Abstract 3302: Comprehensive analysis of B7-H7/HHLA2 expression in pan-solid tumors and its potential significance in anti-tumor immunity

Abstract BACKGROUND: B7-H7/HHLA2 is a B7 family member expressed on select normal epithelium and upregulated in many tumors. B7-H7 suppresses T and NK cell function through an inhibitory receptor, KIR3DL3, and stimulates lymphocyte activation through a distinct receptor, TMIGD2. High levels of B7-H7 expression in tumor cells are frequently associated with poor prognosis and it may represent a novel target for therapeutic interventions. We have previously demonstrated that higher levels of B7-H7 are associated with increased tumor cell killing by a B7-H7-targeting T cell engager. Here, we systematically characterized B7-H7 expression across solid tumor histologies to assess the potential of B7-H7 as a biomarker for use in therapeutic targeting of B7-H7. METHOD: B7-H7 protein expression was assessed by immunohistochemistry (IHC) using a monoclonal antibody that has been widely used in the literature (566.1). Hundreds of tumors across many indications and select normal tissues were characterized for B7-H7 expression. To develop a B7-H7 IHC assay that could be used for patient selection for B7-H7 targeting therapeutics including NPX887, a monoclonal antibody currently in clinical trials, the performance of 566.1 and a commercially available monoclonal antibody (E1U6X) were compared. Quantitative flow cytometry (qFC) was used to measure the number of molecules per cell across cell lines displaying different levels of B7-H7, and concordance of qFC and IHC measurements by E1U6X was established. To investigate the effects of B7-H7 levels on immunomodulation and cell killing, in vitro tumor and immune cell co-culture systems were used. RESULTS: Tumor cells from colorectal, non-small cell lung, pancreatic, renal and gastric cancer displayed the highest protein levels of B7-H7 and defined patient populations of interest for NPX887. Normal B7-H7 expression was restricted to substructures of intestine and lung and showed only faint cytoplasmic staining in the kidney. There was no detectable B7-H7 expression on any circulating immune cells. During assay development, similar patterns of B7-H7 expression were detected using the two monoclonal antibodies, however the optimized assay using E1U6X displayed superior performance. The levels of B7-H7 expression on tumor samples were comparable to tumor cell lines effectively killed by immune cells in the presence of anti-B7-H7 therapeutic antibodies in vitro. CONCLUSION: We demonstrated that normal tissue expression of B7-H7 is restricted to organ substructures within the intestine, lung, and kidney. We observed significant expression of B7-H7 across tumor cells of varying histologies, highlighting patient populations for B7-H7 targeting therapies. The IHC assay using E1U6X was selected for investigational use in ongoing (NPX887-001; NCT06240728) and potential future clinical studies of therapeutic molecules targeting the B7-H7 axis. Citation Format: Yasmin L. Hashambhoy-Ramsay, Karishma Vekaria, Emilien Loeuillard, Melissa A. Hayes, Emma Ya-Ting Wang, Matthew Rausch, Tatiana Novobrantseva, Katalin Kis-Toth, Leena Gandhi. Comprehensive analysis of B7-H7/HHLA2 expression in pan-solid tumors and its potential significance in anti-tumor immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3302.