Optimal adoptive cell therapy (ACT) should contribute to effective cancer 
treatment. The unique ability of natural killer (NK) cells to kill cancer cells 
independent of major histocompatibility requirement makes them suitable as ACT 
tools. Herceptin, an antihuman epidermal growth factor receptor-2 (anti-HER2) 
monoclonal antibody, is used to treat HER2+ breast cancer. However, it has 
limited effectiveness and possible severe cardiotoxicity. Given that Herceptin 
may increase the cytotoxicity of lymphocytes, we explored the possible 
augmentation of NK cell cytotoxicity against HER2+ breast cancer cells by 
Herceptin. We demonstrated that Herceptin could interact with CD16 on NK cells 
to expand the cytotoxic NK (specifically, CD56dim) cell population. 
Additionally, Herceptin increased NK cell migration and cytotoxicity against 
HER2+ breast cancer cells. In a pilot study, Herceptin-treated NK cells shrunk 
lung nodular metastasis in a woman with HER2+ breast cancer who could not 
tolerate the cardiotoxic side effects of Herceptin. Our findings support the 
therapeutic potential of Herceptin-treated NK cells in patients with HER2+ and 
Herceptin-intolerant breast cancer.