Abstract 1906: The role of immunogenic SPANX antigens in distinct cancer stem cell subsets within triple negative breast cancers

Breast cancer is the leading cause of death in women, primarily due to metastatic disease rather than the primary tumor. Median survival with metastatic breast cancer is 3 years, with no statistically significant change in survival in over 20 years. Triple-negative breast cancer (TNBC) lacks estrogen and progesterone receptor expression and ERBB2 amplification and is the most lethal form of breast cancer. It is resistant to endocrine therapy and chemo-resistance and metastasis invariably emerge. Increasing evidence indicates that cancer stem cells (CSCs) are chemo-resistant and initiate metastasis. We have previously identified and characterized distinct subsets of CSCs within TNBC cell lines and patient-derived tumors. Within the surface CD44+ populations, CD24 expression defines two subsets of CSCs: mesenchymal-like CD24neg and epithelial-like CD24low+. Epithelial-like CD24low+ cells are more aggressive than CD24neg cells, with increased self-renewal, migration, invasion, tumor-initiation and metastatic potential. In addition, CD24low+ cells are enriched following chemo- and radiation therapies; greater than 80% of CD24neg cells die and the majority of cells that survive are CD24low+. Hence, therapies that selectively eliminate the CD24low+ population in TNBC have the potential to be of enormous benefit to cancer patients. Therapeutic targeting of surface CD24 using antibodies have not been successful due to the expression of CD24 on many different cell types including B cells. Thus, there is a need for more selective strategies to target CSC populations without affecting normal cells. Here, we show for the first time, increased expression of a family of cancer/testis antigens (CTAs) namely SPANX in the most aggressive CD24low+ CSC population. CTAs belong to a class of testis-derived proteins which are only expressed in germ cells in the male testis, and the expression of CTA genes is entirely silenced in the adult somatic tissues. Thus, SPANX may serve as a selective marker for targeting CSCs. In addition, we demonstrate a functional role for SPANX in mediating CSC phenotype. Knockdown of SPANX decreased the percent of CD24low+ CSCs in TNBCs. It also attenuated the proliferation, migration and invasion of CD24low+ cells. Radiation treatment increased SPANX expression levels and enriched for CD24low+ cells. Loss of SPANX resulted in increased cell death of CD24low+ cells after radiation treatment. Hence, we show that SPANX may be promoters of the most aggressive CSC subset of TNBC. Since SPANX is highly immunogenic, our data provide rationale for further testing of combined radiation and immunotherapy approaches in the treatment of this deadly cancer. It also supports the use of protective and therapeutic SPANX vaccines against the most aggressive CSC subset in TNBC. Citation Format: Lauren Shahin, Elena Cubedo, Ebony Coats, Jeff Boyd, Diana Azzam. The role of immunogenic SPANX antigens in distinct cancer stem cell subsets within triple negative breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1906. doi:10.1158/1538-7445.AM2017-1906