LAUSR

Background: Ductal carcinoma in situ (DCIS) consists of a heterogeneous group that can be subclassified by molecular signatures. High grade DCIS is commonly associated with periductal inflammation, a feature associated with increased risk for microinvasion. We previously assessed the immune microenvironment of DCIS and reported that HER2+ DCIS has increased numbers of tumor infiltrating lymphocytes (TILs), with significant expression of PD-L1. CTLA-4 is a supplementary immunomodulatory marker, which serves as a negative regulator of T-cell response. Targeted therapy is currently available against both PD-L1 and CTLA-4, and studies suggest an enhanced response with combination therapy as these therapies act though different mechanisms. In this study we evaluated the expression of CTLA-4 in both TILs and tumor cells in DCIS. Design: The study population consisted of 74 cases of DCIS treated with surgical excision from 2008-2012. The molecular subtypes of DCIS were determined based on ER and HER2 expression. Tissue microarrays were constructed (3 cores/case) to account for tumor heterogeneity. The TMAs were immunostained for CTLA-4 and the expression CTLA-4 was assessed. Results: Of the 74 cases, 41 were classified as Luminal A (ER+/HER2-), 17 as Luminal B (4 as Luminal B HER2- [ER+/HER2-/high Ki-67] and 13 as Luminal B HER2+ [ER+/HER2+]), 11 as HER2+ (ER-/HER2+) and 5 as basal (ER-/HER2-). Overall, the DCIS tumor cells expressed CTLA-4 in 58% of Luminal B (HER2+ 53% [7/13]; HER2- 75% [3/4]) and 36% of HER2+ (4/11), while only 39% of Luminal A (16/41), and one basal DCIS (1/5) did. In addition, the presence of CTLA-4 positive TILs was strongly associated with HER2+ expression (Luminal B HER2+=21.8%, HER2+=13.4%), compared to HER2- DCIS (Luminal B HER2-=8.5%, Luminal A=7.3%, Basal=6.4%) (p=0.026). Conclusion: 1. CTLA-4 expression in TILs is increased in HER2+ DCIS. 2. A significant percent of tumor cells express CTLA-4 in all DCIS subtypes. Our results further advance our understanding of the role of the immune microenvironment in DCIS. Additional studies are needed to further elucidate the complex interplay between the host immune system and DCIS, which may offer insights into breast carcinoma progression and possible use of novel immune-based therapies. Citation Format: Jorge E. Novo, Lauren Rosen, Julianne M. Ubago, Jennifer Pincus, Luis Blanco, Kalliopi P. Siziopikou. Cytotoxic T lymphocyte antigen 4 (CTLA-4) expression in ductal carcinoma in situ (DCIS) of the breast [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 625.