Abstract 4093: Synergy of CTL tumor cytotoxicity with myxoma oncolytic virotherapy

Abstract

Abstract Background: Checkpoint blockade (CKB) are in clinical trials in breast cancer, but responses may be limited by the low tumor micro-immune profile and somatic mutation burden. Oncolytic viruses may function by activating the immune microenvironment, and improve clinical responses when combined with CKB against cancers like melanoma. In this study, we evaluate the synergy of cytotoxic T lymphocytes (CTLs) combined with oncolytic myxoma virus in an in vitro model of human breast cancer. Method: BMLF1-CTLs were generated from human HLA-A*02 PBMCs, which were stimulated with antigen presenting cells pulsed with the EBV BMLF1 HLA-A*02 peptide (GLCTLVAML). MCF-7 was engineered to express the full-length BMLF1 gene from EBV. To assess the impact of myxoma virus infection on CTL cytotoxicity, target MCF-7 cells (+/- BMLF1) were co-cultured with BMLF1-CTLs (E:T ratio=5:1) and myxoma virus vMyx-M135KO-GFP (vMyx135KO), at multiplicity of infection (MOI) ranges from 0.1 to 10 ffu/cell, in vitro for 48 hours. Cytotoxicity was measured by flow cytometry with propidium iodide (PI). Cell surface expression of MHC class I and class II were measured by flow cytometry. Statistical comparisons were performed using an unpaired t-test and variation among and between groups was calculated using ANOVA. Statistical significance was defined as p<0.05. Results: At the highest MOI (10 ffu/cell), cell surface MHC class I expression on MDA-MB-231 was decreased 38-fold (p<0.01), compared to untreated control. MCF-7 was more sensitive to MHC class I downregulation (MOI 1 ffu/cell, 37.3-fold reduction, p<0.002). No significant change of cell surface expression of MHC class II was detected in either breast cancer cell lines at any MOI. In MCF-7-BMLF1- cells, baseline cytotoxicity remained low in both CTL and vMyx135KO (MOI=0.1 ffu/cell) only treatments (14% vs. 10%). With antigen expression (MCF-7-BMLF1+), CTL cytotoxicity increase to 21% (p=0.02). At a MOI of 0.1 ffu/cell, vMyx135KO viral infection further sensitized cells to CTL cytotoxicity (21% vs. 36%, p<0.001) Conclusions: Oncolytic myxoma virus infection can decrease the MHC class I expression in human breast cancer cell lines, but only at high multiplicities of infection. At low level of virus, myxoma virus infection can enhance CTL cytotoxcity of breast cancer cell line. Citation Format: Meixuan Chen, Ana L. Matos, Padhmavathy Yuvaraj, Laura Belmont, Grant McFadden, Karen S. Anderson. Synergy of CTL tumor cytotoxicity with myxoma oncolytic virotherapy [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 4093.