Increased Circulating Activated T-Cells in Lung Cancer*

T-cell activation (Tac) antigens, which are closely associated with the receptors for interleukin 2 (IL 2) and expressed on activated human T-lymphocytes, are found on a small percentage of normal peripheral T-cells. Elevated levels of Tac antigen-positive (Tac + ) cells were observed in a high proportion of patients with untreated primary lung cancer assessed by using monoclonal anti-Tac antibody. The mean percentage of Tac + cells in peripheral blood lymphocytes was 13.1 ± 6.4 percent in patients with primary lung cancer (n = 67), as compared with 4.3 ± 1.9 percent in normal controls (n = 30) (p + cells. Our results suggest that T-cell-mediated active immune mechanisms against malignant cancer cells are operative in patients with lung cancer, resulting in an increase in activated T-cells in the peripheral blood, although it remains to be elucidated whether these activated T-cells exert a favorable or unfavorable effect on their host.