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Cell lines with natural killer (NK) activity grown from native spleen cells cultured in medium conditioned by spleen cells proliferating in the presence of concanavalin A (Con A) were characterized. One NK cell line was cloned and assayed on several human and mouse NK-sensitive targets to analyze whether target specificities segregate upon cloning. Results showed that NK clones display target specificities identical to NK cells in normal spleen. This suggests that NK cells have no clonally distributed specific receptors to a given target. They may, however, have receptors which recognize identical antigens on all NK-sensitive targets or may possess multiple receptors for different target specificities. NK lines could not be demonstrated to possess activity in antibody-dependent cell-mediated cytotoxicity, nor did they effect mutual lysis. In the presence of Con A, NK cells exhibited dramatically enhanced lysis of NK-sensitive targets but only a slight increase in lysis of NK-insensitive targets. This indicates that the degree of lysis of an NK target is a function of two variables: effector binding to the target and target sensitivity to lysis. Furthermore, it suggests that the affinity of the putative antigen receptors on NK effectors must be rather weak. Cell surface marker analysis reveals that NK cell lines are Thy 1.2+, Lt-1-2-, T200+, asialo GM1+, and asialo GM2+. These markers distinguish NK cells from cytolytic thymus-derived lymphocytes, without resolving the question of classification within a give hematopoietic cell lineage.

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