Abstract 4106: A novel synthetic long peptide vaccine composition targeting multiple PD-L1 T-cell epitopes exhibits anti-tumor efficacy in a syngeneic mouse colorectal cancer model

Abstract Background Cancer immunotherapy targeting immune checkpoints has shown promising clinical anti-tumor efficacy over the past decades. Monoclonal antibodies against PD-1 or PD-L1 have demonstrated clinical efficacy in a wide spectrum of cancer types. Despite the clinical success of therapies using immune checkpoint inhibitors (ICI), only a subset of patients shows long-lasting tumor remission, while resistance and toxicities to these agents remain to be addressed. This supports the need to explore alternative immunotherapeutic approaches or additional therapies for synergistic combinations to augment the anti-tumor efficacy for cancer patients receiving ICI therapies. Methods As an active immunotherapy approach, we developed a blend of four novel synthetic peptide vaccines each containing one PD-L1 T-cell epitope with individual covalent conjugation of the carrier protein, keyhole limpet haemocyanin (KLH), which is to break immune tolerance towards self-molecules. The four PD-L1 epitopes range from 30 to 40 amino acids in length, which target different domains of mouse PD-L1 protein. The PD-L1 peptides were synthesized using the technique of solid-phase peptide synthesis and purified by high performance liquid chromatography. All naked peptides were conjugated to KLH protein by thiol-maleimide reaction, followed by dialysis for purification. The peptide vaccines were formulated with a Toll-like receptor 9 agonist CpG ODN 1826 and aluminum hydroxide gel for animal immunization. Results These synthetic long PD-L1 peptide vaccines are highly immunogenic in a syngeneic BALB/c mouse model for a cell-mediated immune response when admixed and formulated with adjuvants. The vaccine composition is well tolerated by the mice with no evidence of toxicity and autoimmunity. In vivo administration of the vaccine formulation can elicit a positive PD-L1-specific T cell response in splenocytes with an IFN-γ ELISPOT assay. Moreover, in vivo vaccination of these PD-L1 peptide vaccines in a CT26 syngeneic mouse preventive colorectal cancer model exhibits a potent anti-tumor activity, with a 54% tumor growth inhibition (p0. 001) in tumor volume compared to that of the adjuvant-control group at the end of the study. Conclusions Taken together, our data demonstrate a proof-of-concept that T cell-based PD-L1 peptide vaccines are a promising therapeutic strategy for inducing anti-tumor immunity to combat cancers. Citation Format: Kenneth Nansheng Lin, Melvin Toh. A novel synthetic long peptide vaccine composition targeting multiple PD-L1 T-cell epitopes exhibits anti-tumor efficacy in a syngeneic mouse colorectal cancer model abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 4106.