Until recently, the main limiting factor to the clinical application and efficacy of NK cells was the difficulty in obtaining sufficient cell numbers for adoptive transfer. Development of novel methods of expanding primary human NK cells ex vivo has renewed interest in NK cells for immunotherapy for cancer. [2][3][4][5][6] Expanded NK cells have enhanced expression of activating receptors, 4,7,8 that in turn improves their antitumor cytotoxicity. Where activating receptors did not sufficiently elicit an antitumor response, researchers augmented the antitumor effect of NK cells by expression of chimeric Ag receptors. 9-11 Ultimately, the success of NK cell adoptive immunotherapy for cancer depends not only on target recognition but also on homing of NK cells to the tumor target in vivo. Thus, the effector cells must express the appropriate chemokine receptors.Cancer cells express a wide array of chemokines and chemokine receptors that are instrumental in tumor survival 12 and metastatic spread. 13 Lymph nodes, particularly the tumor-draining nodes, are the foci of metastatic spread of tumors for a vast number of cancer types. 13,14 The expression of CCR7, a member of the G protein-coupled receptor family, on lymphocytes directs their homing to lymph node, coordinates primary immune responses, and induces peripheral immune tolerance. 15 CCR7 expression on tumor cells has been reported and shown to play a pivotal role in lymph node metastasis of various cancers such as breast, 16 pancreatic, 17 thyroid, 18 and colorectal 19 cancers; oral squamous cell carcinoma 20 ; melanoma 21 ; and lymphoma. 22 Lymph node involvement in these cancers is generally associated with poor prognosis.Peripheral NK cells express a variety of chemokine receptors such as CXCR1, CXCR3, and CXCR4 with subsets expressing CCR1, CCR4, CCR5, CCR6, CCR7, CCR9, CXCR5, and CXCR6. Expression of CCR7 on NK cells can facilitate homing to lymph nodes, which, in the context of adoptive immunotherapy for various cancers, would offer a main advantage in targeting lymph node metastases. However, CD56 bright CD16 ÏȘ NK cells, which primarily secrete cytokines, express CCR7, but CD56 dim CD16 Ï© NK cells, which are primarily responsible for cytotoxicity, do not. 23 In a previous study, we reported that expanded NK cells are predominantly of CD56 Ï© CD16 bright phenotype and did not express CCR7. 7 In this study, we sought to express CCR7 on expanded NK cells ex vivo to facilitate lymph node homing on adoptive transfer. Although investigators have used viral vectors to gene modify NK cell lines 10,24 and primary NK cells, 9,25 because of safety concerns over integrating viral vectors there has been a recent shift in emphasis toward nonviral methods of gene transfer, particularly nonintegrating, mRNA-based electroporation approaches. 11 However, electroporation of NK cells has been difficult in that the transfection efficiency and viability of NK cells are low, and high-throughput electroporation methods for gene modifying clinically relevant NK cell numbers are ...