The recombinant immunotoxin anti-Tac(Fv)-Pseudomonas exotoxin 40 is cytotoxic toward peripheral blood malignant cells from patients with adult T-cell leukemia.
 Anti-Tac(Fv)-PE40 is a recombinant single-chain immunotoxin containing the heavy and light variable regions of the anti-Tac monoclonal antibody fused to a mutant form of Pseudomonas exotoxin (PE).
 Anti-Tac binds to the p55 subunit of the human interleukin 2 (IL-2) receptor, and anti-Tac(Fv)-PE40 kills human or monkey cell lines that contain either the intact IL-2 receptor or its p55 subunit alone.
 To assess the usefulness of anti-Tac(Fv)-PE40 in treatment of IL-2 receptor-positive leukemia, we tested peripheral blood mononuclear cells from six patients with adult T-cell leukemia.
 In each of the six patients, anti-Tac(Fv)-PE40 was extremely cytotoxic to the malignant cells.
 Metabolic activity and sensitivity of the fresh cells improved when a small amount of IL-2 (10 units per ml) was present during incubation.
 The toxin concentration necessary to inhibit protein synthesis by 50% after 16-hr incubation of cells with immunotoxin varied from 1.6 to 16 ng/ml (2.5-25 x 10(-11) M).
 In every case, binding was by means of the Tac antigen because anti-Tac(Fv)-PE40 cytotoxicity was prevented by adding excess anti-Tac antibody.
 Moreover, anti-Tac alone or an inactive mutant of anti-Tac(Fv)-PE40 without ADP-ribosylation activity had very little cytotoxic activity.
 Peripheral blood mononuclear cells from normal controls, from a patient with Tac-negative leukemia, and from adult T-cell leukemia patients without significant peripheral blood involvement were not sensitive to anti-Tac(Fv)-PE40.
 These results indicate that anti-Tac(Fv)-PE40 is a potent cytotoxin against adult T-cell leukemia cells in vitro and warrants clinical testing.
