Abstract A052: Characterization of immune regulatory molecules B7-H4, PD-L1, and ICOS in micro-satellite stable (MSS) and micro-satellite instable (MSI) endometrial tumors
Authors: Deshpande Amit , Growdon Whitfield B. , Hirsch Heather , Zi Tong , Grange Chris , Reeves Jason , Stall Jennifer , Rueda Bo , Sathyanarayanan Sriram 
Published: November 2016

Abstract
Background: Endometrial cancer is the most frequent tumors of the female reproductive system. While 5-year survival rates are greater than 90% for patients with low histological grade endometrial tumors, survival rates drop for patients with high grade endometrial tumors. With the recent success in checkpoint targeting therapies, particularly in genomic instable tumors, we wanted to characterize the immune infiltrate in a cohort of endometrial tumors. B7-H4 and PD-L1 are known members of checkpoint inhibitory pathways that regulate T cell activity, and have been shown to play a role in promoting immunogenic tolerance in tumors. Both these receptors have been shown to be expressed by tumor cells and tumor-associated macrophages (TAM). ICOS (Inducible CO-Stimulator molecule), a member of the CD28 superfamily expressed primarily on T lymphocytes, has been identified as a potentially key molecule in providing optimal anti-tumor benefit following anti-CTLA-4 therapy. Preclinical data have confirmed that engagement of the ICOS pathway plays a crucial role in mediating anti-tumor responses to checkpoint inhibitors. Published reports have shown that PD-L1 expression is strongly associated with MSI status and can enrich for response to anti-PD-1 therapy. To understand the role of these molecules in mediating immune tolerance we profiled the expression of B7-H4, PD-L1, and ICOS in a cohort of low grade and high grade endometrial patients and correlated with MSI and MSS status.
Methods: FFPE tissues from a cohort of 92 patients with endometrial cancer, collected at the Massachusetts General Hospital, were analyzed for expression of B7-H4, PD-L1, and ICOS using established immunohistochemistry (IHC) protocols. Microsatellite instability status was assessed for approximately 50 tumors using an IHC assay for expression of the mismatch repair (MMR) genes, MLH1, MSH2, MSH6 and PMS2. Additionally, ICOS expression on intratumoral T cells was evaluated using RNA sequencing data collected from Cancer Genome Atlas (TCGA).
Results: Staining of endometrial cancer samples showed that while a majority of endometrial tumors expressed B7-H4, only a small subset expressed PD-L1. No significant overlap was observed in tumors expressing both inhibitory ligands suggesting that these mechanisms are mutually exclusive in the generation of immune tolerance process. The MSI status was determined by evaluating the expression of miss-match repair genes (MMR) by IHC as a surrogate for genomic stability. We observed that PD-L1 expression was strongly associated with microsatellite instable (MSI) tumors (∼40%) with little-to-no expression of PD-L1 observed in the microsatellite stable (MSS) patients. In both sub-sets, PD-L1 expression was observed predominantly on the infiltrating tumor cells. In contrast, B7-H4 was observed to be in both MSI and MSS sub-set of tumors, however higher levels of B7-H4 (2+/3+) were observed in MSI compared to the MSS subset. In addition to inhibitory ligands we evaluated expression of ICOS on the immune infiltrates. High infiltration of ICOS+ cells was observed primarily in the MSI subset. This is consistent with published literature showing high numbers of T cells are associated with the MSI phenotype. Similar data was observed with the POLE mutant tumors that also have high levels of mutations. Together, these data support development and use of B7-H4 or ICOS targeted therapeutics for treatment of MSI sub-set of endometrial tumors.
Citation Format: Amit Deshpande, Whitfield B. Growdon, Heather Hirsch, Tong Zi, Chris Grange, Jason Reeves, Jennifer Stall, Bo Rueda, Sriram Sathyanarayanan. Characterization of immune regulatory molecules B7-H4, PD-L1, and ICOS in micro-satellite stable (MSS) and micro-satellite instable (MSI) endometrial tumors [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr A052.