Impaired binding of perforin on the surface of tumor cells is a cause of target cell resistance against cytotoxic effector cells

Exocytosis of perforin, subsequent binding of perforin to the target cell m embrane, and formation of lytic pores form an important pathway involved in the induction of tumor cell death by cytotoxic effector cells. Here we des cribe a novel escape mechanism employed by tumor cells to protect themselve s from granule-mediated cell death: We were able to demonstrate that the re sistance of the human leukemia cell line ML-2 to natural killer (NK)-cell-m ediated killing is not caused by impaired NK-cell activation but by resista nce against effector molecules contained in the granules of cytotoxic cells . No resistance was observed against other pore-forming agents like complem ent and streptolysin O, By using the NK-susceptible leukemia cell line K562 , we could show that the induction of cell death by cytotoxic granules can be blocked completely by anti-perforin antibodies, indicating that perforin is essentially involved in this process. Flow cytometric data revealed tha t an impaired binding of perforin on the tumor cell membrane is mainly resp onsible for target cell resistance, because perforin turned out to bind wel l on K562 cells but Is not able to attach to the surface of ML-2 cells. Aft er impaired binding of perforin was identified as a potential mechanism of tumor cell resistance, leukemia cells from 6 patients with acute myeloid le ukemia (AML) were examined. As predicted, AML cells that failed to bind per forin on their surface demonstrated complete resistance toward NK-cell-medi ated cytotoxicity. Thus, perforin resistance could represent an important t umor escape mechanism that should be considered when cytotoxic effector cel ls are used for cellular immunotherapy. (C) 2000 by The American Society of Hematology.