Abstract Background: KEYNOTE-522 established neoadjuvant pembrolizumab plus chemotherapy as standard care for early-stage TNBC, improving pathological complete response (pCR) rates. However, up to 45% of patients (pts) do not achieve pCR, and mechanisms driving differential responses are unclear. This study aimed to identify biological pathways distinguishing pCR from non-pCR pts at baseline (BL) and to characterize how these pathways evolve during treatment (DT) and post-surgery (PS). Methods: Blood samples from TNBC pts were collected at BL, DT, and PS. Peripheral blood mononuclear cells (PBMCs) were isolated by density gradient centrifugation. Single-cell RNA sequencing was performed using the Parse Biosciences platform. Gene Set Enrichment Analysis identified enriched pathways using the Molecular Signatures Database (MSigDB) hallmark (50 gene sets) and Reactome (1,787 gene sets) collections. Comparisons were made between pCR and non-pCR patients at each timepoint and within groups (DT vs. BL). Results: Among 18 sequenced pts, 11 achieved pCR. A total of 883,183 cells across 17 cell types were analyzed. At BL, responders showed elevated interferon (IFN) and cytokine signaling, mainly from lymphocytes, along with chemokine upregulation in classical monocytes (CM) and increased DAP12 expression in CD8+ T cells. During treatment, responders demonstrated a myeloid-driven TNFα/NFκB inflammatory response and upregulated GPCR signaling in CM. In contrast, non-responders exhibited ineffective myeloid IFN signaling and downregulated TNFα/NFκB activity across all cell types DT. Conclusions: pCR is distinguished by a coordinated lymphocyte and myeloid immune activation. Non-responders exhibit ineffective signaling and suppressed inflammatory pathways. These divergent mechanisms highlight potential biomarkers to guide patient stratification and therapeutic optimization in TNBC. Citation Format: Nickolas Stabellini, Prerana B. Parthasarathy, Iluja Gautam, Patricia A. Rayman, Monaben Patel, Paul G. Pavicic Jr, Adam Moen, Brian Race, Eden Mundell, Amanda Trevino, Jennifer Powers, Tyler Joseph Alban, Jennifer Ko, Bahar Moftakhar, Takae Mizukami, Cynthia Owusu, Timothy A. Chan, Alberto J. Montero, C. Marcela Diaz-Montero. Biological pathways of response to neoadjuvant chemo-immunotherapy in triple-negative breast cancer (TNBC) abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7768.
Stabellini et al. (2026) studied this question.