Abstract 3536: Synergistic effects of combined PD-L1 and B7-H3 fusion protein vaccination in a mouse model of pancreatic cancer

Abstract Background: Expressing immune checkpoint ligands is a mechanism for cancer cells to escape immune surveillance. Inhibiting these checkpoints effectively disrupts their escape mechanisms, improving cancer treatment outcomes. Various strategies have been developed to induce immune checkpoint inhibition. In this study, we developed cancer vaccines targeting checkpoint regulators, PD-L1 and B7-H3, and studied the anti-cancer activity in a syngeneic mouse cancer model using Panc02 cancer cells, known for their expression of both PD-L1 and B7-H3. Methods: Vaccines targeting PD-L1 and B7-H3 were recombinant proteins consisting of immune enhancers fused with mouse PD-L1 and B7-H3 extracellular domains, respectively, and produced in E.coli. C57BL/6 mice were vaccinated multiple times with vaccine protein(s) adjuvanted with Poly(I:C) and Freund's Adjuvant. Vaccine-induced humoral responses were measured by ELISA using serum samples collected from the mice. Anti-cancer activity was investigated using mice vaccinated four times followed by subcutaneous implantation of Panc02 cells and additional four vaccinations. Tumor growth was recorded. Tumor-infiltrated immune cells were characterized by immunofluorescence staining. Results: PD-L1 and B7-H3 vaccines induced specific antibodies against PD-L1 and B7-H3. Serum samples collected from PD-L1-vaccinated mice displayed neutralizing activity, blocking the binding of PD-L1 to PD-1. In mice inoculated with Panc02 cells, vaccination with PD-L1, B7-H3, and PD-L1/B7-H3 mixture inhibited tumor growth by 10%, 30%, and 66%, respectively. In the tumors, vaccination with the PD-L1/B7-H3 mixture increased CD4+ T cells significantly, indicating that vaccine-induced immune cell infiltration plays a critical role in tumor inhibition. Conclusions: Our results demonstrate that vaccination is an effective strategy for inducing neutralizing antibodies that inhibit immune checkpoints in cancer treatment. The most significant tumor growth inhibition observed with the combination of PD-L1 and B7-H3 vaccination underscores the necessity of simultaneously targeting multiple checkpoint regulators in cancers that express these proteins. Citation Format: Kenneth Nansheng Lin, Melvin Toh, Hong Wang. Synergistic effects of combined PD-L1 and B7-H3 fusion protein vaccination in a mouse model of pancreatic cancer [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 3536.