Effector cells in carcinomatous pleural
effusions of patients with primary or secondary lung cancer were examined for natural killer (NK) activity against K562 cells in a 4-h chromium release assay, and for mitogenic responses and lymphocyte subpopulation constitutions. NK activity of lymphocyte-rich mononuclear cells isolated from carcinomatous pleural effusions by centrifugation on a discontinuous gradient of Ficoll-Hypaque was markedly low in seven of 4O patients studied, and absent in the other 33 cases. NK activity of peripheral blood mononuclear cells from the patients was lower than that of cells from normal donors, but always higher than that of effusion cells from the same patients. N K cells in the peripheral blood and in pleural effusions had some characteristics in common, in that they lacked a capacity to bind sheep erythrocytes, were nonadherent to Sephadex G-IO beads and nylon wool, and belonged to large granular lymphocytes. On the other hand, nonmalignant effusions of patients" with congestive heart failure had significant N K activity. The effector cells in the effusions included a higher frequency of T cells than those in the peripheral blood of the same patients. Proliferative responses to phytohemagglutinin and concanavalin A of effusion cells were comparable to those of normal blood cells and were higher than those of blood cells from the same patients. The reason for low N K activity and high mitogenic response in carcinomatous pleural effusions is as yet undefined.