or T lymphocytes). Interestingly, the neutrophil-enriched subtype exhibited a marked splenomegaly which remained through successive transplants and correlated with systemic immune activation. Moreover, our in vivo experiments revealed that treatment responses varied according to the immune profile of the tumor. Neutrophil-enriched tumors exhibited dramatic tumor regression upon epirubicin+anti-PD-1 treatment, accompanied by a reduction in spleen size and enhanced cytotoxic T cell infiltration (n=17). In contrast, macrophage-enriched tumors displayed limited immune remodeling and responded preferentially to anti-PD-1 monotherapy with persistent immunosuppressive features (n=11). In univariate analysis using a ROC-derived cutoff, patients with a larger spleen size had nearly fourfold higher odds of achieving pathological complete response (pCR) to chemo-immunotherapy (OR = 3.99, 95% CI: 1.76-9.80, p = 0.0015) mirroring the murine outcomes. Ki-67 (OR=1.03, 95% CI :1.01-1.04, p=0.004), Breast SUV max (OR=1.13, 95% CI :1.01-1.27, p=0.045) and family history (OR=0.43, 95%CI : 0.20-0.91, p=0.028) emerged as top associated markers with pCR in univariate analysis. Importantly, in multivariate logistic regression adjusting for key clinical and biological covariates, spleen vertical size remained an independent predictor of pCR (OR = 1.06, 95% CI: 1.01-1.13, p = 0.033). The final multivariate model demonstrated excellent discriminative power, achieving an area under the ROC curve (AUC) of 0.91 These findings highlight spleen size as a clinically relevant and independent predictor of response to chemo-immunotherapy in triple-negative breast cancer. Conclusion: The MMTV-R26 Met mouse model recapitulates the immune heterogeneity of human TNBC, identifying splenomegaly as a hallmark of neutrophil-enriched tumors with heightened sensitivity to chemo-immunotherapy. This phenotype translates to improved response in patients with larger spleens, establishing spleen size as a novel, actionable biomarker for immunotherapy benefit. The MMTV-R26 Met mouse model provides a robust platform for personalizing TNBC treatment and advancing biomarker-driven clinical strategies. Citation Format: J. MONATTE, A. TASSIN DE NONNEVILLE, N. CORVAISIER, O. CASTELLANET, R. FERRARA, P. MICHEA, J. P. BORG, F. MAINA, A. GONCALVES, F. LAMBALLE. Splenomegaly and response to chemo-immunotherapy in Triple-Negative Breast Cancer: Findings from MMTV-R26Met mouse model and patients 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 PS2-09-12.