HIGH LYTIC ACTIVITY AGAINST HUMAN LEUKEMIA-CELLS AFTER ACTIVATION OF ALLOGENEIC NK CELLS BY IL-12 AND IL-2

IL-12 is a novel cytokine with interesting features regarding its pote ntial usefulness in peripheral blood stem cell transplantation and leu kemia immunotherapy. We used cryopreserved leukemia cells of 18 patien ts with acute myelogenous (n = 14) or lymphocytic (n = 4) leukemia to investigate the effect of IL-12, alone or in combination with IL-2, on the cytolytic activity of NK cells against human leukemia targets. Ef fector cells were peripheral blood mononuclear cells from healthy dono rs which were depleted from CD3(+) T cells by immunomagnetic separatio n. CDS-negative effector cells (mainly CD56(+) NK cells) were treated for 24 h with various concentrations of IL-2 (100 U/ml to 1000 U/ml) a nd IL-12 (1 U/ml to 100 U/ml). Cytotoxicity was measured in a 4 h Cr-5 1-release assay. Whereas a two-fold enhancement of cytotoxic activity was observed after incubation with optimal doses of IL-2 or IL-12, the combination of both cytokines (500 U/ml IL-2, 100 U/ml IL-12) increas ed the lytic activity more than six-fold. This effect was accompanied by increased expression of cellular adhesion molecules (CD2, CD18) and CD25 on CD56(+) effector cells. Of 18 leukemias investigated, five we re completely resistant to lysis by effector cells activated with IL-2 or IL-12 alone. In three of these five cases, however, high cytolytic activity was observed after coincubation with IL-2 and IL-12. In comp arison to allogeneic NK cells, autologous cells of three patients in r emission demonstrated significantly lower cytotoxic activity. No killi ng of nonmalignant cells (PMA blasts) by allogeneic NK cells was obser ved. Our data demonstrate that IL-12 can enhance or even induce MHC-un restricted cytotoxicity of IL-2-activated allogeneic natural killer ce lls. Since IL-12 has also been shown to have stem-cell mobilizing capa cities, it could be used for the recruitment of both stem cells and an tileukemic effector cells in the context of peripheral blood stem cell transplantation.