Abstract A17: HHLA2 is a new immune checkpoint expressed in pediatric Hodgkin lymphoma

Purpose: To identify new pharmacologically targetable immune checkpoints in pediatric Hodgkin lymphoma. Background: The role of PD-1/PD-L1 axis in Hodgkin lymphoma (HL) has led to FDA approval for use of inhibitors of this pathway in chemotherapy-refractory HL. There are numerous additional B7/CD28 immune checkpoints that may similarly represent useful targets but have not yet been evaluated in HL. HHLA-2 is the newest member of the B7/CD28 family of immune checkpoint regulators that typically help tumor immune escape via inhibition T-cell activity and tumor killing. The prevalence of HHLA-2 has been demonstrated with expression on a wide range of adult cancers, with associated poor outcomes. HHLA-2 has yet to be evaluated in pediatric cancers, except for osteosarcoma where it was found to have increased expression and associated with worse five-year EFS. The purpose of this study is to characterize the expression pattern and clinical significance of HHLA-2 in pediatric HL using immunohistochemistry. Methods: Patient tumor samples from prior Children’s Oncology Group clinical trials with matched patient outcome data containing over 300 patient samples were obtained. 95% confidence intervals were calculated based on a range of observed prevalence of immune checkpoint expression for this initial pilot cohort of 100 samples. Using immunohistochemistry, paraffin-embedded samples were tested for the expression of HHLA-2 and B7x. Samples were also stained for CD30 to better delineate Reed Sternberg cells (RS) from the remainder of the tumor microenvironment. Immune checkpoint staining was compared to known positive controls of A204 cell line for HHLA-2 and SKBR3 for B7x, and negative controls of 3T3 cells. Expression intensity was scored by a pediatric pathologist. Results: The initial tissue microarray contained 128 unique HL cases; only 121 tissue samples were evaluable as HL tissue. Samples from 52 patients demonstrated positive HHLA-2 staining (43%); however, there was no identifiable B7x staining. HHLA-2 staining ranged from weak to moderate, with 23% (12/52) of positive samples demonstrating moderate staining. RS staining was observed in 77% of samples (40/52), with the remaining samples containing positively staining lymphocytes. Conclusion: This project is innovative in its characterization of HHLA-2 as a novel immune checkpoint target in pediatric HL. With these preliminary results we will validate these results with the remaining 200 patient samples and explore the relationship of expression with standard tumor characteristics, including stage and patient outcomes such as ESF and OS. These results can help discover new prognostic biomarkers and guide future treatment as therapeutic antibodies are currently being developed. We hope that the information from this project will be used to support new clinical trials for pediatric patients with Hodgkin lymphoma. Citation Format: Scott Moerdler, Damini Chand, Michelle Ewart, XingXing Zang, Peter Cole. HHLA2 is a new immune checkpoint expressed in pediatric Hodgkin lymphoma [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr A17.