Abstract 4563: PD-L1 expression associated with treatment responses in colorectal cancer patients with XELOX/FOLFOX chemotherapy: Potential of checkpoint blockage and natural killer cell-based immunotherapy
Authors: SIU Elaine H.L. , NG Simon S.M. , CHAN Anthony W.H. , LAU James Y.W. , LO Kwok Wai , CHONG Charing C.N. , LAI Paul B.S. , CHAN Stephen L. , LAW Marcus H.N. , NG Linda W.C. , CHEUNG S.T. 
Published: August 2018

Abstract
Chemo-resistance is a major obstacle in human cancer treatment. Immunotherapies including checkpoint blockage and cell-based therapy have been clinically validated for cancer treatment. The present study aimed to investigate the role of PD-L1 and potential of immunotherapy for chemo-resistant cancers. In a pilot study, we have examined the expression of PD-L1 in colorectal cancer (CRC) patients who received XELOX (capecitabine plus oxaliplatin) or FOLFOX (fluorouracil plus oxaliplatin) chemotherapy. Detection rate of PD-L1 expression by immunohistochemistry was higher in CRC patients who demonstrated poor response (5/10, 50%) compared to good responders (0/5, 0%) on XELOX/FOLFOX chemotherapy. Chemo-resistant CRC cell lines (HT29 and HCT116) were used to examine the role of PD-L1. Cells were plated out, selected under fluorouracil and oxaliplatin in serial, and referred as fluorouracil-oxaliplatin-resistant (FOR) cells. Half maximal inhibitory concentration (IC50) for respective drug was determined by MTT assay. HT29 FOR, compared to parental cells, showed chemo-resistance (359.4 and 10.1 fold increase to fluorouracil and oxaliplatin respectively) and similarly for HCT116 FOR (8.0 and 9.0 fold increase respectively). Hepatocellular carcinoma Hep3B and nasopharyngeal carcinoma C666-1 cells that have been selected for fluorouracil chemo-resistant (FR) were included. All the chemo-resistant cell lines showed increased expression of PD-L1 by real-time qPCR compared to their respective parental cells (HT29 FOR: 6.1 fold; HCT116 FOR: 1.9 fold; Hep3B FR: 3.1 fold; C666-1 FR: 4.4 fold). To explore the potential of immunotherapy, natural killer (NK) cells were isolated from peripheral blood PBMC of healthy donors. NK cells were expanded and activated ex vivo in stem cell growth medium supplemented with interleukins. NK co-culture assay was performed and apoptosis of cancer cells examined by flow cytometry. Parental cancer cells were sensitive to cytotoxic killing by NK cells compared to control no NK treatment (36.0% vs 2.8% apoptotic cells, P=0.001), and PD1 antibody (Nivolumab) treatment promoted NK-mediated cytotoxicity (52.9%, P=0.034). Notably, chemo-resistant cancer cells were similarly sensitive to cytotoxic killing by NK cells compared to control (27.1% vs 3.4%, P=0.002), and PD1 antibody (Nivolumab) treatment further enhanced NK-mediated cytotoxicity (52.8%, P=0.013). In summary, PD-L1 expression was associated with poor response for XELOX/FOLFOX chemotherapy in CRC patients. Chemo-resistant cancer cells showed elevated PD-L1 expression, nonetheless, susceptible to NK-mediated cytotoxicity and checkpoint blockage by PD1 antibody augmented the treatment response. Further studies are warranted to examine immunotherapy for chemo-resistant cancers.
Citation Format: Elaine H.L. SIU, Simon S.M. NG, Anthony W.H. CHAN, James Y.W. LAU, Kwok Wai LO, Charing C.N. CHONG, Paul B.S. LAI, Stephen L. CHAN, Marcus H.N. LAW, Linda W.C. NG, S.T. CHEUNG. PD-L1 expression associated with treatment responses in colorectal cancer patients with XELOX/FOLFOX chemotherapy: Potential of checkpoint blockage and natural killer cell-based immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4563.