Abstract 700: Induction of cancer-specific T-cell responses in patients immunized with P10s-PADRE vaccine

Abstract Introduction: HR+/HER2− breast cancer is the most common form of breast cancer in the United States. HR+/HER2− tumors are known for a low number of tumor-infiltrating lymphocytes (TILs) and considered less immunogenic than other breast cancer subtypes. We have demonstrated that vaccination with P10s-PADRE, a carbohydrate-mimetic-based peptide, cancer vaccine in combination with standard-of-care chemotherapy in HR+/HER2− breast cancer patients led to an increase in TILs and stromal CD3+ T cells. The current study was performed to determine the vaccine specificity and anti-tumor functionality of T cells in treated patients. Methods: Peripheral blood mononuclear cells (PBMCs) collected at the baseline and after vaccination were used in T-cell assays by multi-color ELISpot, examining Th1, Th2, and cytotoxic responses. PBMCs were also interrogated for the vaccine-induced changes in immune gene expression by next-generation RNA-seq analysis. Results: We observed a significant increase in IFN-g, but not in IL-5 or IL-10, responses in post-immune PBMCs after stimulation with P10s, P10s-PADRE and polyclonal stimulation of T cells by anti-CD3/CD28 antibodies. Stimulation with cancer cell lysate resulted in a strikingly high IFN-g response in post-immune samples. Determining the trio of IFN-g, GzB, and IL-2 together with CD4/CD8 cell proliferation assays of consequent post vaccination specimens established the dynamics of effector T-cell populations. RNA-seq data clearly distinguished post-treatment samples by the increase in immune cell activation, cytokine response, and antigen presentation. Conclusions: The data indicate that immunization of HR+/HER2− breast cancer patients with P10s-PADRE in combination with chemotherapy leads to specific activation of T-cells that recognize breast cancer cells. Improving immunogenicity of such immunologically cold tumors could increase the effectiveness of standard therapeutic approaches. Citation Format: John J. Lee, Bernice C. Nounamo, Fariba Jousheghany, Issam Makhoul, Eric R. Siegel, Thomas Kieber-Emmons, Behjatolah Monzavi-Karbassi. Induction of cancer-specific T-cell responses in patients immunized with P10s-PADRE vaccine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 700.