NK-92 serial killers - characterization of potential limitations to cytotoxic capacity

Abstract The NK cell line NK-92 and transgenic variants are receiving attention as adoptive transfer immunotherapies to treat a range of malignancies. However, since NK-92s are themselves tumors, they require irradiation prior to transfer and are potentially susceptible to attack by patients’ immune systems. In the present study, we investigated NK-92 serial killing against various tumor cells as well as the effects of gamma-irradiation and ligation of the death receptor Fas (CD95) on NK-92 cells and the susceptibility of NK-92s to attack by primary NK cells. To evaluate serial killing, we used a 51Cr-release assay with very low effector to target (E:T) Raji, Daudi or K562 tumor cell ratios at 2, 4, 6, and 8 hour time points. NK-92s that were maintained in high concentrations of recombinant IL-2 were able to kill as many as 14 Raji cells per NK-92 cell in 8 hours at an E:T of 1:32. NK-92 cells retained high cytotoxic activity immediately after irradiation with 10 Gy but lost >50% activity 24 hours after just 7 Gy irradiation. Despite strong expression of CD95, NK-92s maintained viability after 24 hours of Fas-ligation with an anti-CD95 IgM as compared to cells treated with an isotype control antibody. While NK-92s survived 16 hours of exposure to unstimulated primary NK cells, they were killed when exposed to IL-2 activated primary LAK cells. In conclusion, we recommend that adoptive transfer of irradiated NK-92 cells into patients be made as quickly as possible post-irradiation of the NK-92s. Genetic modification of NK-92 cell lines to secrete IL-2 as an autocrine growth factor may need reconsideration because in vivo activation of endogenous LAK cells by IL-2 could reduce the number of NK-92 cells within tumors and thus decrease therapeutic efficacy.