HDAC6 plays a non-canonical role in the regulation of anti-tumor immune responses, dissemination, and invasiveness of breast cancer.

Despite the outstanding clinical results of immune checkpoint blockade (ICB) in melanoma and other cancers, clinical trials in breast cancer have reported low responses to these therapies. Current efforts are now focused on improving the treatment efficacy of ICB in breast cancer using new combination designs such as molecularly targeted agents, including histone deacetylase inhibitors (HDACi). These epigenetic drugs have been widely described as potent cytotoxic agents for cancer cells. In this work, we report new non-canonical regulatory properties of ultra-selective HDAC6i over the expression and function of epithelial-mesenchymal transition pathways and the invasiveness potential of breast cancer. These unexplored roles position HDAC6i as attractive options to potentiate ongoing immunotherapeutic approaches. These new functional activities of HDAC6i involved regulation of the E-cadherin/STAT3 axis. Pre-treatment of tumors with HDAC6i induced critical changes in the tumor microenvironment, resulting in improved effectiveness of ICB and preventing dissemination of cancer cells to secondary niches. Our results demonstrate for the first time that HDAC6i can both improve ICB antitumor immune responses and diminish the invasiveness of BC with minimal cytotoxic effects, thus departing from the cytotoxicity-centric paradigm previously assigned to HDACi.