Human lymphokine-activated killer (LAK) cells: Identification of two types of effector cells

We analyzed the antigenic phenotype of lymphokine-activated killer (LAK) effector cells. Human blood lymphocytes were cultured for 3 days with 100 U/ml recombinant interleukin 2 (rIL 2), subpopulations isolated with monoclonal antibodies and a fluorescence-activated cell sorter (FACS) and assayed for cytotoxic activity against ⁵¹chromium labeled noncultured melanoma tumor cells. Initial experiments compared the LAK effector function of CD5⁺ T lymphocytes vs CD5^- cells (predominantly CD16⁺ NK cells). The mean percent specific release at a 10:1 effector:target (E:T) ratio was 25% ± 16 for CD5^- cells, 10% ± 6 for CD5⁺ cells, and 22% ± 9 for unsorted cells. In contrast, when lymphocyte subpopulations were isolated before rIL 2 culture (LAK precursors), CD5^- cells but not CD5⁺ cells developed LAK activity (28% ± 12 vs 1% ± 1, mean percent specific release, 10:1 E:T ratio), confirming our previous results showing that only CD16⁺ cells were LAK precursors. The discrepancy between LAK effector and precursor phenotypes suggested that LAK precursors acquired CD5 determinants during rIL 2 culture; however, double label immunofluorescence of rIL 2 cultured CD16⁺ cells showed that this was not the case. The data suggested that in the presence of other cell types, some T lymphocytes may develop LAK activity, but purified blood T lymphocytes do not develop LAK function when cultured with rIL 2 alone. We also analyzed LAK effector function in lymphocyte subpopulations defined by CD4 and CD8 antigens. The data showed that lymphocytes with a low density expression of CD8 and no expression of CD4 were enriched for LAK effector cells, whereas CD4⁺ and CD8^- had less activity than unsorted cells. Lymphocytes with a high density expression of CD8 had activity similar to unsorted cells. We also assessed the contribution of Leu-7 (HNK-1) granular lymphocytes to LAK effector function. After culture with IL 2, lymphocytes were depleted of Leu-7⁺ cells by antibody and complement treatment and then were sorted into CD5⁺ and CD5^- fractions. The cytotoxic activity of Leu-7^- CD5⁺ cells was a mean 5% ± 5 vs a mean 14% ± 8 for the total CD5⁺ population (20:1 E:T ratio). The activity of Leu-7^- CD5^- was slightly less than the total CD5^- fraction (21% ± 9 vs 28% ± 14, 10:1 E:T ratio). In conclusion, LAK effector function was highest in non-T cell (CD5^- CD16⁺) populations and some activity was also present in T cell populations (CD5⁺ and predominantly Leu-7⁺).