Abstract PO5-02-05: Assessing Vaccine-Mediated Cellular Immune Responses in Patients Receiving Combined Vaccine and Chemotherapy Treatment

Abstract Background: We showed recently that immunizing breast cancer patients with the P10s-PADRE vaccine in combination with chemotherapy activates both humoral and cellular immune responses. Induction of specific antibody response was easily determined by measuring anti-P10s antibodies, however, we observed increase in serum interferon-gamma (IFNg) and tumor necrosis factor-alpha (TNFα) as well as activation of Natural Killer (NK) and T-cells, accurate assessing of which proved complicated due to the non-specific impact of chemotherapy. This study was undertaken to further distinguish the vaccine-induced cellular immune responses from the immune effects induced by chemotherapy, and to evaluate their potential contributions to anti-tumor activity. Methods: Peripheral blood mononuclear cells (PBMCs) from twenty-five subjects who received a combination of P10s-PADRE and standard-of-care chemotherapy were utilized for gene expression analysis and various immune assays. PBMC samples collected before and after treatment were subjected to flow cytometry and ELISpot assays, as well as bulk and single-cell RNA sequencing analyses. For functional assays PBMCs were stimulated with P10s, P10s-PADRE, anti-CD3/CD28, and tumor cell lysates. Differential gene expression analysis, gene set analysis, Hiearchical Clustering, and principal component analysis were performed. Results: Non-specific (anti-CD3/CD28) and specific (peptide, cancer cells) stimulation of PBMCs significantly activated immune cells in post-immune samples compared to pre-immune samples as measured by cytokine secretion, with IFNg being the dominant cytokine secreted. Flow cytometry showed activation of both natural killer and CD8+ cells measured by a significant increase in CD69 expression. These data plus detection of Ab response suggests induction of both humoral and cellular immune responses. However, bulk RNA-seq performed on 24 patients indicate a dramatic reduction of B, T, and NK cells in the post-immune samples. Deconvolution reanalysis of the bulk sequencing data together with complementary scRNA-seq analysis of PBMCs of select subjects supports vaccine-mediated activation of both CD8+ and NK cells and suggests NK cells as a source of detected serum increase in IFNg and TNFα. Conclusions: The P10s-PADRE vaccine elicits substantial and quantifiable cellular and humoral immune responses, despite the deleterious effects of chemotherapy on effector immune cells involved. The evidence indicates that the activation of both CD8+ and NK cells is facilitated by the vaccine. It is probable that the NK cells are the primary contributors of IFNγ and TNFα. These findings carry considerable implications for the structuring of advanced phase trials and the potential application of the vaccine in treating other forms of malignancies. Citation Format: Donald Johann, Fariba Jousheghany, Emily Blitz, Bernice Nounamo, Sam Makhoul, Eric Siegel, Thomas Kieber-Emmons, Behjatolah Monzavi-Karbassi. Assessing Vaccine-Mediated Cellular Immune Responses in Patients Receiving Combined Vaccine and Chemotherapy Treatment [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-02-05.