Abstract 459: PD-L1 expression in breast cancer cell lines after IFNγ treatment varies significantly between molecular subtypes.
Authors: Luddy Kimberly , Soliman Hatem 
Published: August 2014

Abstract
Background
The molecular subtypes of breast cancer described by Charles Perou provides greater insight into the underlying biology of different breast cancers. An important aspect of this biology is the ability of a particular breast cancer to evade the host immune response through activation of immunosuppressive pathways. Understanding the relationship between molecular subtypes and potential for immune evasion is important in determining which breast tumors can be targeted by immunomodulatory treatments. We focused specifically on the surface expression of the programmed death ligand 1 (PDL1) on breast cancer cell lines within different molecular subtypes. Binding of PD1 on T cells by its ligand PDL1 causes an inhibitory cascade resulting in T cell anergy and apoptosis. Expression of PDL1 on tumor cells creates an immunosuppressive environment reducing the number of tumor specific cytotoxic T cells. A potent inducer of PDL1 expression is interferon γ (IFN γ), which is frequently present in the tumor microenvironment. We sought to study differences in PDL1 expression between luminal (MCF7, AU565), basal A (BT20, HCC1143), and basal B (MDA231, HCC38) breast cancer cell lines provided by CK Zhang at ATCC.
Methods
PDL1 protein expression was analyzed in an ATCC assay ready breast cancer cell panel. Surface and intracellular PDL1 expression +/- IFN γ for 48hrs was measured by flow cytometry (BD Biosciences) on a LSRII with mean fluorescence intensity (MFI). The cells were treated with 50 μM of epigallocatechin gallate (EGCG) (Sigma), a STAT1 inhibitor, to explore the role of STAT1 signaling in the expression of PDL1. Levels of intracellular phosphoSTAT1 were also measured by flow (Beckman Coulter) in these experiments.
Results
Basal cells had the greatest baseline MFI values (mean 1817) vs luminal (mean 0). All cells increased PDL1 expression with IFN γ, but the basal B cells demonstrated the largest absolute increases by far (HCC38 MFI =13948 vs. MCF7 MFI =764). This was in part due to greater total PDL1 expression, but also a larger proportion of total expressed PDL1 localizing to the surface in comparison to the other molecular subtypes. Finally, treatment with EGCG was able to significantly inhibit expression of PDL1 and lowered levels of pSTAT1 especially in the HCC38 cell lines.
Conclusion
Our data suggests that basal type breast cancer cells (especially basal B) express greater levels of PDL1 both constitutively and in response to IFN γ. It appears that the STAT1 pathway is involved in mediating this effect in response to IFN γ. Prospective knowledge of a breast tumor's ability to phenotypically express PDL1 based on molecular subtype will allow for better selection of patients for future anti-PDL1 antibody clinical trials.
Citation Format: Kimberly Luddy, Hatem Soliman. PD-L1 expression in breast cancer cell lines after IFNγ treatment varies significantly between molecular subtypes. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 459. doi:10.1158/1538-7445.AM2013-459