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Reply to Maguer-Satta et al.: Revisiting the mammosphere assay

The letter from Maguer-Satta et al. (1) highlights the potential for CD10 to be used as a marker for mammary stem cells based on their work illustrating that FACS-sorted CD10+ cells form bipotent adherent colonies and mammospheres in nonadherent culture (2). In our work (3), we used immunomagnetic beads to isolate luminal and basal/myoepithelial populations through a similar sorting strategy. Although both studies reinforce the notion that epithelial cell adhesion molecule (EpCAM) and CD10 enrich for cells with different functional and phenotypic characteristics (4), Maguer-Satta et al. (1) note a potential difference regarding stem cell activity. In their study, CD10+ cells enriched for mammosphere formation, whereas we reported increased spheres from EpCAM+CD10− cells in both adherent and nonadherent conditions. Additionally, consistent with others, we found that bipotent progenitor activity was not restricted solely to basal cells using both in vitro and in vivo assays; hence, we refrained from making conclusions about whether CD10 or EpCAM cells might enrich for stem cells. Rather, we focused on how functional and phenotypic differences in EpCAM+ and CD10+ cells contribute to human breast cancer heterogeneity.