CD6-Directed Immunotherapy Targets Breast Cancer Stem Cell Function and Enhances Immune-Mediated Cytotoxicity in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is associated with recurrence, metastasis, and limited durable responses to immunotherapy, in part due to persistence of breast cancer stem cells (BCSCs). We investigated whether CD6-directed immunotherapy with the monoclonal antibody UMCD6 enhances immune-mediated killing and alters function of BCSC in stem cell-enriched TNBC models. The SUM-149 and SUM-159 cell lines were analyzed for CD6 ligand expression, cocultured with human peripheral blood mononuclear cells (PBMCs) treated with UMCD6, pembrolizumab, or isotype control, and assessed by live-cell cytotoxicity imaging, flow cytometry, soft agar colony formation, and extreme limiting dilution sphere assays. Both TNBC lines co-expressed the CD6 ligands CD44, CD166/ALCAM, and CD318/CDCP1. UMCD6 significantly increased PBMC-mediated apoptosis and reduced tumor cell survival in both models, with greater activity than pembrolizumab under these in vitro conditions. In surviving SUM-159 cells, UMCD6 reduced the ALDH+ population without significantly altering CD44+CD24− frequency, indicating preferential effects on a distinct stem-like compartment. Functionally, UMCD6 decreased anchorage-independent colony formation and reduced sphere-forming frequency from 1/33.6 to 1/68.3 cells (P = 0.0186). These findings identify the CD6 ligand axis as a therapeutic vulnerability in BCSC-enriched TNBC and support further preclinical evaluation of CD6-directed immunotherapy as a strategy to enhance antitumor immunity while limiting tumor-initiating capacity.