The carboxyl terminal amino acid residues of Pseudomonas aeruginosa exotoxin A involved in cell toxicity and pathogenesis, characterized by a neutralizing human monoclonal antibody.

Human monoclonal antibody HI-1A4 (IgG3, lambda) neutralized a toxicity caused by pseudomonal exotoxin A (Ex-A) in cell culture and in vivo, and was effective in experimental Pseudomonas aeruginosa infections in mice. HI-1A4 inhibited an Ex-A catalyzed ADP-ribosylation of elongation factor 2 but did not inhibit an incorporation of toxin into a target cell at all. One molecule of HI-1A4 neutralized at least 2 molecules of Ex-A. HI-1A4 retained its binding activity at pH 4.0. The epitope region for HI-1A4 was demonstrated to be a carboxyl terminal end of amino acid residues 591-613 of Ex-A. HI-1A4 might bind to Ex-A carboxyl terminal region outside a target cell, be incorporated into cells as a complex with Ex-A, and inhibit the intracellular function in which the carboxyl terminal part of Ex-A was involved, resulting in the interruption of intoxication of Ex-A.