Background. Lymphokine‐activated killer (LAK) cell function can be generated in peripheral blood mononuclear cells (PBMC) after brief exposure of high dose interleukin‐2 (IL‐2) over the course of 1 or 2 days' culture in plain culture medium (IL‐2–pulsed PBMC). The aim of the present study was to investigate the ability of granulocyte‐macrophage–colony stimulating factor (GM‐CSF) to augment LAK induction in low dose IL‐2–pulsed PBMC derived from patients with cancer undergoing immunotherapy with IL‐2.

Methods. Peripheral blood mononuclear cells were collected from patients with cancer receiving a 5‐day cycle of local (intraperitoneal or intrapleural) infusions with IL‐2. The cells were incubated with IL‐2 in the presence or absence of GM‐CSF for 1 hour and then tested as effectors against allogeneic tumor cells and LAK‐sensitive cell lines.

Results. Granulocyte‐macrophage–colony stimulating factor at doses between 10 and 100 ng/ml was synergized with low dose IL‐2 (100 IU/ml) in the generation of LAK activity in PBMC. Lymphokine‐activated killer cell‐mediated cytotoxicity derived from PBMC cultures incubated with IL‐2 and GM‐CSF was significantly higher (up to three‐fold) compared with that generated with IL‐2 alone. The GM‐CSF‐induced enhanced LAK activity was maintained when tested at day 5. GM‐CSF increased the percentages of IL‐2 receptor (R) positive (⁺) and CD8⁺ cells in the IL‐2–pulsed PBMC. In contrast to CD56+ cells, highly purified CD8+ cells isolated from PBMC pulsed with IL‐2 and GM‐CSF responded with increased LAK activity, thus representing the cell‐type that mediates the augmenting effect of GM‐CSF. Major histocompatibility complex (MHC) molecules or the CD3 surface antigens were not involved in the GM‐CSF–mediated enhancement of LAK induction because anti‐MHC class I and class II monoclonal antibodies (MoAb) or MoAb against the CD3 molecules remained without any effect in this system. The GM‐CSF‐mediated LAK‐enhancement was IL‐2–dependent because MoAb against IL‐2 receptor completely inhibited the generation of LAK activity.

Conclusions. The use of GM‐CSF for the enhancement of IL‐2–induced LAK activity in 1 hour cultures may improve clinical results in cancer immunotherapy. In addition, implementation of this procedure could eliminate the high cost of cell culture which usually accompanies IL‐2/LAK cell therapy as well as eliminate the known toxic side effects associated with this kind of therapy.