Cancer stem cells are prevalent in the basal-like 2 and mesenchymal triple-negative breast cancer subtypes .

Triple-negative breast cancer (TNBC) is an aggressive subtype with the most unfavorable clinical outcomes, in part due to tumor heterogeneity, treatment resistance, and tumor relapse. The TNBC subtypes [basal-like 1 (BL1), basal-like 2 (BL2), mesenchymal (M), and luminal androgen receptor (LAR)] are biologically and clinically distinct entities that respond differently to local and systemic therapies. Therefore, we need to have a better understanding of cancer stemness relating to drug-resistant populations in the TNBC subtypes. Breast cancer stem cell (BCSC) distribution was investigated using an integrated flow cytometry approach with the ALDEFLUOR™ assay (ALDH) and CD24/CD44 antibodies. In total, 27 commercially available cell lines derived from normal and malignant mammary tissue were characterized into differentiated tumor cells and/or BCSC subpopulations (ALDHCD44CD24 enriched mesenchymal-like BCSCs, ALDHnon-CD44CD24 enriched epithelial-like BCSCs, and highly purified ALDHCD44CD24 BCSCs). BCSCs were not only enriched in estrogen receptor (ER) negative (mean, 49.6% 6.9% in ER+) and TNBC cell lines (51.3% 2.1% in Luminal A), but certain BCSC subpopulations (e.g., enriched mesenchymal-like BCSCs) were also significantly more common in the M (64.0% 6.2% in BL1; 64.0% 0% in LAR) and BL2 (77.4% 6.2% in BL1; 77.4% 0% in LAR; 77.4% 10.4% in TNBC UNS) TNBC subtypes. In contrast, ALDH status alone was not indicative of ER status or BC subtype. Taken together, these findings demonstrate the enrichment of potentially treatment-resistant BCSC subpopulations in the M and BL2 triple-negative breast cancer subtypes.