Differences in peripheral blood immune cells composition and pathway expression between non-Hispanic Black and non-Hispanic White females at different disease states of breast cancer.

e12564 Background: Racial differences in breast cancer are well established. Compared to non-Hispanic white (NHW) women, non-Hispanic Black (NHB) women have a lower diagnosis rate but suffer higher mortality of ER+/HER2- breast cancer. Immune fitness and function play a critical role in cancer control and patient outcome. We hypothesize that the peripheral immune system differs between NHB and NHW women. To test this hypothesis, we performed single-cell RNA sequencing (scRNA-seq) and compared peripheral blood mononuclear cell (PBMCs) molecular profiles between NHB and NHW women in three groups: high-risk lesion (HRL), ductal carcinoma in situ (DCIS), and treatment-naïve primary ER+/HER2- invasive BC (BC). Methods: PBMCs were isolated from age-matched NHB (HRL:10, DCIS:6, BC:14) and NHW women (HRL:10, DCIS:4, BC:13). scRNA-seq was performed using the 10x Genomics 5’ v2 platform. Data processing included quality control, normalization, scaling, and clustering with Seurat (v5.1.0). Cell types were identified via reference-guided annotation with Azimuth (v0.5.0) and canonical markers. Pathway enrichment of differentially expressed genes between NWB and NHW was performed with Gene Set Enrichment Analysis (GSEA). Results: 436,793 high-quality single PBMCs were identified, including subtypes of CD4 and CD8 T cells, B cells, NK cells, monocytes, and dendritic cells (DC). In the HRL group, NHB patients had higher proportions of CD4-Tcm Tfh (central memory follicular helper T cells) and lower proportions of ASDC (AXL+SIGLEC6+ DC), DC2 (CD1C+_A conventional DC), DC3 (CD1C+_B conventional DC), and classical monocytes. In the DCIS group, NHB patients exhibited higher proportions of activated regulatory T cells (ID2). In the BC group, NHB showed higher proportions of CD4-Tcm Th0 (central memory helper 0 T cells) and lower proportions of classical monocytes. GSEA revealed that in the BC cohort, NHB patients had enrichment of inflammatory and hypoxia pathways in B cells, CD4 T cells, and NK cells, and enrichment of interferon response pathways in monocytes. Conclusions: This study revealed differences in PBMC composition and gene expression between NHB and NHW in women with HRL, DCIS, and BC. Further evaluation of the differences by race in the systemic immune response during breast cancer development is warranted.