Abstract Introduction: We demonstrated that vaccination of HR+/HER2− breast cancer subjects with P10s-PADRE cancer vaccine in combination with standard-of-care chemotherapy led to generation of antibodies (Ab), activation of Natural Killer (NK) and T cells, increases in IFNγ and TNFα, and increases in TILs and stromal CD3+ T cells. The current study was conducted to assess the vaccine's efficacy, particularly focusing on tumor shrinkage and pathologic Complete Response (pCR). Additionally, it aims to explore the association of specific peripheral-blood immune responses, such as antibody levels and NK and T-cell activation, with pCR and objective reduction in primary tumor size. Methods: Blood samples collected at baseline and after treatment of at least 25 subjects were used in characterization of immune responses. Serum and plasma samples were used in ELISA, cytokine, and glycan-array assays to measure anti-peptide and anti-glycan antibodies and blood-cytokine levels. Peripheral blood mononuclear cells (PBMCs) were interrogated by RNA-seq, flow cytometry, and ELISpot assays. Results: We observed that high levels of anti-peptide and anti-GD2 antibody responses correlated with pCR. Ab response was positively correlated with differential expression of CD56, GZMA, IFNγ, NKp46, and CD8b, among which GZMA and NKp46 showed a statistically significant association. Vaccine-induced functional cellular immune responses (measured by differential expression of IFNγ, GZMA, CD8, CD69, NKp46, CD56, and PRF1) correlated with reduction of primary tumor size. We also observed substantial differences among immune responders vs non-responders in immune gene expression and in cytokine production in post-immune PBMCs after specific and polyclonal stimulation. Conclusions: The data suggest that immunization of HR+/HER2− breast cancer patients with P10s-PADRE in combination with chemotherapy leads to both humoral and cellular immune responses. While induction of a robust antibody response is a good marker for potential clinical efficacy, vaccine-associated increases of the ratio of CD8+ cells and the expression of CD69, CD8b, GZMA, IFNγ, NKp46, and PRF1 seem to serve as additional efficacy indicators. Future studies are underway to validate these findings and establish reliable prognostic biomarkers. Citation Format: Fariba Jousheghany, John P. Lee, Bernice Nounamo, Vinay Raj, Jennifer L. Faulkner, Eric Siegel, Issam Makhoul, Thomas Kieber-Emmons, Behjatolah Monzavi-Karbassi. Characterizing peripheral immune correlates of tumor response in breast cancer patients immunized with the P10s-PADRE vaccine [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(6_Suppl):Abstract nr 3860.
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