317. PD-L1 DYNAMICS DURING THE CISPLATIN-INDUCED ACQUIRED RESISTANCE PROCESS IN ESOPHAGEAL SQUAMOUS CELL CARCINOMA

Abstract Background Immunotherapy combined with chemotherapy has been the first-line treatment and the most widely-used neoadjuvant regimen in esophageal squamous cell carcinoma (ESCC). Resistance to chemotherapy is an inevitable clinical problem, however, the impact of chemoresistance on immunotherapy efficacy remains unclear. This study aimed to investigate the impact of chemoresistance formation on PD-L1 dynamic changes both in vivo and in vitro during the construction of cisplatin-resistant ESCC models, with the goal of exploring the potential influence of chemoresistance on immunotherapy. Methods For in vivo model, fluorescent-labeled ESCC cells (AKR) were implanted into C57 mice, followed by cisplatin administration for six cycles. Surviving tumor cells were isolated using flow cytometry, and reinjected to establish cisplatin-resistant model. Tumors were harvested to evaluate PD-L1 expression by immunohistochemistry. For in vitro model, ESCC cell lines (TE-1, KYSE150) received low-dose cisplatin for 48h. After three cycles, resistance index (RI, RI=IC50 of drug-expose cell/IC50 of parental cell) was determined by CCK8 assay. The process was repeated with escalating cisplatin doses until resistant cell lines were established (RI>5). PD-L1 expression was measured using RT-qPCR/Western-Blot after each generation. Results In vivo (Figure A), cisplatin-resistant tumor exhibited elevated PD-L1 expression compared to control group (H-score: 22.17 vs 2.09, p<0.001). In vitro (Figure B), PD-L1 expression exhibited an initial increase, followed by a gradual decrease during the process of cisplatin-induced resistance. Specifically, in early stages of cisplatin-resistance in TE-1 cells, PD-L1 mRNA expression rose 4.9 times, consistently paralleled by an elevation in protein level. Subsequently, PD-L1 expression gradually decreased in both mRNA and protein levels. KYSE150 cells exhibited a trend of upregulation in PD-L1 expression in the early stages of cisplatin resistance but later decreased, returning to parental cell levels. Conclusion Under in vivo conditions, acquired cisplatin-resistance in ESCC cells significantly upregulated PD-L1 expression, indicating a potential mechanism for PD-L1-mediated immune escape and its impact on immunotherapy efficacy. However, under in vitro conditions, no consistent trend in PD-L1 expression was observed during the acquisition of cisplatin resistance. This suggests a potentially crucial role of the immune system involved in the chemoresistance regulatory mechanism. Further investigations are needed to explore the impact of chemoresistance on tumor immunology.