BCR-ABL Does Not Prevent Apoptotic Death Induced by Human Natural Killer or Lymphokine-Activated Killer Cells

Abstract

The erythromyeloid cell line, K562, the most sensitive target in human natural killer (NK) cell mediated cytotoxicity, is derived from a chronic myeloid leukemia (CML) patient and expresses the characteristic reciprocal translocation t(9;22). The resulting BCR-ABL fusion protein has been shown to mediate the unusual resistance of K562, and other BCR-ABL expressing lines, to apoptosis induced by a variety of agents (irradiation, UV light, cytotoxic drugs). Here we show that human NK and lymphokine-activated killer (LAK) cells, when tested at low effector to target ratio, can readily induce apoptotic death in K562 cells. This was accompanied with classical DNA oligonucleosomal fragmentation, an unexpected finding given the reported lack of such fragmentation when apoptosis is induced in K562 by chemical agents, after downregulation of BCR-ABL. Apoptosis was assessed by several means: morphological studies, 125I-DNA versus 51Cr release, DNA agarose gel electrophoresis, and results were always concordant, with a delayed kinetics for DNA oligonucleosomal fragmentation. Similar data were obtained with a pluripotent human hematopoietic cell line, UT-7, infected with a defective amphotropic p210 BCR-ABL retrovirus. The BCR-ABL expressing subclone UT-7/9, while being no longer sensitive to cytotoxic drugs or to tumor necrosis factor, a lytic mediator to which UT-7 cells are sensitive, underwent apoptotic death when exposed to LAK effector cells to the same degree as the parental UT-7 line. With these targets, DNA oligonucleosomal fragmentation occurred concomitantly with isotope release. Results obtained with several inhibitors of exocytosis strongly suggest that cytotoxic granules mediate NK and LAK cell-induced apoptotic death. In conclusion, NK and LAK cell-induced apoptotic signals, unlike those activated by chemotherapeutic agents, are unaffected by the antiapoptotic action of BCR-ABL. This unique property may support the observed curative effect of allogeneic bone marrow transplantation in CML.