Soluble Inflammatory Mediators Proteinase-3 and Neutrophil Elastase Are Targeted by PR1-Specific Immunotherapy After Cellular Uptake and Cross Presentation of PR1 Peptide by Breast Cancer Cells
Authors: Alatrash Gheath , Mittendorf Elizabeth , Sergeeva Anna , Sukhumalchandra Pariya , Qiao Na , Quintanilla Kathryn , Zhang Mao , Sijie Lu , Clise-Dwyer Karen , Molldrem Jeffrey J. 
Published: October 2019

Abstract
Abstract 2089
The human leukocyte antigen (HLA)-A2 restricted nonapeptide PR1 (VLQELNVTV) was shown to be immunogenic in leukemia. A phase I/II clinical trial has been initiated with PR1 peptide vaccine and to date has demonstrated clinical efficacy, including complete remission and immunologic responses in patients with acute (AML) and chronic (CML) myeloid leukemia, as well as myelodysplastic syndrome. PR1 is derived from the serine proteases proteinase-3 (P3) and neutrophil elastase (NE), which are normally found within neutrophil azurophil granules and are released into the inflammatory milieu. We have shown that P3 and NE are taken up and cross presented by antigen presenting cells and that their cross presentation elicits PR1 immunity. Because P3 and NE are present in breast cancer biopsies, we hypothesized that P3/NE may be taken up by breast cancer cells and cross presented to PR1-CTL. We recently demonstrated that the breast cancer cell lines MDA-MB-231, MDA-MB-453, MCF-7 and HER18 do not endogenously express NE and that NE is taken up by these cell lines. In this report, using PCR, western blot and flow cytometry, we show that P3 also is NOT endogenously expressed by the breast cancer cell lines MDA-MB-231, MDA-MB-453, MCF-7 or HER18. Using confocal microscopy, we demonstrate that P3 is taken up by these breast cancer cell lines within 10 minutes of pulsing and localizes to LAMP-2 containing lysosomal vesicles by 4 hours, suggesting its processing for presentation by (HLA)-I (i.e. HLA-A2). Using 8F4, the novel PR1-HLA-A2 monoclonal antibody, we show that PR1 is cross presented from P3 by 3 of 4 HLA-A2+ breast cancer cell lines (MDA-MB-231, MDA-MB-453-A2+, MCF-7), and from NE by 1 of 4 breast cancer cell lines (MDA-MB-231). Next, we studied whether PR1 presentation made cells susceptible to PR1-specific killing by PR1-CTL and the 8F4 monoclonal antibody. We show that following 12-hour pulsing of the MDA-MB-231 cell line with NE or P3, PR1 CTLs killed up to 31% and 38% of the NE- or P3-pulsed breast cancer cells respectively, vs. <1% of ovalbumin (ova)-pulsed MDA-MB-231cells. Additionally, in a complement mediated cytotoxicity assay using 8F4 antibody, pulsing of MDA-MB-231 cells with P3 led to 60% cytotoxicity (vs. 40% in ova-pulsed cells). In conclusion, this study shows that 1) PR1 is cross presented by breast cancer cells following uptake of soluble P3 and NE and 2) PR1 expression makes breast cancer a target of PR1-specific immunotherapy. If uptake of P3 or NE, present in the inflammatory milieu of other solid tumors, also leads to PR1 cross presentation, then PR1-based immunotherapy may be useful to treat other non-hematopoietic tumors. These results support a new paradigm linking inflammation and innate immunity to adaptive immune responses to cancer.

Disclosures:
No relevant conflicts of interest to declare.