A single cell marker of active NK cytotoxicity: only a fraction of target binding lymphocytes are killer cells.

The frequency of cytotoxic NK cells has been directly measured. The cytotoxic assay is performed by preparing conjugates of effector cells bound to target cells and assessing under the microscope target cell lysis by the trypan blue dye exclusion test. With Ficoll-Hypaque purified cells, it was found that 11--13% of lymphocytes formed conjugates with K562 or Molt-4 target cells, but only 3--4% of the total lymphocytes actually killed. Thus, the NK cell's ability to bind targets cannot be equated to the ability to kill. The nature of the noncytotoxic binding lymphocytes is discussed. The kinetics of lysis by NK cells show a random distribution of killing in relationship to time and plateaus at 3--4 hours. The frequency of killing is the same for a wide variety of populations tested at different times. We conclude that some of the target binding cells are antigen specific NK cells that can be converted to cytotoxic natural killer cells.