Lymphoproliferative activity of Pseudomonas exotoxin A is dependent on intracellular processing and is associated with the carboxyl-terminal portion.

Pseudomonas aeruginosa exotoxin A (PE) represents a microbial superantigen that requires processing by accessory cells in order to induce the proliferation of V beta 8-bearing murine T lymphocytes. In this study, we have observed that PE requires intracellular processing by a protease in order to induce lymphoproliferation. Pepstatin A, an inhibitor of acid proteases, inhibited PE-induced lymphoproliferation, whereas leupeptin, an inhibitor of serine and thiol proteases, had no effect on PE-induced lymphoproliferation. A number of mutant forms of PE were examined for their ability to induce lymphoproliferation. The mutant form which lacks amino acids 5 to 224 of the receptor-binding domain, PE43, was capable of inducing murine thymocytes to proliferate in the presence of accessory cells. However, neither PEgly276, a mutant toxin which undergoes a different intracellular processing pattern than wild-type PE, nor PE589, a mutant toxin which lacks amino acids 590 to 613 at the carboxyl terminus, was able to induce thymocyte proliferation. In addition, the lymphoproliferation induced by the PE43 mutant form of PE could also be inhibited by pepstatin A. Therefore, our data indicate that intracellular processing by a proteolytic enzyme which is inhibited by pepstatin A is critical for PE-induced lymphoproliferation. Furthermore, the lymphoproliferative activity of PE is associated with the carboxyl-terminal portion of PE.