Cellular Uptake of Soluble Neutrophil Elastase Increases Cyclin E (CCNE) Isoform Expression and Significantly Augments Susceptibility of Breast Cancer Cells to Cytolysis by CCNE-Specific Cytotoxic T Lymphocytes
Authors: Mittendorf Elizabeth , Alatrash Gheath , Qiao Na , Sukhumalchandra Pariya , Lu Sijie , Quintanilla Kathryn , Clise-Dwyer Karen , Molldrem Jeffrey J. 
Published: October 2019

Abstract
Abstract 2090
We have shown that cytotoxic T lymphocytes (CTL) with specificity for the cyclin E (CCNE) derived HLA-A2-restricted peptide CCNE144-152 (ILLDWLMEV) specifically lyse myeloid and lymphoid leukemia in proportion to CCNE overexpression. Full length (FL) CCNE is also overexpressed in most solid tumors, including breast cancer where it is a poor prognostic factor. In addition, leukemia and many breast cancers express tumor-specific low molecular weight (LMW) isoforms of CCNE that result from post-translational processing of the FL protein. Neutrophil elastase (NE), derived from the primary granules of neutrophils, cleaves FL CCNE into LMW forms and NE has been identified in breast cancer tissue. Therefore, we hypothesized that CCNE may also be a breast cancer tumor antigen because the CCNE144-152 peptide is contained within the overexpressed FL and LMW forms, and that effective T cell immunity could be amplified by increased availability of LMW forms within tumor cells exposed to NE. The link between innate immunity, inflammation, and tumor immunity is poorly understood, and this mechanism could explain a role for tumor-infiltrating inflammatory cells in breast cancer. To test this, we elicited CCNE-CTL from peripheral blood lymphocytes of HLA-A2+ healthy donors by weekly stimulation with CCNE-pulsed T2 cells and low-dose IL-2. After 21 days, cytotoxicity of target cells by the lymphocytes was tested with a standard 4-hour calcein AM-based assay. The CCNE-CTL specifically lysed T2 cells pulsed with CCNE (30%) but not non-pulsed T2 cells (0%) at an effector:target (E:T) ratio of 20:1 (p < 0.01). Next, we tested whether CCNE-CTL killed HLA-A2+ MDA-MB-231 (231) breast cancer cells. First, we confirmed that FL CCNE and LMW forms were expressed in 231 cells, while low expression of FL CCNE and no expression of the LMW forms was observed in benign epithelial cells by Western blot. Next, we showed that CCNE-CTL mediated 49% lysis of 231 breast cancer cells at an E:T ratio of 20:1. To look for in vivo evidence of CCNE recognition, we studied peripheral blood lymphocytes from breast cancer patients by flow cytometry with CCNE/HLA-A2 tetramers and anti-CD8 antibodies. In 3 of 4 breast cancer patients we identified CCNE-CTL, with no detectable CCNE-CTL in healthy controls. Together, these results confirm that CCNE is also a tumor antigen in breast cancer. NE, which cleaves CCNE and is expressed in breast cancer tissue, may be produced endogenously by breast cancer cells, or exogenously by inflammatory cells in the tumor microenvironment. Therefore, we studied 231 cells and three other breast cancer cell lines (MDA-MB-453, MCF-7, and HER18) for NE expression. RT-PCR performed with NE-specific primers showed that none of the cells expressed NE mRNA and Western blot showed no NE protein expression, suggesting that NE in tumor tissue derives from neutrophils or other inflammatory cells. To determine whether NE is taken up by breast cancer cells, we used flow cytometry to show that 231 cells pulsed with soluble NE took up an increasing amount of NE and was maximal by 24 hours when intracellular NE expression in 231 cells was comparable to that of HL60 leukemia cells that express high levels of NE. In addition, LMW isoforms of CCNE were increased in NE-pulsed 231 cells, by Western blot. Importantly, CCNE-CTL specific lysis of NE-pulsed 231 cells was 2-fold higher compared to nonpulsed 231 (46% versus 22% at E:T 10:1, p = 0.01). Taken together, these data show that overexpressed CCNE144-152 is a novel breast cancer peptide antigen. Furthermore, exogenous NE is taken up by breast cancer cells, increasing LMW forms of CCNE and enhancing the susceptibility of breast cancer cells to CCNE-CTL-mediated cytolysis. This study links a specific enzyme secreted by neutrophils in the innate immune response to tumors to a specific adaptive immune response against breast cancer, and it suggests that immunotherapy targeting CCNE is warranted.

Disclosures:
No relevant conflicts of interest to declare.