Assessment of physiologic natural killer cell cytotoxicity in vitro

Here, we describe an improved (51)chromium release assay (CRA) to compare donor natural killer (NK) cell activity. To validate the assay, we analyzed sample preparation, incubation, and cryopreservation of NK cells. The effector-to-target ratio was corrected for the percentage of NK cells. A logarithmic curve was fitted to the data of the CRA for calculation of the maximum activity. The specific lysis was standardized to a reference sample and normalized to the mean specific lysis of the reference. We found that a longer time span involved with both the addition and the removal of DMSO increased the recovery of NK cell activity. Freezing and thawing reduced the cytotoxicity of NK cells but sustained the relative differences that were seen between freshly prepared NK cells. In contrast, medium incubation of thawed cells markedly increased the cytotoxic potential but also deranged these relative differences. Those were widely equalized when cells were stimulated with IL-2. In conclusion, we established a standardized assay with cryopreserved peripheral blood mononuclear cells as an appropriate tool for investigation of individual physiologic NK cell activity. This assay may help to predict donor NK cell activity in vivo, to reconcile conflicting data about NK cells obtained in transplantation studies.