Immunotoxins show rapid entry of diphtheria toxin but not ricin via the T3 antigen

We compared immunotoxins made with ricin and diphtheria toxin (DT) and with two monoclonal antibodies against different T cell-specific antigens, CD5 (T101) and CD3 (UCHT1). Only one reagent, UCHT1 linked to DT (UCHT1-DT), had exceptional properties. UCHT1-DT killed human peripheral T cells and T leukemia cells (Jurkat) at 2 to 10 pM, a concentration 10- to 100-fold lower than UCHT1-ricin and 10 to 500 times lower than native DT. The toxicity was blocked 50- to 100-fold by excess UCHT1 antibody. Human multipotent stem cells were not killed at up to 2000 pM UCHT1-DT. UCHT1-DT shows greater selectivity between T cells and stem cells than UCHT1-ricin, and may better prevent graft-vs-host disease in allogeneic bone marrow transplantation. The kinetics of UCHT1-DT were extremely rapid. UCHT1-DT inhibited Jurkat cell protein synthesis faster than DT and had a different ratio of lag time to inactivation rate. UCHT1-DT killed 90% of Jurkat cells within 2 hr at concentrations nontoxic to human stem cells. In contrast, UCHT1-ricin required more than 18 hr to kill one log of Jurkat cells. Another monoclonal antibody, T101, against the 65 kD CD5 antigen on Jurkat cells was linked to DT and ricin, and was compared with the UCHT1 immunotoxins. UCHT1-DT was 100 times more potent and five to 10 times faster than T101-DT and T101-ricin. Standardization to other antibodies with regard to the number of molecules bound per cell shows that UCHT1-DT is 10 to 100 times faster than previously reported immunotoxins. The role of the T3 antigen in transporting DT to the cytosol is discussed