PD-L1 Expression in Triple-Negative Breast Cancer

Early-phase trials targeting the T-cell inhibitory molecule programmed cell death ligand 1 (PD-L1) have shown clinical efficacy in cancer. This study was undertaken to determine whether PD-L1 is overexpressed in triplenegative breast cancer (TNBC) and to investigate the loss of PTEN as a mechanism of PD-L1 regulation. The Cancer Genome Atlas (TCGA) RNA sequencing data showed significantly greater expression of the PD-L1 gene in TNBC (n1⁄4 120) compared with non-TNBC (n1⁄4 716; P < 0.001). Breast tumor tissue microarrays were evaluated for PD-L1 expression, which was present in 19% (20 of 105) of TNBC specimens. PD-L1þ tumors had greater CD8þ T-cell infiltrate than PD-L1 tumors (688 cells/mm vs. 263 cells/mm; P < 0.0001). To determine the effect of PTEN loss on PD-L1 expression, stable cell lines were generated using PTEN short hairpin RNA (shRNA). PTEN knockdown led to significantly higher cell-surface PD-L1 expression and PD-L1 transcripts, suggesting transcriptional regulation. Moreover, phosphoinositide 3-kinase (PI3K) pathway inhibition using the AKT inhibitor MK-2206 or rapamycin resulted in decreased PD-L1 expression, further linking PTEN and PI3K signaling to PD-L1 regulation. Coculture experimentswere performed to determine the functional effect of altered PD-L1 expression. Increased PD-L1 cell surface expression by tumor cells induced by PTEN loss led to decreased T-cell proliferation and increased apoptosis. PD-L1 is expressed in 20% of TNBCs, suggesting PD-L1 as a therapeutic target in TNBCs. Because PTEN loss is one mechanism regulating PD-L1 expression, agents targeting the PI3K pathway may increase the antitumor adaptive immune responses. Cancer Immunol Res; 2(4); 361–70. 2014 AACR. Introduction Triple-negative breast cancer (TNBC), which constitutes 10% to 20% of all breast tumors, is characterized by a lack of expression of estrogen receptor (ER), progesterone receptor (PR), and HER2/neu (HER2; refs. 1, 2). TNBCs are generally high-grade, aggressive tumors with a high rate of distant metastasis and poorer disease-specific survival than other breast cancer subtypes (1, 3). The poor outcomes occur even though standard chemotherapy regimens have activity against these tumors. Studies evaluating chemotherapy in the neoadjuvant setting have demonstrated that TNBC has higher rates of pathologic complete response than other tumor types; however, there is a paradoxical shortening of progression-free and overall survival (4). Therefore, novel therapeutic strategies are needed to improve the management of patients with TNBC. There is significant heterogeneity within TNBC. A study analyzing gene expression profiles identified six TNBC subtypes, one of which was an immunomodulatory subtype enriched for genes involved in immune cell processes including immune cell signaling, cytokine signaling, antigen processing and presentation, and signaling through core immune signal transduction pathways (2). In a meta-analysis integrating published gene expression data with clinicopathologic data, investigators developed gene expression modules related to key biologic processes in breast cancer. For TNBC, only the immune response module was associated with clinical outcome (5). Loi and colleagues recently reported a prognostic role of tumor-infiltrating lymphocytes (TIL) in TNBC in a large prospective clinical trial (6), and in a study looking specifically at CD8þ intratumoral lymphocytes, Liu and colleagues found that TNBC had higher rates of CD8þ T-cell infiltration, which was an independent favorable prognostic factor (7). Taken together, these data suggest that immunotherapy may have a role in the management of patients with TNBC. A promising approach to augmenting antitumor immunity is blockade of immune checkpoints. One example is CTLassociated antigen 4 (CTLA-4), a T-cell inhibitory receptor that is expressed on activated CD8þ T cells. CTLA-4 attenuates the T-cell immune response by counteracting the activity of the T-cell costimulatory receptor CD28 (8, 9). Ipilimumab, a monoclonal antibody targeting CTLA-4, has received approval from the U.S. Food and Drug Administration for Authors' Affiliations: Departments of Surgical Oncology, Pathology, Bioinformatics and Computational Biology, Breast Medical Oncology, Immunology, Melanoma Medical Oncology, Genitourinary Medical Oncology, and Stem Cell Transplantation and Cellular Therapy, The University of Texas MD Anderson Cancer Center, Houston, Texas; and Department of Urology, The Mayo Clinic, Rochester, Minnesota Note: Supplementary data for this article are available at Cancer Immunology Research Online (http://cancerimmunolres.aacrjournals.org/). Corresponding Author: Elizabeth A. Mittendorf, Department of Surgical Oncology, The University of Texas MD Anderson Cancer Center, 1400 Pressler Street, Unit 1484, Houston, TX 77030. Phone: 713-792-2362; Fax: 713-745-1462; E-mail:eamitten@mdanderson.org doi: 10.1158/2326-6066.CIR-13-0127 2014 American Association for Cancer Research. Cancer Immunology Research www.aacrjournals.org 361 on November 2, 2017. © 2014 American Association for Cancer Research. cancerimmunolres.aacrjournals.org Downloaded from Published OnlineFirst January 10, 2014; DOI: 10.1158/2326-6066.CIR-13-0127