A recombinant single-chain immunotoxin composed of anti-Tac variable regions and a truncated diphtheria toxin.

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Abstract
To kill human or primate cells expressing the p55 subunit of the interleukin 2 receptor, we have constructed a single-chain immunotoxin. DNA sequences encoding the first 388 amino acids of diphtheria toxin (DT) were fused to DNA elements encoding the antigen-binding portion (variable region or Fv) of the anti-Tac monoclonal antibody. The antigen-binding portion consists of 116 amino acids of the heavy-chain variable region connected by a 15-amino acid linker to 106 amino acids of the variable region of the light chain. The single-chain immunotoxin DT388-anti-Tac(Fv) was expressed in Escherichia coli and found in inclusion bodies. The monomeric form was then purified to near homogeneity with a high yield (3-5 mg/liter). Monomeric DT388-anti-Tac(Fv) was highly cytotoxic to cell lines bearing the p55 subunit of the human interleukin 2 receptor but not to cells without this subunit. DT388-anti-Tac(Fv) was also effective in killing proliferating human T cells produced in a mixed leukocyte reaction.