Hemacytotoxicity and natural killer lytic index: New parameters to evaluate natural killer cell immunity for clinical use in cancer

Cytotoxicity assays with patient peripheral blood mononuclear cell (PBMC)-derived natural killer (NK) cells are useful in evaluating the innate immunity of patients with cancer. However, the size of the NK cell population in PBMC preparations may have significant effects on the assay outcome. Therefore, the present study examined the effect of NK cell frequency in a cytotoxicity system to investigate NK cell immunity in post-surgical colorectal cancer patients. For this, hemacytotoxicity was assessed using PBMC preparations, and lymphocyte subset populations were analyzed in samples obtained from 47 patients and 45 healthy volunteers. In addition, a new theoretical parameter, the ‘NK lytic index’, was termed to represent the per-cell cytotoxicity and compensate for the NK cell frequency effect during PBMC preparations. Notably, the patterns of hemacytotoxicity and NK lytic index did not coincide in follow-up studies with consecutive patients following surgical intervention. In addition, it was determined that NK cell NKG2D expression influences NK lytic index, but not hemacytotoxicity. Transforming growth factor (TGF)-β-bound lymphocytes influenced hemacytotoxicity and NK lytic index. These findings indicate that total cell activity (hemacytotoxicity) is not a sum of per-cell activities (NK lytic indexes), suggesting that clinicians should employ NK lytic index in addition to hemacytotoxicity in order to precisely determine how to enhance NK cell immunity in patients with cancer, either focusing on recovering the number of NK cells or boosting NK cell activity in single cell levels, or both.