P2‐36: Activation of G protein‐coupled estrogen receptor (GPER) regulates ILC2

shows a poor prognosis with less than 10% of 5-year survival rate. Recent clinical studies show programmed cell death 1 (PD-1) inhibitors have therapeutic potential, however the distribution of PD-L1 expression and tumor infiltrating lymphocytes (TILs) in MPM have not been elucidated. Aims: To assess PD-L1 expression and investigate density of TILs and spatial characteristics in MPM. Methods: Thirty-seven patients with MPM who underwent surgery in Tokyo Medical and Dental University Hospital between 2016-2019 were included. Pathological review and PD-L1 staining were performed for all patients. Recent invention of OPAL multiplex IHC (OPAL) enable to detect a variety of cell surface markers simultaneously and show its spatial information. OPAL with FFPE especially for patients who had recurrent MPM treated with nivolumab was performed for identifying localization of cell mediated immunity markers (CD4, CD8, FoxP3) and Immune checkpoint molecules (PD-L1, VISTA). Results: PD-L1 stain was positive in 22 patients and negative in 15 patients. In OPAL analysis, the density of CD8 positive T-cell was significantly higher than those of CD4 and FoxP3 positive T-cell, respectively (p<0.05). No statistical difference was observed between the density of CD4 and FoxP3 positive T-cell. In the calculated distance between tumor and T-cell, the distance from PD-L1 positive tumor showed prominently closer to CD8 positive T-cell than those to CD4 and FoxP3 positive T-cell, respectively (p<0.05). Conclusions: Expression of PD-L1, as well as features of TILs (higher CD8 positive T-cell), were partially revealed in MPM using OPAL.