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Background: Studies of immune checkpoint inhibitors e.g., anti-PD-1PD-L1 antibodies in ovarian cancer have demonstrated isolated responses in tumors ith clear cell histology. In order to assess the immunogenicity of clear cell ovarian cancer CCOC , e evaluated the presence of CD3+, CD8+ and CD4+ tumor-infiltrating lymphocytes TILs and the expression of PD-1 and PD-L1 in CCOCs as compared to high grade serous ovarian cancers HGSOCs . These findings ere also correlated ith presence of ARID1A loss, microsatellite instability MSI and endometriosis in CCOCs. Methods: In this pilot study, 30 CCOCs ere evaluated and compared ith 53 HGSOCs. Immunohistochemistry IHC as performed for CD3, CD4, CD8, PD-1 and PD-L1 using standard protocols. PD-1 and PD-L1 ere evaluated in both intraepithelial and peritumoral immune cells; PD-L1 as also evaluated in tumor cells. MSI status and ARID1ABAF250a loss ere determined via IHC. Results: Of the 30 CCOCs, 3 10 percent exhibited MSI, 8 26.6 percent exhibited loss of ARID1A, and 22 73.3 percent ere associated ith personal history andor histologic evidence of endometriosis. CCOCs ith MSI had a higher number of CD3+ TILs mean 52.33 vs 33, to-tailed p=0.3 and higher number of PD1+TILs 22 vs. 3.56, plt0.001 compared to microsatellite stable MSS CCOCs. Furthermore, CCOCs ith MSI had a higher number of CD3+ TILs mean 52.33 vs 36.2, to-tailed p=0.25 and higher number of PD-1+TILs 22 vs. 3.45, plt0.001 compared to HGSOCs. Unlike CCOCs ith MSI, there as no significant difference in the number of TILs or PD-1PD-L1 expression beteen MSS-CCOCs and HGSOCs. Furthermore, CCOCs ith ARID1A loss and CCOCs ith endometriosis did not exhibit higher number of TILs or PD-1 or PD-L1 expression than the remaining CCOCs or than HGSOCs. Conclusions: CCOCs ith MSI may represent a subset of CCOCs that is more immunogenic and may thus respond favorably to immune checkpoint inhibition. Apart from MSI-CCOCs, e did not detect any difference beteen CCOCs and HGSOCs in terms of number of TILs and PD-1PD-L1 expression. Endometriosis and ARID1A loss ere not associated ith higher number of TILs or PD-1PD-L1 expression.,J Clin Oncol 34, 2016 suppl; abstr 5514 00:00.0,Gynecologic Cancer