Abstract: Acquisition of CD9 protein on the cell surface of NK cells confers an immunosuppressive phenotype to the NK cells, making them less effective in immunotherapy. CD9 can be transferred from tumor cells to NK cells present in the tumor environment through the process of trogocyotosis. Methods of enhancing NK cell anti-tumor activity can include evaluating NK receptor ligand expression within the tumor microenvironment(s) for patients eligible to receive NK cell immunotherapy.