Identification of human natural killer soluble cytotoxic factors (NKCF) derived from NK-enriched lymphocyte populations: specificity of generation and killing.

Activated natural killer (NK) cells were induced by stimulation of normal peripheral blood lymphocytes with a B cell line for 5 days or by interferon activation for 1 hr. They were enriched for large granular lymphocytes (LGL) by separation on Percoll density gradients. Supernatants were obtained from incubation mixtures of LGL and NK-sensitive target cells (MOLT-4 or K562). Although some lysis could be detected at 16 hr, optimal NK-sensitive target cell lysis by this supernatant was evident after 40 hr incubation by using a microcytotoxicity assay and trypan blue exclusion. Supernatants derived by co-culture of NK targets with either NK-depleted cell populations or NK cells alone were not cytolytically active. Target cell requirements for both the generation and cytotoxic activity of NKCF were specific. That is, only the NK-sensitive target cells MOLT-4 and K562, and not the NK-resistant cells YAC-1, EL4, IM9, and Raji, could induce release of, or be lysed by, NKCF. Furthermore, the requirement for both Ca2+ and Mg2+ in the generation of NKCF suggests that both target cell binding and initiation of programming for lysis are required for its release. NKCF lytic potential was stable for 2 mo at -20 degrees C and partially stable at 60 degrees C. NKCF activity could be substantially removed by prior absorption on the NK-sensitive target cells, K562 or MOLT-4, but not Raji or IM9 (resistant) cells. Therefore, it appears that the specificity of this cellfree lytic factor depends on its recognition of, or its being recognized by, appropriate target cells. The properties of NKCF will enable the study of the mechanism involved in the lethal hit of NK cell-mediated cytotoxicity.