Abstract 3808: Regulation of PD-L1+ tumor associated macrophages by collagen expressing myCAFs and luminal progenitors in endocrine-resistant breast cancer

Abstract Endocrine resistance is a major clinical obstacle that undermines the effectiveness of hormone-based therapies and drives disease progression. Around 15-20% of ER+ breast cancers are intrinsically endocrine resistant and an additional 40-50% acquire resistant during treatment, leading to metastatic progression. The five-year survival rate of these patients drops to 20-25%, while many of them progress to TNBC, a very aggressive subset of breast cancer.Recently, we demonstrated that PD-L1+ tumor-associated macrophages (TAMs) are a central mediator for endocrine resistance and are recruited by Dll1-mediated Notch signaling. However, regulation of the PD-L1+TAMs in endocrine resistance is not completely understood. Single-cell RNA sequencing (scRNA-seq) followed by cell chat communication analysis depicts that PD-L1+ TAMs are regulated by myCAFs (Myofibroblasts, a type of cancer-associated fibroblasts) and LPs (Luminal progenitors). CD61+ luminal progenitor cells are linked to tumor heterogeneity and enriched with tumorigenic potential, serving as a cell of origin for breast cancers. Mechanistic studies reveal myCAFs are enriched in collagen, which is further corroborated in endocrine-resistant breast cancer patients’ samples and ER+ mouse mammary tumors stained with trichrome and SMA staining. Closer investigation also shows that these SMA+ myCAFs are in close proximity to PD-L1+ TAMs, aiding juxtracrine signaling. Further analysis highlights that myCAFs which are high in collagen signaling, are mediating interaction with PD-L1+ TAMs through APP-CD74 and Col1a1/Col4a1/Col4a2/Col6a3 crosstalk. Detailed analysis on CD61+ LPs, on the other hand, by scRNA-seq show that LPs crosstalk to PDL1+ TAM through APP-CD74 signaling as well. APP (Amyloid precursor protein) is a transmembrane protein that is often correlated with worse clinical outcomes. Functionally, PD-L1+ TAMs augments LP activity, as seen by increasing sphere number. Detailed investigation shows that these PD-L1+TAMs are CD74+ and IFN-γ responsive. Moreover, co-culture of PD-L1+ TAMs treated with FDA approved drug Milatuzumab, a CD74 inhibitor, resulted in fewer and smaller tumor spheres, underscoring the crucial function of CD74 in mediating this crosstalk. Analysis of downstream regulators confirm that CD8+ T cells are exhausted, suggesting a strongly immune-suppressive TME. Together, we identified a complex cell signaling network between myCAF-luminal progenitor expressing APP and collagens with CD74+/PD-L1+ TAMs in endocrine resistant tumors, identifying novel actionable vulnerabilities in these breast cancer patients. Citation Format: Nirmal Das, Cheng-Han Hsieh, Rumela Chakrabarti. Regulation of PD-L1+ tumor associated macrophages by collagen expressing myCAFs and luminal progenitors in endocrine-resistant breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3808.