Analysis of the murine lymphokine-activated killer (LAK) cell phenomenon: dissection of effectors and progenitors into NK- and T-like cells.

Murine as well as human lymphokine-activated killer (LAK) cells have been reported to have several characteristics of T lymphocytes and to be clearly distinct from natural killer (NK) cells. The present study of murine LAK cells showed that cytotoxic cells generated in the presence of interleukin 2 IL 2 were heterogeneous with respect to cell surface markers of progenitor as well as effector cells. Negative selection of cells with antibodies and complement or positive selection by fluorescence-activated cell sorting unequivocally showed that LAK effector cells consisted of at least two clearly distinct populations, the relative contribution of which was dependent on donor organ and target cells studied. Approximately 40% of the cytotoxic activity of spleen-derived effector cells active against the NK-resistant targets EL-4 or MCA-5 was eliminated by treatment with antibodies to the NK-markers asialo-GM1 and NK 1 (NK-LAK). Approximately 60% of cytotoxic activity was associated with cells expressing the T cell marker Lyt-2, lacked NK 1, and was lacking or expressed only small amounts asialo-GM1 (T-LAK). The NK-LAK cells were of greater importance for the cytotoxic activity against the standard NK target YAC-1, although T-LAK cells also excerted significant cytotoxicity against this cell line. Limiting dilution analysis estimated that the minimal frequency of precursors developing into cells with cytotoxic activity against EL-4 was 1/6700 in spleen and 1/4200 in peripheral blood. The frequency of cells developing into cytotoxic effectors against YAC-1 cells was 1/3700 and 1/1450 in spleen and peripheral blood, respectively. Depletion of progenitor cells from spleen or peripheral blood expressing NK 1 or Lyt-2 by treating the cells with antibodies to these structures and complement indicated that NK-1-expressing cells were the dominating progenitor of the LAK cells irrespective of target cells used. Culture of murine lymphoid cells from spleen or peripheral blood with high concentrations of IL 2 results in the emergence of two different killer cell populations with phenotypic similarities to NK and T cells, respectively, both being able to kill targets resistant to resting NK cells. In contrast to numerous earlier reports, we concluded that LAK cells are heterogeneous with respect to surface markers, with a major population of LAK cells apparently representing IL 2-activated cells expressing cell surface markers associated with NK cells.