When peripheral blood lymphocytes (PBL) are incubated with interleukin 2 (IL 2), a novel cytotoxic lymphocyte subpopulation, termed lymphokine activated killers (LAK), arises. LAK are functionally defined as IL 2 responsive cells demonstrating major histocompatibility antigenunrestricted cell-mediated cytotoxicity against fresh solid tumors and other natural killer cell-resistant and -sensitive tumor targets in the absence of prior antigen priming.

Flow cytometric analysis of IL 2 activated PBL using forward and right angle light scatter and fluorescence intensity identified the emergence of a large, optically dense, autofluorescent cell population which paralleled the generation of LAK activity. These unique IL 2 induced lymphocytes have been named giant autofluorescent lymphocytes (GAL). These cells are readily distinguished from the small nonfluorescent lymphocytes (SNL) observed in fresh PBL, unstimulated cultured PBL, and those cells remaining after incubation with IL 2 which have not acquired GAL characteristics. In this investigation, LAK cultures were sorted on days 4, 5, and 6 into GAL and SNL populations and were tested for oncolytic activity against the natural killer-resistant Daudi and RC-1 tumor targets. Against these targets, lymphocytes from non-IL 2 activated PBL or the sorted SNL population expressed less than 2% of the oncolytic activity (measured in lytic units) exhibited by GAL effectors. The SNL and GAL populations were cultured in IL 2 for up to 48 h following the sorting procedure and then reassayed for tumor cytolytic activity. During this culture period, GAL but not SNL continued to express LAK killing against natural killer-resistant tumor targets. Using γ-irradiation to prevent further cell cycling, it was shown that the functional half-life of the LAK effector was approximately 8.5 h. Therefore, the cytotoxicity expressed by the sorted GAL population after 48 h in culture (equivalent to five functional half-lives) must be expressed by progeny of the originally plated lymphocytes. These results indicate that in addition to the LAK effector, the GAL population contains a self-sustaining, recycling intermediate responsible for generating new LAK.

Our data indicate that analysis of IL 2 activated PBL using GAL light-scattering properties has application in phenotyping LAK, monitoring of cellular kinetics, cell sorting, and enrichment of the LAK effector population, and in the clinical monitoring of IL 2 therapy.

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Supported in part by NIH Grants NS18775 and HL30411 (L. E. D.) and VAMC Grant 111340132 (G. P. H.).

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