Abstract PS3-12-24: Immune Landscape in Tumor Draining Lymph Nodes after Neoadjuvant Chemoimmunotherapy in Early Triple Negative Breast Cancer

Abstract Background: The integration of immune checkpoint inhibitors (ICIs) with neoadjuvant chemotherapy has significantly improved clinical outcomes for patients with early triple-negative breast cancer (TNBC). However, approximately one-third of patients still fail to achieve a major pathological response, facing a substantially increased risk of recurrence and death. While immune correlates within the tumor microenvironment and peripheral blood have been extensively studied, the immunological dynamics within tumor-draining lymph nodes (TDLNs) - key sites for the initiation and coordination of anti-tumor immunity - remain poorly understood. Here we present our ongoing investigation into the immune architecture of TDLNs following chemoimmunotherapy to uncover predictive signatures of response and mechanisms of resistance. Methods: Sentinel lymph nodes from early TNBC patients undergoing neoadjuvant chemoimmunotherapy are analyzed using the PhenoCycler multiplex imaging platform, deploying a comprehensive 50-marker panel. This approach enables deep spatial and phenotypic profiling of adaptive immune response, including T-cell priming pathways. We quantify immune cell density, diversity, and spatial organization, with a focus on cellular neighborhood analysis. In addition, we employ an innovative tracking method to identify anti-PD1-bound cells, providing direct insights into the pharmacodynamic effects of PD-1 blockade within TLDNs. Results: To date, we have analyzed TDLNs from four stage II TNBC patients - two with complete pathologic response (pCR, residual cancer burden (RCB)=0) and two with residual disease (non-pCR, RCB=1). Preliminary results reveal a trend toward higher frequency of CD8+ (2.7% vs. 1.4% of CD45+ cells) and CD4+ T cells (19.9% vs. 8.5% of CD45+ cells) within the paracortical regions of pCR patients. Furthermore, the composition of CD8+ T cells differed, with increased proportions of precursor-exhausted (Tpex) (16.0% vs. 2.7% of CD8+ cells), and terminally exhausted (Tex) (10.2% vs. 1.1% of CD8+ cells) subsets observed in pCR cases. Ongoing analyses include mapping of key immune populations, such as plasmacytoid dendritic cells (pDC), conventional dendritic cells (cDC1 and cDC2), Th1, Th2, and T follicular helper (Tfh) cells, allowing us to compute cell-to-cell distances and generate individualized neighborhood profiles to identify spatial signatures associated with treatment response. Conclusion: Our high dimensionality spatially resolved analysis provides a powerful window into the immunological landscape of TDLNs in TNBC. By preserving tissue architecture and capturing cellular interactions, we aim to unravel how TDLNs contribute to effective anti-tumor immunity or harbor resistance in the context of chemoimmunotherapy. Preliminary findings suggest distinct immune patterns in patients achieving pCR vs. those with residual disease. A larger cohort, including comprehensive spatial and activation marker analysis, will be presented at the conference. Citation Format: A. Oseledchyk, P. Caloba, S. Uzun, K. Schaeuble, S. Münst, W. P. Weber, C. Kurzeder, M. Vetter, H. Läubli, M. Binder, A. Zippelius. Immune Landscape in Tumor Draining Lymph Nodes after Neoadjuvant Chemoimmunotherapy in Early Triple Negative Breast Cancer 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 PS3-12-24.