[Infiltrative and cytolytic activities of lymphokine-activated killer (LAK) cells against a human glioma mass: ultrastructural analysis using a three-dimensional multicellular spheroid model].

In the present study, we have investigated not only the infiltrative and cytotoxic activities of lymphokine-activated killer (LAK) cells on a tumor mass, but also the ultrastructural cell-to-cell interaction between LAK effector cells and target tumor cells during the cytolytic process within a three-dimensional solid tumor. A multicellular tumor spheroid (MTS) of a human malignant glioma cell line (U-251MG) was utilized as a solid tumor model. LAK cells were generated from peripheral blood lymphocytes (PBL) of a healthy donor after 4-day culture in the presence of interleukin-2 (IL-2). MTSs of 500 microns diameter were cocultivated with either LAK cells or non-activated PBL, and then time-sequential kinetic, morphological, and ultrastructural examinations were carried out. It was demonstrated that the number of viable tumor cells present within MTSs gradually decreased in parallel with the increase in the number of LAK cells. Morphological analyses revealed that LAK cells directly infiltrated toward the inner areas of MTSs and caused a progressive tumor destruction. In contrast, PBL hardly exhibited such activities. Ultrastructurally, it was found that the infiltrating LAK effector cells were composed of heterogeneous subpopulations, T-like cells and large granular lymphocyte (LGL)-like cells, and that both types of lymphocytes tightly adhered to the tumor cells and extended their cytoplasmic extensions deeply into the targets which underwent a progressive degeneration. Concerning the cellular interaction, it was found that these two kinds of LAK cells displayed some distinct ultrastructural feature in the process of target cell killing. Particularly, it should be stressed that LGL-like LAK cells exhibited a significant development of the intracytoplasmic secretory granules, suggesting an association with the lethal hit of target cell lysis.