Abstract Background: HOXB13 gene expression has been extensively studies in hormone receptor positive (HR+) post-menopausal breast cancer patients. Recently, we demonstrated that HOXB13 is expressed in a subset of triple negative breast cancer (TNBC) patients and that ectopic expression of HOXB13 in a mouse mammary model of TNBC confers a tumoral growth advantage by impeding antitumor T-cell immunity. Here, we performed comparative spatial transcriptomic profiling of the immune microenvironment of HOXB13+ and HOXB13- human TNBCs. Methods: Using the Nanostring CosMx platform, we performed 1000-plex spatial transcriptomic profiling of 15 treatment-naïve human HOXB13+ TNBCs and 8 treatment-naïve HOXB13- TNBCs. Cells were segmented with Cellpose, then filtered based on RNA counts. Sparse areas of tissue were filtered entirely to retain structured tissue. Expression was normalized, log-transformed, and scaled for dimensionality reduction with PCA and UMAP. Cell-typing was performed using an immune-oncology reference profile. Cell types were validated via marker genes identified by differential expression. Differential expression analysis was conducted with generalized linear mixed modeling. Analytical neighborhood enrichment analysis for each cell type was conducted using a spatial neighborhood graph constructed by Delaunay triangulation. Results: Using spatial transcriptomics, we find two immune “neighborhoods” within TNBC patients denoted by an association of multiple cell types that preferentially are found adjacent to one another. The first “cytotoxic” neighborhood is associated with CD8+ T cells, antigen presenting cells, and CD4+ T cells all of which preferentially are found near one another but are not found in proximity to other immune cell types. The second “plasmablast-rich” neighborhood is comprised mostly of plasmablasts with B cells and plasma cells also present. HOXB13+ patients had significantly more plasmablasts (p= 0.017) than HOXB13- patients and these plasmablasts were typically found associated with B cells or plasma cells in a plasmablast-rich neighborhood and not associated with CD8+ T cells or any of the other cytotoxic neighborhood members. Conclusions: A plasmablast-rich immune neighborhood is more frequently seen in HOXB13+ rather than HOXB13- TNBCs, and such a neighborhood is not associated with CD8+ T cells. We have recently shown that HOXB13 expression in a preclinical model of TNBC impedes anti-tumor CD8 T-cell immunity. Our findings raise the possibility that plasmablasts may directly suppress CD8+ T cells or that the formation of these plasmablast-rich neighborhoods occupies a niche that comes at the cost of anti-tumor/cytotoxic neighborhoods. Understanding the molecular underpinnings of this process could provide for novel immunotherapeutic strategies. Citation Format: B. Flood, C. Nawrocki, L. Neiman, K. Treuner, Y. Zhang, D. Sgroi. Spatial transcriptomics of TNBCs show an association between HOXB13 expression and formation of a plasmablast-rich neighborhood 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 PS4-07-08.
Flood et al. (Tue,) studied this question.