Cytotoxic activities of recombinant immunotoxins composed of Pseudomonas toxin or diphtheria toxin toward lymphocytes from patients with adult T-cell leukemia

Abstract

We have previously shown that the recombinant single-chain immunotoxin anti-Tac(Fv)-PE40, composed of the variable domains of the anti-Tac monoclonal antibody in a single-chain form joined to a derivative of Pseudomonas exotoxin (PE), is cytotoxic toward malignant cells from adult T-cell leukemia (ATL) patients. Using this assay, we have now compared the activity of anti-Tac(Fv)-PE40 with that of an improved version, anti-Tac(Fv)-PE40KDEL which contains an altered carboxyl terminus, and also with two chimeric toxins made with diphtheria toxin (DT). One of these is a fusion of amino acids 1-388 of DT with anti-Tac(Fv) and is termed DT388-anti-Tac(Fv). The other, DT388-IL2, contains interleukin 2 (IL2) at the carboxyl terminus of the same DT derivative. We incubated these toxins with malignant ATL peripheral blood mononuclear cells (PBMCs) for 1-3 days and then measured [3H]leucine incorporation. We found that anti-Tac(Fv)-PE40KDEL was the most cytotoxic agent and was followed in decreasing order of activity by anti-Tac(Fv)-PE40, DT388-anti-Tac(Fv), and finally DT388-IL2. Trypan blue staining showed that inhibition of protein synthesis correlated with cell death. Time course studies showed that the recombinant toxins containing anti-Tac(Fv) were cytotoxic even if exposed to the cells for only one hour. After intravenous injection into mice, the half-life of anti-Tac(Fv)-PE40 or anti-Tac(Fv)-PE40KDEL was 30 minutes. Normal PBMCs were resistant to all four toxins. Recombinant immunotoxins made with anti-Tac merit further study as potential reagents in the treatment of ATL.