IMMU-06. Development of a Pediatric Cancer Surfaceome Atlas to identify molecular targets across pediatric malignancies.

Introduction Cell surface proteins make up the majority of targeted and immune therapies for cancer. While numerous targets have been characterized in the adult setting, prediction of pediatric-specific surface antigens has been limited. With the Pediatric Cancer Surfaceome Atlas (PCSA), we sought to leverage pediatric molecular atlas resources to characterize the surfaceome of pediatric cancer, generate an atlas of surface antigens, and develop a pipeline for prospective surface antigen identification. Methods Tumor specific antigens were identified using a specificity metric that considers the difference between bulk-RNA-sequenced pediatric tumor specimens (n = 1896) and pediatric normal specimens from GTEx and external studies (n = 2633). This specificity metric integrates information from four distinct differential expression algorithms: an empirical bayes linear model, a generalized linear model, a Mann-Whitney test, and a Tumor-Specificity Index. Targets were then filtered and prioritized by integrating information about their characterized oncogenicity, mutational profiles, clinical prognostic utility, and expression distribution. This standardized pipeline was then built into a web application (CARTART) for prospective target identification. Results Across pediatric malignancies, a total of 625 cancer-specific surface antigens were identified with 24.3% of these antigens being targeted in the cancer setting. Of note, characterized targets like IL13RA2, B7-H3, GPC2, and CLDN6 were identified. To further validate these bulk-identified targets, we analyzed single-cell RNA sequencing profiles from 11 pediatric malignancies notably finding that cancer-specific surface antigens were specific to malignant cells when compared to normal cell types (p < 0.01). To mitigate possible off-­tumor toxicity, we also identified 287 distinct antigen pairs for logic-gated CAR development and characterized the mutational, functional, and genetic traits of these targets. Conclusions These findings highlight the potential utility of the PCSA and CARTART as exploratory resources for surface target identification in the pediatric cancer setting and as tools for prospective identification of subtype-specific targets across pediatric tumors.