Abstract PO4-16-04: Decoding the immune landscape of breast cancer (BC) during pregnancy (PrBC): Impact of hormone receptors (HR) and tumor-infiltrating lymphocytes (TILs) phenotype on gene expression signatures

Abstract Introduction: PrBC is an uncommon malignancy with aggressive behavior. Its pathogenesis involves distinct immune mechanisms associated with maternal-fetal tolerance and tumor-host immunoediting. PrBC displays specific patterns of TILs with increased CD8+ cells. Gaining a comprehensive understanding of the molecular processes underlying this immune synergy is crucial for enhancing PrBC patients’ clinical management. Here, we sought to identify dysregulated immune-related genes in PrBC and explore their association with HR status and TILs. Methods: A total of n=75 PrBC (age range 26-43 years) and n=67 age-matched early-onset breast cancer (EOBC) in non-pregnant women (controls; age range 28-43 years) were selected from our Institutional registry. For all cases TILs were quantified according to the International TILs Working Group recommendations and profiled by IHC for CD4 and CD8. RNA was extracted from representative FFPE tissue blocks to perform the expression analysis of 395 genes involved in tumor-immune interactions using a targeted NGS panel (Oncomine™ Immune Response Research Assay, Thermofisher). Samples with >1,000,000 mapped reads and >800,000 valid reads were considered adequate. R package DESeq2 software 1.38.3 was used for sequencing depth differences normalization and differential gene expression analysis. Differentially expressed genes (DEGs) were identified based on a significant p-value (p< 0.05). Results: The comparison between PrBC and EOBC revealed a total of n=7 DEGs. All of these genes were upregulated and belonged to distinct superfamilies, including Cancer/Testis (CT) Antigen (MAGEA1/3, XAGE1B), Interferons/Cytokines (IFNA17, IFNB1), Chemokines (CXCL13), and Immunoglobulin (PECAM1/CD31). Notably, the upregulation of Chemokines in respect to EOBC was observed exclusively in HR+ PrBC, whereas triple-negative (TN) PrBC did not exhibit this pattern compared to the control group. Hence, the upregulation of ALOX15B, an enzyme involved in fatty acid peroxidation, was specific to TN PrBC. The immune signatures showed significant variations between PrBC and EOBC also based on TILs density and subpopulations. Indeed, the upregulation of CT genes (MAGEA1, XAGE1B) was exclusively observed in PrBC cases characterized by low TILs levels and prevalence of CD8+ or CD4+ cells. Finally, CD4+ TILs were absent or low in PrBC with upregulated Interferons, Cytokines, Chemokines, and Immunoglobulin genes, but present in cases with increased expression of CT and KLRF1 (NK cells). Conclusion: These findings highlight the heterogeneity and distinct molecular characteristics of PrBC and EOBC. The upregulation of specific immune-related gene families, such as CT and Chemokines, in different PrBC subtypes may suggest their potential role as actionable biomarkers (e.g. MAGEA, a well-known oncogene and potential immunotherapy target). The differences in immune signatures and TILs subpopulations further emphasize the importance of the immune microenvironment in PrBC biology and behavior. Future studies could delve deeper into the clinical implications of these gene expression patterns and explore their relevance in personalized treatment strategies for PrBC. Citation Format: Konstantinos Venetis, Elham Sajjadi, Chiara Frascarelli, Mariia Ivanova, Marianna D'Ercole, Concetta Blundo, Massimo Giroda, Eugenia Di Loreto, Giovanna Scarfone, Stefano Ferrero, Paolo Veronesi, Viviana Galimberti, Fedro Alessandro Peccatori, Nicola Fusco, Elena Guerini-Rocco. Decoding the immune landscape of breast cancer (BC) during pregnancy (PrBC): Impact of hormone receptors (HR) and tumor-infiltrating lymphocytes (TILs) phenotype on gene expression signatures [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-16-04.