Enhancement of antitumor potency of extracellular vesicles derived from natural killer cells by IL-15 priming

Purpose: Natural killer (NK) cells are the key subset of innate-immunity lymphocytes; they possess antitumor activities and are used for cancer immunotherapy. In a previous study, extracellular vehicles (EVs) from NK92MI cells were isolated and exploited for their ability to kill human cancer cells in vitro and in vivo (multiple injection methods). Here, the potential of NK-cell derived EVs (NK-EVs) for immunotherapy was improved by priming with interleukin (IL)-15. Methods: NK-EVs were isolated from the culture medium without or with IL-15 (NK-EVSIL-15) by ultra centrifugation and were purified via density gradient ultracentrifugation. In addition, NK-EVs and NK-EVSIL-15 were characterized by transmission electron microscopy, nanoparticle-tracking analysis, and western blotting. Flow cytometry, bioluminescence imaging, and a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay were performed for apoptosis, protein expression, cell proliferation, and cytotoxicity analyses. Furthermore, xenograft tumor bearing mice were injected with PBS, NK-EVs, or NK-EVSIL-15 is intravenously five times. Tumor growth was monitored using calipers and bioluminescence imaging. Toxicity of the nanoparticles was evaluated by measuring the body weight of the mice. Results: NK-EVSIL-15 showed significantly higher cytolytic activity toward human cancer cell lines (glioblastoma, breast cancer, and thyroid cancer) and simultaneously increased the expression of molecules associated with NK-cell cytotoxicity. When compared with NK-EVs, NK-EVSIL-15 significantly inhibited the growth of glioblastoma xenograft cells in mice. In addition, both NK-EVs and NK-EVSIL-15 were not significantly toxic to either normal cells or mice. Conclusion: IL-15 may improve the immunotherapeutic effects of NK-EVs, thus improving the applications of NK-EVs in the future.