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Abstract 4040: PD-L1 expression level and CD8+/FoxP3+T cell ratio in breast cancer and prognosis

Introduction: PD-L1 can bind many receptors, including PD-1, which is expressed on activated T cells and dendritic cells. This interaction contributes to the suppression of the T cell-mediated anti-tumor response. While CD8+ T lymphocytes play a key role in tumor-specific cellular-adaptive immunity, their actions may be counteracted by FoxP3+ cells, a major suppressor of CD8+ T cells. The aims of this study were to examine the expression level of PD-L1 as well as quantification of CD8+ and FoxP3+ cell infiltration in breast tumors and their associations with prognosis. Methods: Tumor samples from 96 breast cancer patients were obtained from the University of Chicago Breast Cancer tissue bank, and were classified into four molecular subtypes based on immunohistochemical (IHC) staining: luminal A (ER+/PR+, HER2-), luminal B (ER+/PR+, HER2+), Her2-enriched (ER-, HER2+), and basal-like (ER-, HER2-, EGFR+ and/or CK5/6+). IHC for PD-L1, CD8 and FoxP3 was performed on tissue microarrays. PD-L1 expression was scored as negative, weak positive, moderate, and strong. CD8+ and FoxP3+ cells were counted manually and recalculated for 1 mm2. The Kaplan-Meier method was used to estimate probabilities of overall survival (OS), breast cancer-specific survival (BCSS), and distant disease-free survival (DDFS). Results: The study cohort was comprised of 64.6% luminal A, 7.3% luminal B, 5.2% Her2-enriched, and 22.9% basal-like cases. The mean age at diagnosis was 58.2 years and 58% had node-negative breast cancer. 15% were grade 1, 42% grade 2, and 35% grade 3. PD-L1 expression was scored as follows: negative 9.4%, weak positive 50%, moderate 31.2%, and strong positive 9.4%. PD-L1 expression was not significantly associated with OS, BCSS and DDFS in the total cohort, but was associated with OS in the basal-like subtype (p=0.03). PD-L1 high-expressing tumors had, on average, greater FoxP3+ cell infiltration. The highest FoxP3+ cells infiltration was observed in the basal-like subtype and the lowest was in the luminal B subtype (p=0.02). The CD8+/FoxP3+ ratio was lowest for the basal-like subtype (mean: 2.3), but there was no significant association with OS, BCSS and DDFS for the total cohort or for each subtype. Conclusions: The PD-L1 expression pattern and CD8+/FoxP3+ T cell ratio were different among the breast cancer molecular subtypes and were not statistically associated with the prognosis in the total cohort. The presence of and interactions among immune cell subsets in the tumor microenvironment for each breast cancer subtype are important in understanding the immunologic response and its effect on outcome. Obtaining a more comprehensive picture of the tumor immune microenvironment and maximizing the information from tumor tissues require a multiplexed three-dimensional (3D) staining approach. Future studies will evaluate multiplex 3D immunofluorescence of multiple immune biomarkers in breast tumors. Citation Format: Masaya Hattori, Galina Khramtsova, Lise Sveen, Toshio Yoshimatsu, Dezheng Huo, Olufunmilayo I. Olopade. PD-L1 expression level and CD8+/FoxP3+ T cell ratio in breast cancer and prognosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4040.