BIOM-107. EVALUATION OF B7-H3 IMMUNOHISTOCHEMISTRY IN HIGH-GRADE GLIOMAS FOR CHIMERIC ANTIGEN RECEPTOR T-CELL PREDICTIVE TESTING

Abstract INTRODUCTION Chimeric antigen receptor (CAR) T-cell therapy efficacy continues to improve in solid tumors, including in glioblastoma (GBM). B7-H3 (CD276) is an attractive tumor-associated antigen expressed in glioblastoma and many other solid cancers. Several B7-H3 CAR T-cell clinical trials are ongoing, including a Phase I trial in patients with recurrent GBM (NCT05474378), currently enrolling at our institution. Given clinical need and reported heterogeneity of expression, reliable assays are required to accurately evaluate B7-H3 expression in GBM and other high-grade glioma (HGG) patient tissues. METHODS Using archived formalin-fixed paraffin-embedded tissue from our institution, we performed immunohistochemistry using standard automated methods in the Stanford Clinical Histology Lab on Leica Bond III instruments using a rabbit monoclonal anti-B7-H3 antibody (Abcam, ab219648) at a dilution of 1:4000 with pH9 retrieval buffer (ER2). Immunostaining was independently scored by three expert pathologists, and a consensus score was derived. RESULTS A total of 267 HGG samples representing 160 unique patients were included in the validation cohort. A subset were molecularly confirmed GBMs, while the remainder showed high-grade histologic features. Eighty percent of HGGs showed at least moderate staining, and greater than 80% of samples were scored concordantly. Most HGGs showed membranous and cytoplasmic staining, with strong expression also observed on endothelial cells. Epithelioid and giant-cell histological subtypes showed predominantly membranous staining. Single infiltrating tumor cells often showed diminished staining intensity. Most normal tissue controls were negative, with some exceptions, including germinal centers, alveolar macrophages, and placenta. CONCLUSIONS To our knowledge, this is the largest cohort of HGGs evaluated for B7-H3 expression by immunohistochemistry. Our results demonstrate that most HGGs show moderate to strong B7-H3 protein expression, and these results are reproducible across independent pathologists. Future directions of this work will include correlation of pre-treatment B7-H3 expression by immunohistochemistry with response to CAR T-cell therapy.