Abstract 2317: PD-1 and PD-L1 expression patterns and DNA mismatch repair status for precision management of patients with gastric cancer

BACKGROUND: Programmed cell death protein 1 (PD-1) and its ligand (PD-L1) downregulate T cell activation and are related to immune tolerance. Recently, microsatellite unstable (MSI) colorectal cancers have been shown to have good therapeutic response to PD-1 antibody immunotherapy, possibly due to release exhaustion of their ability to recognize high number of tumor neo-antigens. The aim of this study was to clarify the significance of PD-1 and PD-L1 expression, and to analyze the relationship between MSI status and MLH1 hypermethylation status in gastric cancer. METHODS: A total of 105 patients who underwent curative gastrectomy for gastric cancer were included in this study. The PD-1, PD-L1, HER2, and mismatch repair proteins (MLH1/PMS2/MSH2/MSH6) expression were examined by immunohistochemistry. MSI status were examined by analyzing three mononucleotide repeat markers. MLH1 methylation was evaluated using a highly sensitive fluorescence-based PCR assay, as we reported previously (JNCI 2009). KRAS/BRAF/PIK3CA mutations were determined by conventional Sanger sequencing. Infection of Helicobacter Pylori (H. Pylori) and EB virus (EBV) were determined by amplifying H. Pylori and EBV specific sequence by PCR, followed by conventional Sanger sequencing. RESULTS: PD-1 and PD-L1 were expressed in 40% (38/95) and 33% (31/95) gastric cancers, respectively. HER2 was expressed in 15% (14/96) gastric cancers. Infection of H.Pylori and EBV were observed in 71% (70/98) and 8% (8/98) gastric cancers, respectively. MSI was observed in 13% (13/98) gastric cancers. Among 13 MSI cancers, 11 cases (85%) showed extensive methylation in the MLH1 promoter region, suggesting their sporadic manifestation. KRAS (exon 2), BRAF (exon 15) and PIK3CA (exon 9 & 20) mutations were observed in 5% (5/98), 0% (0/98), and 4% (4/98), respectively. Among these, one case harbored simultaneous KRAS and PIK3CA mutation. Gastric cancers with KRAS/PIK3CA were more frequently MSI-positive (5/8, p = 0.003). Among eight EBV infected cancers, only one case showed MSI and MLH1 methylation. PD-1 expression was significantly correlated with PD-L1 expression (p = 0.04). Although both PD-1 and PD-L1 expression were associated with MSI (p = 0.007 and p = 0.008, respectively), only PD-L1 expression was significantly correlated with gastric cancers with MLH1 methylation (p = 0.01). While expression of immune checkpoint molecules were specific for tumors with MMR deficiency, HER2 expression was exclusively obs