Enumeration andCharacterization of Human Killer and Natural Killer Cells by a Modified Single‐Cell Assay

Human natural killer (NK) and killer (K) cells were assayed in a modified single‐cell cytotoxicity assay using poly‐L‐lysine‐coated cover slips. When human Chang liver cells were used as targets, 20% of the lymphocytes formed conjugates and 2% were active NK cells. When anti‐Chang antibodies were present, the proportion of target‐binding cells (TBC) increased to 30% and that of the cytotoxic effector cells (comprising NK + K) to 6%. With the mouse mastocytoma cells (P815), which are not susceptible to NK, similar proportions of lymphocytes formed conjugates, and 6–9% were active as K cells. By an in situ rosetting assay a significant fraction of the TBC and cytotoxic effector cells bound either C3b or C3bi in both systems, with a certain predominance of C3bi‐binding cells among the K cells. However, by indirect immunofluorescence, significantly more OKT3⁺ cells than OKM1⁺ cells were TBC or cytotoxic in the Chang cell system, whereas the OKT3⁺/OKM1⁺ ratios for both TBC and cytotoxic cells were 1:1 in the mouse mastocytoma system. The results indicate that TBC, NK and K cells are heterogeneous with respect to surface marker expression and that effector cells of different phenotypes predominate in different target systems.