Abstract PS3-12-20: Activated PMN-MDSC phenotype associates with immune-related adverse events to immunotherapy in triple negative breast cancer

Abstract Introduction/Background: Triple-negative breast cancer (TNBC) is an aggressive subtype, that comprises approximately 15-20% of all breast cancers. Currently, the standard for care in early-stage TNBC, as established the KEYNOTE-522 (KN-522) trial is combined neoadjuvant cytotoxic chemotherapy plus pembrolizumab. The trial demonstrated that the addition of pembrolizumab, significantly improved pathologic complete response rates and survival outcomes. However, pembrolizumab is also associated with immune-related adverse events (irAEs), leading to early treatment discontinuation. In the KN-522 trial, irAEs occurred in 34% of patients. Identifying predicative biomarkers may inform new strategies for intervention and management in patients likely to develop irAEs. Therefore, this study aimed to identify circulating immune determinants of irAEs to immunotherapy in TNBC. Methods: Peripheral blood was collected at multiple time points from 41 patients with TNBC undergoing neoadjuvant therapy with the KN-522 regimen. Blood was collectedat: baseline (BL), on cycle 3 day 1, and after surgery. Peripheral blood mononuclear cells (PBMCs) were isolated via density gradient centrifugation and profiled using high-dimensional flow cytometry to assess immune cell composition and phenotypes. For transcriptomic analysis, single-cell RNA sequencing was performed using the Parse Biosciences platform (pipeline split-pipe v1.2.0). The data was then analyzed with the Seurat pipeline to identify differentially expressed genes and investigate cell-state dynamics across samples. In parallel, plasma cytokine levels were quantified using the Olink platform to assess circulating biomarkers associated with irAE development. Results: Twelve of the 40 patients enrolled in the study (30%) experienced a documented irAE, including hypothyroidism (n = 4), hepatitis (n = 3), adrenal insufficiency (n = 2), colitis (n = 1), myocarditis (n = 1), and hemophagocytic lymphohistiocytosis (n = 1). At baseline (BL), flow cytometry revealed an accumulation of PMN-MDSCs with an activated/platelet-associated phenotype (CD16- CD62p+ CD42b+; 42.1% vs. 1.24%) among irAR+ patients. In contrast irAE- patients had significantly higher proportions of immature non-platelet-associated APC like traits PMN-MDSC (CD80+, CD86+, HLDR+; 90.4% vs. 40.7%). Pathway analysis showed irAE+ patients at BL have enriched interferon-alpha/gamma response, TNFα signaling via NF-κB, IL2–STAT5 signaling. On-treatment, irAE+ patients exhibited either sustained or amplified enrichment of these pathways. Proteomics analysis showed a correlation between irAE and the presence of the following pro-inflammatory cytokines: IL-10, CLEC7A, IL-1 alpha and CXCL1. Conclusion(s): Together these multi-omic analysis indicates that patients who develop irAEs may have a distinctive immune signature characterized by increased frequencies of activated PMN-MDSCs, and an inflammatory milieu conductive to overly activated CD4+ T cells, and B cells. Citation Format: J. Basit, P. Parthasarathy, I. Gautam, N. Stabellini, P. Rayman, M. Patel, P. Pavicic, J. Powers, A. Moen, B. Race, E. Mundell, A. Trevino, T. Chan, B. Moftakhar, T. Mizukami, C. Owusu, T. Alban, V. Makarov, A. Montero, M. Diaz-Montero. Activated PMN-MDSC phenotype associates with immune-related adverse events to immunotherapy in triple negative breast cancer abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-12-20.